Vertebrate and invertebrate herbivores often co-occur and are both important drivers of plant abundance and distribution. Nevertheless, many studies often focused on their effects on plants in isolation, either vertebrate or invertebrate herbivores, few studies have embraced both. In a semi-arid grassland in northeastern China, we conducted a three-year field manipulative experiment to investigate how vertebrate herbivores (domestic sheep) and invertebrate insect herbivores (grasshoppers) can interact to affect plant performance of a dominant grass Leymus chinensis. In the study system, sheep and grasshoppers have widely different diet selection--sheep prefer forb species whereas grasshoppers prefer the dominant L. chinensis grass.
Results/Conclusions
Three-year's sheep grazing affect neither plant growth nor absolute abundance of L. chinensis grass. Grazing significantly increased grasshopper abundance, as well as their herbivory rate on L. chinensis grass by three-fold in the grazed sites. Nevertheless, once we assigned the ambient abundance level of grasshoppers (grasshopper abundance in the ungrazed control sites) into the sheep grazed sites, both plant growth and absolute abundance of L. chinensis show a significant increase compared to the ungrazed sites. This is because the relatively low abundance grasshoppers in the control sites have failed to control the dominant grass L. chinensis and allowed it to expand, after the removal of forb competitors by sheep in the grazed sites. These results indicate that sheep and grasshoppers interact to control plant performance of L. chinensis grass in our system. Namely, sheep not only indirectly affected L. chinensis by feeding on the forb competitors, but also indirectly increase the abundance of grasshoppers and strengthen their top-down effects on L. chinensis grass. Our study suggests that a deeper understanding how herbivore species can interact with each other is crucial for better understand the mechanisms of top-down effects of herbivores.