Interspecific differences in area-limitation versus isolation-limitation produce non-nested mammal communities on Maine’s small islands

Differences between island community compositions can be the result of differences in the ways species respond to island area and isolation. One species may be better at both dispersing to and surviving on islands than another species, making the latter’s distribution nested within the former’s. Alternatively, one species may be more area-limited and another more isolation-limited, creating non-nested distributions. While these patterns have been anecdotally observed before, we sought to directly test for these differences in the ratio of area-limitation to isolation-limitation between species, and whether these differences produced non-nested patterns across area and isolation. To do so, we visited 40 islands in Maine, USA, placing camera traps to detect different land mammal taxa. We modeled each taxon’s presences and absences with simple logistic models incorporating area and isolation, comparing AICc scores using Markov Chain Monte Carlo to quantify support for interspecific differences in the ratio of area- to isolation-limitation and to find the probability of nested and non-nested distributions between taxa. We found that American red squirrels (Tamiasciurus hudsonicus) and shrews (Sorex sp.) are comparatively more isolation-limited than the more area-limited white-tailed deer (Odocoileus virginianum) and raccoons (Procyon lotor), and that non-nested distributions are expected between squirrels and raccoons, squirrels and deer, and shrews and raccoons. Most taxa, meanwhile, were nested within the distribution of voles (Arvicolinae). Modeling these distributions and their interspecific differences has uncovered a mosaic of island community types occupying different regions of area/isolation variable-space.