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Low tide figure 20-4

Chthamalus stellatus

Semibalanus balanoides

Chthamalus stellatus

Semibalanus balanoides

The realized niche of the Chthamalus barnacle species is smaller than its fundamental niche because of competition from the Semibalanus species. Although Chthamalus can survive at all levels of the intertidal zone, it is usually crowded out at the lower level by the faster-growing Semibalanus. But Semibalanus cannot survive in the upper level, which is left dry for longer periods.

www.scilinks.org Topic: Competition Keyword: HM60326

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www.scilinks.org Topic: Competition Keyword: HM60326

cy, Maintained by the

ÍIM^C National Science JI v ft Taachers Association

As Figure 20-4 shows, each species of barnacle occupied a distinct band within the intertidal zone. Chthamalus lived higher on the rocks than Semibalanus did. Connell demonstrated that this difference was partly due to competition. When a rock covered with Chthamalus was transplanted to the lower zone, Chthamalus was clearly able to tolerate the conditions in the lower zone. However, Semibalanus would soon settle on the rock and eventually crowd out Chthamalus. Connell concluded that competition restricted the range of Chthamalus. Although Chthamalus could survive lower on the rocks, competition from Semibalanus prevented it from doing so. Higher on the rocks, Chthamalus is free from competition because it can tolerate drying out more than Semibalanus can.

Reduced Niche Size

Recall that a species' niche (NICH) is the role that the species plays in its environment. But this role may vary due to interactions with other species. So, ecologists differentiate between the fundamental niche and the realized niche of a species. The fundamental niche of a species is the range of conditions that it can potentially tolerate and the range of resources that it can potentially use. Often, predation and competition limit the species' use of these ranges. Thus, the realized niche is the part of the niche that the species actually uses, as shown in Figure 20-4.

Character Displacement

Competition has the potential to be an important influence on the nature of a community. The composition of a community may change as competitors win, lose, or evolve differences that lessen the intensity of competition. Natural selection favors differences between competitors, especially where the niches of the competitors overlap. This evolution of differences in a characteristic due to competition is called character displacement. Character displacement is a way of reducing niche overlap.

The beaks of Galápagos Island finches provide an example of character displacement. Many of these closely related finch species eat seeds that come from the small number of plant species on the islands. Furthermore, the size of each bird's beak determines the size of the seeds that the bird can eat. When the average beak size of each species is compared with the average beak size of each of the other species, the greatest difference in beak size is observed between species that share an island. Differences in beak size reduce competition by enabling species that share an island to favor different food sources.

Resource Partitioning

As Charles Darwin noted, competition is likely to be most intense between similar species that require the same resources. When similar species coexist, each species may avoid competition with others by using a specific part of an available resource. This pattern of resource use is called resource partitioning. For example, the pioneering ecologist Robert MacArthur (1930-1972) studied several species of warblers that feed on insects in spruce and fir trees. MacArthur discovered that when more than one species of warbler is foraging within the same tree, each species hunts for insects only in a particular section of the tree. As a result, competition among the species is reduced.

Word Roots and Origins parasite from the Latin word parasitus, meaning "one who eats at the table of another"

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