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

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.

尽管学界早已熟知岛屿生境中生物体型的快速演化，但这一岛屿现象背后的生态机制仍未得到充分阐释。食性是推动形态分化的关键选择压力。本研究以斯凯罗斯壁蜥（Podarcis gaigeae）为研究对象，旨在探究两个核心问题：针对新型食性的选择是否促成了该物种多起独立演化的岛屿巨化事件，以及食性、捕食压力或二者共同作用能否最优解释岛屿巨化现象。我们整合了体型、躯体形态、咬合力，以及实际摄食食谱与可利用猎物类群的相关数据以开展相关研究。多项证据表明食性在岛屿巨化过程中发挥了关键作用：相较于仅由体型差异所能预期的水平，小岛种群壁蜥的头部相对更宽，咬合力也更强。小岛生境中，被摄食的硬质猎物与可获得的硬质猎物占比均高于大陆分布区，且蜥蜴体型与硬质猎物占比呈显著正相关。进一步而言，头部形态分化的核心轴线与食性分化显著相关。最后，仅纳入食性的模型，与同时纳入食性和捕食压力的模型，对体型分化的解释力相当。本研究结果表明，食性是驱动岛屿体型分化的重要生态因子，但也不排除捕食压力存在额外贡献的可能性。