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.