School of Science, Hawkesbury Institute, Western Sydney University, Locked Bag, 1797, Penrith, NSW, 2751, Australia.
School of Life and Environmental Sciences, University of Sydney, 380 Werombi Road, Brownlow Hill, NSW, 2570, Australia.
BMC Ecol. 2020 Feb 18;20(1):11. doi: 10.1186/s12898-020-00279-6.
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.
最适觅食理论解释了动物如何根据不同食物类型的可利用性、营养价值和处理时间来做出觅食决策。杂食动物通过食用多种食物类型来解决这个问题，并相对轻松地改变其饮食。而专家则只吃少数几种食物，如果这些食物不可用，它们可能会挨饿。我们将稳定同位素分析与之前发表的胃内容物和环境数据相结合，研究了三种共生淡水龟在四个湿地中的觅食生态如何因浊度和初级生产者丰度的不同而变化。
我们发现，当初级生产者可用且水清澈时，杂食性的 Emydura macquarii 会消耗多样化的饮食（但主要是丝状绿藻），但当水浑浊且初级生产者稀缺时，它会转而采用更肉食性的饮食，这符合最适觅食理论的预测。相比之下，两种更专门化的肉食性物种Chelodina expansa 和 Chelodina longicollis 在不同湿地之间的饮食没有差异，并且当 E. macquarii 是肉食性时，种间竞争可能会增加。当被迫更多地采用肉食性饮食时，E. macquarii 表现出更高的空胃率，并且雌性龟的身体状况下降，但 Chelodina 物种都没有受到影响。
我们的结果支持了最适觅食理论，但也表明改变饮食的能力并不能保护杂食性动物在其首选食物稀缺时免受觅食成功率降低的影响，这对它们的能量预算有直接影响。我们的结果具有保护意义，因为 Murray-Darling 河流系统中的湿地浊度增加且大型植物丰度低，而所有三种物种都在减少。
