Data from: Food and habitat provisions jointly determine competitive and facilitative interactions among distantly related herbivores

1. Interactions between distantly related herbivores exert powerful influences on ecosystems, but most studies to date have only considered unidirectional effects. Few have simultaneously examined the mutual effects that vertebrate herbivores and insect herbivores have on one another. 2. We conducted a set of manipulative experiments to evaluate the potential competition and facilitation between two pairs of distantly related herbivore taxa: an insect caterpillar (Gynaephora alpherakii) and two large vertebrate herbivores, yak (Bos grunniens) and Tibetan sheep (Ovis aries tibetica). 3. We found that these large herbivores consistently increased the density of caterpillars likely by improving the habitat for caterpillars. The caterpillars, in turn, decreased yak but increased Tibetan sheep foraging time and intake bites by differently changing available food resources of the two large herbivores. Diet preferences of herbivores modified the habitat and food resources, thereby causing a diet-mediated competition between yak and caterpillars, and facilitation between sheep and caterpillars. The vertebrate herbivores non-feeding upon Lamiophlomis rotata and Gentiana straminea, the caterpillars preferred habitat, increased densities of the two plant species, thus favoring the caterpillars. In turn, the caterpillar preference for Kobresia pygmaea, significantly reduced food resources for yak, while promoted food resources (multiple forbs) for sheep. 4. Synthesis. Our study indicates that two different mechanisms (the changes in habitat and food availability) induced by herbivore foraging jointly determine competitive and facilitative interactions between distantly related herbivore species. We also suggest that examining the bidirectional effects between herbivores offers a better understanding of competition and facilitation in terrestrial animal communities.

1. 远缘植食动物之间的相互作用对生态系统具有显著影响，但迄今为止多数研究仅考虑了单向效应，鲜有研究同时考察脊椎动物植食者（vertebrate herbivore）与昆虫植食者（insect herbivore）之间的双向互作效应。2. 我们开展了一系列控制性野外实验，以评估两对远缘植食动物类群之间潜在的竞争与促进关系：昆虫类植食者毛虫（Gynaephora alpherakii），以及两种大型脊椎动物植食者——牦牛（Bos grunniens）与藏系绵羊（Ovis aries tibetica）。3. 研究结果显示，这些大型植食动物通过改善毛虫的栖息生境，持续提升了毛虫的种群密度。反过来，毛虫通过改变两种大型植食动物的可利用食物资源，产生了差异化效应：降低了牦牛的取食时长与取食口数，却提升了藏系绵羊的取食时长与取食口数。植食动物的食性偏好会重塑栖息生境与食物资源，进而引发基于食物媒介的种间相互作用：牦牛与毛虫之间为竞争关系，绵羊与毛虫之间则为促进关系。牦牛和藏绵羊不取食毛虫偏好栖息生境中的独一味（Lamiophlomis rotata）与条纹龙胆（Gentiana straminea），这两种植物的种群密度因此得以提升，从而对毛虫产生有利影响。而毛虫偏好取食矮生嵩草（Kobresia pygmaea），这一行为显著减少了牦牛的可利用食物资源，却提升了绵羊的可利用食物资源（多种杂类草）。4. 研究总结：本研究表明，植食动物取食所引发的两种不同机制（生境与食物可获得性变化）共同决定了远缘植食动物类群之间的竞争与促进相互作用。我们还提出，探究植食动物之间的双向互作效应，能够更深入地理解陆地动物群落中的竞争与促进关系。