<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//TaxonX//DTD Taxonomic Treatment Publishing DTD v0 20100105//EN" "https://biogeography.pensoft.net/nlm/tax-treatment-NS0.dtd">
<article xmlns:tp="http://www.plazi.org/taxpub" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mml="http://www.w3.org/1998/Math/MathML" article-type="research-article" xml:lang="en">
  <front>
    <journal-meta>
      <journal-id journal-id-type="publisher-id">127</journal-id>
      <journal-id journal-id-type="index">urn:lsid:arphahub.com:pub:15f175fc-9690-5d13-a2ea-114d8a2e74bd</journal-id>
      <journal-title-group>
        <journal-title xml:lang="en">Frontiers of Biogeography</journal-title>
        <abbrev-journal-title xml:lang="en">biogeography</abbrev-journal-title>
      </journal-title-group>
      <issn pub-type="epub">1948-6596</issn>
      <publisher>
        <publisher-name>Pensoft Publishers</publisher-name>
      </publisher>
    </journal-meta>
    <article-meta>
      <article-id pub-id-type="doi">10.21425/fob.19.167957</article-id>
      <article-id pub-id-type="publisher-id">167957</article-id>
      <article-categories>
        <subj-group subj-group-type="heading">
          <subject>Research Article</subject>
        </subj-group>
        <subj-group subj-group-type="biological_taxon">
          <subject>Angiospermae</subject>
          <subject>Gymnospermae</subject>
          <subject>Plantae</subject>
        </subj-group>
        <subj-group subj-group-type="scientific_subject">
          <subject>Biogeography</subject>
          <subject>Diversity</subject>
          <subject>Ecology</subject>
          <subject>Functional</subject>
          <subject>Functional ecology</subject>
          <subject>Island biogeography</subject>
          <subject>Phylogenetic</subject>
          <subject>Taxonomic</subject>
        </subj-group>
        <subj-group subj-group-type="geographical_area">
          <subject>Southern Africa</subject>
        </subj-group>
      </article-categories>
      <title-group>
        <article-title>Diverging responses of taxonomic, functional, and phylogenetic plant diversity to biogeographic parameters in arid habitat islands of southern Africa</article-title>
      </title-group>
      <contrib-group content-type="authors">
        <contrib contrib-type="author" corresp="yes">
          <name name-style="western">
            <surname>Bürger</surname>
            <given-names>Alexander M.</given-names>
          </name>
          <email xlink:type="simple">a.buerger@em.uni-frankfurt.de</email>
          <uri content-type="orcid">https://orcid.org/0009-0005-0869-7732</uri>
          <xref ref-type="aff" rid="A1">1</xref>
          <xref ref-type="aff" rid="A2">2</xref>
          <role content-type="http://credit.niso.org/contributor-roles/conceptualization/">Conceptualization</role>
          <role content-type="http://credit.niso.org/contributor-roles/writing-original-draft/">Writing - original draft</role>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/data-curation/">Data curation</role>
          <role content-type="http://credit.niso.org/contributor-roles/formal-analysis/">Formal analysis</role>
          <role content-type="http://credit.niso.org/contributor-roles/investigation/">Investigation</role>
          <role content-type="http://credit.niso.org/contributor-roles/methodology/">Methodology</role>
          <role content-type="http://credit.niso.org/contributor-roles/visualization/">Visualization</role>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Meyer</surname>
            <given-names>Katharina</given-names>
          </name>
          <uri content-type="orcid">https://orcid.org/0009-0000-0434-1492</uri>
          <xref ref-type="aff" rid="A3">3</xref>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/investigation/">Investigation</role>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Eibes</surname>
            <given-names>Pia M.</given-names>
          </name>
          <uri content-type="orcid">https://orcid.org/0000-0002-6727-4204</uri>
          <xref ref-type="aff" rid="A1">1</xref>
          <xref ref-type="aff" rid="A2">2</xref>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/formal-analysis/">Formal analysis</role>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Lauel</surname>
            <given-names>Charlotte</given-names>
          </name>
          <xref ref-type="aff" rid="A1">1</xref>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/investigation/">Investigation</role>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Oldeland</surname>
            <given-names>Jens</given-names>
          </name>
          <uri content-type="orcid">https://orcid.org/0000-0002-7833-4903</uri>
          <xref ref-type="aff" rid="A4">4</xref>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/formal-analysis/">Formal analysis</role>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Schmiedel</surname>
            <given-names>Ute</given-names>
          </name>
          <uri content-type="orcid">https://orcid.org/0000-0003-4059-6585</uri>
          <xref ref-type="aff" rid="A3">3</xref>
          <xref ref-type="aff" rid="A5">5</xref>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/funding-acquisition/">Funding acquisition</role>
          <role content-type="http://credit.niso.org/contributor-roles/investigation/">Investigation</role>
          <role content-type="http://credit.niso.org/contributor-roles/project-administration/">Project administration</role>
          <role content-type="http://credit.niso.org/contributor-roles/supervision/">Supervision</role>
        </contrib>
        <contrib contrib-type="author" corresp="no">
          <name name-style="western">
            <surname>Irl</surname>
            <given-names>Severin D. H.</given-names>
          </name>
          <uri content-type="orcid">https://orcid.org/0000-0002-1734-8607</uri>
          <xref ref-type="aff" rid="A1">1</xref>
          <xref ref-type="aff" rid="A2">2</xref>
          <role content-type="http://credit.niso.org/contributor-roles/conceptualization/">Conceptualization</role>
          <role content-type="http://credit.niso.org/contributor-roles/writing-review-editing/">Writing - review and editing</role>
          <role content-type="http://credit.niso.org/contributor-roles/formal-analysis/">Formal analysis</role>
          <role content-type="http://credit.niso.org/contributor-roles/funding-acquisition/">Funding acquisition</role>
          <role content-type="http://credit.niso.org/contributor-roles/methodology/">Methodology</role>
          <role content-type="http://credit.niso.org/contributor-roles/project-administration/">Project administration</role>
          <role content-type="http://credit.niso.org/contributor-roles/supervision/">Supervision</role>
        </contrib>
      </contrib-group>
      <aff id="A1">
        <label>1</label>
        <addr-line content-type="verbatim">Biogeography and Biodiversity Lab, Institute of Physical Geography, Goethe-University Frankfurt, Frankfurt am Main, Germany</addr-line>
        <institution>Institute of Plant Science and Microbiology, University of Hamburg</institution>
        <addr-line content-type="city">Hamburg</addr-line>
        <country>Germany</country>
        <uri content-type="ror">https://ror.org/00g30e956</uri>
      </aff>
      <aff id="A2">
        <label>2</label>
        <addr-line content-type="verbatim">Center of Interdisciplinary African Studies (ZIAF), Goethe-University Frankfurt, Frankfurt am Main, Germany</addr-line>
        <institution>Center of Interdisciplinary African Studies (ZIAF), Goethe-University Frankfurt</institution>
        <addr-line content-type="city">Frankfurt am Main</addr-line>
        <country>Germany</country>
        <uri content-type="ror">https://ror.org/04cvxnb49</uri>
      </aff>
      <aff id="A3">
        <label>3</label>
        <addr-line content-type="verbatim">Institute of Plant Science and Microbiology, University of Hamburg, Hamburg, Germany</addr-line>
        <institution>School for Climate Studies (SCS), Stellenbosch University</institution>
        <addr-line content-type="city">Stellenbosch</addr-line>
        <country>South Africa</country>
        <uri content-type="ror">https://ror.org/05bk57929</uri>
      </aff>
      <aff id="A4">
        <label>4</label>
        <addr-line content-type="verbatim">Institute for Globally Distributed Open Research and Education (IGDORE), Hamburg, Germany</addr-line>
        <institution>Institute of Physical Geography, Goethe-University Frankfurt</institution>
        <addr-line content-type="city">Frankfurt am Main</addr-line>
        <country>Germany</country>
      </aff>
      <aff id="A5">
        <label>5</label>
        <addr-line content-type="verbatim">School for Climate Studies (SCS), Stellenbosch University, Stellenbosch, South Africa</addr-line>
        <institution>Institute for Globally Distributed Open Research and Education (IGDORE)</institution>
        <addr-line content-type="city">Hamburg</addr-line>
        <country>Germany</country>
      </aff>
      <author-notes>
        <fn fn-type="corresp">
          <p>Corresponding author: Alexander M. Bürger (<email xlink:type="simple">a.buerger@em.uni-frankfurt.de</email>)</p>
        </fn>
        <fn>
          <p><bold>Editors</bold> Marcus Vinicius Cianciaruso</p>
        </fn>
      </author-notes>
      <pub-date pub-type="collection">
        <year>2026</year>
      </pub-date>
      <pub-date pub-type="epub">
        <day>18</day>
        <month>02</month>
        <year>2026</year>
      </pub-date>
      <volume>19</volume>
      <elocation-id>e167957</elocation-id>
      <uri content-type="arpha" xlink:href="http://openbiodiv.net/BB36A64C-AF46-51FF-8546-E80FBA16274D">BB36A64C-AF46-51FF-8546-E80FBA16274D</uri>
      <uri content-type="zenodo_dep_id" xlink:href="https://zenodo.org/record/0">0</uri>
      <history>
        <date date-type="received">
          <day>19</day>
          <month>08</month>
          <year>2025</year>
        </date>
        <date date-type="accepted">
          <day>27</day>
          <month>01</month>
          <year>2026</year>
        </date>
      </history>
      <permissions>
        <copyright-statement>Alexander M. Bürger, Katharina Meyer, Pia M. Eibes, Charlotte Lauel, Jens Oldeland, Ute Schmiedel, Severin D. H. Irl</copyright-statement>
        <license license-type="creative-commons-attribution" xlink:href="http://creativecommons.org/licenses/by/4.0/" xlink:type="simple">
          <license-p>This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.</license-p>
        </license>
      </permissions>
      <abstract>
        <label>Abstract</label>
        <p>Habitat islands conceptually bridge true islands and habitat patches, adding new perspectives to island biogeographic principles that shape community assembly processes. Despite growing interest, empirical studies are scarce and the drivers of biodiversity in these systems, such as different aspects of island shape, isolation and habitat diversity, remain unclear. Our aim was to assess how biogeographic parameters influence taxonomic, functional, and phylogenetic plant diversity on habitat islands using quartz islands in South Africa as our model system. We conducted floristic surveys on 20 quartz islands in the harsh and arid summer-rainfall region around Pofadder (Nama–Karoo, South Africa) to quantify species richness, functional dispersion, and phylogenetic mean pairwise distance. Via remote sensing, we derived island metrics and evaluated their effects on the diversity metrics, using multivariate regression analysis. We fitted negative binomial generalized linear models for species richness and linear models for functional and phylogenetic diversity. Island area emerged as the strongest predictor of species richness, consistent with island biogeographic theory. Connectivity was a significant predictor for functional dispersion and explained most of the variation. The mean pairwise distance was significantly influenced solely by the area of the nearest neighbour island, although its explanatory power was only moderate. Our findings highlight that different facets of diversity respond distinctively to biogeographic parameters. Integrating taxonomic, functional, and phylogenetic perspectives is therefore essential for a comprehensive understanding of biodiversity patterns on habitat islands and for providing valuable insights for conservation.</p>
        <sec sec-type="Highlights" id="sec1">
          <title>Highlights</title>
          <boxed-text position="float">
            <list list-type="bullet">
              <list-item>
                <p>First biogeographic assessment of quartz habitat islands in a summer-rainfall region: 220 vascular plant species were recorded in the Pofadder archipelago (<tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Asteraceae">Asteraceae</tp:taxon-name-part></tp:taxon-name>, <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name> and <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Poaceae">Poaceae</tp:taxon-name-part></tp:taxon-name> most species-rich), and functional traits were measured for 134 perennial species.</p>
              </list-item>
              <list-item>
                <p>Species richness increased with island area (aligning with classic island biogeography) and decreased with increasing shape complexity.</p>
              </list-item>
              <list-item>
                <p>Functional and phylogenetic diversity varied little among islands, suggesting strong environmental filtering and broadly similar community composition on quartz islands.</p>
              </list-item>
              <list-item>
                <p>Functional diversity was significantly structured by island connectivity, whereas island area showed no detectable effect, potentially explained by trait redundancy in species-rich communities; phylogenetic diversity showed little response to biogeographic parameters.</p>
              </list-item>
              <list-item>
                <p>Divergent responses across taxonomic, functional and phylogenetic facets highlight the need for multidimensional biodiversity assessments in habitat islands.</p>
              </list-item>
            </list>
          </boxed-text>
        </sec>
      </abstract>
      <kwd-group>
        <label>Keywords</label>
        <kwd>Diversity metrics</kwd>
        <kwd>dryland ecology</kwd>
        <kwd>edaphic habitats</kwd>
        <kwd>functional traits</kwd>
        <kwd>island biogeography</kwd>
        <kwd>island size</kwd>
        <kwd>isolation</kwd>
        <kwd>Nama-Karoo</kwd>
        <kwd>quartz islands</kwd>
        <kwd>South Africa</kwd>
      </kwd-group>
      <funding-group>
        <funding-statement>Deutsche Forschungsgemeinschaft&#13;
Department of Environment and Nature Conservation (Northern Cape, SA)&#13;
</funding-statement>
      </funding-group>
    </article-meta>
  </front>
  <body>
    <sec sec-type="Introduction" id="sec2">
      <title>Introduction</title>
      <p>Although originally developed for true islands, the equilibrium theory of island biogeography (ETIB, <xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>) can also act as a valuable basic concept for habitat islands (<xref ref-type="bibr" rid="B58">Laurance 2008</xref>), which are spatially separated habitats distinct from the surrounding matrix (<xref ref-type="bibr" rid="B117">Whittaker and Fernández-Palacios 2007</xref>). Several types of habitat islands have been studied, including mountain tops (<xref ref-type="bibr" rid="B59">Lian et al. 2025</xref>), fog oases (<xref ref-type="bibr" rid="B32">Gonzales et al. 2023</xref>), steppe islands (<xref ref-type="bibr" rid="B20">Dembicz et al. 2020</xref>), and inselbergs (<xref ref-type="bibr" rid="B88">Sabino et al. 2025</xref>). Unlike true islands, the terrestrial matrix is not fully uninhabitable; while acting as a strong barrier for some habitat island species (e.g. specialists), it allows others (e.g. generalists) to persist, leading to taxon-dependent permeability (<xref ref-type="bibr" rid="B66">Matthews 2021</xref>). In most habitat islands there is no true mainland, and the matrix can partially act as a species source (<xref ref-type="bibr" rid="B41">Itescu 2019</xref>). The matrix thus functions simultaneously as isolation barrier, partial mainland substitute, and driver of diversity via edge effects (<xref ref-type="bibr" rid="B41">Itescu 2019</xref>; <xref ref-type="bibr" rid="B66">Matthews 2021</xref>).</p>
      <p>Historically, studies usually only assessed taxonomic diversity to test island size and isolation effects (<xref ref-type="bibr" rid="B41">Itescu 2019</xref>; <xref ref-type="bibr" rid="B66">Matthews 2021</xref>). However, focusing solely on species richness overlooks functional composition and trait-based environmental filtering (<xref ref-type="bibr" rid="B9">Cadotte et al. 2011</xref>; <xref ref-type="bibr" rid="B31">Garnier et al. 2016</xref>). In recent decades, many studies have moved beyond purely taxonomic approaches, incorporating functional diversity (<abbrev xlink:title="functional diversity">FD</abbrev>) and phylogenetic diversity (<abbrev xlink:title="phylogenetic diversity">PD</abbrev>) to capture other biodiversity dimensions and drivers (<xref ref-type="bibr" rid="B10">Cadotte et al. 2019</xref>; <xref ref-type="bibr" rid="B12">Cheng et al. 2025</xref>; <xref ref-type="bibr" rid="B113">Villa et al. 2025</xref>). Integrating <abbrev xlink:title="phylogenetic diversity">PD</abbrev> and <abbrev xlink:title="functional diversity">FD</abbrev> improves our understanding of the interplay between competition and environmental filtering in community assembly, although these metrics alone may still not fully disentangle how biotic and abiotic processes shape diversity patterns. Previous large-scale analyses of vegetation plot datasets have demonstrated that phylogenetic and functional diversity are frequently decoupled, contrary to the traditional expectation that closely related species should share similar traits (Večeřa et al. 2023; <xref ref-type="bibr" rid="B35">Hähn et al. 2025</xref>). This decoupling indicates that different ecological and historical assembly processes can shape these two dimensions of diversity. Functional traits may be evolutionarily labile or convergent, and <abbrev xlink:title="functional diversity">FD</abbrev> depends strongly on the subset of traits considered, whereas <abbrev xlink:title="phylogenetic diversity">PD</abbrev> reflects deeper lineage history and the regional evolutionary pool (<xref ref-type="bibr" rid="B35">Hähn et al. 2025</xref>). For example, historical dynamics such as in past refugial areas may have maintained multiple evolutionary lineages and thus preserved high <abbrev xlink:title="phylogenetic diversity">PD</abbrev>, whereas strong and persistent environmental filtering simultaneously reduced <abbrev xlink:title="functional diversity">FD</abbrev> via trait convergence. Conversely, a limited set of lineages might have diversified functionally in situ, increasing <abbrev xlink:title="functional diversity">FD</abbrev> without a comparable rise in <abbrev xlink:title="phylogenetic diversity">PD</abbrev> (<xref ref-type="bibr" rid="B110">Večeřa et al. 2023</xref>). Consequently, many researchers advocate for integrating multiple dimensions of diversity to gain a more comprehensive and multifaceted picture of ecological systems (<xref ref-type="bibr" rid="B64">Marcantonio et al. 2014</xref>; <xref ref-type="bibr" rid="B10">Cadotte et al. 2019</xref>; <xref ref-type="bibr" rid="B18">de Bello et al. 2021b</xref>).</p>
      <p>Nevertheless, the number of studies on habitat islands taking phylogenetic and functional diversity in addition to taxonomic diversity is still very limited (<xref ref-type="bibr" rid="B66">Matthews 2021</xref>). While the effects of environmental variables on plant diversity in specialized habitats have been studied in several systems (<xref ref-type="bibr" rid="B19">Delhaye et al. 2024</xref>; <xref ref-type="bibr" rid="B12">Cheng et al. 2025</xref>), the influence of island biogeographic attributes remains understudied. Beyond isolation and size, factors such as matrix characteristics, edge effects, land use and habitat diversity can strongly influence species assembly (<xref ref-type="bibr" rid="B58">Laurance 2008</xref>; <xref ref-type="bibr" rid="B39">Hortal et al. 2009</xref>; <xref ref-type="bibr" rid="B28">Fahrig 2013</xref>; <xref ref-type="bibr" rid="B25">Eibes et al. 2025</xref>).</p>
      <p><xref ref-type="bibr" rid="B25">Eibes et al. (2025)</xref> showed the importance of various biogeographic factors on taxonomic diversity for quartz islands in the winter-rainfall-dominated Knersvlakte (South Africa). Quartz islands are a unique example of edaphic habitat islands (<xref ref-type="bibr" rid="B24">Eibes et al. 2022</xref>), characterized by a dense cover of angular quartz gravel (<xref ref-type="bibr" rid="B94">Schmiedel and Jürgens 1999</xref>), sharp boundaries with the matrix (<xref ref-type="bibr" rid="B97">Schmiedel et al. 2015</xref>; <xref ref-type="bibr" rid="B23">Eibes et al. 2021</xref>) and their distinct vegetation (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>). They support a high diversity of endemic quartz specialists (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>; <xref ref-type="bibr" rid="B24">Eibes et al. 2022</xref>), which evolved via in-situ speciation on quartz islands (<xref ref-type="bibr" rid="B26">Ellis and Weis 2006</xref>; <xref ref-type="bibr" rid="B6">Boucher et al. 2024</xref>; <xref ref-type="bibr" rid="B124">Zhigila et al. 2024</xref>). This parallels patterns observed on true islands, where high rates of endemism are a typical phenomenon (<xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>; <xref ref-type="bibr" rid="B51">Kier et al. 2009</xref>).</p>
      <p>Quartz islands can be found in several biomes such as Fynbos, Succulent Karoo and Nama–Karoo across South Africa and southern Namibia covering a broad bioclimatic gradient (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>; <xref ref-type="bibr" rid="B96">Schmiedel and Mucina 2006</xref>; <xref ref-type="bibr" rid="B16">Curtis et al. 2013</xref>). The Pofadder region is, together with the Namibian Warmbad region, the only quartz island archipelago located within the summer-rainfall zone of the Nama–Karoo biome (<xref ref-type="bibr" rid="B92">Schmiedel 2002</xref>, <xref ref-type="bibr" rid="B95">2004</xref>). Whereas the winter-rainfall region is dominated by widespread gentle showers and coastal fog, summer rainfall comes in localized thunderstorms, is highly variable, and less predictable, leading to different requirements for plants (<xref ref-type="bibr" rid="B21">Desmet and Cowling 1999</xref>; <xref ref-type="bibr" rid="B72">Mucina et al. 2006b</xref>). While the Succulent Karoo biome (in the winter-rainfall zone) shows a high plant diversity, dominated by dwarf to low-growing leaf-succulent shrubs, the Nama–Karoo biome (mainly summer rainfall) generally has lower species richness and lacks the dominance of leaf succulents (<xref ref-type="bibr" rid="B15">Cowling et al. 1998</xref>).</p>
      <p>Previous studies have mostly focused on the quartz islands in the winter-rainfall or transition zone (<xref ref-type="bibr" rid="B94">Schmiedel and Jürgens 1999</xref>; <xref ref-type="bibr" rid="B96">Schmiedel and Mucina 2006</xref>; <xref ref-type="bibr" rid="B16">Curtis et al. 2013</xref>; <xref ref-type="bibr" rid="B97">Schmiedel et al. 2015</xref>; <xref ref-type="bibr" rid="B24">Eibes et al. 2022</xref>, <xref ref-type="bibr" rid="B25">2025</xref>; <xref ref-type="bibr" rid="B124">Zhigila et al. 2024</xref>). However, few studies have examined quartz islands in the summer-rainfall region (<xref ref-type="bibr" rid="B92">Schmiedel 2002</xref>, <xref ref-type="bibr" rid="B95">2004</xref>), and research on these systems remains scarce, with only limited ongoing work (<xref ref-type="bibr" rid="B125">Meyer et al. 2026</xref>). These quartz islands show a relatively species- and succulent-rich vegetation (comparable to the Succulent Karoo) within the Nama–Karoo biome (<xref ref-type="bibr" rid="B72">Mucina et al. 2006b</xref>), making it of considerable ecological interest. Our aim is to use this terrestrial island system to expand the limited knowledge about the role of various biogeographic parameters for different aspects of plant diversity on habitat islands. We postulate the following hypotheses:</p>
      <list list-type="simple">
        <list-item>
          <p>i. Taxonomic diversity is expected to increase with island area and habitat diversity, as larger islands provide more space and different habitats, facilitating the coexistence of more species (<xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>; <xref ref-type="bibr" rid="B66">Matthews 2021</xref>). In contrast, greater isolation is likely to reduce species richness by limiting the arrival of potential colonizers (<xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>; <xref ref-type="bibr" rid="B66">Matthews 2021</xref>).
</p>
        </list-item>
        <list-item>
          <p>ii. Functional diversity is predicted to increase with island area as well as habitat richness, as larger and more diverse islands can accommodate more species with a broader range of traits, thereby expanding the functional trait space (<xref ref-type="bibr" rid="B29">Ferreira-Arruda et al. 2022</xref>; <xref ref-type="bibr" rid="B99">Schrader et al. 2024</xref>). However, competitive exclusion could prevent functionally similar species from co-occurring on small islands, shifting the pattern toward functional overdispersion (<xref ref-type="bibr" rid="B122">Zhang et al. 2023</xref>). Isolation is expected to shape functional assembly of islands (<xref ref-type="bibr" rid="B78">Ottaviani et al. 2020</xref>) by limiting their colonization (<xref ref-type="bibr" rid="B75">Negoita et al. 2016</xref>). This could lead to clustering in the functional trait space on more isolated quartz islands if functional traits correlate with dispersal abilities. In contrast, traits only slightly connected (e.g. plant height) or not related to dispersal (describing leaves, epidermis and succulence) may not deviate from functional randomness with respect to isolation.
</p>
        </list-item>
        <list-item>
          <p>iii. We expect two alternative patterns for phylogenetic diversity. First, if the traits driving functional diversity are phylogenetically conserved, phylogenetic diversity should respond similarily, increasing with island area and habitat diversity, but decreasing with increasing isolation due to phylogenetic clustering (<xref ref-type="bibr" rid="B11">Carvajal-Endara et al. 2017</xref>; <xref ref-type="bibr" rid="B68">Matthews et al. 2020</xref>; <xref ref-type="bibr" rid="B52">Kim and Lee 2021</xref>). Second, if trait divergence has occurred mainly within a small set of lineages via in-situ functional diversification, functional and phylogenetic diversity may become decoupled and exhibit different relationships with area, habitat diversity, and isolation (<xref ref-type="bibr" rid="B110">Večeřa et al. 2023</xref>).
</p>
        </list-item>
      </list>
    </sec>
    <sec sec-type="materials|methods" id="sec3">
      <title>Materials and methods</title>
      <sec sec-type="Study area" id="sec4">
        <title>Study area</title>
        <p>The Pofadder region is situated in the Northern Cape Province in the northwestern part of South Africa (Fig. <xref ref-type="fig" rid="F1">1a</xref>), close to the Namibian border, and is part of the Bushmanland–Warmbad phytochorion (<xref ref-type="bibr" rid="B92">Schmiedel 2002</xref>, <xref ref-type="bibr" rid="B95">2004</xref>). It lies within one of the hottest areas in southern Africa, with mean monthly maximum temperatures reaching up to 38.3 °C during the summer months. In contrast, winter conditions can be quite cold, with mean monthly minimum temperatures dropping to as low as -0.6 °C (<xref ref-type="bibr" rid="B72">Mucina et al. 2006b</xref>). The climate is hyper-arid, with an extremely low average annual precipitation of approximately 92.7 mm (<xref ref-type="bibr" rid="B21">Desmet and Cowling 1999</xref>). Rainfall primarily occurs in the form of highly unpredictable summer thunderstorms concentrated between January and May (<xref ref-type="bibr" rid="B21">Desmet and Cowling 1999</xref>; <xref ref-type="bibr" rid="B85">Roffe et al. 2021</xref>).</p>
        <fig id="F1">
          <object-id content-type="doi">10.21425/fob.19.167957.figure1</object-id>
          <object-id content-type="arpha">8EAC89E9-590F-51C5-A03C-29E91FC06C7E</object-id>
          <label>Figure 1.</label>
          <caption>
            <p>Overview of the study area. <bold>a</bold>. Location in the Pofadder region, South Africa; <bold>b</bold>. Detailed map of sites with close-ups showing quartz islands detected by our classifications (scale 1:17,500). Blue islands were not surveyed for species but were required to calculate the isolation metrics of the sampled islands; <bold>c</bold>. View from the top of a quartz island towards neighbour islands (© A. M. Bürger); <bold>d</bold>. Close-up of a quartz island soil surface with <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Dinteranthus">Dinteranthus</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="vanzylii">vanzylii</tp:taxon-name-part></tp:taxon-name></italic>, <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Mesembryanthemum">Mesembryanthemum</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="tetragonum">tetragonum</tp:taxon-name-part></tp:taxon-name></italic>, <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Mesembryanthemum">Mesembryanthemum</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="vaginatum">vaginatum</tp:taxon-name-part></tp:taxon-name></italic>, and various <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Poaceae">Poaceae</tp:taxon-name-part></tp:taxon-name> (© A. M. Bürger). Satellite images from Sentinel-2 cloudless (<ext-link xlink:href="https://s2maps.eu" ext-link-type="uri">https://s2maps.eu</ext-link>) by EOX IT Services GmbH (contains modified Copernicus Sentinel data 2024).</p>
          </caption>
          <graphic xlink:href="biogeography-19-001_article-167957__-g001.jpg" id="oo_1543463.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1543463</uri>
          </graphic>
        </fig>
        <p>The Pofadder region is situated within the Nama–Karoo biome (<xref ref-type="bibr" rid="B87">Rutherford and Westfall 1986</xref>) and belongs to the Palaeotropical Floristic Kingdom, in contrast to the Succulent Karoo which is part of the Cape Floristic Kingdom (<xref ref-type="bibr" rid="B46">Jürgens 1991</xref>). The broader vegetation matrix around Pofadder is classified mostly as Bushmanland Arid Grassland, a semi-desert steppe dominated by grass species of <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Stipagrostis">Stipagrostis</tp:taxon-name-part></tp:taxon-name></italic>, locally interspersed with low shrubs such as <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Salsola">Salsola</tp:taxon-name-part></tp:taxon-name></italic> (<xref ref-type="bibr" rid="B72">Mucina et al. 2006b</xref>). Quartz islands are a remarkable feature of the region (Fig. <xref ref-type="fig" rid="F1">1b</xref>), which are covered with angulated quartz pebbles and dominated by low-growing succulent species (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>, for examples see Fig. <xref ref-type="fig" rid="F2">2b</xref>). These habitat islands are edaphically distinct, often characterized by low pH, high salinity, and shallow soils (<xref ref-type="bibr" rid="B97">Schmiedel et al. 2015</xref>; <xref ref-type="bibr" rid="B24">Eibes et al. 2022</xref>). However, <xref ref-type="bibr" rid="B92">Schmiedel (2002)</xref> found evidence suggesting that salinity may play only a minor role in the Pofadder region. Surface properties of quartz islands also differ markedly from the surroundings: the high albedo of white quartz leads up to 8 K lower near-ground temperatures than in adjacent areas (<xref ref-type="bibr" rid="B95">Schmiedel and Jürgens 2004</xref>). These distinctive microclimatic conditions appear to be particularly advantageous for dwarf succulents (<xref ref-type="bibr" rid="B95">Schmiedel and Jürgens 2004</xref>), potentially leading to the unique vegetation on quartz islands, which was described by <xref ref-type="bibr" rid="B71">Mucina et al. (2006a)</xref> as an exclave of the Succulent Karoo biome (termed Aggeneys Gravel Vygieveld and Bushmanland Inselberg Shrubland). The Pofadder region is the only quartz island archipelago showing a high number of hemicryptophytes alongside nano-, micro-, and megachamaephytes, which dominate in all other quartz island archipelagos (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>). Moreover, the Pofadder region is a center of diversity and endemism particularly due to the quartz islands (<xref ref-type="bibr" rid="B46">Jürgens 1991</xref>). In the past, 16 local endemic quartz specialists have been identified for the Warmbad–Bushmanland phytochorion such as the whole genus <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Dinteranthus">Dinteranthus</tp:taxon-name-part></tp:taxon-name></italic> (e.g. <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Dinteranthus">Dinteranthus</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="vanzylii">vanzylii</tp:taxon-name-part></tp:taxon-name></italic> in Fig. <xref ref-type="fig" rid="F2">2b</xref>; <xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>).</p>
        <fig id="F2">
          <object-id content-type="doi">10.21425/fob.19.167957.figure2</object-id>
          <object-id content-type="arpha">050DECFC-2AAF-5B1D-AE53-648540E48E81</object-id>
          <label>Figure 2.</label>
          <caption>
            <p>Flora of quartz islands in the Pofadder region. <bold>a</bold>. Overview of quartz flora composition (220 species) by plant families on the 20 surveyed quartz islands in the Pofadder region (bar colors indicate the taxonomic order of each family); <bold>b</bold>. Selected plant species: <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Gazania">Gazania</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="lichtensteinii">lichtensteinii</tp:taxon-name-part></tp:taxon-name></italic>: annual generalist within the arid zone of southern Africa; <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Hoodia">Hoodia</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="gordonii">gordonii</tp:taxon-name-part></tp:taxon-name></italic>: leafless stem succulent; <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Dinteranthus">Dinteranthus</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="vanzylii">vanzylii</tp:taxon-name-part></tp:taxon-name></italic>: dwarf leaf succulent and local quartz-endemic; and <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Anacampseros">Anacampseros</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="quinaria">quinaria</tp:taxon-name-part></tp:taxon-name></italic>: dwarf succulent with tuberous rootstock (© A. M. Bürger).</p>
          </caption>
          <graphic xlink:href="biogeography-19-001_article-167957__-g002.jpg" id="oo_1543464.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1543464</uri>
          </graphic>
        </fig>
      </sec>
      <sec sec-type="Data sampling" id="sec5">
        <title>Data sampling</title>
        <p>Fieldwork was conducted from February to March 2024. Species presence/absence on 20 quartz islands was assessed using systematic walk-through observations to ensure comprehensive species detection, and following a previous approach applied to the quartz islands of the Knersvlakte (<xref ref-type="bibr" rid="B25">Eibes et al. 2025</xref>), yielding a complete species list per island. Species identification was based on relevant taxonomic literature (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table SS1 for a full list of references), consultation with botanical experts, and comparison with herbarium voucher specimens (Bolus Herbarium at the University of Cape Town (<bold><named-content content-type="dwc:institutional_code" xlink:title="University of Cape Town" xlink:href="https://scientific-collections.gbif.org/institution/bda4f4e2-621f-4fc0-9dca-1a7fd2e0f2b7">BOL</named-content></bold>), Herbarium Hamburgense (<bold><abbrev content-type="institution" xlink:title="Herbarium Hamburgense">HBG</abbrev></bold>), and Natural History Museum Vienna (<bold>W</bold>)). Plant names were harmonized according to the World Checklist of Vascular Plants (<xref ref-type="bibr" rid="B33">Govaerts 2024</xref>) using the R package U.Taxonstand (<xref ref-type="bibr" rid="B123">Zhang and Qian 2023</xref>). Due to the threat of plant poaching, particularly affecting rare and endemic dwarf succulents (<xref ref-type="bibr" rid="B101">Smith et al. 2023</xref>), GPS coordinates will not be published.</p>
      </sec>
      <sec sec-type="Taxonomic diversity" id="sec6">
        <title>Taxonomic diversity</title>
        <p>Taxonomic diversity was quantified using species richness (<abbrev xlink:title="species richness">SR</abbrev>), calculated as the total number of species recorded per island. In total, 220 vascular plant species were recorded across the 20 surveyed quartz islands (Fig. <xref ref-type="fig" rid="F2">2a</xref>).</p>
      </sec>
      <sec sec-type="Functional diversity" id="sec7">
        <title>Functional diversity</title>
        <p>Functional trait data were collected for 134 perennial plant species, following standard practice in trait-based ecology, prioritizing the most abundant species (<xref ref-type="bibr" rid="B18">de Bello et al. 2021b</xref>). Together, these species represent more than 80% of all perennial species recorded across the quartz islands. Trait measurements followed standardized protocols (<xref ref-type="bibr" rid="B80">Pérez-Harguindeguy et al. 2013</xref>) and adapted protocols for succulent species based on Eibes et al. (in prep.). For each species, three individuals from different locations were sampled and used to calculate functional species’ trait means. For a subset of species that were locally abundant but restricted to one or a few islands, only one or two individuals could be collected; these species were also included because they provide valuable information on island-level functional diversity.</p>
        <p>To assess functional diversity we selected the following functional traits: plant height connected to dispersal ability (<xref ref-type="bibr" rid="B106">Thomson et al. 2011</xref>) and representing the balance between the competitiveness of plants and adaptation to habitat harshness (<xref ref-type="bibr" rid="B70">Moles et al. 2009</xref>), specific leaf area indicating net primary production (<xref ref-type="bibr" rid="B30">Garnier et al. 2004</xref>) and negatively correlating with leaf longevity and adaptation to temperature stress (<xref ref-type="bibr" rid="B7">Brasil et al. 2025</xref>), leaf water content regulating leaf trait scaling relationships (<xref ref-type="bibr" rid="B116">Wang et al. 2022</xref>) and correlating with leaf succulence, which is an important adaptation to drought stress and arid climate (<xref ref-type="bibr" rid="B40">Irl et al. 2020</xref>; <xref ref-type="bibr" rid="B81">Pérez-López et al. 2023</xref>), and relative leaf phosphorus content as an important factor for photosynthetic rates (<xref ref-type="bibr" rid="B115">Walker et al. 2014</xref>). In addition, we included the presence of epidermal trichomes, prickles, wax coating, and glands, representing different strategies against radiation, water loss, and herbivory (<xref ref-type="bibr" rid="B37">Hanley et al. 2007</xref>; <xref ref-type="bibr" rid="B89">Samuels et al. 2008</xref>; <xref ref-type="bibr" rid="B48">Karabourniotis et al. 2020</xref>; <xref ref-type="bibr" rid="B105">Tang et al. 2020</xref>) as well as stem succulence resembling adaptation to aridity (<xref ref-type="bibr" rid="B40">Irl et al. 2020</xref>; <xref ref-type="bibr" rid="B81">Pérez-López et al. 2023</xref>).</p>
        <p>Plant height was measured in situ from ground level to the highest point of the photosynthetic tissue, excluding the inflorescence. Specific leaf area (<abbrev xlink:title="Specific leaf area">SLA</abbrev>), defined as leaf area (mm²) divided by dry mass (mg), was calculated in <italic>Fiji</italic> (<xref ref-type="bibr" rid="B90">Schindelin et al. 2012</xref>) from scans (300 dpi) of up to 30 leaves per species using the Optical Leaf macro (<xref ref-type="bibr" rid="B36">Haghshenas 2022</xref>). If highly succulent leaves resulted in scans with insufficient contrast, the leaf outlines were traced. Fresh leaf mass was determined with a precision balance. The leaves were then completely dried until their weight stabilized, and the dry mass was recorded. Relative leaf water content (%) was calculated as one minus the ratio of dry mass (mg) to fresh mass (mg). Relative leaf phosphorus content (%) was analyzed at the Department of Agriculture Western Cape Government (Elsenburg, South Africa). Stem succulence was assessed as a qualitative trait (0 = absent, 1 = moderate, 2 = strong) as well as the four-leaf epidermal traits (0 = absent, 1 = present): trichomes, prickles, wax coating and glands.</p>
        <p>Based on these nine traits and species presence/absence per island we calculated the functional dispersion (<abbrev xlink:title="functional dispersion">FDis</abbrev>) as a measure for <abbrev xlink:title="functional diversity">FD</abbrev>, which is considered robust to outliers and largely independent of species richness. <abbrev xlink:title="functional dispersion">FDis</abbrev> is defined as the mean distance of individual species to the centroid of the community in trait space (<xref ref-type="bibr" rid="B55">Laliberté and Legendre 2010</xref>). Continuous traits were log-transformed (when deviating from normal distribution) and traits were scaled to a 0–1 range to ensure independence from their individual range as suggested by <xref ref-type="bibr" rid="B18">de Bello et al. (2021b)</xref>. To combine quantitative and qualitative traits, a dissimilarity matrix was calculated using the Gower distance implemented in the package gawdis (<xref ref-type="bibr" rid="B17">de Bello et al. 2021a</xref>), applying optimized weighting with a group weight for the epidermal structure traits to avoid a disproportionate influence. The method handles missing values, useful for taxa lacking leaf traits, e.g., leafless <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus" reg="Hoodia">Hoodia</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species" reg="gordonii">gordonii</tp:taxon-name-part></tp:taxon-name></italic> (Fig. <xref ref-type="fig" rid="F2">2b</xref>). <abbrev xlink:title="functional dispersion">FDis</abbrev> was calculated with the <abbrev xlink:title="functional diversity">FD</abbrev> package (<xref ref-type="bibr" rid="B56">Laliberté et al. 2014</xref>), applying Cailliez correction for negative eigenvalues. Although <abbrev xlink:title="functional dispersion">FDis</abbrev> is often treated as independent of species richness, moderate correlations have been reported under certain conditions (<xref ref-type="bibr" rid="B65">Mason et al. 2013</xref>). We therefore tested for a potential richness dependency of <abbrev xlink:title="functional dispersion">FDis</abbrev> but did not detect a significant correlation, indicating that our estimates of functional dispersion were not biased by species richness (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S1). Furthermore, we calculated community‐mean trait values (<abbrev xlink:title="community‐mean trait values">CMs</abbrev>) for each island to get a better view on the functional structure of the plant communities. For quantitative traits, <abbrev xlink:title="community‐mean trait values">CMs</abbrev> were obtained by averaging trait values across all species present on an island. Because abundance data were unavailable, all species were weighted equally, so that each species contributed the same to the island mean irrespective of local dominance. For the categorical epidermal traits, community means were calculated as the proportion of species per island exhibiting the respective trait state. As stem succulence was recorded on an ordinal scale, we treated this trait as a numerical score. This procedure retains the ordered nature of the categories and provides an interpretable measure of average stem succulence within communities, consistent with treating ordinal intensity classes as a functional gradient.</p>
      </sec>
      <sec sec-type="Phylogenetic diversity" id="sec8">
        <title>Phylogenetic diversity</title>
        <p>To reconstruct a phylogeny for all recorded species, we used the U.PhyloMaker package (<xref ref-type="bibr" rid="B45">Jin and Qian 2023</xref>). We used GBOTB.extended.WCVP.tre megatree (<xref ref-type="bibr" rid="B119">Zanne et al. 2014</xref>; <xref ref-type="bibr" rid="B102">Smith and Brown 2018</xref>; <xref ref-type="bibr" rid="B44">Jin and Qian 2022</xref>) as the backbone phylogeny. The phylogeny was constructed using scenario 3 and nodes.type 1, defining how unmatched species are placed into the tree and how non-monophyletic groups are resolved (<xref ref-type="bibr" rid="B123">Zhang and Qian 2023</xref>). The resulting phylogenetic tree was used to calculate the mean pairwise distance (<abbrev xlink:title="mean pairwise distance">MPD</abbrev>) with the <italic>mpd</italic> function from the picante package (<xref ref-type="bibr" rid="B50">Kembel et al. 2010</xref>). The Mean Pairwise Distance (<abbrev xlink:title="mean pairwise distance">MPD</abbrev>) measures the average branch distance between all species pairs in a community and is widely used to quantify phylogenetic diversity (Webb et al. 2002, <xref ref-type="bibr" rid="B109">Tucker et al. 2017</xref>). Moreover, as a divergence measure, it is conceptual similar to <abbrev xlink:title="functional dispersion">FDis</abbrev>. We found no significant correlation between <abbrev xlink:title="mean pairwise distance">MPD</abbrev> and <abbrev xlink:title="species richness">SR</abbrev> in our dataset (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S1).</p>
      </sec>
      <sec sec-type="Remote sensing data" id="sec9">
        <title>Remote sensing data</title>
        <p>To be able to calculate different island parameters, individual quartz islands were mapped by supervised classification of multispectral remote sensing imagery. For this, Sentinel-2 Level-2A surface reflectance imagery was retrieved with the <italic>sen2r</italic> R package (v 1.5-x, <xref ref-type="bibr" rid="B84">Ranghetti et al. 2020</xref>) selecting a single cloud-free tile (16 Dec 2023) from the Copernicus SciHub at 10 m resolution. Six sites were analyzed containing all sampled islands and their surrounding islands. Presence-only classification for quartz islands was performed with the Maximum Entropy Presence Prediction tool in SAGA-GIS (v9.1, <xref ref-type="bibr" rid="B13">Conrad et al. 2015</xref>) using all 12 Sentinel-2 surface-reflectance bands (10 m) and the NDVI (<xref ref-type="bibr" rid="B108">Tucker 1979</xref>) band as continuous predictors. Presence points were supplied to the module, whereas pseudo-absence (background) samples were drawn automatically at 0.1% of the remaining pixels across the scene. We selected the Yoshimasa Tsuruoka iterative-scaling solver (<xref ref-type="bibr" rid="B107">Tsuruoka et al. 2009</xref>) and applied L2 regularization with a weight of 0.01 to minimize overfitting. At two sites we switched to one-class Support Vector Machine (<abbrev xlink:title="Support Vector Machine">SVM</abbrev>), due to low quality of the Maxent classification. Model fitting was carried out in SAGA-GIS (v 9.1, <xref ref-type="bibr" rid="B13">Conrad et al. 2015</xref>) via its LIBSVM wrapper, using a sigmoid kernel. All 13 input bands were z-standardized. The regularization parameter was fixed at ν = 0.5, allowing up to 50% of the support vectors to lie on the margin to curb overprediction (Fig. <xref ref-type="fig" rid="F1">1b</xref>). Model accuracy was evaluated against withheld ground-truth points (confusion matrix metrics: overall accuracy, producer’s/user’s accuracy, F-score, Kappa; Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S2). Based on the GLO-30 DEM from the Copernicus program (European Space Agency and Sinergise 2021), islands were classified into 16 distinct landform classes, including peaks, ridges, valleys, slopes, further subdivided by texture and convexity to retrieve landform data which was used to calculate habitat diversity. Using SAGA-GIS, slope and aspect were calculated with the Morphometry modules, while convexity and texture were derived using the Terrain Surface Classification module based on the algorithm by <xref ref-type="bibr" rid="B43">Iwahashi and Pike (2007)</xref>. If classes were missing or relatively small, they were merged into the thematically closest category.</p>
        <p>For more details see Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: method S1 and <xref ref-type="bibr" rid="B77">Oldeland et al. (2022)</xref>.</p>
      </sec>
      <sec sec-type="Island indices" id="sec10">
        <title>Island indices</title>
        <p>We adapted eleven island parameters for the quartz islands of the Pofadder region based on methods described by <xref ref-type="bibr" rid="B25">Eibes et al. (2025)</xref>. Island polygons and landform data retrieved from our quartz island classification (see remote sensing section) were loaded, transformed into a common coordinate reference system, and validated for topological correctness. Parameters describing island shape and isolation were calculated using the island polygons, whereas habitat diversity indices were derived from the landform data (slope, aspect, convexity, texture). The calculation of each parameter and the class they belong to are shown in Table <xref ref-type="table" rid="T1">1</xref>.</p>
        <table-wrap id="T1" position="float" orientation="portrait">
          <label>Table 1.</label>
          <caption>
            <p>Biogeographic island parameters and their calculations used in this study. <italic>i</italic> describes the index of the targeted island. For neighbour index and proximity index a buffer with radius 1000 m was chosen to cover wind‐dispersal distances (<xref ref-type="bibr" rid="B103">Soons et al. 2004</xref>) and to make the results comparable between the two parameters and among the different classified sites. For further information see Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S3.</p>
          </caption>
          <table>
            <tbody>
              <tr>
                <th rowspan="1" colspan="1">Island Index</th>
                <th rowspan="1" colspan="1">Formula</th>
                <th rowspan="1" colspan="1">Reference</th>
              </tr>
              <tr>
                <td rowspan="1" colspan="3">Island Shape</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Area</td>
                <td rowspan="1" colspan="1">
                  <italic>A<sub>i</sub></italic>
                </td>
                <td rowspan="2" colspan="1"/>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Perimeter</td>
                <td rowspan="1" colspan="1">
                  <italic>P<sub>i</sub></italic>
                </td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Shape Index</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M1">
                    <mml:mfrac>
                      <mml:msub>
                        <mml:mi>P</mml:mi>
                        <mml:mi>i</mml:mi>
                      </mml:msub>
                      <mml:mrow>
                        <mml:mn>2</mml:mn>
                        <mml:msqrt>
                          <mml:mi>π</mml:mi>
                          <mml:mo>·</mml:mo>
                          <mml:msub>
                            <mml:mrow>
                              <mml:mtext/>
                              <mml:mi mathvariant="normal">A</mml:mi>
                            </mml:mrow>
                            <mml:mrow>
                              <mml:mrow>
                                <mml:mi mathvariant="normal">i</mml:mi>
                              </mml:mrow>
                            </mml:mrow>
                          </mml:msub>
                        </mml:msqrt>
                      </mml:mrow>
                    </mml:mfrac>
                  </mml:math>
                </td>
                <td rowspan="1" colspan="1">(<xref ref-type="bibr" rid="B98">Schrader et al. 2019</xref>) based on (<xref ref-type="bibr" rid="B79">Patton 1975</xref>)</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="3">Distance-based Isolation</td>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">Nearest Neighbour Distance</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M2">
                    <mml:munder>
                      <mml:mo>min</mml:mo>
                      <mml:mrow>
                        <mml:mi>j</mml:mi>
                        <mml:mo>≠</mml:mo>
                        <mml:mi>i</mml:mi>
                      </mml:mrow>
                    </mml:munder>
                    <mml:msub>
                      <mml:mi>d</mml:mi>
                      <mml:mrow>
                        <mml:mi>i</mml:mi>
                        <mml:mi>j</mml:mi>
                      </mml:mrow>
                    </mml:msub>
                  </mml:math>
                </td>
                <td rowspan="4" colspan="1"/>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>d<sub>ij</sub></italic> ...edge-to-edge-distance between islands</td>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">5 Nearest Neighbour Mean Distance</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M3">
                    <mml:mfrac>
                      <mml:mn>1</mml:mn>
                      <mml:mn>5</mml:mn>
                    </mml:mfrac>
                    <mml:munder>
                      <mml:mo>∑</mml:mo>
                      <mml:mi>k</mml:mi>
                    </mml:munder>
                    <mml:msub>
                      <mml:mi>d</mml:mi>
                      <mml:mrow>
                        <mml:mi>i</mml:mi>
                        <mml:mi>k</mml:mi>
                      </mml:mrow>
                    </mml:msub>
                  </mml:math>
                </td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>k</italic> ...index of 5 nearest neighbours of island <italic>i</italic></td>
              </tr>
              <tr>
                <td rowspan="1" colspan="3">Area-based Isolation</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Nearest Neighbour Area</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M4">
                    <mml:msub>
                      <mml:mi>A</mml:mi>
                      <mml:mi>i</mml:mi>
                    </mml:msub>
                    <mml:mo>,</mml:mo>
                    <mml:mi>j</mml:mi>
                    <mml:mo>=</mml:mo>
                    <mml:mi>arg</mml:mi>
                    <mml:mo>⁡</mml:mo>
                    <mml:munder>
                      <mml:mo>min</mml:mo>
                      <mml:mrow>
                        <mml:mi>j</mml:mi>
                        <mml:mo>≠</mml:mo>
                        <mml:mi>i</mml:mi>
                      </mml:mrow>
                    </mml:munder>
                    <mml:msub>
                      <mml:mi>d</mml:mi>
                      <mml:mrow>
                        <mml:mi>i</mml:mi>
                        <mml:mi>j</mml:mi>
                      </mml:mrow>
                    </mml:msub>
                  </mml:math>
                </td>
                <td rowspan="3" colspan="1"/>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">5 Nearest Neighbour Mean Area</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M5">
                    <mml:mfrac>
                      <mml:mn>1</mml:mn>
                      <mml:mn>5</mml:mn>
                    </mml:mfrac>
                    <mml:munder>
                      <mml:mo>∑</mml:mo>
                      <mml:mi>k</mml:mi>
                    </mml:munder>
                    <mml:msub>
                      <mml:mi>A</mml:mi>
                      <mml:mi>k</mml:mi>
                    </mml:msub>
                  </mml:math>
                </td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>k</italic> ...index of 5 nearest neighbours of island <italic>i</italic></td>
              </tr>
              <tr>
                <td rowspan="1" colspan="3">Connectivity-based Isolation</td>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">Neighbour Index</td>
                <td rowspan="1" colspan="1"><mml:math id="M6"><mml:munderover><mml:mo>∑</mml:mo><mml:mrow><mml:mi>j</mml:mi><mml:mo>∈</mml:mo><mml:msub><mml:mi>N</mml:mi><mml:mi>i</mml:mi></mml:msub></mml:mrow><mml:mi>n</mml:mi></mml:munderover><mml:mfrac><mml:msub><mml:mi>A</mml:mi><mml:mi>j</mml:mi></mml:msub><mml:msup><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>d</mml:mi><mml:mrow><mml:mi>i</mml:mi><mml:mi>j</mml:mi></mml:mrow></mml:msub><mml:mo>+</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msup></mml:mfrac></mml:math>, <italic>j</italic> ≠ <italic>i</italic></td>
                <td rowspan="2" colspan="1">(<xref ref-type="bibr" rid="B47">Kalmar and Currie 2006</xref>)</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>N<sub>i</sub></italic> ...islands within buffer around island <italic>i</italic></td>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">Proximity Index</td>
                <td rowspan="1" colspan="1"><mml:math id="M7"><mml:munderover><mml:mo>∑</mml:mo><mml:mrow><mml:mi>j</mml:mi><mml:mo>∈</mml:mo><mml:msub><mml:mi>N</mml:mi><mml:mi>i</mml:mi></mml:msub></mml:mrow><mml:mi>n</mml:mi></mml:munderover><mml:mfrac><mml:msub><mml:mi>A</mml:mi><mml:mi>j</mml:mi></mml:msub><mml:mrow><mml:mi>d</mml:mi><mml:mi>j</mml:mi></mml:mrow></mml:mfrac></mml:math>, <mml:math id="M8"><mml:msub><mml:mi>d</mml:mi><mml:mi>j</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:munder><mml:mo movablelimits="true">min</mml:mo><mml:mrow><mml:mi>k</mml:mi><mml:mo>≠</mml:mo><mml:mi>j</mml:mi></mml:mrow></mml:munder><mml:msub><mml:mi>d</mml:mi><mml:mrow><mml:mi>j</mml:mi><mml:mi>k</mml:mi></mml:mrow></mml:msub></mml:math></td>
                <td rowspan="2" colspan="1">(<xref ref-type="bibr" rid="B34">Gustafson and Parker 1994</xref>)</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>N<sub>i</sub></italic> ...islands within buffer around island <italic>i</italic></td>
              </tr>
              <tr>
                <td rowspan="1" colspan="3">Habitat Diversity</td>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">Habitat Diversity Index</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M9">
                    <mml:mfrac>
                      <mml:mrow>
                        <mml:mi>T</mml:mi>
                        <mml:msub>
                          <mml:mi>P</mml:mi>
                          <mml:mi>i</mml:mi>
                        </mml:msub>
                      </mml:mrow>
                      <mml:mrow>
                        <mml:mn>2</mml:mn>
                        <mml:msqrt>
                          <mml:mi>π</mml:mi>
                          <mml:mo>·</mml:mo>
                          <mml:msub>
                            <mml:mrow>
                              <mml:mtext/>
                              <mml:mi mathvariant="normal">A</mml:mi>
                            </mml:mrow>
                            <mml:mrow>
                              <mml:mrow>
                                <mml:mi mathvariant="normal">i</mml:mi>
                              </mml:mrow>
                            </mml:mrow>
                          </mml:msub>
                        </mml:msqrt>
                      </mml:mrow>
                    </mml:mfrac>
                  </mml:math>
                </td>
                <td rowspan="2" colspan="1">(<xref ref-type="bibr" rid="B79">Patton 1975</xref>)</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>TP<sub>i</sub></italic> ...Perimeter + all inner habitat edges</td>
              </tr>
              <tr>
                <td rowspan="2" colspan="1">Habitat Shannon Diversity</td>
                <td rowspan="1" colspan="1">
                  <mml:math id="M10">
                    <mml:mo>−</mml:mo>
                    <mml:munder>
                      <mml:mo>∑</mml:mo>
                      <mml:mrow>
                        <mml:mrow>
                          <mml:mi>c</mml:mi>
                        </mml:mrow>
                      </mml:mrow>
                    </mml:munder>
                    <mml:msub>
                      <mml:mi>p</mml:mi>
                      <mml:mrow>
                        <mml:mi>i</mml:mi>
                        <mml:mi>c</mml:mi>
                      </mml:mrow>
                    </mml:msub>
                    <mml:mo>⋅</mml:mo>
                    <mml:mi>ln</mml:mi>
                    <mml:mo>⁡</mml:mo>
                    <mml:mrow>
                      <mml:mo>(</mml:mo>
                      <mml:msub>
                        <mml:mi>p</mml:mi>
                        <mml:mrow>
                          <mml:mi>i</mml:mi>
                          <mml:mi>c</mml:mi>
                        </mml:mrow>
                      </mml:msub>
                      <mml:mo>)</mml:mo>
                    </mml:mrow>
                    <mml:mo>,</mml:mo>
                    <mml:msub>
                      <mml:mi>p</mml:mi>
                      <mml:mrow>
                        <mml:mi>i</mml:mi>
                        <mml:mi>c</mml:mi>
                      </mml:mrow>
                    </mml:msub>
                    <mml:mo>=</mml:mo>
                    <mml:mfrac>
                      <mml:msub>
                        <mml:mi>A</mml:mi>
                        <mml:mrow>
                          <mml:mi>i</mml:mi>
                          <mml:mi>c</mml:mi>
                        </mml:mrow>
                      </mml:msub>
                      <mml:msub>
                        <mml:mi>A</mml:mi>
                        <mml:mi>i</mml:mi>
                      </mml:msub>
                    </mml:mfrac>
                  </mml:math>
                </td>
                <td rowspan="3" colspan="1">(<xref ref-type="bibr" rid="B100">Shannon 1948</xref>)</td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1"><italic>c</italic> ...different habitat classes detected on island <italic>i</italic></td>
              </tr>
              <tr>
                <td rowspan="1" colspan="1">Habitat Richness</td>
                <td rowspan="1" colspan="1"><italic>n<sub>ic</sub></italic> ...number of classes on island <italic>i</italic></td>
              </tr>
            </tbody>
          </table>
        </table-wrap>
      </sec>
      <sec sec-type="Statistical analysis" id="sec11">
        <title>Statistical analysis</title>
        <p>As an exploratory measure, relationships among diversity indices and between island parameters were quantified using Pearson’s correlation coefficients (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: figs S1, S2) and we used single predictor generalized linear models (<abbrev xlink:title="generalized linear models">GLMs</abbrev>) to assess the influence of island parameters on the diversity measures (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S3). Right‐skewed predictors were log‐transformed [ln(x)] and all predictors z‐standardized.</p>
        <p>Prior to constructing multivariate models, we evaluated several commonly used transformations for each predictor (identity, ln(x), x² and x²+x). For every predictor, the transformation yielding the lowest AIC<sub>c</sub> in a univariate <abbrev xlink:title="linear models">LM</abbrev> was applied, and predictors were subsequently z-standardized. As <abbrev xlink:title="species richness">SR</abbrev> is a count variable and exhibited overdispersion, we fitted a negative binomial generalized linear model (<abbrev xlink:title="generalized linear model">GLM</abbrev>) with a log-link function (<italic>glm.nb</italic>(), MASS package; <xref ref-type="bibr" rid="B112">Venables and Ripley 2002</xref>). For <abbrev xlink:title="functional dispersion">FDis</abbrev> and <abbrev xlink:title="mean pairwise distance">MPD</abbrev>, Gaussian linear models (<abbrev xlink:title="linear models">LM</abbrev>) were applied. Model selection was conducted with <italic>dredge</italic>() (<italic>MuMIn</italic>; <xref ref-type="bibr" rid="B4">Bartoń 2025</xref>), testing all predictor combinations, excluding those containing highly correlated predictors (|r|&gt;0.7). Models were ranked by AIC<sub>c</sub> and the ones with the lowest AIC<sub>c</sub> were selected.</p>
        <p>These final models were subjected to comprehensive diagnostic checks. Pearson residuals were plotted against fitted values to detect heteroscedasticity. For linear models, residual normality was assessed using the Shapiro–Wilk test. Influence diagnostics were performed using Cook’s distance; where Cook’s D indicated strong influence, we further assessed coefficient stability using HC3 heteroscedasticity-consistent standard errors (<xref ref-type="bibr" rid="B63">MacKinnon and White 1985</xref>) and robust regression implemented via the packages sandwich (<xref ref-type="bibr" rid="B120">Zeileis 2004</xref>; <xref ref-type="bibr" rid="B121">Zeileis et al. 2020</xref>) and lmtest (Zeileis and Hothorn 2002). In the <abbrev xlink:title="mean pairwise distance">MPD</abbrev> analysis, one highly influential observation was removed and the model refitted, with robust procedures confirming the stability of the retained predictor. For the negative-binomial <abbrev xlink:title="species richness">SR</abbrev> model, quality of the fit was evaluated using the overdispersion ratio and residual inspection. The standardized regression coefficients of the selected models were calculated with <italic>parameterize</italic>() (effectsize; Ben-Sachar et al. 2020), which were further visualized using the ggplot2 package (<xref ref-type="bibr" rid="B118">Wickham 2016</xref>).</p>
        <p>For <abbrev xlink:title="functional dispersion">FDis</abbrev>, explained variance of the best model was partitioned with varpart() (vegan; <xref ref-type="bibr" rid="B76">Oksanen et al. 2025</xref>). For <abbrev xlink:title="species richness">SR</abbrev>, we used a pseudo-R²-based variance partitioning, following the logic of varpart but implemented through sequential negative binomial <abbrev xlink:title="generalized linear models">GLMs</abbrev>. No variance partitioning was performed for <abbrev xlink:title="mean pairwise distance">MPD</abbrev> because the best model contained only a single predictor. For all variance partitioning approaches, predictors were grouped into Island Shape, Isolation, and Habitat Diversity (see Table <xref ref-type="table" rid="T1">1</xref>). We then modelled <abbrev xlink:title="functional dispersion">FDis</abbrev> as a function of the community mean of each trait separately to assess which traits are most strongly associated with functional dispersion. For every trait, we fitted a linear model, and a quadratic model, using standardized trait values. Linear and quadratic models were compared using a nested ANOVA and AIC; the quadratic form was retained when it improved fit (p &lt; 0.05 in the nested test or lower AIC), otherwise the linear model was selected. For the best-supported model of each trait, we recorded the coefficient of determination (R²) as a measure of explained variance and visualized Trait–<abbrev xlink:title="functional dispersion">FDis</abbrev> relationships. All analyses were conducted in R 4.5.0 (<xref ref-type="bibr" rid="B83">R Core Team 2025</xref>).</p>
      </sec>
    </sec>
    <sec sec-type="Results" id="sec12">
      <title>Results</title>
      <sec sec-type="Taxonomic diversity" id="sec13">
        <title>Taxonomic diversity</title>
        <p>Of the 220 plant species recorded on the quartz islands, roughly one third belonged to <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Asteraceae">Asteraceae</tp:taxon-name-part></tp:taxon-name>, <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name>, or <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Poaceae">Poaceae</tp:taxon-name-part></tp:taxon-name> (Fig. <xref ref-type="fig" rid="F2">2</xref>). Species richness (<abbrev xlink:title="species richness">SR</abbrev>) varied strongly among islands, ranging from 23 to 105 species (mean = 54, sd = 23), i.e. a difference of 82 species between the poorest and richest islands (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S4). The best-supported model contained four parameters (area, shape index, nearest neighbour distance and habitat Shannon diversity, Fig. <xref ref-type="fig" rid="F3">3</xref>) and reduced the deviance in <abbrev xlink:title="species richness">SR</abbrev> by about 80% relative to the null model (Fig. <xref ref-type="fig" rid="F4">4</xref>), indicating an excellent fit. There was no evidence of residual overdispersion (Pearson χ² = 19.94, χ²/df = 1.33, p = 0.174) and Pearson residuals showed no systematic pattern when plotted against fitted values, indicating that model assumptions of the negative binomial regression were met (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S4a).</p>
        <fig id="F3">
          <object-id content-type="doi">10.21425/fob.19.167957.figure3</object-id>
          <object-id content-type="arpha">1BFAAE61-4AC7-559C-92CB-655086990922</object-id>
          <label>Figure 3.</label>
          <caption>
            <p>Forest plot showing the standardized coefficients of the multivariate models with the lowest AIC assessing the effect of island parameters on different aspects of plant diversity. <bold>a</bold>. Species richness (<abbrev xlink:title="species richness">SR</abbrev>) was fitted with a negative-binomial generalized linear model, whereas <bold>b</bold>. Functional dispersion (<abbrev xlink:title="functional dispersion">FDis</abbrev>) and <bold>c</bold>. Phylogenetic mean pairwise distance (<abbrev xlink:title="mean pairwise distance">MPD</abbrev>) were modelled using linear models. All island parameters were standardized. In <bold>a</bold>, coefficients indicate the change in species number per unit change in the predictor; in <bold>b</bold>, beta coefficients indicate that species are more dispersed (positive) or more clustered (negative) in functional trait space; in <bold>c</bold>, beta coefficients show whether species are more phylogenetically dispersed (positive) or clustered (negative). Asterisks indicate significance levels: * p &lt; 0.05, ** p &lt; 0.01, *** p &lt; 0.001.</p>
          </caption>
          <graphic xlink:href="biogeography-19-001_article-167957__-g003.jpg" id="oo_1543465.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1543465</uri>
          </graphic>
        </fig>
        <fig id="F4">
          <object-id content-type="doi">10.21425/fob.19.167957.figure4</object-id>
          <object-id content-type="arpha">6A35899B-B8BD-5886-AED1-ABE08DD16EAE</object-id>
          <label>Figure 4.</label>
          <caption>
            <p>Partitioning of explained deviance (species richness) and explained variance (functional dispersion and mean pairwise distance) from the best multipredictor models (see Fig. <xref ref-type="fig" rid="F3">3</xref>). For <abbrev xlink:title="mean pairwise distance">MPD</abbrev> no variance partitioning was performed as the selected model had just one predictor, nevertheless it was included to complement the figure. Predictors were thematically grouped. Island Shape: area + shape index; Isolation: nearest neighbour distance nearest neighbour area, neighbour index; Habitat diversity: habitat Shannon diversity. Asterisks indicate significance levels of the models: * p &lt; 0.05, ** p &lt; 0.01, *** p &lt; 0.001. Significance was assessed by a likelihood-ratio test for <abbrev xlink:title="species richness">SR</abbrev> (χ²(4): 30.74) and by an F-test for <abbrev xlink:title="functional dispersion">FDis</abbrev> (F<sub>3,16</sub> = 15.8) and <abbrev xlink:title="mean pairwise distance">MPD</abbrev> (F<sub>2,17</sub> = 5,531).</p>
          </caption>
          <graphic xlink:href="biogeography-19-001_article-167957__-g004.jpg" id="oo_1543466.jpg">
            <uri content-type="original_file">https://binary.pensoft.net/fig/1543466</uri>
          </graphic>
        </fig>
        <p>Partitioning of explained deviance showed that island shape (area and shape index) accounted for most of the variation (&gt; 80%), whereas habitat diversity and isolation jointly contributed roughly 15%, with a minor remainder attributable to shared fractions (Fig. <xref ref-type="fig" rid="F4">4</xref>). Both island area and shape index emerged as highly significant predictors of species richness, with area exerting the strongest positive effect while more irregular island shapes (higher shape index) were associated with lower <abbrev xlink:title="species richness">SR</abbrev> (Fig. <xref ref-type="fig" rid="F3">3</xref>). Although shape index is derived from island area and perimeter, the correlation with area remained under the critical threshold (|r| &lt; 0.7). In addition to island shape effects, species richness increased with greater nearest neighbour distance and higher habitat Shannon diversity (Fig. <xref ref-type="fig" rid="F3">3</xref>).</p>
      </sec>
      <sec sec-type="Functional diversity" id="sec14">
        <title>Functional diversity</title>
        <p>Functional dispersion varied moderately among islands, ranging from 0.1187 to 0.1564 (mean = 0.1370, sd = 0.0073), corresponding to a total difference of 0.0377 between the least and most functionally diverse islands (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S4). The best‐supported multi-predictor model (lowest AIC) was highly significant and explained 70% of the variation in <abbrev xlink:title="functional dispersion">FDis</abbrev> (Fig. <xref ref-type="fig" rid="F4">4</xref>). Model diagnostics indicated a good fit: residuals were normally distributed (Shapiro–Wilk p = 0.6284) and Pearson residuals showed no systematic structure (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S4b). Results were robust to influence diagnostics and HC3 heteroscedasticity-consistent standard errors (not shown). Variance partitioning revealed that isolation/connectivity metrics accounted for most of the explained variation in <abbrev xlink:title="functional dispersion">FDis</abbrev> (≈60%), whereas habitat diversity contributed the remaining part (Fig. <xref ref-type="fig" rid="F4">4</xref>). Within the multi-predictor model, island connectivity (neighbour index) exerted the strongest positive effect, while both nearest neighbour distance and habitat Shannon diversity were significantly negatively related to <abbrev xlink:title="functional dispersion">FDis</abbrev> (Fig. <xref ref-type="fig" rid="F3">3b</xref>). When relating community means (<abbrev xlink:title="community means">CM</abbrev>, Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S4) to functional dispersion, several traits showed significant associations with <abbrev xlink:title="functional dispersion">FDis</abbrev> (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S5). Specifically, <abbrev xlink:title="functional dispersion">FDis</abbrev> was significantly related to the <abbrev xlink:title="community means">CM</abbrev> of <abbrev xlink:title="Specific leaf area">SLA</abbrev>, leaf water content, stem succulence, as well as to the proportion of species bearing glands and prickles (all p &lt; 0.05; R² = 0.31–0.39; Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S5). In contrast the <abbrev xlink:title="community means">CM</abbrev> of plant height, leaf P content, and the proportions of hairy and waxy species showed no detectable relationships with <abbrev xlink:title="functional dispersion">FDis</abbrev> (R² ≤ 0.02; Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S5).</p>
        <p>Finally, community means varied among islands but were largely unrelated to the island parameters included in the best <abbrev xlink:title="functional dispersion">FDis</abbrev> model (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S6). Across trait-specific models, only the <abbrev xlink:title="community means">CM</abbrev> of relative leaf water content exhibited a significant dependence on island characteristics, decreasing with increasing habitat Shannon diversity (p = 0.0278; Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S6). All other quantitative and categorical community means showed no significant associations with nearest neighbour distance, neighbour index, or habitat diversity (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S6).</p>
      </sec>
      <sec sec-type="Phylogenetic diversity" id="sec15">
        <title>Phylogenetic diversity</title>
        <p>Phylogenetic mean pairwise distance varied only moderately among islands, ranging from 223.91 to 236.16 (mean = 229.37, sd = 3.42), i.e. a total difference of 12.25 units between the least and most phylogenetically dispersed islands (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S4). Initial model selection including all islands suggested positive effects of island connectivity (neighbour index) and negative effects of nearest neighbour distance on <abbrev xlink:title="mean pairwise distance">MPD</abbrev>. Model diagnostics, however, indicated that one island exerted a disproportionate influence on parameter estimates. After excluding this influential island, the best‐supported model retained only nearest neighbour area as a significant positive predictor of <abbrev xlink:title="mean pairwise distance">MPD</abbrev> (Fig. <xref ref-type="fig" rid="F3">3</xref>c). It explained about one third of the variance in <abbrev xlink:title="mean pairwise distance">MPD</abbrev> (Fig. <xref ref-type="fig" rid="F4">4</xref>). Residuals of the final model were approximately normally distributed (Shapiro–Wilk p = 0.549) and Pearson residuals showed no systematic structure (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S4c). Robust regression and HC3 standard errors confirmed the stability of the effect of nearest neighbour area (not shown).</p>
      </sec>
    </sec>
    <sec sec-type="Discussion" id="sec16">
      <title>Discussion</title>
      <p>This is the first study to quantify taxonomic, functional, and phylogenetic plant diversity on quartz islands in a summer-rainfall region. Species richness was driven primarily by island shape indices, phylogenetic mean pairwise distance was only shaped by area-based isolation, whereas functional dispersion responded to isolation and habitat diversity metrics, demonstrating diverging responses of different diversity facets to biogeographic parameters. By focusing on a quartz‐island archipelago in the summer‐rainfall region, which has been largely overlooked in previous research (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>), our results expand our knowledge of how spatial attributes shape the diversity of habitat islands, specifically quartz islands in southern Africa.</p>
      <sec sec-type="Taxonomic diversity" id="sec17">
        <title>Taxonomic diversity</title>
        <p>Island size showed the strongest effects on species richness (Fig. <xref ref-type="fig" rid="F3">3a</xref>), consistent with classical island biogeography theory (<xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>) and other habitat islands (<xref ref-type="bibr" rid="B67">Matthews 2016</xref>, <xref ref-type="bibr" rid="B66">2021</xref>; <xref ref-type="bibr" rid="B38">Henneron et al. 2019</xref>; <xref ref-type="bibr" rid="B69">Mendez-Castro et al. 2021</xref>). Similar patterns were also found for quartz islands in the winter-rainfall dominated Knersvlakte (<xref ref-type="bibr" rid="B25">Eibes et al. 2025</xref>). Larger areas offer more resources and space, reducing the risk of population extinction (<xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>) and have an enhanced chance of being colonized by erratic dispersal (<xref ref-type="bibr" rid="B69">Mendez-Castro et al. 2021</xref>). Furthermore, area correlates with habitat heterogeneity and ecological niches, increasing the potential species pool suitable for an island (<xref ref-type="bibr" rid="B38">Henneron et al. 2019</xref>). In addition, the shape index exhibited a significant negative effect (Fig. <xref ref-type="fig" rid="F3">3a</xref>), suggesting that high shape complexity can flatten the species–area richness relationship.</p>
        <p>Against our expectations, isolation had a weak but significant positive effect on species richness (Fig. <xref ref-type="fig" rid="F3">3a</xref>), contrasting island biogeography theory (<xref ref-type="bibr" rid="B62">MacArthur and Wilson 1967</xref>; <xref ref-type="bibr" rid="B61">Losos and Parent 2010</xref>) and some previous works in habitat island biogeography (<xref ref-type="bibr" rid="B20">Dembicz et al. 2020</xref>; <xref ref-type="bibr" rid="B66">Matthews 2021</xref>). However, it only accounted for a small portion of the deviance explained (Fig. <xref ref-type="fig" rid="F4">4</xref>). This suggests isolation is a minor factor for species richness on quartz islands, or that other methods may be needed to detect isolation in quartz island systems (e.g., focus on matrix effects; see <xref ref-type="bibr" rid="B25">Eibes et al. 2025</xref>). The minor importance of isolation is consistent with reports of weak to negligible effects in other habitat islands (<xref ref-type="bibr" rid="B38">Henneron et al. 2019</xref>; <xref ref-type="bibr" rid="B60">Liu et al. 2020</xref>) and with findings from true islands, where reported effect sizes of isolation are much smaller than those of area (<xref ref-type="bibr" rid="B8">Buckley 1985</xref>; <xref ref-type="bibr" rid="B42">Itescu et al. 2020</xref>). It would be interesting to study larger sample sizes to see if the positive effect of nearest neighbour distance persists or if it occurred because of some undetected covariations.</p>
        <p>Moreover, habitat Shannon diversity was included in the final model (Fig. <xref ref-type="fig" rid="F3">3a</xref>), though it was based solely on topographic attributes that reflect heterogeneity rather than distinct vegetation units (<xref ref-type="bibr" rid="B104">Stein and Kreft 2015</xref>). Indeed, microhabitats play an important role on quartz islands, where distinct vegetation units occur in close proximity (<xref ref-type="bibr" rid="B94">Schmiedel and Jürgens 1999</xref>). We captured this fine scale heterogeneity using indices of topographic habitat diversity that integrate variation in slope, aspect, surface texture, and microrelief, which modulate wind exposure, solar radiation, and water availability. Although quartz islands in the winter- and summer-rainfall regions share features such as dwarf leaf succulents and predominantly low-growing vegetation, they differ edaphically. Quartz islands in the Pofadder region differed from those in the Knersvlakte with respect to both pH and salinity, being only negligibly more acidic than the surrounding matrix and lacking an elevated salinity (<xref ref-type="bibr" rid="B24">Eibes et al. 2022</xref>; <xref ref-type="bibr" rid="B57">Lauel 2025</xref>; <xref ref-type="bibr" rid="B125">Meyer et al. 2026</xref>). These differences in soil conditions may be related to the regional climatic regime. The Pofadder region experiences more extreme temperatures and unpredictable heavy rainfall (<xref ref-type="bibr" rid="B21">Desmet and Cowling 1999</xref>). The latter can result in horizontal run-off and lateral transport of fine material and soil chemicals, thereby decreasing spatial contrasts in soil conditions between quartz islands and the surrounding matrix (<xref ref-type="bibr" rid="B125">Meyer et al. 2026</xref>).</p>
        <p>In both biomes, soils on quartz islands exhibit higher edaphic aridity than the surrounding soils, generally due to their shallower soil depth, and further enhanced by increased salinity in the Succulent Karoo (Schmiedel 2015) and finer surface soil structure in the Pofadder region (<xref ref-type="bibr" rid="B125">Meyer et al. 2026</xref>). Despite belonging to different floristic kingdoms and biomes, the quartz floras of both regions show convergent adaptation to these arid conditions, particularly through dwarf growth forms and leaf succulence (<xref ref-type="bibr" rid="B94">Schmiedel and Jürgens 1999</xref>). In the Succulent Karoo, <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name> have been particularly successful due to preadaptation to arid conditions, which, in combination with their range-restricted dispersal mechanisms, has led to rapid radiation (<xref ref-type="bibr" rid="B53">Klak et al. 2004</xref>). Accordingly, quartz islands in the Knersvlakte are dominated by <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name>: they account for about 40% of the species pool, and together with <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Asteraceae">Asteraceae</tp:taxon-name-part></tp:taxon-name> and <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Crassulaceae">Crassulaceae</tp:taxon-name-part></tp:taxon-name> they represent roughly two thirds of all species (<xref ref-type="bibr" rid="B24">Eibes et al. 2022</xref>).</p>
        <p>In the Pofadder region, <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name> are also well represented on quartz islands, but are clearly less dominant than in the Succulent Karoo. Here, <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Asteraceae">Asteraceae</tp:taxon-name-part></tp:taxon-name> constitute the most frequent family, followed by <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name> and <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Poaceae">Poaceae</tp:taxon-name-part></tp:taxon-name>, and together these three families account for only about one third of the species pool (Fig. <xref ref-type="fig" rid="F2">2a</xref>). This difference in family composition likely reflects, first, the influence of the regional species pool from which the quartz flora is assembled (<xref ref-type="bibr" rid="B124">Zhigila et al. 2024</xref>), and second, the more extreme climatic regime in the summer-rainfall Nama–Karoo. Cowling et al. (<xref ref-type="bibr" rid="B14">1994</xref>, <xref ref-type="bibr" rid="B15">1998</xref>) suggested that recurring prolonged droughts in this region act as a major constraint on leaf-succulent richness. Although leaf succulents can tolerate low but relatively regular water availability, they show high mortality during extended dry periods (Von Willert 1985). By contrast, many <tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Poaceae">Poaceae</tp:taxon-name-part></tp:taxon-name> species build up large soil seed banks in arid habitats (<xref ref-type="bibr" rid="B86">Romig et al. 2025</xref>) and often follow ephemeral life-history strategies well suited to unpredictable rainfall and droughts (<xref ref-type="bibr" rid="B111">Venable and Brown 1988</xref>), potentially explaining the higher proportion of grasses on quartz islands in the summer-rainfall region.</p>
        <p>In addition, the more extreme temperatures in the Pofadder region may further contribute to the generally lower richness of leaf succulents, as experimental climate warming has caused metabolic impairment and high mortality in southern African quartz-field succulents (<xref ref-type="bibr" rid="B73">Musil et al. 2009</xref>). Together, these climatic and edaphic contrasts likely result in different matrix–island dynamics between quartz islands predominantly influenced by summer versus winter rainfall. This is supported by the presence of a distinct transition zone along island boundaries in the Pofadder region (<xref ref-type="bibr" rid="B125">Meyer et al. 2026</xref>), that has not been observed in the Knersvlakte (<xref ref-type="bibr" rid="B97">Schmiedel et al. 2015</xref>; <xref ref-type="bibr" rid="B23">Eibes et al. 2021</xref>).</p>
      </sec>
      <sec sec-type="Functional diversity" id="sec18">
        <title>Functional diversity</title>
        <p>Across islands, functional dispersion varied only within a narrow range (mean <abbrev xlink:title="functional dispersion">FDis</abbrev> ≈ 0.14, SD ≈ 0.01; Fig. <xref ref-type="fig" rid="F3">3a</xref>), indicating that communities occupy a relatively compact region of trait space (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S4). This pattern points to strong environmental filtering and broadly similar functional composition among islands. Despite the limited spread in <abbrev xlink:title="functional dispersion">FDis</abbrev>, several island characteristics still showed significant effects, suggesting that even subtle differences in trait dispersion are structured by island properties. However, unlike species richness, functional dispersion exhibited no clear positive relationship with island size (Fig. <xref ref-type="fig" rid="F3">3b</xref>). This contrasts with our expectation that larger islands with more topographically diverse habitats would provide more ecological niches and thus support higher functional diversity (<xref ref-type="bibr" rid="B29">Ferreira-Arruda et al. 2022</xref>; <xref ref-type="bibr" rid="B99">Schrader et al. 2024</xref>). A plausible explanation is that opposing processes operate on small versus large islands. On small quartz islands, competitive exclusion may promote the coexistence of species occupying markedly different trait niches, thereby increasing functional dispersion despite low species richness (<xref ref-type="bibr" rid="B122">Zhang et al. 2023</xref>). However, our results are more consistent with a second mechanism. <xref ref-type="bibr" rid="B49">Karadimou et al. (2016)</xref> and <xref ref-type="bibr" rid="B3">Barabás et al. (2022)</xref> argued that functional diversity does not necessarily increase with species richness because trait space can become densely occupied in species-rich communities, reducing dispersion through redundancy. In our system, this mechanism is supported by the narrow range of <abbrev xlink:title="functional dispersion">FDis</abbrev> values across islands, which indicates that most communities share a similar set of trait combinations; additional species occurring on larger islands are therefore likely to be functionally redundant rather than expanding the occupied trait space. High levels of functional redundancy have likewise been reported for the Knersvlakte (<xref ref-type="bibr" rid="B14">Cowling et al. 1994</xref>). Thus, while we anticipated a significant positive area–<abbrev xlink:title="functional dispersion">FDis</abbrev> relationship, our findings highlight that strong environmental filtering and trait redundancy on larger islands can decouple functional diversity from species richness on quartz islands.</p>
        <p>Habitat Shannon diversity showed a significant negative effect in the multiple regression model (Fig. <xref ref-type="fig" rid="F3">3b</xref>), accounting for about 40% of the explained variance in functional dispersion (Fig. <xref ref-type="fig" rid="F4">4</xref>). Similar negative heterogeneity effects have been reported for Frisian islands after disentangling heterogeneity from area (<xref ref-type="bibr" rid="B29">Ferreira-Arruda et al. 2022</xref>) and interpreted via the area–heterogeneity trade-off (<xref ref-type="bibr" rid="B1">Allouche et al. 2012</xref>), whereby higher habitat heterogeneity on small areas reduces effective habitat per species and increases extinction risk. Isolation/connectivity parameters contributed the remaining ~60% of explained variance (Fig. <xref ref-type="fig" rid="F4">4</xref>). In the multi-predictor model, the neighbour index had a strong positive effect, while nearest neighbour distance showed a weaker but significant negative effect (Fig. <xref ref-type="fig" rid="F3">3b</xref>). Higher neighbour index values indicate that surrounding islands are, on average, larger and more closely spaced, implying greater connectivity. Notably, functional dispersion showed no purely random pattern but depended on isolation despite plant height being the only trait in our dataset loosely linked to dispersal ability, and community-mean plant height showing no significant relationship with the isolation metrics (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S6). This suggests that isolation effects on <abbrev xlink:title="functional dispersion">FDis</abbrev> are not driven by directional filtering of dispersal-related trait values captured by our trait set. <xref ref-type="bibr" rid="B74">Musker et al. (2021)</xref> similarly showed that spatial genetic structure and gene flow patterns in contrasting growth forms do not follow simple expectations based on plant size alone, indicating that plant height is a poor proxy for dispersal ability in this system. In addition, most species in our dataset are relatively low-growing (&lt; 1 m), so height differences are not very pronounced. The positive connectivity effect therefore likely reflects broader assembly constraints: better-connected island systems may maintain higher functional dispersion due to more effective coupling to the regional species pool, increasing the likelihood of a more complete filling of functional niches (<xref ref-type="bibr" rid="B2">Baguette et al. 2013</xref>). This may be particularly important in environmentally extreme, specialist-dominated habitats such as quartz islands, where only a limited subset of the regional flora can be established.</p>
        <p>Trait-based analyses indicate that <abbrev xlink:title="functional dispersion">FDis</abbrev> is primarily associated with community means of leaf water content, <abbrev xlink:title="Specific leaf area">SLA</abbrev>, and stem succulence—traits linked to stress tolerance and resource-use strategies in arid, low-productivity environments—as well as with the epidermal traits prickles and glands, which relate to defense against biotic interactions (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S5). In contrast, plant height, relative leaf phosphorus content, and the remaining epidermal traits showed no detectable relationships with <abbrev xlink:title="functional dispersion">FDis</abbrev>. Hump-shaped relationships between community mean traits and functional diversity are often reported, reflecting reduced dispersion when communities are dominated by extreme trait values (<xref ref-type="bibr" rid="B22">Dias et al. 2013</xref>). In our system, however, only epidermal glands and stem succulence showed this pattern, whereas the remaining traits exhibited either U-shaped relationships or no detectable associations with <abbrev xlink:title="functional dispersion">FDis</abbrev>. Because abundance data were unavailable, unweighted community means were calculated from presence–absence data. Incorporating abundance information may modify <abbrev xlink:title="community means">CM</abbrev>–<abbrev xlink:title="functional dispersion">FDis</abbrev> relationships by accounting for species dominance.</p>
      </sec>
      <sec sec-type="Phylogenetic diversity" id="sec19">
        <title>Phylogenetic diversity</title>
        <p>Like <abbrev xlink:title="functional dispersion">FDis</abbrev>, <abbrev xlink:title="mean pairwise distance">MPD</abbrev> varied only moderately across islands (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: table S4). In contrast to <abbrev xlink:title="functional dispersion">FDis</abbrev>, the best <abbrev xlink:title="mean pairwise distance">MPD</abbrev> model explained only about one third of the variation, with the area of the nearest neighbour island as the only significant positive predictor (Fig. <xref ref-type="fig" rid="F3">3c</xref>), while all other isolation metrics had no detectable effect (Fig. <xref ref-type="fig" rid="F4">4</xref>). Although the megatree used to derive phylogenetic distances is unresolved at the species level for parts of our dataset, <abbrev xlink:title="mean pairwise distance">MPD</abbrev> is considered robust to such resolution limits (<xref ref-type="bibr" rid="B82">Qian and Jin 2021</xref>), so this is unlikely to bias the main pattern.</p>
        <p>This suggests that phylogenetic diversity is primarily shaped by the immediate size of the closest potential source area, rather than by broader network connectivity, as suggested for <abbrev xlink:title="functional dispersion">FDis</abbrev>. The absence of stronger isolation signals is in line with studies on other habitat island systems showing weak or random <abbrev xlink:title="phylogenetic diversity">PD</abbrev>–insularity relationships (<xref ref-type="bibr" rid="B54">Klimeš et al. 2024</xref>; <xref ref-type="bibr" rid="B88">Sabino et al. 2025</xref>). Findings of <xref ref-type="bibr" rid="B124">Zhigila et al. (2024)</xref> indicate that quartz communities were originally assembled from preadapted lineages in the surrounding flora, followed by in-situ diversification on the edaphic islands. However, the floristic similarity to the Succulent Karoo and the enclave-like character of the quartz islands within the Nama–Karoo could also reflect palaeoclimatic oscillations that shifted rainfall regimes and the historical distribution of the Succulent Karoo biome (<xref ref-type="bibr" rid="B92">Schmiedel 2002</xref>). Under this scenario, the present-day quartz flora in the summer-rainfall region would represent a relict assemblage, with subsequent in-situ diversification and renewed environmental filtering potentially promoting functional–phylogenetic decoupling.</p>
        <p>Overall, our data are not sufficient to support a clear pattern of either coupling or decoupling between <abbrev xlink:title="functional diversity">FD</abbrev> and <abbrev xlink:title="phylogenetic diversity">PD</abbrev>, unlike the distinct decoupling patterns reported at broader spatial and ecological scales (Večeřa et al. 2023; <xref ref-type="bibr" rid="B35">Hähn et al. 2025</xref>). Considering the univariate models, isolation metrics showed weak, non-significant, and partly opposing tendencies for <abbrev xlink:title="functional dispersion">FDis</abbrev> versus <abbrev xlink:title="mean pairwise distance">MPD</abbrev>, whereas both indices responded in similar directions to island shape and habitat diversity (Suppl. material <xref ref-type="supplementary-material" rid="S1">1</xref>: fig. S3). In addition, <abbrev xlink:title="functional dispersion">FDis</abbrev> was more sensitive to island parameters, yielding a better-fitting model than <abbrev xlink:title="mean pairwise distance">MPD</abbrev>. These patterns indicate differences between functional and phylogenetic diversity, but further studies are needed to unravel the assembly processes and mechanistic links between them.</p>
      </sec>
    </sec>
    <sec sec-type="Conclusion" id="sec20">
      <title>Conclusion</title>
      <p>Quartz islands in South Africa represent a unique and understudied habitat island system that significantly contributes to the biodiversity and endemism of the Karoo (<xref ref-type="bibr" rid="B93">Schmiedel 2004</xref>). This study offers the first biogeographic perspective on quartz islands in the harsh, arid summer-rainfall region, applying a multifaceted approach to examine how island parameters such as area and isolation influence taxonomic, functional, and phylogenetic plant diversity. Because quartz islands occur across multiple archipelagos along a wide bioclimatic gradient, they offer an exceptional natural system for comparative habitat island biogeography. Future research should extend taxonomic, functional and phylogenetic diversity analyses across regions on a larger scale to identify their main drivers and to determine how climate shapes functional trait space and island-biogeographic relationships in otherwise comparable edaphic environments. Furthermore, it would be highly informative to connect phylogeny with functional traits in the quartz flora, to test whether certain trait classes are more phylogenetically conserved than others. This would help interpret diversity patterns and assembly processes, while help to reveal the relationship between functional and phylogenetic diversity.</p>
      <p>Overall, our findings advance habitat-island biogeography and provide ecologically relevant insights into the functioning of species-rich quartz islands. They further highlight conservation priorities on largely privately owned land where threats such as dwarf succulent poaching persist (<xref ref-type="bibr" rid="B101">Smith et al. 2023</xref>). Specifically, conservation should safeguard both large islands—given the strong area effect on species richness—and well-connected networks of intact islands, which appear crucial for maintaining functional diversity. Maintaining connectivity is also likely to support dispersal and gene flow among islands (<xref ref-type="bibr" rid="B91">Schmidt et al. 2025</xref>), thereby enhancing the adaptive potential of quartz island floras under ongoing global change.</p>
    </sec>
    <sec sec-type="Acknowledgements" id="sec21">
      <title>Acknowledgements</title>
      <p>We are grateful to the Department of Environment and Nature Conservation (Northern Cape, SA) for issuing the necessary research permit (permit number: FLORA 0004/2024). Our heartfelt thanks go to the farmers who generously allowed us to conduct fieldwork on their private land. Our thanks go to Sebastian Wilfinger and Fenja Wiese for their assistance in measuring functional plant traits. Special appreciation is extended to the following experts for their invaluable support in plant identification: Cornelia Klak (<tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Aizoaceae">Aizoaceae</tp:taxon-name-part></tp:taxon-name>), Peter V. Bruyns (<tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family" reg="Euphorbiaceae">Euphorbiaceae</tp:taxon-name-part></tp:taxon-name>), and Annelise Le Roux (various taxa). Furthermore, we thank the University of Hamburg, the University of Cape Town, and the Natural History Museum of Vienna for granting us access to their herbarium collections. Lastly, we sincerely thank the handling editor, Marcus Vinicius Cianciaruso, and the two reviewers for their constructive and insightful comments, which substantially improved the manuscript. We are particularly grateful to Rafael Molina-Venegas for additional follow-up discussion during the revision process.</p>
    </sec>
    <sec sec-type="Author contributions" id="sec22">
      <title>Author contributions</title>
      <p>AMB and SDHI conceived the ideas and designed methodology; AMB, KM, CL, and US collected the data; AMB, JO and SDHI analyzed the data with contributions by PME; AMB led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.</p>
    </sec>
    <sec sec-type="Use of AI" id="sec23">
      <title>Use of AI</title>
      <p>OpenAI’s ChatGPT assisted with language editing and provided support in code debugging and structuring.</p>
    </sec>
    <sec sec-type="Funding" id="sec24">
      <title>Funding</title>
      <p>We sincerely thank the German Research Foundation (DFG, project number: 404519812) for funding our Quartz Island II project.</p>
    </sec>
    <sec sec-type="Data availability statement" id="sec25">
      <title>Data availability statement</title>
      <p>The data underpinning the analysis reported in this paper are deposited in the Dryad Data Repository at <ext-link xlink:href="10.5061/dryad.15dv41p8x" ext-link-type="doi">https://doi.org/10.5061/dryad.15dv41p8x</ext-link>.</p>
    </sec>
    <sec sec-type="Competing interests" id="sec26">
      <title>Competing interests</title>
      <p>The authors have declared that no competing interests exist.</p>
    </sec>
  </body>
  <back>
    <ref-list>
      <title>References</title>
      <ref id="B1">
        <mixed-citation>Allouche O, Kalyuzhny M, Moreno-Rueda G, Pizarro M, Kadmon R (2012) Area-heterogeneity tradeoff and the diversity of ecological communities. Proceedings of the National Academy of Sciences of the United States of America 109: 17495–17500. <ext-link xlink:href="10.1073/pnas.1208652109" ext-link-type="doi">https://doi.org/10.1073/pnas.1208652109</ext-link></mixed-citation>
      </ref>
      <ref id="B2">
        <mixed-citation>Baguette M, Blanchet S, Legrand D, Stevens VM, Turlure C (2013) Individual dispersal, landscape connectivity and ecological networks. Biological Reviews 88: 310–326. <ext-link xlink:href="10.1111/brv.12000" ext-link-type="doi">https://doi.org/10.1111/brv.12000</ext-link></mixed-citation>
      </ref>
      <ref id="B3">
        <mixed-citation>Barabás G, Parent C, Kraemer A, Van de Perre F, De Laender F (2022) The evolution of trait variance creates a tension between species diversity and functional diversity. Nature Communications 13: 2521. <ext-link xlink:href="10.1038/s41467-022-30090-4" ext-link-type="doi">https://doi.org/10.1038/s41467-022-30090-4</ext-link></mixed-citation>
      </ref>
      <ref id="B4">
        <mixed-citation>Bartoń K (2025) MuMIn: Multi-Model Inference. R package version 1.48.11. <ext-link xlink:href="10.32614/CRAN.package.MuMIn" ext-link-type="doi">https://doi.org/10.32614/CRAN.package.MuMIn</ext-link></mixed-citation>
      </ref>
      <ref id="B5">
        <mixed-citation>Ben-Shachar MS, Lüdecke D, Makowski D (2020) effectsize: Estimation of effect size indices and standardized parameters. Journal of Open Source Software 5: 2815. <ext-link xlink:href="10.21105/joss.02815" ext-link-type="doi">https://doi.org/10.21105/joss.02815</ext-link></mixed-citation>
      </ref>
      <ref id="B6">
        <mixed-citation>Boucher FC, Verboom GA, Gallien L, Ellis AG (2024) Multiple reproductive barriers maintain species boundaries in stone plants of the genus <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus">Argyroderma</tp:taxon-name-part></tp:taxon-name></italic>. Botanical Journal of the Linnean Society 204: 187–197. <ext-link xlink:href="10.1093/botlinnean/boad046" ext-link-type="doi">https://doi.org/10.1093/botlinnean/boad046</ext-link></mixed-citation>
      </ref>
      <ref id="B7">
        <mixed-citation>Brasil SNR, de Carvalho CE, de Araujo FS, Loiola MIB (2025) Leaf traits as predictors of climatic adaptation and distributional range in wide- and narrow-range species. Flora 326: 152708. <ext-link xlink:href="10.1016/j.flora.2025.152708" ext-link-type="doi">https://doi.org/10.1016/j.flora.2025.152708</ext-link></mixed-citation>
      </ref>
      <ref id="B8">
        <mixed-citation>Buckley RC (1985) Distinguishing the effects of area and habitat type on island plant species richness by separating floristic elements and substrate types and controlling for island isolation. Journal of Biogeography 12: 527–535. <ext-link xlink:href="10.2307/2844908" ext-link-type="doi">https://doi.org/10.2307/2844908</ext-link></mixed-citation>
      </ref>
      <ref id="B9">
        <mixed-citation>Cadotte MW, Carscadden K, Mirotchnick N (2011) Beyond species: functional diversity and the maintenance of ecological processes and services. Journal of Applied Ecology 48: 1079–1087. <ext-link xlink:href="10.1111/j.1365-2664.2011.02048.x" ext-link-type="doi">https://doi.org/10.1111/j.1365-2664.2011.02048.x</ext-link></mixed-citation>
      </ref>
      <ref id="B10">
        <mixed-citation>Cadotte MW, Carboni M, Si X, Tatsumi S (2019) Do traits and phylogeny support congruent community diversity patterns and assembly inferences? Journal of Ecology 107: 2065–2077. <ext-link xlink:href="10.1111/1365-2745.13247" ext-link-type="doi">https://doi.org/10.1111/1365-2745.13247</ext-link></mixed-citation>
      </ref>
      <ref id="B11">
        <mixed-citation>Carvajal-Endara S, Hendry AP, Emery NC, Davies TJ (2017) Habitat filtering not dispersal limitation shapes oceanic island floras: species assembly of the Galápagos archipelago. Ecology Letters 20: 495–504. <ext-link xlink:href="10.1111/ele.12753" ext-link-type="doi">https://doi.org/10.1111/ele.12753</ext-link></mixed-citation>
      </ref>
      <ref id="B12">
        <mixed-citation>Cheng Y, Li D, Maaroufi NI, You J, Zhou W, Liu W, Qi D, Liu X, Wang Y, Pan X, Zhang W, Yang J, Zhou S, Song Z (2025) Environmental filtering effect drives the plant species distribution in alpine grasslands on the Qinghai-Tibetan Plateau. Ecology and Evolution 15: e71599. <ext-link xlink:href="10.1002/ece3.71599" ext-link-type="doi">https://doi.org/10.1002/ece3.71599</ext-link></mixed-citation>
      </ref>
      <ref id="B13">
        <mixed-citation>Conrad O, Bechtel B, Bock M, Dietrich H, Fischer E, Gerlitz L, Wehberg J, Wichmann V, Böhner J (2015) System for Automated Geoscientific Analyses (SAGA) v. 2.1.4. Geoscientific Model Development 8: 1991–2007. <ext-link xlink:href="10.5194/gmd-8-1991-2015" ext-link-type="doi">https://doi.org/10.5194/gmd-8-1991-2015</ext-link></mixed-citation>
      </ref>
      <ref id="B14">
        <mixed-citation>Cowling RM, Esler KJ, Midgley GF, Honig MA (1994) Plant functional diversity, species diversity and climate in arid and semi-arid southern Africa. Journal of Arid Environments 27: 141–158. <ext-link xlink:href="10.1006/jare.1994.1054" ext-link-type="doi">https://doi.org/10.1006/jare.1994.1054</ext-link></mixed-citation>
      </ref>
      <ref id="B15">
        <mixed-citation>Cowling RM, Rundel PW, Desmet PG, Esler KJ (1998) Extraordinary high regional-scale plant diversity in southern African arid lands. Diversity and Distributions 4: 27–36. <ext-link xlink:href="10.1046/j.1365-2699.1998.00026.x" ext-link-type="doi">https://doi.org/10.1046/j.1365-2699.1998.00026.x</ext-link></mixed-citation>
      </ref>
      <ref id="B16">
        <mixed-citation>Curtis OE, Stirton CH, Muasya AM (2013) A conservation and floristic assessment of poorly known species rich quartz–silcrete outcrops within Rûens Shale Renosterveld (Overberg, Western Cape), with taxonomic descriptions of five new species. South African Journal of Botany 87: 99–111. <ext-link xlink:href="10.1016/j.sajb.2013.03.017" ext-link-type="doi">https://doi.org/10.1016/j.sajb.2013.03.017</ext-link></mixed-citation>
      </ref>
      <ref id="B17">
        <mixed-citation>de Bello F, Botta-Dukát Z, Lepš J, Fibich P (2021a) Towards a more balanced combination of multiple traits when computing functional differences between species. Methods in Ecology and Evolution 12: 443–448. <ext-link xlink:href="10.1111/2041-210X.13537" ext-link-type="doi">https://doi.org/10.1111/2041-210X.13537</ext-link></mixed-citation>
      </ref>
      <ref id="B18">
        <mixed-citation>de Bello F, Carmona CP, Dias ATC, Götzenberger L, Moretti M, Berg MP (2021b) Handbook of Trait-Based Ecology: From Theory to R Tools. Cambridge University Press, Cambridge, 296 pp. <ext-link xlink:href="10.1017/9781108628426" ext-link-type="doi">https://doi.org/10.1017/9781108628426</ext-link></mixed-citation>
      </ref>
      <ref id="B19">
        <mixed-citation>Delhaye G, Dimitrakopoulos PG, Adamidis GC (2024) Interspecific trait differences drive plant community responses on serpentine soils. Journal of Ecology 112: 2887–2900. <ext-link xlink:href="10.1111/1365-2745.14429" ext-link-type="doi">https://doi.org/10.1111/1365-2745.14429</ext-link></mixed-citation>
      </ref>
      <ref id="B20">
        <mixed-citation>Dembicz I, Moysiyenko II, Kozub Ł, Dengler J, Zakharova M, Sudnik-Wójcikowska B (2020) Steppe islands in a sea of fields: Where island biogeography meets the reality of a severely transformed landscape. Journal of Vegetation Science 32: e12930. <ext-link xlink:href="10.1111/jvs.12930" ext-link-type="doi">https://doi.org/10.1111/jvs.12930</ext-link></mixed-citation>
      </ref>
      <ref id="B21">
        <mixed-citation>Desmet PG, Cowling RM (1999) The climate of the Karoo – a functional approach. In: Dean WRJ, Milton S (Eds) The Karoo: Ecological Patterns and Processes. Cambridge University Press, Cambridge, 3–16. <ext-link xlink:href="10.1017/CBO9780511541988.004" ext-link-type="doi">https://doi.org/10.1017/CBO9780511541988.004</ext-link></mixed-citation>
      </ref>
      <ref id="B22">
        <mixed-citation>Dias ATC, Berg MP, de Bello F, Van Oosten AR, Bíla K, Moretti M (2013) An experimental framework to identify community functional components driving ecosystem processes and services delivery. Journal of Ecology 101: 29–37. <ext-link xlink:href="10.1111/1365-2745.12024" ext-link-type="doi">https://doi.org/10.1111/1365-2745.12024</ext-link></mixed-citation>
      </ref>
      <ref id="B23">
        <mixed-citation>Eibes PM, Oldeland J, Irl SDH, Twerski A, Kühne N, Schmiedel U (2021) Partitioned beta diversity patterns of plants across sharp and distinct boundaries of quartz habitat islands. Journal of Vegetation Science 32: e13036. <ext-link xlink:href="10.1111/jvs.13036" ext-link-type="doi">https://doi.org/10.1111/jvs.13036</ext-link></mixed-citation>
      </ref>
      <ref id="B24">
        <mixed-citation>Eibes PM, Schaffrath F, Oldeland J, Thormählen W, Schmiedel U, Irl SDH (2022) Testing the concept of edaphism for the quartz island flora of the Knersvlakte, South Africa. South African Journal of Botany 151: 555–564. <ext-link xlink:href="10.1016/j.sajb.2022.10.015" ext-link-type="doi">https://doi.org/10.1016/j.sajb.2022.10.015</ext-link></mixed-citation>
      </ref>
      <ref id="B25">
        <mixed-citation>Eibes PM, Schmiedel U, Oldeland J, Irl SDH (2025) Specific island biogeographic and landscape features shape plant diversity and habitat specialism on edaphic quartz islands in an arid ocean. Scientific Reports 15: 9982. <ext-link xlink:href="10.1038/s41598-025-94562-5" ext-link-type="doi">https://doi.org/10.1038/s41598-025-94562-5</ext-link></mixed-citation>
      </ref>
      <ref id="B26">
        <mixed-citation>Ellis AG, Weis AE (2006) Coexistence and differentiation of ‘flowering stones’: the role of local adaptation to soil microenvironment. Journal of Ecology 94: 322–335. <ext-link xlink:href="10.1111/j.1365-2745.2005.01074.x" ext-link-type="doi">https://doi.org/10.1111/j.1365-2745.2005.01074.x</ext-link></mixed-citation>
      </ref>
      <ref id="B27">
        <mixed-citation>European Space Agency, Sinergise (2021) Copernicus Global Digital Elevation Model. <ext-link xlink:href="10.5069/g9028pqb" ext-link-type="doi">https://doi.org/10.5069/g9028pqb</ext-link> [Visited on 01/01/2024].</mixed-citation>
      </ref>
      <ref id="B28">
        <mixed-citation>Fahrig L (2013) Rethinking patch size and isolation effects: the habitat amount hypothesis. Journal of Biogeography 40: 1649–1663. <ext-link xlink:href="10.1111/jbi.12130" ext-link-type="doi">https://doi.org/10.1111/jbi.12130</ext-link></mixed-citation>
      </ref>
      <ref id="B29">
        <mixed-citation>Ferreira-Arruda T, Guerrero-Ramírez NR, Denelle P, Weigelt P, Kleyer M, Kreft H (2022) Island area and historical geomorphological dynamics shape multifaceted diversity of barrier island floras. Ecography 45: e06238. <ext-link xlink:href="10.1111/ecog.06238" ext-link-type="doi">https://doi.org/10.1111/ecog.06238</ext-link></mixed-citation>
      </ref>
      <ref id="B30">
        <mixed-citation>Garnier E, Cortez J, Billès G, Navas ML, Roumet C, Debussche M, Laurent G, Blanchard A, Aubry D, Bellmann A, Neill C, Toussaint JP (2004) Plant functional markers capture ecosystem properties during secondary succession. Ecology 85: 2630–2637. <ext-link xlink:href="10.1890/03-0799" ext-link-type="doi">https://doi.org/10.1890/03-0799</ext-link></mixed-citation>
      </ref>
      <ref id="B31">
        <mixed-citation>Garnier E, Navas M-L, Grigulis K (2016) Plant functional diversity: organism traits, community structure, and ecosystem properties. Oxford University Press, Oxford, 255 pp. <ext-link xlink:href="10.1093/acprof:oso/9780198757368.001.0001" ext-link-type="doi">https://doi.org/10.1093/acprof:oso/9780198757368.001.0001</ext-link></mixed-citation>
      </ref>
      <ref id="B32">
        <mixed-citation>Gonzales FN, Craven D, Armesto JJ (2023) Islands in the mist: A systematic review of the coastal lomas of South America. Journal of Arid Environments 211: 104942. <ext-link xlink:href="10.1016/j.jaridenv.2023.104942" ext-link-type="doi">https://doi.org/10.1016/j.jaridenv.2023.104942</ext-link></mixed-citation>
      </ref>
      <ref id="B33">
        <mixed-citation>Govaerts R [Ed.] (2024) WCVP: World Checklist of Vascular Plants. Royal Botanic Gardens, Kew. <ext-link xlink:href="http://sftp.kew.org/pub/data-repositories/WCVP/" ext-link-type="uri">http://sftp.kew.org/pub/data-repositories/WCVP/</ext-link> [Visited on 15/01/2025]</mixed-citation>
      </ref>
      <ref id="B34">
        <mixed-citation>Gustafson EJ, Parker GR (1994) Using an index of habitat patch proximity for landscape design. Landscape and Urban Planning 29: 117–130. <ext-link xlink:href="10.1016/0169-2046(94)90022-1" ext-link-type="doi">https://doi.org/10.1016/0169-2046(94)90022-1</ext-link></mixed-citation>
      </ref>
      <ref id="B35">
        <mixed-citation>Hähn GJA, Damasceno G, Alvarez-Davila E, Aubin I, Bauters M, Bergmeier E, Biurrun I, Bjorkman AD, Bonari G, Botta-Dukát Z, Campos JA, Čarni A, Chytrý M, Ćušterevska R, de Gasper AL, De Sanctis M, Dengler J, Dolezal J, El-Sheikh MA, Finckh M, Galán-de-Mera A, Garbolino E, Gholizadeh H, Golub V, Haider S, Hatim MZ, Hérault B, Homeier J, Jandt U, Jansen F, Jentsch A, Kattge J, Kessler M, Khanina L, Kreft H, Küzmič F, Lenoir J, Moeslund JE, Mucina L, Naqinezhad A, Noroozi J, Pérez-Haase A, Phillips OL, Pillar VD, Rivas-Torres G, Ruprecht E, Sandel B, Schmidt M, Schmiedel U, Schnitzer S, Schrodt F, Šilc U, Sparrow B, Sporbert M, Stančić Z, Strohbach B, Svenning J-C, Tang CQ, Tang Z, Vibrans AC, Violle C, Waller D, Wana D, Wang H-F, Whitfeld T, Zizka G, Sabatini FM, Bruelheide H (2025) Global decoupling of functional and phylogenetic diversity in plant communities. Nature Ecology &amp; Evolution 9: 237–248. <ext-link xlink:href="10.1038/s41559-024-02589-0" ext-link-type="doi">https://doi.org/10.1038/s41559-024-02589-0</ext-link></mixed-citation>
      </ref>
      <ref id="B36">
        <mixed-citation>Haghshenas A (2022) Optical Leaf Area (version 1.0.0). Zenodo. <ext-link xlink:href="10.5281/zenodo.6462380" ext-link-type="doi">https://doi.org/10.5281/zenodo.6462380</ext-link></mixed-citation>
      </ref>
      <ref id="B37">
        <mixed-citation>Hanley ME, Lamont BB, Fairbanks MM, Rafferty CM (2007) Plant structural traits and their role in anti-herbivore defence. Perspectives in Plant Ecology, Evolution and Systematics 8: 157–178. <ext-link xlink:href="10.1016/j.ppees.2007.01.001" ext-link-type="doi">https://doi.org/10.1016/j.ppees.2007.01.001</ext-link></mixed-citation>
      </ref>
      <ref id="B38">
        <mixed-citation>Henneron L, Sarthou C, de Massary JC, Ponge JF (2019) Habitat diversity associated to island size and environmental filtering control the species richness of rock-savanna plants in neotropical inselbergs. Ecography 42: 1536–1547. <ext-link xlink:href="10.1111/ecog.04482" ext-link-type="doi">https://doi.org/10.1111/ecog.04482</ext-link></mixed-citation>
      </ref>
      <ref id="B39">
        <mixed-citation>Hortal J, Triantis KA, Meiri S, Thébault E, Sfenthourakis S (2009) Island species richness increases with habitat diversity. The American Naturalist 174: 205–217. <ext-link xlink:href="10.1086/645085" ext-link-type="doi">https://doi.org/10.1086/645085</ext-link></mixed-citation>
      </ref>
      <ref id="B40">
        <mixed-citation>Irl SDH, Obermeier A, Beierkuhnlein C, Steinbauer MJ (2020) Climate controls plant life-form patterns on a high-elevation oceanic island. Journal of Biogeography 47: 2261–2273. <ext-link xlink:href="10.1111/jbi.13929" ext-link-type="doi">https://doi.org/10.1111/jbi.13929</ext-link></mixed-citation>
      </ref>
      <ref id="B41">
        <mixed-citation>Itescu Y (2019) Are island-like systems biologically similar to islands? A review of the evidence. Ecography 42: 1298–1314. <ext-link xlink:href="10.1111/ecog.03951" ext-link-type="doi">https://doi.org/10.1111/ecog.03951</ext-link></mixed-citation>
      </ref>
      <ref id="B42">
        <mixed-citation>Itescu Y, Foufopoulos J, Pafilis P, Meiri S (2020) The diverse nature of island isolation and its effect on land bridge insular faunas. Global Ecology and Biogeography 29: 262–280. <ext-link xlink:href="10.1111/geb.13024" ext-link-type="doi">https://doi.org/10.1111/geb.13024</ext-link></mixed-citation>
      </ref>
      <ref id="B43">
        <mixed-citation>Iwahashi J, Pike RJ (2007) Automated classifications of topography from DEMs by an unsupervised nested-means algorithm and a three-part geometric signature. Geomorphology 86: 409–440. <ext-link xlink:href="10.1016/j.geomorph.2006.09.012" ext-link-type="doi">https://doi.org/10.1016/j.geomorph.2006.09.012</ext-link></mixed-citation>
      </ref>
      <ref id="B44">
        <mixed-citation>Jin Y, Qian H (2022) V.PhyloMaker2: an updated and enlarged R package that can generate very large phylogenies for vascular plants. Plant Diversity 44: 335–339. <ext-link xlink:href="10.1016/j.pld.2022.05.005" ext-link-type="doi">https://doi.org/10.1016/j.pld.2022.05.005</ext-link></mixed-citation>
      </ref>
      <ref id="B45">
        <mixed-citation>Jin Y, Qian H (2023) U.PhyloMaker: an R package that can generate large phylogenetic trees for plants and animals. Plant Diversity 45: 347–352. <ext-link xlink:href="10.1016/j.pld.2022.12.007" ext-link-type="doi">https://doi.org/10.1016/j.pld.2022.12.007</ext-link></mixed-citation>
      </ref>
      <ref id="B46">
        <mixed-citation>Jürgens N (1991) A new approach to the Namib Region – I: phytogeographic subdivision. Vegetatio 97: 21–38. <ext-link xlink:href="10.1007/bf00033899" ext-link-type="doi">https://doi.org/10.1007/bf00033899</ext-link></mixed-citation>
      </ref>
      <ref id="B47">
        <mixed-citation>Kalmar A, Currie DJ (2006) A global model of island biogeography. Global Ecology and Biogeography 15: 72–81. <ext-link xlink:href="10.1111/j.1466-822x.2006.00205.x" ext-link-type="doi">https://doi.org/10.1111/j.1466-822x.2006.00205.x</ext-link></mixed-citation>
      </ref>
      <ref id="B48">
        <mixed-citation>Karabourniotis G, Liakopoulos G, Nikolopoulos D, Bresta P (2020) Protective and defensive roles of non-glandular trichomes against multiple stresses: structure–function coordination. Journal of Forestry Research 32: 553–567. <ext-link xlink:href="10.1007/s11676-019-01034-4" ext-link-type="doi">https://doi.org/10.1007/s11676-019-01034-4</ext-link></mixed-citation>
      </ref>
      <ref id="B49">
        <mixed-citation>Karadimou EK, Kallimanis AS, Tsiripidis I, Dimopoulos P (2016) Functional diversity exhibits a diverse relationship with area, even a decreasing one. Scientific Reports 6: 35420. <ext-link xlink:href="10.1038/srep35420" ext-link-type="doi">https://doi.org/10.1038/srep35420</ext-link></mixed-citation>
      </ref>
      <ref id="B50">
        <mixed-citation>Kembel SW, Cowan PD, Helmus MR, Cornwell WK, Morlon H, Ackerly DD, Blomberg SP, Webb CO (2010) Picante: R tools for integrating phylogenies and ecology. Bioinformatics 26: 1463–1464. <ext-link xlink:href="10.1093/bioinformatics/btq166" ext-link-type="doi">https://doi.org/10.1093/bioinformatics/btq166</ext-link></mixed-citation>
      </ref>
      <ref id="B51">
        <mixed-citation>Kier G, Kreft H, Tien ML, Jetz W, Ibisch PL, Nowicki C, Mutke J, Barthlott W (2009) A global assessment of endemism and species richness across island and mainland regions. Proceedings of the National Academy of Sciences of the United States of America 106: 9322–9327. <ext-link xlink:href="10.1073/pnas.0810306106" ext-link-type="doi">https://doi.org/10.1073/pnas.0810306106</ext-link></mixed-citation>
      </ref>
      <ref id="B52">
        <mixed-citation>Kim H, Lee CB (2021) On the relative importance of landscape variables to plant diversity and phylogenetic community structure on uninhabited islands, South Korea. Landscape Ecology 36: 209–221. <ext-link xlink:href="10.1007/s10980-020-01134-1" ext-link-type="doi">https://doi.org/10.1007/s10980-020-01134-1</ext-link></mixed-citation>
      </ref>
      <ref id="B53">
        <mixed-citation>Klak C, Reeves G, Hedderson T (2004) Unmatched tempo of evolution in southern African semi-desert ice plants. Nature 427: 63–65. <ext-link xlink:href="10.1038/nature02243" ext-link-type="doi">https://doi.org/10.1038/nature02243</ext-link></mixed-citation>
      </ref>
      <ref id="B54">
        <mixed-citation>Klimeš A, Molina-Venegas R, Carta A, Chytrý M, Conti L, Götzenberger L, Hájek M, Horsák M, Jiménez-Alfaro B, Klimešová J, Méndez-Castro FE, Zelený D, Ottaviani G (2024) Weak phylogenetic effect on specialist plant assemblages and their persistence on habitat islands. Journal of Biogeography 51: 1723–1733. <ext-link xlink:href="10.1111/jbi.14833" ext-link-type="doi">https://doi.org/10.1111/jbi.14833</ext-link></mixed-citation>
      </ref>
      <ref id="B55">
        <mixed-citation>Laliberté E, Legendre P (2010) A distance-based framework for measuring functional diversity from multiple traits. Ecology 91: 299–305. <ext-link xlink:href="10.1890/08-2244.1" ext-link-type="doi">https://doi.org/10.1890/08-2244.1</ext-link></mixed-citation>
      </ref>
      <ref id="B56">
        <mixed-citation>Laliberté E, Legendre P, Shipley B (2014) FD: measuring functional diversity from multiple traits, and other tools for functional ecology. R package version 1.0-12.3. <ext-link xlink:href="10.32614/CRAN.package.FD" ext-link-type="doi">https://doi.org/10.32614/CRAN.package.FD</ext-link></mixed-citation>
      </ref>
      <ref id="B57">
        <mixed-citation>Lauel C (2025) Vegetation-ecological characterization of quartz islands in the Pofadder region. Master’s Thesis, Goethe University Frankfurt, Frankfurt am Main, Germany.</mixed-citation>
      </ref>
      <ref id="B58">
        <mixed-citation>Laurance WF (2008) Theory meets reality: how habitat fragmentation research has transcended island biogeographic theory. Biological Conservation 141: 1731–1744. <ext-link xlink:href="10.1016/j.biocon.2008.05.011" ext-link-type="doi">https://doi.org/10.1016/j.biocon.2008.05.011</ext-link></mixed-citation>
      </ref>
      <ref id="B59">
        <mixed-citation>Lian WH, Zhao WS, Wang PD, Han JR, Lu CY, Hu CJ, Shi GY, Chen F, Dong L, Zhou T, Li WJ (2025) Habitat island biogeography of mountaintop plant and soil microbiomes: similar patterns driven by different mechanisms. Global Ecology and Conservation 59: e03574. <ext-link xlink:href="10.1016/j.gecco.2025.e03574" ext-link-type="doi">https://doi.org/10.1016/j.gecco.2025.e03574</ext-link></mixed-citation>
      </ref>
      <ref id="B60">
        <mixed-citation>Liu J, Matthews TJ, Zhong L, Liu J, Wu D, Yu M (2020) Environmental filtering underpins the island species–area relationship in a subtropical anthropogenic archipelago. Journal of Ecology 108: 424–432. <ext-link xlink:href="10.1111/1365-2745.13272" ext-link-type="doi">https://doi.org/10.1111/1365-2745.13272</ext-link></mixed-citation>
      </ref>
      <ref id="B61">
        <mixed-citation>Losos JB, Parent CE (2010) The Speciation–Area Relationship. In: Losos JB, Ricklefs RE (Eds) The Theory of Island Biogeography Revisited. Princeton University Press, Princeton, 415–438. <ext-link xlink:href="10.1515/9781400831920.415" ext-link-type="doi">https://doi.org/10.1515/9781400831920.415</ext-link></mixed-citation>
      </ref>
      <ref id="B62">
        <mixed-citation>MacArthur RH, Wilson EO (1967) The theory of island biogeography. Princeton University Press, Princeton, 244 pp.</mixed-citation>
      </ref>
      <ref id="B63">
        <mixed-citation>MacKinnon JG, White H (1985) Some heteroskedasticity-consistent covariance matrix estimators with improved finite sample properties. Journal of Econometrics 29: 305–325. <ext-link xlink:href="10.1016/0304-4076(85)90158-7" ext-link-type="doi">https://doi.org/10.1016/0304-4076(85)90158-7</ext-link></mixed-citation>
      </ref>
      <ref id="B64">
        <mixed-citation>Marcantonio M, Rocchini D, Ottaviani G (2014) Impact of alien species on dune systems: a multifaceted approach. Biodiversity and Conservation 23: 2645–2668. <ext-link xlink:href="10.1007/s10531-014-0742-2" ext-link-type="doi">https://doi.org/10.1007/s10531-014-0742-2</ext-link></mixed-citation>
      </ref>
      <ref id="B65">
        <mixed-citation>Mason NWH, de Bello F, Mouillot D, Pavoine S, Dray S (2013) A guide for using functional diversity indices to reveal changes in assembly processes along ecological gradients. Journal of Vegetation Science 24: 794–806. <ext-link xlink:href="10.1111/jvs.12013" ext-link-type="doi">https://doi.org/10.1111/jvs.12013</ext-link></mixed-citation>
      </ref>
      <ref id="B66">
        <mixed-citation>Matthews TJ (2021) On the biogeography of habitat islands: the importance of matrix effects, noncore species, and source–sink dynamics. Quarterly Review of Biology 96: 73–104. <ext-link xlink:href="10.1086/714482" ext-link-type="doi">https://doi.org/10.1086/714482</ext-link></mixed-citation>
      </ref>
      <ref id="B67">
        <mixed-citation>Matthews TJ, Guilhaumon F, Triantis KA, Borregaard MK, Whittaker RJ (2016) On the form of species–area relationships in habitat islands and true islands. Global Ecology and Biogeography 25: 847–858. <ext-link xlink:href="10.1111/geb.12269" ext-link-type="doi">https://doi.org/10.1111/geb.12269</ext-link></mixed-citation>
      </ref>
      <ref id="B68">
        <mixed-citation>Matthews TJ, Rigal F, Kougioumoutzis K, Trigas P, Triantis KA (2020) Unravelling the small-island effect through phylogenetic community ecology. Journal of Biogeography 47: 2341–2352. <ext-link xlink:href="10.1111/jbi.13940" ext-link-type="doi">https://doi.org/10.1111/jbi.13940</ext-link></mixed-citation>
      </ref>
      <ref id="B69">
        <mixed-citation>Mendez-Castro FE, Conti L, Chytrý M, Jiménez-Alfaro B, Hájek M, Horsák M, Zelený D, Malavasi M, Ottaviani G (2021) What defines insularity for plants in edaphic islands? Ecography 44: 1249–1258. <ext-link xlink:href="10.1111/ecog.05650" ext-link-type="doi">https://doi.org/10.1111/ecog.05650</ext-link></mixed-citation>
      </ref>
      <ref id="B125">
        <mixed-citation>Meyer K, Bürger AM, Eibes PM, Oldeland J, Irl SDH, Schmiedel U (2026) Species composition shifts across quartz island boundaries in the summer rainfall Nama-Karoo. South African Journal of Botany 190: 414–423. <ext-link xlink:href="10.1016/j.sajb.2026.01.043" ext-link-type="doi">https://doi.org/10.1016/j.sajb.2026.01.043</ext-link></mixed-citation>
      </ref>
      <ref id="B70">
        <mixed-citation>Moles AT, Warton DI, Warman L, Swenson NG, Laffan SW, Zanne AE, Pitman A, Hemmings FA, Leishman MR (2009) Global patterns in plant height. Journal of Ecology 97: 923–932. <ext-link xlink:href="10.1111/j.1365-2745.2009.01526.x" ext-link-type="doi">https://doi.org/10.1111/j.1365-2745.2009.01526.x</ext-link></mixed-citation>
      </ref>
      <ref id="B71">
        <mixed-citation>Mucina L, Jürgens N, le Roux A, Rutherford MC, Schmiedel U, Esler KJ, Powrie LW, Desmet PG, Milton SJ (2006a) Succulent Karoo Biome. In: Mucina L, Rutherford MC (Eds) The vegetation of South Africa, Lesotho and Swaziland. South African National Biodiversity Institute, Pretoria, Strelitzia 19: 220–299.</mixed-citation>
      </ref>
      <ref id="B72">
        <mixed-citation>Mucina L, Rutherford MC, Palmer AR, Milton SJ, Scott L, Lloyd JW, van der Merwe B, Hoare DB, Bezuidenhout H, Vlok JHJ, Euston-Brown DIW, Powrie LW, Dold AP (2006b) Nama–Karoo Biome. In: Mucina L, Rutherford MC (Eds) The vegetation of South Africa, Lesotho and Swaziland. South African National Biodiversity Institute, Pretoria, Strelitzia 19: 324–347.</mixed-citation>
      </ref>
      <ref id="B73">
        <mixed-citation>Musil CF, van Heerden PDR, Cilliers CD, Schmiedel U (2009) Mild experimental climate warming induces metabolic impairment and massive mortalities in southern African quartz field succulents. Environmental and Experimental Botany 66: 79–87. <ext-link xlink:href="10.1016/j.envexpbot.2008.11.008" ext-link-type="doi">https://doi.org/10.1016/j.envexpbot.2008.11.008</ext-link></mixed-citation>
      </ref>
      <ref id="B74">
        <mixed-citation>Musker SD, Ellis AG, Schlebusch SA, Verboom GA (2021) Niche specificity influences gene flow across fine-scale habitat mosaics in Succulent Karoo plants. Molecular Ecology 30: 175–192. <ext-link xlink:href="10.1111/mec.15721" ext-link-type="doi">https://doi.org/10.1111/mec.15721</ext-link></mixed-citation>
      </ref>
      <ref id="B75">
        <mixed-citation>Negoita L, Fridley JD, Lomolino MV, Mittelhauser G, Craine JM, Weiher E (2016) Isolation-driven functional assembly of plant communities on islands. Ecography 39: 1066–1077. <ext-link xlink:href="10.1111/ecog.01551" ext-link-type="doi">https://doi.org/10.1111/ecog.01551</ext-link></mixed-citation>
      </ref>
      <ref id="B76">
        <mixed-citation>Oksanen J, Simpson GL, Blanchet FG, Kindt R, Legendre P, Minchin PR, O’Hara RB, Solymos P, Stevens MHH, Szoecs E, Wagner H, Barbour M, Bedward M, Bolker B, Borcard D, Carvalho G, Chirico M, De Caceres M, Durand S, Evangelista H, FitzJohn R, Friendly M, Furneaux B, Hannigan G, Hill MO, Lahti L, McGlinn D, Ouellette M-H, Ribeiro Cunha E, Smith T, Stier A, ter Braak CJF, Weedon J, Borman T (2025) vegan: Community Ecology Package. R package version 2.6–10. <ext-link xlink:href="10.32614/CRAN.package.vegan" ext-link-type="doi">https://doi.org/10.32614/CRAN.package.vegan</ext-link></mixed-citation>
      </ref>
      <ref id="B77">
        <mixed-citation>Oldeland J, Eibes PM, Irl SDH, Schmiedel U (2022) Do image resolution and classifier choice impact island biogeographical parameters of terrestrial islands? Transactions in GIS 26: 2004–2022. <ext-link xlink:href="10.1111/tgis.12920" ext-link-type="doi">https://doi.org/10.1111/tgis.12920</ext-link></mixed-citation>
      </ref>
      <ref id="B78">
        <mixed-citation>Ottaviani G, Keppel G, Götzenberger L, Harrison S, Opedal ØH, Conti L, Liancourt P, Klimešová J, Silveira FAO, Jiménez-Alfaro B, Negoita L, Doležal J, Hájek M, Ibanez T, Méndez-Castro FE, Chytrý M (2020) Linking plant functional ecology to island biogeography. Trends in Plant Science 25: 329–339. <ext-link xlink:href="10.1016/j.tplants.2019.12.022" ext-link-type="doi">https://doi.org/10.1016/j.tplants.2019.12.022</ext-link></mixed-citation>
      </ref>
      <ref id="B79">
        <mixed-citation>Patton DR (1975) A diversity index for quantifying habitat edge. Wildlife Society Bulletin 3: 171–173.</mixed-citation>
      </ref>
      <ref id="B80">
        <mixed-citation>Pérez-Harguindeguy N, Díaz S, Garnier E, Lavorel S, Poorter H, Jaureguiberry P, Bret-Harte MS, Cornwell WK, Craine JM, Gurvich DE, Urcelay C, Veneklaas EJ, Reich PB, Poorter L, Wright IJ, Ray P, Enrico L, Pausas JG, De Vos AC, Buchmann N, Funes G, Quétier F, Hodgson JG, Thompson K, Morgan HD, Ter Steege H, Van Der Heijden MGA, Sack L, Blonder B, Poschlod P, Vaieretti MV, Conti G, Staver AC, Aquino S, Cornelissen JHC (2013) New handbook for standardised measurement of plant functional traits worldwide. Australian Journal of Botany 61: 167–234. <ext-link xlink:href="10.1071/bt12225" ext-link-type="doi">https://doi.org/10.1071/bt12225</ext-link></mixed-citation>
      </ref>
      <ref id="B81">
        <mixed-citation>Pérez-López AV, Lim SD, Cushman JC (2023) Humboldt review: tissue succulence in plants: carrying water for climate change. Journal of Plant Physiology 289: 154081. <ext-link xlink:href="10.1016/j.jplph.2023.154081" ext-link-type="doi">https://doi.org/10.1016/j.jplph.2023.154081</ext-link></mixed-citation>
      </ref>
      <ref id="B82">
        <mixed-citation>Qian H, Jin Y (2021) Are phylogenies resolved at the genus level appropriate for studies on phylogenetic structure of species assemblages? Plant Diversity 43: 255–263. <ext-link xlink:href="10.1016/j.pld.2020.11.005" ext-link-type="doi">https://doi.org/10.1016/j.pld.2020.11.005</ext-link></mixed-citation>
      </ref>
      <ref id="B83">
        <mixed-citation>R Core Team (2025) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.</mixed-citation>
      </ref>
      <ref id="B84">
        <mixed-citation>Ranghetti L, Boschetti M, Nutini F, Busetto L (2020) “sen2r”: An R toolbox for automatically downloading and preprocessing Sentinel-2 satellite data. Computers &amp; Geosciences 139: 104473. <ext-link xlink:href="10.1016/j.cageo.2020.104473" ext-link-type="doi">https://doi.org/10.1016/j.cageo.2020.104473</ext-link></mixed-citation>
      </ref>
      <ref id="B85">
        <mixed-citation>Roffe SJ, Fitchett JM, Curtis CJ (2021) Quantifying rainfall seasonality across South Africa on the basis of the relationship between rainfall and temperature. Climate Dynamics 56: 2431–2450. <ext-link xlink:href="10.1007/s00382-020-05597-5" ext-link-type="doi">https://doi.org/10.1007/s00382-020-05597-5</ext-link></mixed-citation>
      </ref>
      <ref id="B86">
        <mixed-citation>Romig KB, James DK, Maxwell CJ, Bestelmeyer BT, Brown JR, Salley SW, Faist AM (2025) Hidden biodiversity: Dryland soil seed banks across ecological sites and states. Journal of Arid Environments 227: 105307. <ext-link xlink:href="10.1016/j.jaridenv.2024.105307" ext-link-type="doi">https://doi.org/10.1016/j.jaridenv.2024.105307</ext-link></mixed-citation>
      </ref>
      <ref id="B87">
        <mixed-citation>Rutherford MC, Westfall RH (1986) Biomes of southern Africa – an objective categorization. Memoirs of the Botanical Survey of South Africa 54: 1–98.</mixed-citation>
      </ref>
      <ref id="B88">
        <mixed-citation>Sabino GP, Pinheiro F, Cabral JS, Koch I, Marcusso GM, Tavares MM, Cunha IM, Kamimura VA (2025) Islanded islands: dual isolation drives distinctive and threatened floras of Neotropical maritime inselbergs. Journal of Vegetation Science 36: 1–14. <ext-link xlink:href="10.1111/jvs.70037" ext-link-type="doi">https://doi.org/10.1111/jvs.70037</ext-link></mixed-citation>
      </ref>
      <ref id="B89">
        <mixed-citation>Samuels L, Kunst L, Jetter R (2008) Sealing plant surfaces: cuticular wax formation by epidermal cells. Annual Review of Plant Biology 59: 683–707. <ext-link xlink:href="10.1146/annurev.arplant.59.103006.093219" ext-link-type="doi">https://doi.org/10.1146/annurev.arplant.59.103006.093219</ext-link></mixed-citation>
      </ref>
      <ref id="B90">
        <mixed-citation>Schindelin J, Arganda-Carreras I, Frise E, Kaynig V, Longair M, Pietzsch T, Preibisch S, Rueden C, Saalfeld S, Schmid B, Tinevez JY, White DJ, Hartenstein V, Eliceiri K, Tomancak P, Cardona A (2012) Fiji: an open-source platform for biological-image analysis. Nature Methods 9: 676–682. <ext-link xlink:href="10.1038/nmeth.2019" ext-link-type="doi">https://doi.org/10.1038/nmeth.2019</ext-link></mixed-citation>
      </ref>
      <ref id="B91">
        <mixed-citation>Schmidt SA, Schmiedel U, Carstens F, Rau AL, Rudolph-Bartsch B (2025) Diversity on a small scale: phylogeography of the locally endemic dwarf succulent genus <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus">Oophytum</tp:taxon-name-part></tp:taxon-name></italic> (<tp:taxon-name><tp:taxon-name-part taxon-name-part-type="family">Aizoaceae</tp:taxon-name-part></tp:taxon-name>) in the Knersvlakte of South Africa. Annals of Botany 135: 735–756. <ext-link xlink:href="10.1093/aob/mcae207" ext-link-type="doi">https://doi.org/10.1093/aob/mcae207</ext-link></mixed-citation>
      </ref>
      <ref id="B92">
        <mixed-citation>Schmiedel U (2002) The quartz fields of southern Africa: flora, phytogeography, vegetation, and habitat ecology. PhD Thesis, University of Cologne, Cologne, Germany.</mixed-citation>
      </ref>
      <ref id="B93">
        <mixed-citation>Schmiedel U (2004) The phytogeography of the obligate quartz field flora of southern Africa. Biodiversity &amp; Ecology 2/ Schumannia 4: 181–205.</mixed-citation>
      </ref>
      <ref id="B94">
        <mixed-citation>Schmiedel U, Jürgens N (1999) Community structure on unusual habitat islands: quartz-fields in the Succulent Karoo, South Africa. Plant Ecology 142: 57–69. <ext-link xlink:href="10.1023/a:1009818210799" ext-link-type="doi">https://doi.org/10.1023/a:1009818210799</ext-link></mixed-citation>
      </ref>
      <ref id="B95">
        <mixed-citation>Schmiedel U, Jürgens N (2004) Habitat ecology of southern African quartz fields: studies on the thermal properties near the ground. Plant Ecology 170: 153–166. <ext-link xlink:href="10.1023/B:VEGE.0000021661.56381.67" ext-link-type="doi">https://doi.org/10.1023/B:VEGE.0000021661.56381.67</ext-link></mixed-citation>
      </ref>
      <ref id="B96">
        <mixed-citation>Schmiedel U, Mucina L (2006) Vegetation of quartz fields in the Little Karoo, Tanqua Karoo and eastern Overberg (Western Cape Province, South Africa). Phytocoenologia 36: 1–44. <ext-link xlink:href="10.1127/0340-269x/2006/0036-0001" ext-link-type="doi">https://doi.org/10.1127/0340-269x/2006/0036-0001</ext-link></mixed-citation>
      </ref>
      <ref id="B97">
        <mixed-citation>Schmiedel U, Kühne N, Twerski A, Oldeland J (2015) Small-scale soil patterns drive sharp boundaries between succulent “dwarf” biomes (or habitats) in the arid Succulent Karoo, South Africa. South African Journal of Botany 101: 129–138. <ext-link xlink:href="10.1016/j.sajb.2015.05.001" ext-link-type="doi">https://doi.org/10.1016/j.sajb.2015.05.001</ext-link></mixed-citation>
      </ref>
      <ref id="B98">
        <mixed-citation>Schrader J, Moeljono S, Keppel G, Kreft H (2019) Plants on small islands revisited: the effects of spatial scale and habitat quality on the species–area relationship. Ecography 42: 1405–1414. <ext-link xlink:href="10.1111/ecog.04512" ext-link-type="doi">https://doi.org/10.1111/ecog.04512</ext-link></mixed-citation>
      </ref>
      <ref id="B99">
        <mixed-citation>Schrader J, Wright IJ, Kreft H, Dixon K, Gallagher RV, Andrew SC, Weigelt P, Westoby M (2024) Trait filtering in island floras: a conceptual framework. Journal of Biogeography 51: 1596–1606. <ext-link xlink:href="10.1111/jbi.14703" ext-link-type="doi">https://doi.org/10.1111/jbi.14703</ext-link></mixed-citation>
      </ref>
      <ref id="B100">
        <mixed-citation>Shannon CE (1948) A mathematical theory of communication. Bell System Technical Journal 27: 379–423. <ext-link xlink:href="10.1002/j.1538-7305.1948.tb01338.x" ext-link-type="doi">https://doi.org/10.1002/j.1538-7305.1948.tb01338.x</ext-link></mixed-citation>
      </ref>
      <ref id="B101">
        <mixed-citation>Smith GF, Figueiredo E, Victor J, Klopper RR (2023) Plant poaching in southern Africa is aided by taxonomy: is a return to Caput bonae spei inevitable? Taxon 72: 717–723. <ext-link xlink:href="10.1002/tax.12882" ext-link-type="doi">https://doi.org/10.1002/tax.12882</ext-link></mixed-citation>
      </ref>
      <ref id="B102">
        <mixed-citation>Smith SA, Brown JW (2018) Constructing a broadly inclusive seed plant phylogeny. American Journal of Botany 105: 302–314. <ext-link xlink:href="10.1002/ajb2.1019" ext-link-type="doi">https://doi.org/10.1002/ajb2.1019</ext-link></mixed-citation>
      </ref>
      <ref id="B103">
        <mixed-citation>Soons MB, Heil GW, Nathan R, Katul GG (2004) Determinants of long-distance seed dispersal by wind in grasslands. Ecology 85: 3056–3068. <ext-link xlink:href="10.1890/03-0522" ext-link-type="doi">https://doi.org/10.1890/03-0522</ext-link></mixed-citation>
      </ref>
      <ref id="B104">
        <mixed-citation>Stein A, Kreft H (2015) Terminology and quantification of environmental heterogeneity in species-richness research. Biological Reviews 90: 815–836. <ext-link xlink:href="10.1111/brv.12135" ext-link-type="doi">https://doi.org/10.1111/brv.12135</ext-link></mixed-citation>
      </ref>
      <ref id="B105">
        <mixed-citation>Tang T, Li CH, Li D Sen, Jing SX, Hua J, Luo SH, Liu Y, Li SH (2020) Peltate glandular trichomes of <italic><tp:taxon-name><tp:taxon-name-part taxon-name-part-type="genus">Colquhounia</tp:taxon-name-part> <tp:taxon-name-part taxon-name-part-type="species">vestita</tp:taxon-name-part></tp:taxon-name></italic> harbor diterpenoid acids that contribute to plant adaptation to UV radiation and cold stresses. Phytochemistry 172: 112285. <ext-link xlink:href="10.1016/j.phytochem.2020.112285" ext-link-type="doi">https://doi.org/10.1016/j.phytochem.2020.112285</ext-link></mixed-citation>
      </ref>
      <ref id="B106">
        <mixed-citation>Thomson FJ, Moles AT, Auld TD, Kingsford RT (2011) Seed dispersal distance is more strongly correlated with plant height than with seed mass. Journal of Ecology 99: 1299–1307. <ext-link xlink:href="10.1111/j.1365-2745.2011.01867.x" ext-link-type="doi">https://doi.org/10.1111/j.1365-2745.2011.01867.x</ext-link></mixed-citation>
      </ref>
      <ref id="B107">
        <mixed-citation>Tsuruoka Y, Tsujii J, Ananiadou S (2009) Stochastic gradient descent training for L1-regularized log-linear models with cumulative penalty. In: Su K-Y, Su J, Wiebe J, Li H (Eds) Proceedings of the Joint Conference of the 47<sup>th</sup> Annual Meeting of the ACL and the 4<sup>th</sup> International Joint Conference on Natural Language Processing of the AFNLP. Association for Computational Linguistics, Suntec, Singapore, 477–485. <ext-link xlink:href="10.3115/1687878.1687946" ext-link-type="doi">https://doi.org/10.3115/1687878.1687946</ext-link></mixed-citation>
      </ref>
      <ref id="B108">
        <mixed-citation>Tucker CJ (1979) Red and photographic infrared linear combinations for monitoring vegetation. Remote Sensing of Environment 8: 127–150. <ext-link xlink:href="10.1016/0034-4257(79)90013-0" ext-link-type="doi">https://doi.org/10.1016/0034-4257(79)90013-0</ext-link></mixed-citation>
      </ref>
      <ref id="B109">
        <mixed-citation>Tucker CM, Cadotte MW, Carvalho SB, Davies TJ, Ferrier S, Fritz SA, Grenyer R, Helmus MR, Jin LS, Mooers AO, Pavoine S, Purschke O, Redding DW, Rosauer DF, Winter M, Mazel F (2017) A guide to phylogenetic metrics for conservation, community ecology and macroecology. Biological Reviews 92: 698–715. <ext-link xlink:href="10.1111/brv.12252" ext-link-type="doi">https://doi.org/10.1111/brv.12252</ext-link></mixed-citation>
      </ref>
      <ref id="B110">
        <mixed-citation>Večeřa M, Axmanová I, Chytrý M, Divíšek J, Ndiribe C, Velasco Mones G, Čeplová N, Aćić S, Bahn M, Bergamini A, Boenisch G, Biurrun I, Bruun HH, Byun C, Catford JA, Cerabolini BEL, Cornelissen JHC, Dengler J, Jansen F, Jansen S, Kattge J, Kozub Ł, Kuzemko A, Minden V, Mitchell RM, Moeslund JE, Mori AS, Niinemets Ü, Ruprecht E, Ruņina S, Šilc U, Soudzilovskaia NA, van Bodegom PM, Vassilev K, Weiher E, Wright IJ, Lososová Z (2023) Decoupled phylogenetic and functional diversity in European grasslands. Preslia 95: 413–445. <ext-link xlink:href="10.23855/preslia.2023.413" ext-link-type="doi">https://doi.org/10.23855/preslia.2023.413</ext-link></mixed-citation>
      </ref>
      <ref id="B111">
        <mixed-citation>Venable DL, Brown JS (1988) The selective interactions of dispersal, dormancy, and seed size as adaptations for reducing risk in variable environments. The American Naturalist 131: 360–384. <ext-link xlink:href="10.1086/284795" ext-link-type="doi">https://doi.org/10.1086/284795</ext-link></mixed-citation>
      </ref>
      <ref id="B112">
        <mixed-citation>Venables WN, Ripley BD (2002) Modern applied statistics with S. 4<sup>th</sup> edn. Springer, New York. <ext-link xlink:href="10.1007/978-0-387-21706-2" ext-link-type="doi">https://doi.org/10.1007/978-0-387-21706-2</ext-link></mixed-citation>
      </ref>
      <ref id="B113">
        <mixed-citation>Villa P, Berton A, Bolpagni R, Caccia M, Castellani MB, Dalla Vecchia A, Gallivanone F, Lastrucci L, Piaser E, Coppi A (2025) Exploring spectral and phylogenetic diversity links with functional structure of aquatic plant communities. Remote Sensing of Environment 318: 114582. <ext-link xlink:href="10.1016/j.rse.2024.114582" ext-link-type="doi">https://doi.org/10.1016/j.rse.2024.114582</ext-link></mixed-citation>
      </ref>
      <ref id="B114">
        <mixed-citation>von Willert DJ, Brinckmann E, Scheitler B, Eller BM (1985) Availability of water controls Crassulacean acid metabolism in succulents of the Richtersveld (Namib Desert, South Africa). Planta 164: 44–55. <ext-link xlink:href="10.1007/bf00391034" ext-link-type="doi">https://doi.org/10.1007/bf00391034</ext-link></mixed-citation>
      </ref>
      <ref id="B115">
        <mixed-citation>Walker AP, Beckerman AP, Gu L, Kattge J, Cernusak LA, Domingues TF, Scales JC, Wohlfahrt G, Wullschleger SD, Woodward FI (2014) The relationship of leaf photosynthetic traits – Vcmax and Jmax – to leaf nitrogen, leaf phosphorus, and specific leaf area: a meta-analysis and modeling study. Ecology and Evolution 4: 3218–3235. <ext-link xlink:href="10.1002/ece3.1173" ext-link-type="doi">https://doi.org/10.1002/ece3.1173</ext-link></mixed-citation>
      </ref>
      <ref id="B116">
        <mixed-citation>Wang Z, Huang H, Wang H, Peñuelas J, Sardans J, Niinemets Ü, Niklas KJ, Li Y, Xie J, Wright IJ (2022) Leaf water content contributes to global leaf trait relationships. Nature Communications 13: 5525. <ext-link xlink:href="10.1038/s41467-022-32784-1" ext-link-type="doi">https://doi.org/10.1038/s41467-022-32784-1</ext-link></mixed-citation>
      </ref>
      <ref id="B117">
        <mixed-citation>Whittaker RJ, Fernández-Palacios JM (2007) Island biogeography: ecology, evolution, and conservation. 2<sup>nd</sup> edn. Oxford University Press, Oxford. <ext-link xlink:href="10.1093/oso/9780198566113.001.0001" ext-link-type="doi">https://doi.org/10.1093/oso/9780198566113.001.0001</ext-link></mixed-citation>
      </ref>
      <ref id="B118">
        <mixed-citation>Wickham H (2016) ggplot2: elegant graphics for data analysis. Springer International Publishing, Cham. <ext-link xlink:href="10.1007/978-3-319-24277-4" ext-link-type="doi">https://doi.org/10.1007/978-3-319-24277-4</ext-link></mixed-citation>
      </ref>
      <ref id="B119">
        <mixed-citation>Zanne AE, Tank DC, Cornwell WK, Eastman JM, Smith SA, FitzJohn RG, McGlinn DJ, O’Meara BC, Moles AT, Reich PB, Royer DL, Soltis DE, Stevens PF, Westoby M, Wright IJ, Aarssen LW, Bertin RI, Calaminus A, Govaerts R, Hemmings F, Leishman MR, Oleksyn J, Soltis PS, Swenson NG, Warman L, Beaulieu JM (2014) Three keys to the radiation of angiosperms into freezing environments. Nature 506: 89–92. <ext-link xlink:href="10.1038/nature12872" ext-link-type="doi">https://doi.org/10.1038/nature12872</ext-link></mixed-citation>
      </ref>
      <ref id="B120">
        <mixed-citation>Zeileis A (2004) Econometric computing with HC and HAC covariance matrix estimators. Journal of Statistical Software 11: 1–17. <ext-link xlink:href="10.18637/jss.v011.i10" ext-link-type="doi">https://doi.org/10.18637/jss.v011.i10</ext-link></mixed-citation>
      </ref>
      <ref id="B121">
        <mixed-citation>Zeileis A, Köll S, Graham N (2020) Various versatile variances: an object-oriented implementation of clustered covariances in R. Journal of Statistical Software 95: 1–36. <ext-link xlink:href="10.18637/jss.v095.i01" ext-link-type="doi">https://doi.org/10.18637/jss.v095.i01</ext-link></mixed-citation>
      </ref>
      <ref id="B122">
        <mixed-citation>Zhang A, Cadotte MW, Wu D, Yu M (2023) What drives phylogenetic and trait clustering on islands? Landscape Ecology 38: 1339–1350. <ext-link xlink:href="10.1007/s10980-023-01629-7" ext-link-type="doi">https://doi.org/10.1007/s10980-023-01629-7</ext-link></mixed-citation>
      </ref>
      <ref id="B123">
        <mixed-citation>Zhang J, Qian H (2023) U.Taxonstand: an R package for standardizing scientific names of plants and animals. Plant Diversity 45: 1–5. <ext-link xlink:href="10.1016/j.pld.2022.09.001" ext-link-type="doi">https://doi.org/10.1016/j.pld.2022.09.001</ext-link></mixed-citation>
      </ref>
      <ref id="B124">
        <mixed-citation>Zhigila DA, Elliott TL, Schmiedel U, Muasya AM (2024) Do phylogenetic community metrics reveal the South African quartz fields as terrestrial-habitat islands? Annals of Botany 133: 833–849. <ext-link xlink:href="10.1093/aob/mcae027" ext-link-type="doi">https://doi.org/10.1093/aob/mcae027</ext-link></mixed-citation>
      </ref>
    </ref-list>
    <sec sec-type="supplementary-material">
      <title>Supplementary materials</title>
      <supplementary-material id="S1" position="float" orientation="portrait" xlink:type="simple">
        <object-id content-type="doi">10.21425/fob.19.167957.suppl1</object-id>
        <object-id content-type="arpha">12AEEF66-41CF-5F7E-B069-BD046A4F5571</object-id>
        <label>Supplementary materials
 Supplementary material 1</label>
        <caption>
          <p>Supplementary tables S1–S6, figs S1–S5 and method S1 (.docx)</p>
        </caption>
        <media xlink:href="biogeography-19-001_article-167957__-s001.docx" mimetype="application" mime-subtype="vnd.openxmlformats-officedocument.wordprocessingml.document" position="float" orientation="portrait" id="oo_1543467.docx">
          <uri content-type="original_file">https://binary.pensoft.net/file/1543467</uri>
        </media>
      </supplementary-material>
    </sec>
  </back>
</article>
