Are Positive Interactions Species-Specific?

Abstract

Positive interactions among plants, or facilitations, have been demonstrated in a wide variety of communities around the world (see reviews by DeAngelis et al. 1986, Hunter and Aarssen, 1988, Bertness and Callaway 1994, Callaway 1995, Callaway and Walker in press). Neighboring plant species compete with one another for resources, but they may also provide benefits for each other such as shade, higher nutrient levels, more available moisture, soil oxygenation, protection from herbivores, more favorable soil microflora, transfer of resources and fixed carbon via mycorrhizae, and increased pollinator visits (Callaway 1995). The species-specificity of positive interactions among plants whether or not benefactor species are highly interchangeable is central to understanding the general role of positive interactions in plant communities. In other words, are the positive effects of plants simply due to general changes in the biophysical environment; that which can be imitated by inanimate objects like rocks, microtopography, or experimental shade cloth? Or can facilitation depend on the species, with some species eliciting strong positive effects and other morphologically similar species producing neutral or negative effects? The species specificity of positive interactions is pertinent to our general concepts of plant communities (see Gleason 1926, Goodall 1963, Shipley and Keddy 1987, Austin 1990, Collins et al. 1993, Callaway 1997). If positive interactions are often species-specific, then many plant communities may be more interdependent than currently thought. Here, I have explored the specificity of positive interactions among plants by examining the literature and asking the following questions: 1) Are beneficiary species non-randomly associated with potential benefactors? 2) Are positive mechanisms produced by species-specific plant traits? 3) Can potential benefactors have similar positive effects, but different negative effects? Are beneficiary species non-randomly associated with potential benefactors?