Hypotheses testing
Our goal was to derive the predictions of the island biogeographic theory from the different roles that community-level processes—speciation, community-level interactions, and extinction—might have at each island given their ontogenetic state (young, mature, senescent) (Figure 1A). In our system, the role of immigration is negligible, given that almost all Canarian subterranean species are single-island endemic species that have evolved within each island (Oromí et al. 1991).
Our approach was to first provide a description of the trait space of each island using n-dimensional hypervolumes. We described those hypervolumes in terms of functional richness, which we used as a proxy of total niche occupancy, and functional evenness, which reflects the homogeneity of the distribution of species within this niche space. Then, we looked at the role of individual species in each island. First, we estimated each species contribution to the islands’ hypervolume; second, we explored the relationship between species’ functional distances and niche/geographical occupancy using partial mantel permutational tests. Lastly, we explored whether the presence of strong species interactions leads to a non-random distribution of functional traits in each island using null modelling. In this last step, we connected the macroscopic description of the island states from step 1, with the community-level processes disentangled in steps 2 and 3. The hypotheses specifically tested at each of the four steps are provided below, along with a detailed description of the methods.