Differential speciation rates, colonization time, and niche conservatism affect community assembly across adjacent biogeographical regions

Aim: To test the importance of evolutionary and biogeographic processes in shaping the assembly of local frog communities in two adjacent regions (hereafter, coastal and inland regions) with different historical signatures. We asked two main questions: i) why does the coastal region harbor more frog species than the inland region? and ii) how do these processes affect the relationship between the spatial variation of taxonomic, phylogenetic, and functional diversities within and across these regions? 

Location: southeastern Brazil Taxon: Anurans

Methods: We generated time-calibrated phylogenies to estimate the relative timing of colonization, rates of speciation, extinction, and dispersal between regions. We tested the phylogenetic signal in reproductive modes. These traits were also used to examine variation in functional composition across sites. We calculated metrics of phylogenetic community structure that capture the relationships near the root and tips of the tree. Finally, we tested the relationships between the spatial variation of multiple diversity dimensions and topographic complexity, Pleistocene and contemporary climate gradients for three spatial extents: (i) only coastal sites; (ii) only inland sites; and (iii) the two regions combined.

Results: The structure of communities was related to the region in which they are located, with regional pool size being two times greater for the coastal than inland region. This pattern seems to reflect both a higher speciation rate and earlier colonization time in the coastal than in the inland region. Reproductive modes within frog genera were less variable than among families, indicating phylogenetic signal. This pattern influenced local community assembly within the inland region due to the absence of species with direct development, tadpoles in bromeliads, or eggs and tadpoles in streams in this region.

Main Conclusions: Macroevolutionary dynamics, such as colonization time, differences in speciation rates, and niche conservatism generate the disparity in species richness and assembly patterns of local communities between regions, but not within regions, in which local communities were more similar to each other.