Current concepts of the role of interspecific 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.

https://www.cell.com/trends/ecology-evolution/pdf/0169-5347(94)90088-4.pdf

Positive interactions in communities.

Interactions among organisms take place within a complex milieu of abiotic and biotic processes, but we generally study them as solitary phenomena. Complex combinations of negative and positive interactions have been identified in a number of plant communities. The importance of these two processes in structuring plant communities can best be understood by comparing them along gradients of abiotic stress, consumer pressure, and among different life stages, sizes, and densities of the interacting species. Here, we discuss the roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation in plant communities.

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Competition and facilitation: a synthetic approach to interactions in plant communities

Experimental evidence for positive interactions, or facilitation, among plants has increased markedly during the last 10 years. Experiments documenting facilitation have been conducted in many diverse ecological systems, which suggests that positive interactions may be fundamental processes in plant communities. Here, I review the evidence for facilitation, the mechanisms by which facilitation operates, and the effects facilitation has on community structure. Facilitative mechanisms may act simultaneously with resource competition or allelopathy, and the overall effect of one species on another may be the product of multiple, complex interactions. Positive interactions may also determine community spatial patterns, permit coexistence, enhance diversity and productivity, and drive community dynamics. Once viewed as anecdotal and idiosyncratic, facilitation is now contributing to a more complete understanding of community structure and dynamics.

Positive interactions among plants

Summary 1 Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized It is timely to consider the progress made by research in this field 2 We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant‐plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory We then consider future directions for facilitation research 3 With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies 4 There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity 5 Certain ecological modelling approaches (eg individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes 6 Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms

/pdf/facilitation-in-plant-communities-the-past-the-present-and-572accn7oh.pdf

Facilitation in plant communities: the past, the present, and the future

If plants cannot simultaneously acclimate to shade and drought because of physiological trade-offs, then plants are expected to be less tolerant to shading under drier conditions. One observation that, at first sight, seems incompatible with this idea is the fact that the establishment of new plants in dry areas is often restricted to shady sites under the canopy of other plants, called “nurse plants.” We use a graphical model to resolve this paradox. The model visualizes how facilitative patterns can be understood from the simultaneous effects of plant canopies on microsite light and moisture, and the growth responses of establishing seedlings to those factors. The approach emphasizes the fact that positive and negative effects of plant canopies always occur simultaneously. In the presented light–water model, facilitation only occurs when the improvement of plant water relations under the canopy exceeds the costs caused by lower light levels. This may be true under dry conditions, whereas in less dry sit...

The interplay of facilitation and competition in plant communities

A lthough the importance of beneficial interactions as a factor of evolution was emphasized early in this century (Kropotkin 1902), the study of competition and predation have dominated ecology, and especially community ecology, during the last three decades (Strong et al. 1984). Recently, however, there has been a surge of interest in beneficial interactions between species (Boucher 1985, Vandermeer 1984), perhaps as a consequence of a waning faith in the importance of interspecific competition (Schoener 1982). Most of the obvious examples of beneficence (mutualism and commensalism) involve animal-animal, animal-fungus, plant-fungus, or plantanimal pairs. These interactions typically involve different trophic levels, and hence organisms that do not normally compete for resources. Plants living in close proximity commonly make demands on the same resources. Plants that benefit from their neighbors, therefore, are likely also to compete with them. That potential competitors may also benefit each other forms the basis of intercropping systems (Francis 1986), such as mixtures of clovers and sycamore seedlings (Haines et al. 1978) or of beans and corn (Boucher and Espinosa 1982).

L. W. Aarssen

Papers

Plants Helping PlantsNew evidence indicates that beneficence is important in vegetation