II.5 Competition and Coexistence in Plant Communities
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II.5 Competition and Coexistence in Plant Communities Ray Dybzinski and David Tilman OUTLINE 1. Introduction to competition and stable coexistence 2. Competition for nutrients 3. Competition for light 4. Competition and temperature 5. From models to reality: Future challenges Numerous species commonly compose natural plant assemblages from the poles to the equator, and a wealth of classic ecological experiments have demonstrated that these often compete strongly with one another for resources such as nutrients or light. Theoretical ecologists have demonstrated that competing species are only expected to stably coexist (i.e., coexist in the long run) when each is protected from local extinction by density- or frequency-dependent processes that benefit it when rare, or equivalently, when the net negative effects of intraspeci fic (within-species) competition exceed those of interspeci fic (between-species) competition. Competition for nutrients, such as nitrogen and phosphorus, is ‘‘size symmetric’’ because smaller and larger individuals are potentially equal competitors on a per-biomass basis. In contrast, shoot competition for light is ‘‘size asymmetric’’ because taller individuals are advantaged irrespective of biomass. We describe the different modeling approaches that this difference requires. However, in either case, stable coexistence requires trade-offs such that species that are better competitors for one limiting nutrient or in one light environment are necessarily worse competitors for a second limiting nutrient or in a second light environment . As one important example of how these models might account for the stable coexistence of numerous species across landscapes, we consider the effects of habitat heterogeneity in mean growing season temperature coupled with trade-offs in performance among species at different mean temperatures. Finally, given our theoretical understanding, we close with a discussion of the current challenges to and opportunities for advancing our empirical understanding of competition and coexistence in the real world. GLOSSARY coexistence. The indefinite persistence of two or more species. The empirically relevant sort of coexistence is termed ‘‘stable coexistence’’ in which species will continue to persist in the face of perturbations in their abundances. It is important to note that species that co-occur may or may not be stably coexisting ; it is possible that one or more species are on their way to local extinction at a time scale that might appear slow to a casual observer. competition. Most broadly, an interaction between individuals in which neither benefits. Here, we are considering exploitation competition for limiting resources in which the resource consumed or intercepted by one individual is no longer available to the second individual, thereby decreasing its fitness. exclusion. A condition in which a species is driven to local extinction as a consequence of a competitive interaction. founder control. A condition in which the dominant species in a competitive interaction is the species that is initially most abundant. interspecific competition. Competition among individuals of different species. intraspecific competition. Competition among individuals of the same species. invader/invasion. In the context of theoretical ecology, an invader is a species introduced at arbitrarily small abundance to a habitat of a resident species at equilibrium. The question is asked: will the invader increase in abundance? Note that this use of the term is different from the sense in which an ‘‘invader’’ may be a foreign species with negative ecological consequences. local extinction. A condition in which a species is no longer present within a defined habitat area. Local extinction is very different from the common use of the term ‘‘extinction,’’ in which a species is no longer present anywhere. resource. Broadly, something that may be consumed by one individual such that it is no longer available to another organism. Relevant resources for plants are nutrients, such as nitrogen, phosphorus, potassium , and micronutrients, along with light, water, and carbon dioxide. 1. INTRODUCTION TO COMPETITION AND STABLE COEXISTENCE Plant species, be they trees in tropical or temperate forests, forbs in prairies or tundra, or algae in lakes or oceans, frequently compete with other plant species for various limiting resources, such as nitrate, phosphate, and light (Harper, 1977). Plants are also impacted by interactions such as mutualism, predation, herbivory, and disease. Each of these interactions has the potential to influence both the types of habitats in which species occur and their abundances in those habitats. Here we focus on mechanisms of competition for nutrients and light and their influence on coexistence and competitive exclusion among competing plant species. As developed in the pioneering work of Lotka and Volterra, competition between two species, or interspeci...