Temporal variability and nestedness in California grassland species composition.

Nestedness occurs when species-poor assemblages contain a subset of the species that occur in more species-rich communities and is a commonly observed pattern in spatial data sets. Examination of nested distribution patterns across time rather than space are rarely conducted, even though they may have important implications for species coexistence. Nested temporal assemblages can occur when most species respond similarly to interannual variation in conditions. In contrast, assemblages might be non-nested when different sets of species occur in different years, either because of different resource requirements or as a result of competitive exclusion. Using eight years of plant occurrence data at 71 sites in California grasslands, we found strong signals of temporal nestedness with most species favored by similar conditions. High-quality years enabled the expansion of both grasses and forbs into locales where they were not found during poor-quality years. Native annual forb, exotic annual forb, and exotic annual grass species richness were all greatest in cool, wet years following hot, dry years. Together, these analyses support the hypothesis that, in the absence of community members that specialize on poor-quality years, interannual environmental variation can cause communities to form nested subsets across time much as they do across space.