THE EVOLUTIONARY ROLE OF INDIRECT EFFECTS IN COMMUNITIES

Indirect effects occurring through chains of direct species interactio ns have been shown to be important in many communities. If indirect ef fects have strong ecological effects, then they must also have strong evolutionary effects through natural selection. We consider how indire ct effects, both positive and negative, combine with positive and nega tive direct effects to influence evolution between interacting species . Any one species can evolve in response to direct interactions with o ther species by changing its ''response'' to the presence of the other species or changing its ''effect'' on the species. We argue that the direction (positive or negative) of the direct and indirect effects wi ll determine whether traits related to ''response'' or ''effect'' are most likely to be subject to natural selection. Further, covariance pa tterns of species in the same community should increase when direct an d indirect effects between species act in the same direction, resultin g in greater compartmentalization of such communities.