Direct and Indirect Effects of Facilitation and Competition in Ecological Communities: Population Persistence, Coexistence, and Species Diversity Along the Stress Gradients

Abstract

The stress gradient hypothesis (SGH) predicts that pairwise species interactions within a single trophic level transition from negative to positive as the relative importance of facilitation to competition increases along the gradient of environmental severity. The SGH has inspired much empirical research testing the prediction, and the results have been utilized to develop more refined predictions. I develop a theoretical framework using mathematical models to incorporate these refined SGH predictions and investigate their population- and community-level consequences on how competition-facilitation transitions along the stress gradient affect population persistence, species coexistence, and species diversity in ecological communities. The framework suggests that population size and persistence of benefactor species as well as the sign and strength of interactions are important prerequisite for their facilitative and competitive interactions to have impacts on the population size, dynamics, and persistence of beneficiary species. At community level, in addition to the direct effect of benefactor species, indirect facilitation and indirect competition can play important roles in determining the coexistence of beneficiary species. The framework predicts that indirect competition tends to act against species coexistence and reduce species diversity in stressful environments, whereas indirect facilitation tends to promote species coexistence and foster high species diversity in benign environments. Moreover, it is shown that the indirect effects of benefactor species can be decomposed into density- and trait-mediated indirect effects, in a similar way to indirect effects mediated by trophic interactions. When competition-facilitation transition follows the classical SGH prediction, benefactor species generally promote the coexistence and diversity of beneficiary species both in stressful and in benign environments. A refined SGH prediction suggests stronger facilitation for stress-intolerant superior competitors. Incorporating such individual-level interactions tends to cause coexistence breakdown and the reduction of species diversity. Another refined SGH prediction is that benefactors exert competitive, rather than facilitative, effect in severest environments. Assuming such interactions results in that the range of stress factors permitting coexistence shifts toward more benign environmental conditions. Testing these population- and community-level predictions in the field may be useful to further our understanding on how facilitative species interactions contribute to the organization of ecological communities.KeywordsCompetitionFacilitationStress gradient hypothesisIndirect competitionIndirect facilitation