Chapter 11 Summary

CONCEPT 11.1 Competition occurs between species that share the use of a resource that limits the growth, survival, or reproduction of each species.

• Organisms compete for resources such as food, water, light and space, all of which can be consumed or otherwise used to the point of depletion.
• When organisms compete, they reduce the availability of resources.
• If resource levels become sufficiently low, the intensity of competition can increase.
• Competition for resources is common in natural communities.

CONCEPT 11.2 Competition, whether direct or indirect, can limit the distributions and abundances of competing species.

• The most common form of competition is exploitation competition, which occurs when species compete indirectly as they share the use of a limiting resource. Species also compete directly for access to resources in interference competition.
• Competition is often asymmetrical, affecting one competitor more strongly than the other.
• Competition can occur between closely or distantly related species.
• Competition can determine the abundance of competing species.
• Competition can determine both where a species lives within a local habitat and the extent of its geographic distribution.

CONCEPT 11.3 Competing species are more likely to coexist when they use resources in different ways.

• The competitive exclusion principle states that if competing species use the same limiting resource in the same way, they cannot coexist.
• Field studies have revealed examples of resource partitioning, in which competing species use one or more shared resources in different ways.
• In the Lotka–Volterra competition model, the effects of competition are modeled by modifying the logistic equation.
• Graphical analyses of the Lotka–Volterra competition model suggest a refinement of the competitive exclusion principle: the more similar two competing species are in their use of resources, the more likely it is that one will drive the other to extinction.

CONCEPT 11.4 The outcome of competition can be altered by environmental conditions, species interactions, disturbance, and evolution.

• The outcome of competition can be altered by changes in the physical conditions of the environment, or by changes in the competitors’ interactions with other species.
• Periodic disturbances that remove a superior competitor can allow an inferior competitor to persist.
• Evolutionary change within a population of an inferior competitor that causes it to become a superior competitor can reverse the outcome of competition.
• In character displacement, competition causes the forms of competing species to evolve to become more different from each other over time, thereby reducing the intensity of competition.