Interspecific Competition
In interspecific competition, individuals of two or more species share a resource in short supply, thus reducing the fitness of both. As with intraspecific competition, competition between species can involve either exploitation or interference. Six types of interactions account for most instances of interspecific competition: (1) consumption, (2) preemption, (3) overgrowth, (4) chemical interaction, (5) territorial, and (6) encounter.
Competition Model 13.2–13.3
The Lotka–Volterra equations describe four possible outcomes of interspecific competition. species 1 may outcompete species 2; species 2 may outcompete species 1. Both of these outcomes represent competitive exclusion. The other two outcomes involve coexistence. One is unstable equilibrium, in which the species that was most abundant at the outset usually outcompetes the other. A final possible outcome is stable equilibrium, in which two species coexist but at a lower population level than if each existed without the other.
Experimental Tests 13.4
Laboratory experiments with species interactions support the Lotka–Volterra model.
Competitive Exclusion 13.5
Experiment results led to the formulation of the competitive exclusion principle—two species with exactly the same ecological requirements cannot coexist. This principle has stimulated critical examinations of competitive relationships outside the laboratory, especially of how species coexist and how resources are partitioned.
Nonresource Factors 13.6
Environmental factors such as temperature, soil or water pH, relative humidity, and salinity directly influence physiological processes related to growth and reproduction but are not consumable resources that species compete over. By differentially influencing species within a community, these nonresource factors can influence the outcome of competition.
Environmental Variability 13.7
Environmental variability may give each species a temporary advantage. It allows competitors to coexist, whereas under constant conditions one would exclude the other.
Multiple Factors 13.8
In many cases, competition between species involves multiple resources. Competition for one resource often influences an organism’s ability to access other resources.
Environmental Gradients 13.9
As environmental conditions change, so may the relative competitive ability of species. Shifts in competitive ability can result either from changes in the carrying capacities related to a changing resource base or from changes in the physical environment that interact with resource availability. Natural environmental gradients often involve the covariation of multiple factors—both resource and nonresource factors—such as salinity, temperature, and water depth.
Niche 13.10
A species’ fundamental niche compresses or shifts when competition restricts the species’ type of food or habitat. In some cases, the realized niche may not provide optimal conditions for the species. In the absence of competition, the species may experience competitive release, and its niche may expand.
Resource Partitioning 13.11
Many species that share the same habitat coexist by partitioning available resources. When each species exploits a portion of the resources unavailable to others, competition is reduced. Resource partitioning is often viewed as a product of the coevolution of characteristics that function to reduce competition. Interspecific competition can reduce the fitness of individuals. If certain phenotypes within the population function to reduce competition with individuals of other species, those individuals will encounter less competition and increased fitness. The result is a shift in the distribution of phenotypes (characteristics) within the competing population(s). When the shift involves features of the species’ morphology, behavior, or physiology, it is referred to as character displacement.
Complexity of Competition 13.12
Competition is a complex interaction that seldom involves the interaction between two species for a single limiting resource. Competition involves a variety of environmental factors that directly influence survival, growth, and reproduction—factors that vary in both time and space.
Wolves and Coyotes Ecological Issues & Applications
The decline of gray wolf populations throughout much of North America have been paralleled by a dramatic expansion in the range of coyotes. Evidence from areas in which wolves have been reintroduced suggests that the expansion of coyotes was in part a result of competitive release from wolf populations over the past century.
