Interactions among plants and evolution, Thorpe et al. 2011

This review explores the question of whether plant-plant interactions drive evolutionary changes. “If such evolution is common, plant communities are not random assemblages of species.” The topic is under-studied compared to plant interactions with other groups.

Research on plant–consumer, plant–pollinator and plant–disperser interactions has been central to understanding the complex mutualistic and co-dependent interactions among species that structure communities. However, with some notable exceptions, interactions among plants have not been emphasized as processes that contribute to selection and evolution [Thorpe 2011: 730].

“The simplest interactions among plants are direct interactions, such as facilitation, resource competition and allelopathy” [Thorpe 2011: 731]. Facilitation is when one plant protects an adjacent plant, such as from drought and heat by providing shade, for example, or from browsing by being thorny or toxic to herbivores and surrounding the facilitated plant. Allelopathy refers to plants’ release of toxic substances that suppress the growth of another organism, including other plants. In natural communities, any given plant may be interacting with several different plants at the same time.

Competition for sunlight, water, and nutrients drives niche differentiation, or the carving out by species of particular spaces or timing within an ecosystem to obtain a share of limited resources. “The exceptionally rich body of ecological literature on the niche is based in part on the idea that competition can drive the evolution of niche differentiation, thus allowing species to coexist” [Thorpe 2011: 732].

Thorpe et al. refer to an example from a 1976 article by Parrish & Bazzaz , who “found that resource partitioning, as estimated from spatial overlap among root systems, was higher in stable prairie communities with a long community history than in early successional old-field communities composed of species without a common history” [Thorpe 2011: 731]. In other words, plants with a long coexistence history more efficiently divvy up resources than do species lacking a common community history.

The primary hypothesis for positive diversity–ecosystem function relationships has been niche ‘complementarity’, the idea that different species or functional groups occupy niches different enough from each other to more fully utilize resources or space, increasing and stabilizing productivity, and making it more difficult for other species to enter the community [Thorpe 2011: 733].

The authors are somewhat inconclusive, however, about what drives niche complementarity (resource partitioning).

We do not yet know whether complementarity is produced by interactions causing evolutionary shifts in niche space (and thus coexistence and more complete resource use) or by sorting of the existing species pool [Thorpe 2011: 733].

Plants can also adapt to one another’s allelopathic substances over time, a fact that contributes to the argument that plant-plant interactions produce evolutionary changes. “Recent experiments raise the possibility that some invaders may exude allelochemicals that are relatively ineffective against neighbors in natural communities, but highly inhibitory to plants in invaded communities” [Thorpe 2011: 734].