Understanding the factors governing the stability of ecosystem function in variable environments is a central focus of ecology. Ecosystem function can be directly altered by changes in environmental factors like resource availability and indirectly altered via associated shifts in species competitive hierarchies. Reshuffled species abundances and turnover in species identity can stabilize ecosystem function across environmental conditions, but may also precipitate species losses and declines in ecosystem function. Here we explore these relationships in the context of California grasslands, which are characterized by both high climate variability and population fluctuations. We synthesize results from a series of experiments in which we manipulated environmental drivers (grazing intensity and rainfall availability) to assess effects on species identity, functional diversity and ecosystem functions. We address two overarching questions. First, is functional diversity maintained under some environmental conditions and lost under others? Second, as environmental conditions vary over time, does functional loss under some environmental conditions affect the capacity to respond as these conditions change? We hypothesized that disproportionate environmental effects on functional diversity loss will result in an ecosystem function “ratchet” that decreases stability in variable environments.
We found that functional diversity buffered declines in ecosystem function during dry years but did little to enhance ecosystem function in wet years. This effect was strongest for cover and mild for production. However, our experiments indicated that it was easier to lose functional diversity than to gain it. Moderate grazing and fall drought were associated with higher functional diversity in our system. In areas with initially high functional diversity, successive wet years lead to a rapid decline in functional diversity, whereas functional diversity was maintained but did not increase over successive dry years. We found similar patterns using both hard traits (e.g., SLA, SRL) and soft traits (e.g., functional groups). Across space, moderately grazed, dry plots exhibited high variability in trait representation and diversity, whereas all wet plots converged to low diversity, resource acquisitive communities. Follow-up species manipulations indicate that this is the result of environment-species interactions in which resource-acquisitive grasses dominant in wet years whereas forbs increase in years with dry falls due to competitive release. Taken together, our results indicate that functional tradeoffs can enhance ecosystem function, but that functional loss under high-resource conditions can ultimately reduce a system’s capacity to respond in low-resource years.