The dual nature of community variability

Community variability has a dual nature. On the one hand, there is compositional variability, changes in the relative abundance of component species. On the other hand, there is aggregate variability, changes in summary properties such as total abundance, biomass, or production. Although these two aspects of variability have received much individual attention, few studies have explicitly related the compositional and aggregate variability of natural communities. In this paper, we show how simultaneous consideration of both aspects of community variability might advance our understanding of ecological communities. We use the distinction between compositional and aggregate variability to develop an organizational framework for describing patterns of community variability. At their extremes, compositional and aggregate variability combine in four different ways: (1) stasis, low compositional and low aggregate variability; (2) synchrony, low compositional and high aggregate variability; (3) asynchrony, high compositional and high aggregate variability; and (4) compensation, high compositional and low aggregate variability. Each of these patterns has been observed in natural communities, and can be linked to a suite of abiotic and biotic mechanisms. We give examples of the potential relevance of variability patterns to applied ecology, and describe the methodological developments needed to make meaningful comparisons of aggregate and compositional variability across communities. Finally, we provide two numerical examples of how our approach can be applied to natural communities.