Species diversity is expected to promote stability in ecosystem functions such as productivity. One mechanism that may contribute to stability is compensatory dynamics. Compensatory dynamics, which occur when an increase in density (biomass) of one species is associated with a decrease in another, are indicated by negative species covariances. These may be driven by competition or negatively correlated species responses to environmental drivers. Despite the widespread assumption that species covary negatively and compensate to maintain stability in ecosystem function, several recent papers have shown generally positive species covariances. Moreover, few studies have addressed how environmental factors influence compensatory dynamics. They have also failed to show whether compensatory dynamics lead to greater stability in ecosystem function. Using 18 years of data from four experimental plant community types at the KBS LTER site, we asked three questions:
1) Do plants show compensatory dynamics in productivity over space or time?
2) Do compensatory dynamics depend on community type?
3) Do compensatory dynamics consistently lead to stability in productivity?
We found that species covariances were significantly negative in all four community types, indicating compensatory dynamics. Annual-dominated communities (maintained by annual tillage) showed stronger compensatory dynamics than perennial-dominated communities. Fertilized communities also had stronger compensatory dynamics than unfertilized communities. Stability in aboveground productivity also depended on community type, increasing in annual-dominated and fertilized communities. However, the expected positive relationship between compensatory dynamics and stability disappeared in annual-dominated and fertilized communities. Instead, annual tillage and fertilization appear to have stabilized communities through changing community composition. Specifically, dominant species were more abundant, more stable, and covaried more negatively with other community members in annual-dominated and fertilized communities. Thus, the often-assumed relationship between compensatory dynamics and stability was environment and community context dependent. As global climate change, habitat degradation, and biological invasions continue to alter ecosystems, it will become increasingly important to understand the prevalence of compensatory dynamics and their importance as a stabilizing mechanism.