THE ROLES OF HARSH AND FLUCTUATING CONDITIONS IN THE DYNAMICS OF ECOLOGICAL COMMUNITIES

Harsh conditions (e.g., mortality and stress) reduce population growth rates directly; secondarily, they may reduce the intensity of interac tions between organisms. Near-exclusive focus on the secondary effect of these forms of harshness has led ecologists to believe that they re duce the importance of ecological interactions, such as competition, a nd favor coexistence of even ecologically very similar species. By exa mining both the costs and the benefits, we show that harshness alone d oes not lessen the importance of species interactions or limit their r ole in community structure. Species coexistence requires niche differe nces, and harshness does not in itself make coexistence more likely. F luctuations in environmental conditions (e.g., disturbance, seasonal c hange, and weather variation) also have been regarded as decreasing sp ecies interactions and favoring coexistence, but we argue that coexist ence can only be favored when fluctuations create spatial or temporal niche opportunities. We argue that important diversity-promoting roles for harsh and fluctuating conditions depend on deviations from the as sumptions of additive effects and linear dependencies most commonly fo und in ecological models. Such considerations imply strong roles for s pecies interactions in the diversity of a community.