Data from: Darwin’s finches and their diet niches: the sympatric coexistence of imperfect generalists

Adaptive radiation can be strongly influenced by interspecific competition for resources, which can lead to diverse outcomes ranging from competitive exclusion to character displacement. In each case, sympatric species are expected to evolve into distinct ecological niches, such as different food types, yet this expectation is not always met when such species are examined in nature. The most common hypotheses to account for the coexistence of species with substantial diet overlap rest on temporal variation in niches (often diets). Yet spatial variation in niche overlap might also be important, pointing to the need for spatiotemporal analyses of diet and diet overlap between closely related species persisting in sympatry. We here perform such an analysis by characterizing the diets of, and diet overlap among, four sympatric Darwin's ground finch species at three sites and over 5 years on a single Galápagos island (Santa Cruz). We find that the different species have broadly similar and overlapping diets – they are to some extent generalists and opportunists – yet we also find that each species retains some ‘private’ resources for which their morphologies are best suited. Importantly, use of these private resources increased considerably, and diet overlap decreased accordingly, when the availability of preferred shared foods, such as arthropods, was reduced during drought conditions. Spatial variation in food resources was also important. These results together suggest that the ground finches are ‘imperfect generalists’ that use overlapping resources under benign conditions (in space or time), but then retreat to resources for which they are best adapted during periods of food limitation. These conditions likely promote local and regional coexistence.

适应辐射（Adaptive radiation）极易受到资源种间竞争（interspecific competition）的影响，该过程可产生从竞争排除（competitive exclusion）到性状替换（character displacement）等多样结果。在任一情形下，同域物种（sympatric species）理论上应演化出截然不同的生态位（ecological niches），例如不同的食物类型，但野外实地调查中该预期往往难以完全达成。解释食性高度重叠（diet overlap）物种共存现象的主流假说，大多立足于生态位（通常为食性）的时间变异（temporal variation）。然而生态位重叠的空间变异（spatial variation）或许同样关键，这提示我们需要对维持同域分布的近缘物种间的食性及食性重叠开展时空分析（spatiotemporal analyses）。本研究针对加拉帕戈斯群岛（Galápagos Islands）圣克鲁斯岛（Santa Cruz）上的4种同域分布达尔文地雀（Darwin's ground finch），在3个采样点开展了为期5年的调查，以此表征其食性及种间食性重叠程度，完成了此类时空分析。研究结果显示，不同物种的食性整体相似且存在广泛重叠：它们在一定程度上属于泛化摄食者与机会主义者，但同时每个物种也保留有部分专属资源，这些资源与其形态特征高度适配。尤为关键的是，当干旱条件下节肢动物（arthropods）这类偏好共享食物的可获得性下降时，物种对专属资源的利用量会显著提升，食性重叠程度也随之降低。食物资源的空间变异同样具有重要影响。综合上述结果可知，达尔文地雀属于‘非完美泛化物种’：在环境适宜的条件下（空间或时间维度上），它们会利用存在重叠的共享资源；而在食物匮乏时期，则会转而取用与其形态适配的专属资源。这类生存策略或可促进物种在局域及区域尺度上实现共存。