Drivers of beta diversity in Macaronesian spiders in relation to dispersal ability

Aim Our aim was to determine the relative contribution of species replacement and richness differences to overall beta diversity in Macaronesian spiders, the influence of several biogeographical drivers in shaping such dissimilarity patterns, and how these change according to the dispersal ability of spiders. Location Four Macaronesian archipelagos: the Azores, Madeira, Selvagens and the Canary Islands. Methods Each spider species was assigned to a group relative to its ballooning propensity (frequent, occasional or rare), used as a surrogate of dispersal ability, based on its family membership. Beta diversity was decomposed for each group, by disentangling all compositional differences (overall beta diversity, βtotal) into two components, species replacement (βrepl) and species richness differences (βrich). The effects of island area, environmental heterogeneity, geological age, distance to mainland and inter-island distances on βrepl and βrich were tested by partial Mantel tests and hierarchical partitioning of variation for each mobility group. Results The archipelagos studied had similar intra-archipelagic richness differences, but species replacement was lower within the Azores for the three groups of spiders. The variation in community composition among the archipelagos was determined by species replacement for frequently ballooning spiders, while richness differences dominated for less mobile spiders. Island area was more important for species with higher mobility, while less mobile species were mostly affected by the distance to mainland and inter-island distances. Environmental heterogeneity had an effect, mostly on richness differences, across the three spider groups. Time had only a weak effect on species replacement for less mobile species. Main conclusions The partition of βtotal into βrepl and βrich identified different causes of beta-diversity patterns as driven by the dispersal ability of spiders. Dispersal-limited species responded more strongly to isolation than more mobile species. In contrast, the latter were influenced more by island area. Thus our findings emphasize the importance of interspecific traits and dispersal limitation for structuring species assemblages on islands.