Data from: Mammal-induced trophic cascades in invertebrate food webs are modulated by grazing intensity in subalpine grassland

1. Even though mammalian herbivores can exert strong indirect effects on other animals by altering the vegetation, the study of trophic cascades retains a focus on apex predators and their top-down forces. Bottom-up trophic interaction chains induced by mammalian herbivores, particularly in invertebrate food webs, remain largely unexplored. 2. We tested whether effects of mammalian herbivores on the vegetation ricochet back up several trophic levels of the invertebrate food web. We further tested two alternative hypotheses: the strength of herbivore-induced indirect interactions either increases with plant productivity because of a concurrent higher grazing intensity, or it decreases because of a higher plant tolerance to grazing. 3. We progressively excluded large, medium, and small herbivorous mammals from replicated plots of 6 m² in productive, intensively grazed short-grass vegetation and less productive, less intensively grazed tall-grass vegetation of subalpine grasslands. We measured vegetation quantity, quality, structure, and composition, and determined the abundance of invertebrate herbivores, detritivores, omnivores, and predators. We used Structural Equation Modelling to test vegetation-mediated cascading effects of the different mammalian herbivores across different trophic groups of invertebrates. 4. In the short-grass vegetation, mammals caused changes in vegetation quantity and thickness. These changes directly affected detritivorous and predatory invertebrate abundance, yet indirectly affected predatory and omnivorous invertebrates through a bottom-up trophic cascade via changes in herbivorous invertebrate abundance. In the tall-grass vegetation, mammal-induced changes in vegetation quality and composition affected detritivorous invertebrates and in turn omnivorous invertebrates, but these cascading effects were weaker than those in the short-grass vegetation. Smaller mammals were at least as important as large mammals in structuring the invertebrate food web. 5. Our results demonstrate that differently sized mammalian herbivores can trigger trophic cascades in the local invertebrate food web. Our findings further support the hypothesis that herbivore-induced indirect interactions are stronger in more productive systems because of higher foraging intensity, as opposed to the hypothesis that a higher grazing tolerance of plants should dampen herbivore-induced indirect interactions in productive systems.

1. 尽管哺乳动物植食动物可通过改变植被对其他动物产生强烈的间接效应，但当前关于营养级联（trophic cascades）的研究仍聚焦于顶级捕食者（apex predators）及其下行调控力量。由哺乳动物植食动物驱动的上行营养交互链（bottom-up trophic interaction chains），尤其是在无脊椎动物食物网（invertebrate food webs）中，仍未得到充分探索。
2. 本研究旨在检验哺乳动物植食动物对植被的影响是否会沿无脊椎动物食物网的多个营养级向上回弹传递。我们进一步验证了两种对立假说：一是植食动物诱导的间接交互强度会随植物生产力升高而增强，因为伴随更高的放牧强度；二是其强度会因植物对放牧的耐受性提升而减弱。
3. 我们在亚高山草地的两类重复样地（每个样方面积为6平方米）中开展实验，分别为生产力较高、放牧强度较大的短草植被区，以及生产力较低、放牧强度较小的高草植被区，并逐步排除大型、中型和小型植食哺乳动物。我们测定了植被的数量、质量、结构与组成，并量化了植食性、食腐性、杂食性和捕食性无脊椎动物的丰度。我们采用结构方程模型（Structural Equation Modelling），检验不同体型哺乳动物植食动物通过植被介导的级联效应，及其在无脊椎动物各营养类群间的传递。
4. 在短草植被区，哺乳动物活动改变了植被的数量与厚度。这些变化直接影响了食腐性和捕食性无脊椎动物的丰度，同时还通过植食性无脊椎动物丰度的变化形成上行营养级联，间接作用于捕食性和杂食性无脊椎动物。在高草植被区，哺乳动物诱导的植被质量与组成变化影响了食腐性无脊椎动物，进而作用于杂食性无脊椎动物，但这类级联效应弱于短草植被区。相较于大型哺乳动物，小型哺乳动物在构建无脊椎动物食物网方面的重要性至少不相上下。
5. 我们的研究结果表明，不同体型的哺乳动物植食动物均可在本地无脊椎动物食物网中触发营养级联。本研究结果进一步支持下述假说：因觅食强度更高，植食动物诱导的间接交互在生产力更高的生态系统中更强；而非“植物对放牧的耐受性越高，越会削弱高生产力系统中植食动物诱导的间接交互”这一假说。