Ecological explanations to island gigantism: dietary niche divergence, predation, and size in an endemic lizard

Abstract

Although rapid evolution of body size on islands has long been known, the ecological mechanisms behind this island phenomenon remain poorly understood. Diet is an important selective pressure for morphological divergence. Here we investigate if selection for novel diets has contributed to the multiple independent cases of island gigantism in the Skyros wall lizard (Podarcis gaigeae) and if diet, predation, or both factors best explain island gigantism. We combined data on body size, shape, bite force, and realized and available diets to address this. Several lines of evidence suggest that diet has contributed to the island gigantism. The larger islet lizards have relatively wider heads and higher bite performance in relation to mainland lizards than would be expected from size differences alone. The proportions of consumed and available hard prey are higher on islets than mainland localities, and lizard body size is significantly correlated with the proportion of hard prey. Furthermore, the main axis of divergence in head shape is significantly correlated with dietary divergence. Finally, a model with only diet and one including diet and predation regime explain body size divergence equally well. Our results suggest that diet is an important ecological factor behind insular body size divergence, but could be consistent with an additional role for predation.

The island rule: An assessment of biases and research trends

“…While it has been supported for enigmatic taxa such as spring tails (Ulrich & Fiera, ) or cave‐dwelling plant hoppers (Hoch, Bourgoin, Stelbrink, & Wessel, ), several studies have questioned the rule's validity, e.g. for lizards (Itescu, Karraker, Raia, Pritchard, & Meiri, ; Meiri, ; Runemark, Sagonas, & Svensson, ).…”

Parental behavior drives large bite force in an insular skink population

“…The island rule and island syndrome hypotheses represent the changes of phenotypic or life-history traits between mainland and island relatives (Lomolino, 2005;Novosolov et al, 2013). Body gigantism on island may due to ecological release, such as lower intraspecific competition, resources competition or predation pressure (Li et al, 2011;Runemark, Sagonas & Svensson, 2015). The larger body size in Orchid Island and Green Island may fit the island syndrome compared to the small size in Kenting population.…”

Predator size divergence depends on community context

“…; Runemark et al . ). The composition and strength of selection pressures acting on a trait vary among geographic locations and communities.…”

“…Geographic variations in such traits in a species can be affected by differences in abiotic factors, such as climatic conditions (Masaki 1967;Blanckenhorn & Demont 2004), in biotic factors such as the presence/absence of competitors and predators (Schluter 2000;Rundle et al 2003;Herczeg et al 2009;Pfennig & Pfennig 2009), and in the composition of species and/or species' character states with which the species interact (e.g. prey) (Ryan et al 2007;Runemark et al 2015). The composition and strength of selection pressures acting on a trait vary among geographic locations and communities.…”

“…To understand geographic divergence in body size, we need to clarify impacts of varying selection pressures from abiotic and biotic factors in a community context. However, how changes in multiple selective factors among geographic locations and communities have contributed to adaptive body size divergence is not fully demonstrated (Rundle et al 2003;Herczeg et al 2009;Runemark et al 2015).…”