The effects of long-term nitrogen loading on grassland insect communities

Just as long-term nitrogen loading of grasslands decreases plant species richness and increases plant biomass, we have found that nitrogen loading decreases insect species richness and increases insect abundances. We sampled 54 plots that had been maintained at various rates of nitrogen addition for 14 years. Total insect species richness and effective insect diversity, as well as herbivore and predator species richness, were significantly, negatively related to the rate of nitrogen addition. However, there was variation in trophic responses to nitrogen. Detritivore species richness increased as nitrogen addition increased, and parasitoids showed no response. Insect abundances, measured as the number of insects and insect biovolume (an estimate of biomass), were significantly, positively related to the rate of nitrogen addition, as were the abundances of herbivores and detritivores. Parasitoid abundance was negatively related to the rate of nitrogen addition. Changes in the insect community were correlated with changes in the plant community. As rates of nitrogen addition increased, plant species richness decreased, plant productivity and plant tissue nitrogen increased, and plant composition shifted from C₄ to C₃ grass species. Along this gradient, total insect species richness and effective insect diversity were most strongly, positively correlated with plant species richness. Insect biovolume was negatively correlated with plant species richness. Responses of individual herbivores varied along the nitrogen gradient, but numbers of 13 of the 18 most abundant herbivores were positively correlated with their host plant biomass. Although insect communities did not respond as strongly as plant communities, insect species richness, abundance, and composition were impacted by nitrogen addition. This study demonstrates that long-term nitrogen loading affects the entire food chain, simplifying both plant and insect communities.