Wild bee-plant network of Madeira Island with a comparison to Porto Santo (Madeira Archipelago, Portugal)

The aim of this study was to analyse wild bee-plant interactions in Madeira Island and construct a network structure covering the largest parts of the island and most altitudes and habitat types. The interaction data were sampled at 179 localities from 1989 to 2012 mainly by the authors; additionally, data from the collections of the Laboratório Agrícola da Madeira, Camacha (ICLAM) and the Museu de História Natural do Funchal, Madeira, Portugal, were used. Twelve wild-bee species (the complete species pool of permanently established Madeiran wild-bee species) were collected or observed on 81 plant taxa (belonging to 30 plant families). The total dataset from Madeira Island includes 637 data points of bee-plant interactions. We assigned the data set to the main thermotypes – infra- / thermo-Mediterranean zone as well as meso-Mediterranean / temperate zone – and constructed two networks. In contrast to most of the mainland networks of temperate and Mediterranean ecosystems with more wild-bee than plant species, the bee-plant networks demonstrate high asymmetry, with many more plant than bee species. This phenomenon has also been documented, e.g., in Porto Santo (Madeira Archipelago). Wild-bee species and plant species were highly interconnected within the network, showing that even under difficult and strong climatic conditions with unequal resource availabilities, there were enough alternatives for obtaining food resources. Ten of the wild-bee species are euryanthic, with only two species (the introduced Hylaeus pictipes and the endemic Andrena maderensis) showing stenanthy. The high individual numbers and euryanthic behaviour of most wild-bee species met the precondition for network stability. The plant families with the highest importance as food resources are in 
the orders Asteraceae (51%), Brassicaceae (14%), Fabaceae (8%) and Boraginaceae (8%). These offer different flower types (actinomorphic, zygomorphic, Asteroideae and Cichorioideae types) and a variability of flower types for wild-bee species of different body sizes and foraging behaviour. Similar to the results of the bee-plant network of Porto Santo, most of the foraged plant species were endemic or native species. Introduced and cultivated plant species mainly showed minor importance. We found no disturbances of the network by the introduced masked bee Hylaeus s. signatus and the probably introduced bumblebee Bombus r. ruderatus. Differences and similarities between the bee-plant networks of Madeira Island (infra- / thermo-Mediterranean zone) and the corresponding areas of the much older island Porto Santo (xeric- / infra-Mediterranean zone) are discussed in this paper