Nitrogen (N) deposition is one of the major drivers of global diversity loss, yet the mechanisms behind this decline remain controversial. The responses of herbivores, the key determinant of plant community structure and functions, may be an important, yet overlooked, mechanism to explain N-induced plant diversity alterations. In a semi-arid grassland in northeastern China, we conducted a four-year field manipulative experiment to investigate how N deposition can affect plant diversity by affecting the top-down effects of Euchorthippus grasshoppers on plants. Euchorthippus grasshoppers are the dominant insect herbivores; they mainly feed on the dominant grass Leymus chinensis in the study system.
Results/Conclusions
N deposition significantly decreased evenness, but increased productivity of plant communities compared to the control sites. N deposition also dramatically decreased the abundance of Euchorthippus grasshoppers, as well as their herbivory rate on the dominant grass L. chinensis, by four-fold. Nevertheless, once we assigned the ambient abundance level of grasshoppers (grasshopper abundance in the un-treated control sites) into the N deposition sites, the decreases in plant diversity was disappeared. This is mainly because the relatively high abundance grasshoppers in the control sites have suppressed the abundance of the dominant grass L. chinensis and released competition. These results indicate that N deposition decreased plant diversity via suppressing the abundance of the dominant herbivore species, which is the key determinant of plant community structure via its top-down effect on the dominant plant species. Our study suggests that a deeper understanding of how plant-herbivore interactions response to N deposition is crucial for better understand the mechanisms of global N-induced diversity loss.