Environmental and geomorphological drivers of frog diversity on islands worldwide

Island biogeography models primarily rely on island physical features and isolation to explain their biodiversity patterns. While newer models have incorporated functional traits to understand plant distribution, few empirical studies have tried to disentangle geometric constraints from niche‐based processes to predict multiple diversity facets of island animals. Frogs are dispersal‐limited organisms with narrow physiological requirements, and little is known about how ecological and geomorphological factors dictate their distribution on islands. Here, we tested how climate, productivity, environmental heterogeneity, isolation, and island area influence frog species richness, functional dispersion (FDis), and evolutionary distinctiveness (ED) on islands worldwide using structural equation models. Quantile regression was used to further explore the influence of island size and isolation on diversity facets. We found a positive association of island area and climate (i.e. temperature) with diversity metrics, while isolation had no effect in most of them. Notably, the influence of island area, but not isolation, was more pronounced on highly diverse islands. The relative importance of predictor variables differed between tropical and temperate islands and across facets: geometric constraints were more important for determining species richness and ED in all islands and in tropical islands, while niche‐related variables dictated FD in all and both tropical and temperate islands. The low tolerance of frogs for crossing seawater may explain the lack of an isolation effect.