Indirect effect of nitrogen enrichment modified invertebrate herbivory through altering plant community composition in an alpine meadow

Nitrogen enrichment may affect ‘community invertebrate herbivory’ (hereafter ‘herbivory’) directly by changing plant species’ specific herbivory, or indirectly by altering the composition of natural plant communities. Here, we investigated how community composition altered the community herbivory in natural ecosystems and compared the relative importance of direct and indirect effects of nitrogen addition on community herbivory. We conducted a 7-year nitrogen addition experiment in an alpine meadow to evaluate the effects of fertilization on both herbivory frequency and severity, and we divided plants into four functional groups to investigate how changes in plant community functional composition affect community herbivory frequency. To separate the relative importance of direct and indirect effects of fertilization on community herbivory frequency, we build a serious of generalized models to select variables and used SEM methods to estimate relative contributions of the direct and indirect effects. We found that nitrogen addition increased community herbivory frequency, but not community herbivory severity in our 7-year nitrogen addition experiment. Although the most parsimonious model for explaining the variation in community herbivory frequency included fixed average of herbivory frequency and nitrogen addition, community fixed average of herbivory was the best single predictor for community herbivory frequency in our study. Changes in fixed average of herbivory frequency mediated by plant community composition (indirect effect) outperformed changes in species-specific herbivory frequency (direct effect) under fertilization in driving community herbivory frequency. Our research suggested that indirect effects caused by changes in plant community composition played a more important role in invertebrate herbivory under the condition of anthropogenic nitrogen enrichment.