SOME IMPLICATIONS OF DIRECT POSITIVE INTERACTIONS FOR COMMUNITY SPECIES-DIVERSITY

Direct positive interactions (mutualisms and commensalisms) are genera lly accepted as important processes in communities. They appear to be most common in environments with relatively high physical disturbance, stress, or predation, where associated species can increase the growt h and survival of other species unable to survive in isolation. Althou gh ecologists have documented direct positive interactions among speci es for decades, there is less known about how these interactions affec t community species diversity patterns. In this paper, we present a qu alitative theoretical model that considers how direct positive interac tions affect community species diversity. The model uses,as its basis, familiar unimodel species diversity models (i.e., ''compensatory mort ality'' and ''intermediate disturbance'' hypothesis) to understand whe re direct positive interactions are likely to be important. Initially, it predicts that direct positive interactions increase species divers ity by facilitating species that might not normally survive under very high physical disturbance, stress, or predation. In addition, it sugg ests that, under intermediate physical disturbance, stress, or predati on, facilitator species that might normally be competitively excluded are released from competition. We suggest that facilitator species may then create new interaction webs that would not be possible in their absence. To illustrate these ideas, we describe a case study taken fro m a New England salt marsh community where a gradient in physical cond itions occurs. In this community, direct positive interactions, and th eir indirect effects, are predicted to increase the species diversity by at least 35%. This empirical case study and model show that by inco rporating direct positive interactions into ecological experiments and theory, it is possible to expand our understanding of the mechanisms responsible for community species diversity patterns.