Optimal foraging theory explains how animals make foraging decisions based on the availability, nutritional content, and handling times of different food types. Generalists solve this problem by consuming a variety of food types, and alter their diets with relative ease. Specialists eat few food types, and may starve if those food types are not available. We integrated stable isotope analyses with previously-published stomach contents and environmental data to investigate how the foraging ecologies of three sympatric freshwater turtle species vary across four wetlands that differ in turbidity and primary producer abundance. We found that the generalist Emydura macquarii consumes a varied diet (but mostly filamentous green algae) when primary producers are available and water is clear, but switches to a more carnivorous diet when the water is turbid and primary producers are scarce, following the predictions of optimal foraging theory. In contrast, two more-specialized carnivorous species, Chelodina expansa and Chelodina longicollis, do not differ in diet across wetlands, and interspecific competition may increase where E. macquarii is carnivorous. When forced to be more carnivorous, E. macquarii exhibits higher rates of empty stomachs, and female turtles have reduced body condition, but neither Chelodina species are affected. Our results provide support for optimal foraging theory, but also show that the ability to change diet does not protect the generalist from experiencing lower foraging success when its preferred food is rare, with direct consequences for their energy budgets. Our results have conservation implications because wetlands in the Murray–Darling river system are increasingly turbid and have low macrophyte abundance, and all three species are declining.

中文翻译：

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当多面龟偏爱的食物稀缺时，猎物转换并不能保护多面龟免受生物能后果的影响


最佳觅食理论解释了动物如何根据不同食物类型的可用性、营养成分和处理时间做出觅食决策。通才通过食用多种食物来解决这个问题，并且相对容易地改变他们的饮食。专家吃的食物类型很少，如果没有这些食物类型，他们可能会挨饿。我们将稳定同位素分析与先前发表的胃内容物和环境数据相结合，以研究三种同域淡水龟物种的觅食生态在四个浊度和初级生产者丰度不同的湿地中有何不同。我们发现，当初级生产者可用且水清澈时，通才 Emydura macquarii 会消耗多种饮食（但主要是丝状绿藻），但当水浑浊且初级生产者稀缺时，会转向更多的肉食性饮食，如下预测最优觅食理论。相比之下，两种更专业的肉食性物种，Chelodina expansa 和 Chelodina longicollis，在整个湿地的饮食上没有差异，并且在 E. macquarii 是肉食性的地方，种间竞争可能会加剧。当被迫变得更加食肉时，E. macquarii 表现出更高的空腹率，雌性海龟的身体状况也有所下降，但两种 Chelodina 物种都没有受到影响。我们的研究结果为最佳觅食理论提供了支持，但也表明，改变饮食的能力并不能保护通才在其偏爱的食物稀有时避免觅食成功率较低，从而直接影响其能量预算。我们的结果具有保护意义，因为墨累-达令河水系的湿地越来越浑浊，大型植物丰度较低，而且所有三种物种都在减少。