Prey-switching does not protect a generalist turtle from bioenergetic consequences when its preferred food is scarce

Background: 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 switch between them
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.   Results: 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 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.   Conclusions: 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.

背景：最优觅食理论（Optimal Foraging Theory）阐释了动物如何依据不同食物的可获得性、营养成分及处理时长做出觅食决策。广食性物种通过摄取多种食物类型并能相对轻松地在不同食物间切换来应对该问题；而特食性物种仅依赖少数食物类型，若这些食物匮乏则可能面临饥饿风险。本研究将稳定同位素分析（Stable Isotope Analyses）与已发表的胃容物数据及环境数据相结合，旨在探究3种同域分布淡水龟类的觅食生态在4个浊度与初级生产者（primary producer）丰度存在差异的湿地间的变化规律。
结果：本研究发现，广食性物种麦氏长颈龟（Emydura macquarii）在初级生产者存在且水体清澈时，会摄食多样的食物（但以丝状绿藻为主）；而当初级生产者匮乏时，其会转向肉食性更强的食谱，这符合最优觅食理论的预测。与之形成对比的是，2种特化肉食性龟类——宽壳长颈龟（Chelodina expansa）和长颈长颈龟（Chelodina longicollis），其食谱在不同湿地间并无显著差异；当麦氏长颈龟转为肉食性时，种间竞争可能会加剧。当被迫更多依赖肉食时，麦氏长颈龟的空胃率更高，且雌性个体的身体状况会下降，但上述2种长颈龟均未受到此类影响。
结论：本研究结果不仅为最优觅食理论提供了实证支持，同时也表明，即便广食性物种具备改变食谱的能力，当其偏好的食物稀缺时，其觅食成功率仍会下降，这会直接影响它们的能量收支。本研究结果具有保护学意义：墨累-达令河水系的湿地正日益浑浊且大型水生植物丰度偏低，而本次研究涉及的3种龟类种群均呈下降趋势。