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. 尽管草食性哺乳动物可通过改变植被对其他动物产生强烈的间接效应，但当前营养级联效应（trophic cascades）的研究仍聚焦于顶级捕食者（apex predators）及其下行调控作用力。由草食性哺乳动物介导的上行营养交互链，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. 本研究首先验证草食性哺乳动物对植被的影响是否会向上传递至无脊椎动物食物网的多个营养级；同时检验两种对立假说：草食动物诱导的间接交互强度，要么因伴随的更高放牧强度随植物生产力提升而增强，要么因植物对放牧的耐受性更高而减弱。3. 本研究在亚高山草地的两类样地中开展控制实验：一类为生产力较高、放牧强度大的短草植被样地，另一类为生产力较低、放牧强度较小的高草植被样地，每个样地面积为6平方米。我们通过逐步排除的方式，分别隔绝样地内的大型、中型与小型草食性哺乳动物。随后测定了植被的数量、质量、结构与组成，并统计了无脊椎草食动物、食腐动物、杂食动物与捕食者的丰度；同时采用结构方程模型（Structural Equation Modelling），检验不同体型草食性哺乳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情况。4. 在短草植被样地中，哺乳动物引发了植被数量与厚度的变化，这些变化直接影响了食腐与捕食性无脊椎动物的丰度，并通过改变无脊椎草食动物的丰度形成上行营养级联，间接作用于捕食性与杂食性无脊椎动物。在高草植被样地中，哺乳动物诱导的植被质量与组成变化影响了食腐无脊椎动物，进而作用于杂食性无脊椎动物，但此类级联效应弱于短草样地。此外，小型哺乳动物在构建无脊椎动物食物网中的作用至少与大型哺乳动物相当。5. 本研究结果表明，不同体型的草食性哺乳动物均可在当地无脊椎动物食物网中触发营养级联效应；进一步支持了“草食动物诱导的间接交互强度在更高产的系统中更强，这是因为取食强度更高”的假说，而非“植物更高的放牧耐受性会抑制高产系统中草食动物诱导的间接交互”的假说。