Basic Growth Analysis

Abstract

Current concepts of the role of inter-specific interactions in communities have been shaped by a profusion of experimental studies of interspecific competition over the past few decades. Evidence for the importance of positive interactions-facilitations-in community organization and dynamics has accrued to the point where it warrants formal inclusion into community ecology theory, as it has been in evolutionary biology. E arly ecological theory included both positive and negative interactions among species as important driving forces in the structure and organization of natural communitiesl.2. More recently, the role of competition in natural communities has received considerable attention (see Refs 3,4), while positive interactions have received little attention and are largely ignored in current models of community organizatior@. We broadly define positive interactions as all non-consumer interactions among two or more species that positively affect at least one of the species involved; thus, we include fac-ultative and obligatory facilitations and mutualisms. Whereas the ecology and evolutionary biology of mutualisms has attracted recent attention7, the role that they play in the structure and organization of natural communities has not. The lack of recent attention paid to the role of positive interactions in communities is at least partly due to their uncritical acceptance by early ecologists and the preoccupation of contemporary community ecologists with competition (but see Ref. 8). In addition, much of the early development of ecology which highlighted positive interactions pre-dated the common use of field experiments in ecology and thus received little critical testingg. Moreover, fascination with competition has focused attention on communities where competition is conspicuous , potentially distracting ecol-ogists from even recognizing positive interactions. Consequently, while facili-tative and/or positive interactions are part of most working ecologists' conventional wisdom, and while anecdotal examples can be shown in most communities, the general importance of positive interactions to community diversity, structure and productivity is rarely acknowledged. Modern ecology's view of positive interactions is particularly puzzling given the prominent role that they play in many communities and their importance as evolutionary forces. Few ecologists would deny the potential importance of mycorrhizal associations in forests and coral-zooxanthellae associations in coral reefs, even though our understanding of the community impact of these associations is almost entirely speculative. Moreover, while the evolutionary role of positive interactions has become clear over the past decade (e.g. the evolution of eukaryotic cells, and flowering plants and their pollinators), positive interactions remain absent from general models of community dynamics and organization. How can an evolutionary play featuring strong positive interactions take place on an ecological stage where positive interactions are insignificant? Textbooks, however , strongly support our contention that positive interactions are currently largely overlooked by community ecol-ogists. Whereas ecology textbooks earlier this century devoted as much attention to positive interactions as they did to competitive ones, modern textbooks hardly mention positive interactions in a community contextl'l. Recent theoretical (e.g. Refs 11,12) and empirical (e.g. Refs 13,14) work has suggested that positive indirect interactions and feedback mechanisms in food-webs may be common important forces in natural communities. In this article, however, we focus on the direct, non-trophic positive interactions that early ecologists suggested were critical aspects of community dynamics and organiz-ation1,2. Direct positive interactions occur when neighbors modify physical and/or biotic conditions and lead to positive effects. Although these positive interactions have been largely ignored by theoretical ecologists, evidence from a wide range of communities has begun to emerge during the past five to ten years indicating that direct positive interactions may be common, predictable and pervasive forces in natural communities and in physically harsh environments in particular. Here, we examine a small sub sample of this evidence and re-evaluate the role of direct positive interactions in ecological communities. Do positive interactions affect recruitment? Positive interactions during recruitment in desert plants were hypothesized thirty years ago, based on spatial patterns suggesting that neighbors buffer one another from potentially limiting physical stresses'5.16. But ecologists have tended to view these interactions as idiosyn-cratic features of deserts rather than examples of general principle and, until recently, have not experimentally tested these ideas. Nurse-plant effects and positive density-dependent recruit survivor-ship, however, have recently been found in other harsh physical environments,