SOME IMPLICATIONS OF DIRECT POSITIVE INTERACTIONS FOR COMMUNITY SPECIES DIVERSITY

Direct positive interactions (mutualisms and commensalisms) are generally 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 growth and survival of other species unable to survive in isolation. Although ecologists have documented direct positive interactions among species for decades, there is less known about how these interactions affect community species diversity patterns. In this paper, we present a qualitative theoretical model that considers how direct positive interactions affect community species diversity. The model uses, as its basis, familiar unimodel species diversity models (i.e., “compensatory mortality” and “intermediate disturbance” hypothesis) to understand where direct positive interactions are likely to be important. Initially, it predicts that direct positive interactions increase species diversity by facilitating species that might not normally survive under very high physical disturbance, stress, or predation. In addition, it suggests that, under intermediate physical disturbance, stress, or predation, 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 from a New England salt marsh community where a gradient in physical conditions occurs. In this community, direct positive interactions, and their indirect effects, are predicted to increase the species diversity by at least 35%. This empirical case study and model show that by incorporating direct positive interactions into ecological experiments and theory, it is possible to expand our understanding of the mechanisms responsible for community species diversity patterns.