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III.4 Facilitation and the Organization of Plant Communities Ragan M. Callaway OUTLINE 1. Introduction 2. What mechanisms cause positive interactions 3. Can we predict when positive or negative interactions may be important? 4. What do positive interactions mean for community theory? Current plant community ecology, as presented in most textbooks, often promotes the perspective that communities are produced only by the traits of populations and that assemblages of different plant species exist primarily because each shares adaptations to particular abiotic conditions. To some degree, this perspective leads to the conclusion that plant communities are simply a handy typological construct. However, a large body of research accruing during the last 30 years demonstrates that many if not most plant communities have fascinating interdependent characteristics, and although they are not ‘‘organic entities,’’ it is clear that many species create conditions that are crucial for the occurrence and abundance of other species. This research is the focus of this chapter. GLOSSARY continuum. A distribution of many species along a gradient in which each species appears to be distributed randomly with respect to other species facilitation. The positive effect of one species on another holistic communities. The idea that species within a community are highly interdependent, forming organism-like units hydraulic lift. The process by which some plant species passively move water from deep in the soil profile, where water potentials are high, to more shallow regions where water potentials are low indirect interactions. Interactions between two species that are modified by a third species individualistic communities. The idea that communities are fundamentally groups of populations that occur together primarily because they share adaptations to the same abiotic environment; communities do not have organism-like qualities niche complementarity. The condition in which different niches result in variation in the utilization of resources or space 1. INTRODUCTION As the discipline of ecology emerged from its biogeographic origins in the early 1900s, two strikingly polar views on the nature of plant communities vied for recognition, and the conflict established a precedent for ecological thought today. Initially, the view of Frederic Clements was ascendant with most ecologists accepting the idea that [T]he community is an organic entity. As an organism the community arises, grows, and dies. Furthermore, each community is able to reproduce itself, repeating with essential fidelity the stages of its development . . . comparable in its chief features with the life history of an individual plant. (Clements , F. E. 1916. Plant Succession. Washington, DC: The Carnegie Institution, Publication 242) This holistic perspective, however, was replaced in the middle of the 1900s by new ideas promoted by Henry Gleason. In this new individualistic world view, the community ‘‘is merely the resultant of two factors, the fluctuating and fortuitous immigration of plants and an equally fluctuating and variable environment . . . not an organism, scarcely even a vegetational unit, but merely a coincidence’’ (Gleason, H. A. 1917. The structure and development of the plant association. Bulletin of the Torrey Botanical Club 44: 463–481). It would be hard to dream up two more diametrically contrasting perspectives for how species are organized into groups. Texts have a strong individualistic flavor, but most ecologists are fully aware that the nature of plant communities is more nuanced than the hyperdichotomy of individualistic versus organismal communities. Perceiving such nuances is important, but the dominant individualistic perception of plant community organization has probably left lingering but strong effects on the way we conduct research, leading to a great deal of information on negative interactions such as predation, competition for resources, and allelopathy. However, this dominant perception has probably impeded the progress of empirical research on facilitation and indirect interactions among plants. Understanding the nature of communities is not just an academic issue. Whether or not communities have weak or strong tendencies toward independent or interdependent assembly has strong implications for conservation. For example, the view that plant species are fully individualistic and interchangeable in communities has been used to advocate active human involvement in ‘‘shaping and synthesizing new ecosystems , even in the ‘natural’ environment’’ (italics mine; Johnson and Mayeux, 1992). This may be reasonable if maintaining functional plant communities is simply a matter of finding a suite of populations that can grow in a particular set of conditions. But if interactions among plants are more complex and interdependent, as suggested by research on facilitation, indirect effects of herbivores and mycorrhizae, and networks of direct and indirect interactions within the plant community, shaping and synthesizing new communities may not work. Conservationists typically...


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