Standing Litter Modifies Top-Down Effects of Large Herbivores on a Grassland Plant Community

Large herbivores can exert pronounced top-down effects on plant communities in grassland ecosystems. Previous studies highlighted the importance of the composition and traits of living plants in regulating the impact of herbivores on plant community. However, there has been little consideration of whether and how plant litter, a ubiquitous “after-life” plant component, affects the outcome of herbivore grazing on grasslands. Here, we conducted a large-scale field experiment in temperate grasslands of northeastern China to investigate how standing plant litter influenced top-down effects of large herbivores (sheep; Ovis aries) on plant species richness, evenness, community composition, and productivity. We found that, in the presence of standing litter, sheep grazing significantly reduced living biomass of forbs by 56%, but have no effects on biomass of the dominant grass, Leymus chinensis. However, in the absence of standing litter, sheep shifted their diet preference from forbs to the grass L. chinensis, leading to a 36% decrease in the biomass of L. chinensis and a 21% decrease in total biomass. Such changes in foraging pressure on plant species led to competitive release that in turn significantly altered plant community composition and increased species evenness. Synthesis and applications. Our results demonstrate that standing litter can alter foraging behaviors of large herbivores and modifying the outcome of their top-down effects on plant community properties in grasslands. These cryptic but perhaps ubiquitous interactions between litter and herbivores may help us better understand the organization and dynamics of plant communities in the grazed grasslands, with important implications for developing effective management and conservation plans.