Ecosystems are changing at an unprecedented rate, particularly in response to global change and species invasions. At the same time, there is a growing understanding that environmental variability is inherent in natural systems and fundamental to many ecological processes. Stable species coexistence, for example, can be structured by environmental variability (fluctuation-dependent coexistence). Scaling up, tradeoffs between species can promote ecosystem stability because a decline in one species is compensated for by the rise of another (compensatory dynamics). Here we use a 38-year record of annual plant dynamics at a California serpentine grassland, Jasper Ridge, that has experienced nitrogen (N) deposition. Using the first half of the timeseries (with a limited signature of N deposition), we first test fluctuation-dependent coexistence and compensatory dynamics as inter-related mechanisms of species and ecosystem stability. We build pairwise models to assess coexistence among dominant species and relate associated species and trait tradeoffs to their implications for ecosystem stability. Second, we compare these patterns to those that emerge in the second half of the timeseries (with higher N) to explore directional resource enrichment effects on mechanisms of species and ecosystem stability.
Results/Conclusions
Species abundances varied widely over the 38-year timeseries. Species largely coexisted, with minimal niche overlap (≤ 0.2, where 0 is none and 1 is complete niche overlap). Species differences resulted in strong compensatory dynamics, particularly at long (10 year) timescales. Differing species responses to long-term climate patterns largely structured these compensatory dynamics, although differential rates of return between annual and perennial species in response to gopher disturbance also enhanced compensation. Early in the time series the community exhibited large oscillations between native forbs with conservative growth traits and native forbs with more resource acquisitive traits, as well as parallel oscillations between conservative forbs and non-native annual grasses. Later in the time series oscillations between conservative and acquisitive forbs were more muted with an overall increase in acquisitive forbs. There was a lasting increase in annual grass abundance, and many native conservative forbs exhibited suppressed growth rates when rare when annual grasses dominated. Taken together, our results suggest that N enrichment erodes both fluctuation-dependent coexistence and compensatory dynamics as mechanisms of species and ecosystem stability. Further, the long timescales at which these dynamics operate underscore that the full effects of enrichment on stability may take time to emerge.