Species reordering increases community variability driven by chronic nutrient addition

Reordering of abundances among species is a common response in communities whether affected by anthropogenic drivers or natural disturbance. However, understanding how competitive relationships drive community dynamics under global environmental change remains limited, primarily due to uncertainties related to changes in species interactions and the scarcity of long-term observations. By combining long-term data and time series analysis tools, we quantified the compositional dynamics and causal interactions among functional groups of an arid grassland community under chronic nutrient enrichment for 15 years following wildfire. We hypothesized that chronic nutrient addition would promote species reordering among dominant grasses and subordinate annual forbs after wildfire, thereby increasing biomass and compositional variation over the long term. Contrary to expectations, while the abundance of the dominant grass Bouteloua eriopoda (black grama) declined immediately after the wildfire, the increase in annual forbs under N addition did not occur until a decade later. Convergent cross-mapping revealed that annuals were causally influenced by black grama abundance and maintained relatively lower abundance in control plots. However, with N addition, this causal interaction from black grama to annuals disappeared. Accordingly, temporal variability of biomass and community composition increased as the abundance of annuals rose. Combined with evidence of precipitation response, these results imply that the competitive advantage of perennial plants over annual forbs could serve as a stabilizing mechanism for community variability by limiting the response of annuals to precipitation fluctuations. However, this stabilizing process is disrupted by the cumulative effects of chronic nitrogen addition. This long-term experiment provides new insights into the destabilizing effects of community reordering, without changes in species richness, in response to anthropogenic nutrient loading.