More and bigger lizards reside on islands with more resources

Abstract

AbstractWhile global patterns in body size evolution in island vertebrates have been described extensively, the ecological processes that generate these patterns are not well understood. Here we used variation among lizard populations occupying an archipelago to test hypotheses about body size evolution. We examined 35 populations of Aegean wall lizards (Podarcis erhardii, Lacertidae), a species widely distributed across the Balkan mainland and hundreds of Aegean islands. We evaluated measures of resource availability (island area and seabird density), intraspecific competition (lizard abundance) and predation risk (presence of rats, carnivorous mammals, vipers, and birds of prey) as possible factors affecting lizard body size. Lizard body size increased with island size overall, as well as with seabird colony density, suggesting a role for increased food resources and especially seabird subsidies in the evolution of body size. Lizards were larger where lizard population density was higher, suggesting a possible role for intraspecific competition in the evolution of body size. In our sample, rats, carnivorous mammals, and vipers did not have obvious effects on lizard body size, and lizards were smaller on islands with resident birds of prey. Males were larger than females on average, yet sexual dimorphism did not vary consistently with measures of resource availability and competition. Overall, our results suggest that local resource levels predict population density and body size in these lizards, and that nesting seabirds in particular can substantially affect lizard body size.