Effects of facilitation on community stability and dynamics: synthesis and future directions

P>1. New insights into the mechanisms and outcomes of facilitation have led to important advances in our understanding of ecological patterns and processes. However, the effects of facilitation on non-successional community dynamics have yet to be developed into a general theory. 2. By synthesizing spatial and temporal relationships between biotic interactions and environmental severity, a new model of facilitation-driven community dynamics is presented that applies to any facilitative mechanism related to abiotic stress or resource limitation. 3. In general, facilitation tends to stabilize community dynamics in moderately severe environments, due to a buffering effect of increased facilitation during more severe periods and enhanced competitive effects in milder conditions. In contrast, a strong negative relationship between environmental severity and facilitative strength in highly severe environments leads to a destabilizing effect of facilitation on community dynamics. 4. If only mature plants have significant facilitative effects, developmental lags may be introduced that decouple environmental fluctuations and community dynamics, decreasing the effects of facilitation on community stability. Additionally, dual regulation of facilitation by environmental and demographic factors decouples abundance from climate and produces periodic local extinctions. In general, the interplay of facilitation and competition produce highly variable dynamics in moderate-severity environments, whereas qualitatively similar results were found in high-severity environments regardless of facilitative mechanism or model parameters. 5. Additional variation in community dynamics can be explained by the combination of effect and response functional traits of species within a community. The relative abundance and proportion of species within a community falling into one of four different effect and response categories provide an effective framework for predicting responses to climatic variation and biotic interactions. 6.Synthesis. Facilitation either increases or decreases community stability in predictable ways as a function of empirically identifiable environmental gradients. Effects of developmental lags, complex controls of facilitative mechanisms and species' functional traits explain additional variation in community dynamics that can be applied to a broad array of ecosystems in which facilitation occurs.