The Role of Positive Interactions in Communities: Lessons from Intertidal Habitats

Abstract

Positive interactions that result from neighbors buffering one another from stressful conditions are predictably important community forces in physically stressful habitats. Here, we examine the generality of this hypothesis in marine intertidal communities. Intertidal communities have historically played a large role in the development of community ecology since they occur across pronounced physical gradients and are easily manipulated. Positive interactions, however, have not been emphasized in studies of intertidal communities. We first review studies of intertidal marsh plant communities that suggest that positive interactions play a dominant role in the structure and dynamics of these common assemblages. We then present the results of an experimental manipulation on New England rocky shores that suggests that group benefits are as important in maintaining the upper intertidal limits of dominant spaceholders on rocky shores as the negative forces of competition and predation are in maintaining lower distributional limits. We conclude by discussing the generality and implications of our results. We argue that biogeographic biases have limited appreciation of the role played by positive interactions in intertidal communities. Most of the work that has formed the foundation of marine intertidal ecology was done in cool temperate habitats, whereas positive interactions driven by the amelioration of thermal or desiccation stresses are likely more important in warmer climates. We further argue that many important positive feedbacks operate at large spatial scales, not conducive to experimental study, and thus have escaped critical attention and general acceptance. We suggest that recognizing the role of positive interactions in communities may be key to understanding population and community processes in physically stressful habitats, many large-scale landscape processes, and uncovering long-suspected linkages between biodiversity and community stability.