Nitrogen enrichment alters biodiversity of plant communities at multiple scales

Small-scale experimental studies demonstrating nitrogen (N) enrichment reduces plant diversity have led to the widespread conclusion that anthropogenic N enrichment is a major threat to global biodiversity. However, these conclusions ignore influence of spatial scale on biodiversity. While changes at larger scales are most relevant to inform policy and management, it is not known how N enrichment affects biodiversity at scales larger than a small plot. Here, we synthesize data from 18 experiments across North America to show that N enrichment differentially affects biodiversity of herbaceous plant communities at multiple spatial scales. We show that N enrichment reduces biodiversity within small plots but frequently enhances biodiversity by increasing variation in the identities of species present in different plots. Although divergence in species composition among plots was rarely sufficient to completely compensate for N induced reductions in biodiversity within plots, it often reduced the amount of species loss at larger scales. Our results demonstrate that a focus on the effects of N enrichment on biodiversity at small spatial scales may often overestimate declines in regional biodiversity. Instead, it is necessary to incorporate both local and regional scale processes to predict how N enrichment may change biodiversity at scales relevant to national and global policy.