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.