509 to ecologists the robins niche becomes important

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509 To ecologists, the robin’s niche becomes important. A niche includes all aspects of the physical and biological environment that are important to a species.Where does the species live? With what other organisms does it share space and other resources? How does it interact with other species? A species’ niche includes not only where it lives—its habitat—but its functional role within the community of organisms with which it lives. The intensity of interactions between populations may be slight or great. We are scarcely aware of the populations of invertebrates that live beneath stones and scraps of wood in our backyards, and we have little influence on them so long as we leave them undisturbed. We share space and little else. At the other extreme, humans and most of their crop plants are mutually interdependent. Most humans today subsist mainly on crop plants or animals that eat crop plants. Most crops are so changed from their original populations that they could not exist without constant human care and protection. Humans and their crop plants are mutually, totally interdependent. Let’s take a closer look at the interactions between populations. When Populations Compete, Both Are Harmed There are at least two kinds of competition with which organisms must contend. Indi- viduals must often compete with others of their kind for food, other resources, space, and mates. We explored competition within species in Chapter 14. In this chapter, we will look at the other kind of competition—that between species. Competition between species, too, is sometimes for space, resources, and food (but, by the very definition of species, never for mates). The effects of competition appear to be straightforward. In the 1930s, the Russian ecologist G.F.Gause conducted numerous laboratory experiments on populations of two different species of Paramecium grown on culture medium. When grown alone, each species’ population showed the expected S-shaped population growth curve.When grown together,both species initially expressed the S-shaped growth pattern,but,as time passed, one species survived and the other became extinct. For the next 30 years, similar labo- ratory experiments demonstrated the same results in other protozoa, yeasts, hydras, water fleas, fruit flies, grain beetles, and duckweeds. So pervasive were the results that, in 1960, the ecologist Garret Hardin put forth the competitive exclusion principle :Two species cannot coexist while exploiting the same limiting resource. But what about natural populations? Does the competitive exclusion principle apply outside the laboratory? Examples are harder to come by. Intense competition between two populations is, after all, a transitory condition. Whenever one population excludes the other, all traces of competition are gone. Species introductions, however, yield evi- dence that the principle still applies. In the late 1940s, the Hawaii Department of Agri- culture released numerous potential insect parasites and predators to control crop-eating pests.Three species of tiny wasps were particularly useful in attacking Hawaiian fruit flies.
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