Species interactions are a cornerstone of ecological research wherein the effects of an individual of one species on another individual, frequently a different species, are studied. Within versus between species interactions are also commonly contrasted as a means to infer relative importance, but the majority of theory advances, at least at the community level, are associated with interactions between individuals of different species. Interactions can range from positive to negative, and effects are measured at all levels of development, or life history stages, of an organism. Positive interactions have been extensively studied in both population and community ecology. Facilitation, however, is a relatively specific term that has evolved primarily to describe positive plant–plant interactions (see Defining Facilitation). Facilitation, or positive interactions, is a relatively recent subset of these species interactions in general, including related processes, such as competition, mutualism, and parasitism. Facilitation is best viewed as the antithesis of the plant competition literature, as it shares many of the main attributes, both in terms of scope and approach, and arose as a comparator to this research. Facilitation studies mainly refer to positive plant–plant interactions, as the term was proposed in the plant literature and extensively used to describe interactions that include a positive effect of one species on another. Mutualism and parasitism research is often plant–insect based and formally identifies the reciprocal effect in the interaction, that is, (+, +) in mutualism and (+,−) in parasitism, whereas facilitation studies are generally (+,0) or (+,?), with the second effect often unreported. Interactions that include at least one negative interaction are usually described as competition in the plant literature and do not apply the term facilitation (although the frequency of both being discussed concomitantly is increasing). Hence, the term facilitation, owing to historical use, describes the subset of interactions that are (+,0) and is mostly specific to within plants, although its usage is expanding. The research on facilitation has most likely peaked, similar to plant competition studies, in that facilitation has been clearly established as an important process in the formation of plant communities. Additional studies simply demonstrating facilitation are increasing unlikely to be present in the literature. That said, the implications to theory and other, more nuanced aspects of interaction, such as context dependence, shifting balances, and importance of the environment, as they relate to facilitation, are still largely unexplored. In the early 21st century the most contentious debates, with respect to facilitation, center on either disagreement concerning what a community is and whether research should be conducted at this scale or on how to use environmental gradients (i.e., stress) most effectively. Both of these topics are described herein, with readings also included on Historical Background, Experimental and Analytical Approaches, Evolution, other taxa, and Applications.
Evolutionary Responses to Conditionality in Species Interactions across Environmental Gradients
