The dual nature of community variability

Community variability has a dual nature. On the one hand, there is composit ional variability, changes in the relative abundance of component species. On the other hand, there is aggregate variability, changes in summary prope rties such as total abundance, biomass, or production. Although these two a spects of variability have received much individual attention, few studies have explicitly? related the compositional and aggregate variability of nat ural communities. In this paper, we show how simultaneous consideration of both aspects of community variability might advance our understanding of ec ological communities. We use the distinction between compositional and aggregate variability to d evelop an organizational framework for describing patterns of community var iability. At their extremes, compositional and aggregate variability combin e in four different ways: (I) stasis, low compositional and low aggregate v ariability; (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 t hese patterns has been observed in natural communities, and can be linked t o a suite of abiotic and biotic mechanisms. We give examples of the potenti al relevance of variability patterns to applied ecology, and describe the m ethodological developments needed to make meaningful comparisons of aggrega te and compositional variability across communities. Finally, we provide tw o numerical examples of how our approach can be applied to natural communit ies.