Body size structure of Pleistocene mammalian communities: what support is there for the "island rule"?

Islands are often regarded by scientists as living laboratories of evolution and an optimal context for the study of forces influencing evolution and diversification. Two main issues have been attentively scrutinized and debated: the loss of biodiversity and the peculiar changes undergone by island settlers, primarily changes in size of endemic vertebrates. Over time, several hypotheses have been formulated to explain the causal mechanism of body size modification. Faunas of those islands where mainland taxa migrate more than once provide the most interesting data to answer the question of whether or not trends of insular taxa result from a predictable response to differences in competition and availability of niches between insular and mainland environments. To contribute to the debate, the body size structure of the Pleistocene mammalian faunas from two Mediterranean islands, Sicily and Crete, were analyzed and compared with the structure of coeval mainland faunas. The results obtained suggest that: (i) size of endemic species does not directly depend on the area of islands; (ii) evolution and size of endemic species seems somewhat affected by the degree of isolation (constraining colonization from mainland) and physiography (sometimes permitting adaptive radiation); (iii) in unbalanced insular communities, the shift in size of non-carnivorous species largely depends on the nature of competing species; and (iv) body size of carnivorous species mainly depends on the size of the most available prey. Consequently, it is rational to suppose that the body size of insular mammals mainly results from the peculiar biological dynamics that characterizes unbalanced insular communities. Ecological interaction, particularly the intraguild competition, is the major driver behind the evolution of insular communities, leading towards an optimization of energy balance through a change in body size of endemic settlers.