Climate, predation, and the controls of island lizard abundance and community structure

Alternative ecological theories make divergent predictions about the relationship between predators and their prey. If predators exert top-down ecosystem control, increases in predation should diminish prey abundance and could either diminish or enhance community diversity of prey species. However, if bottom-up ecosystem controls predominate, predator populations should track underlying variation in prey diversity and abundance, which ultimately should reflect available energy. Past research, both across islands and comparing islands with the mainland, has frequently invoked the importance of predation in regulating lizard abundance and diversity, suggesting an important role of top-down control when predators are present. However, others have posited a stronger role of food limitation, via competition or bottom-up forces. If top-down control predominates, then negative correlations between prey abundance and predator occurrence should emerge within and among islands. Using data from eBird, we inferred landscape-level presence data for bird species on the islands of Jamaica and Hispaniola. By summing occurrence probabilities of all known anole-predator birds, we estimated total avian predation pressure and combined these estimates with anole community data from a mark-recapture study that spanned spatial and climatic gradients on both islands. Avian predators and anole lizards were both affected by climate, with total predator occurrence, anole abundance and anole species richness increasing with mean annual temperature. Anole abundance and predator occurrence showed a curvilinear relationship, where abundance and predator occurrence increased together until predator occurrence became sufficiently high that anole abundance was negatively impacted. This indicates that bottom-up ecosystem controls drive richness of both anoles and their predators, mitigating the negative effects predators might have on their prey, at least until predator occurrence reaches a threshold. We did not detect consistent evidence of predator occurrence reducing anole community richness. These findings support past research showing that islands with more predators tend to have lower prey abundances, but it does not seem that these top-down forces are strongly limiting species coexistence. Instead, bottom-up forces linked with climate may be more important drivers of diversity in both lizards and their avian predators on these islands.