Summary of Chapter Concepts

Competition occurs when individuals experience limited resources. Competition can either be intraspecific or interspecific and occurs when there is a limited resource. Resources can either be renewable or nonrenewable and they can be generated either from within an ecosystem or from outside an ecosystem. Leibig’s law of the minimum states that a population will increase until the most limiting resource prevents further growth, although we now appreciate that different resources can have interactive effects on population growth. The competitive exclusion principle states that two species cannot coexist indefinitely when they are both limited by the same resource.

The theory of competition is an extension of logistic growth models. The simplest models consider competition for a single resource and consider the zero population growth isoclines of two competing species. Using these models, we can make predictions regarding the conditions under which two species can win a competitive outcome or coexist. Under the more realistic situation of multiple limiting resources, we can have coexistence of multiple species of competitors when each species is limited by a different resource.

The outcome of competition can be altered by abiotic conditions, disturbances, and interactions with other species. If a species is competitively superior but not tolerant of extreme abiotic conditions, it will not be able to dominate areas that experience such conditions. Similarly, competitively superior species that cannot persist with frequent disturbances, such as fire, cannot come to dominate inferior competitors. In the same way, superior competitors that are more vulnerable to herbivores or predators cannot outcompete inferior competitors because they are preferentially harmed or killed.

Competition can occur through exploitation or direct interference, or it may be apparent competition. Exploitative competition occurs when one species consumes enough of a resource that another species can no longer persist. In contrast, interference competition occurs when a species defends a resource and prevents other individuals from consuming it. Interference competition includes aggressive interactions among species and allelopathy. Sometimes species appear to be competing because the presence of one species has a negative effect on the population of the other. In cases of apparent competition, the underlying mechanism is not competition, but another type of interaction such as predation, herbivory, or parasitism.