NICHE The ecological niche of a species is its total way of life or its role in an ecosystem. It includes all physical, chemical, and biological conditions a species needs to live and reproduce in an ecosystem. Specialist species have narrow niches. They may be able to live in only one type of habitat, tolerate only a narrow range of climatic and other environmnetal conditions, or use only one or a few types of food. Examples of specialists are tiger salamanders, which can breed only in fishless ponds so their laryae won’t be eaten. General species have broad niches. Flies, cockroaches, mice, rats, white-tail deer, raccoons, and humans are all generalist species. Interactions Between Species WAYS SPECIES INTERACT When any two species in an ecosystem have some activities or requirements in common, they may interact to some degree. The principal types of species interactions are interspecific competition, predation, parasitism, mutualism, and comensalism. Three of these interactions--parasitism, mutualism, and commensalism--are symbiotic relationships in which two or more kinds of organisms live together in an intimate association, with members of one or both species benefiting from the association. In mutualism and commensalism neither species is harmed by the interaction. COMPETITION BETWEEN SPECIES FOR LIMITED RESOURCES Fundamental niche: the full potentail range of physical, chemical, and biological factors it could use if there were no competition from other species. In most ecosystems each species faces competition from one or more of the same limited resources (such as food, sunlight, water, soil nutrients, or space) it needs. Because of such interspecific competition, parts of the fundamental niches of different species overlap, one species may occupy more of its fundamental niche than the other species. This is done through two types of competitive interactions. In interference competition one species may limit anther’s access to some resource, regardless of whether the resource is abundant or scarce. For example, one species of hummingbird may defend patches of spring wildflowers from which it gets nectar by chasing away individuals of other hummingbird species. Coral animals kill other nearby species of coral by poisoning and growing over them. In pure exploitation competition two competing species have equal access to a specific resource but differ in how quickly or efficiently they exploit it. In this way one species gets more of the resource and thus hampers the growth, reproduction, or survival of the other species. When shared resources are abundant this type or interspecific competition does not occur. Another process that reduces the degree of fundamental niche overlap is resource partitioning, the dividing up of scarce resources so that species with similar requirements use them at different times, in different ways, or in different places (Figure 4-38). In effect, they “share the wealth,” with each competing species occupying a realized niche, the portion of the fundamental niche that a species actually occupies. For example, hawks and owls feed on similar prey, but hawks hunt during the day and owls hunt at night. Experiments have shown that no two species can occupy exactly the same fundamental niche indefinitely in a habitat where there is not enough of a particular resource to meet the needs of both species. This is called the competitive exclusion principle. As a result, one of the competing species must migrate to another area (if possible), shift its feeding habits or behavior, suffer a sharp population decline, or become extinct. PREDATION The most obvious form of species interaction in food chains and webs is predation. Members of a predator species feed on parts or all of an organism of a prey species, but do not live on or in the prey. Together the two kinds of organisms, such as lions and zebras, are said to have a predator-prey relationship. Predator-prey relationships can also include carnivore-prey, herbivore-plant, and parasite-host interactions. Other predators, called scavengers, feed on dead organisms that were either killed by other organisms or died naturally. Prey species have various protective mechanisms. Some can run fast; others have highly developed sight or sense. Some have protective shells (turtles) or thorns (cacti). Some prey species give off chemicals that smell (skunk and skunk cabbages). The caterpillars of the monarch butterfly eat milkweed, which contains chemicals that poison some of the milkweed’s predators, but not monarch caterpillars. These poisons are incorporated into the caterpillar’s body, and after its metamorphosis the adult monarch butterfly is foul-tasting and poisonous to some of its predators. Other butterfly species, such as the viceroy, are protected by looking like the monarch, a protective device known as mimicry. PARASITISM Another type of predator-prey interaction is parasitism. A parasite is a predator that preys on another organism--it’s host- by living on or in the host for all or most of the host’s life. The parasite is smaller than its host and draws nourishment from and gradually weakens the host, sometimes killing it. Some parasites move from one host to another, as fleas and ticks do. Others spend their adult lives attached to a single host. Examples are mistletoe, which feeds on oak tree branches, and tapeworms, which feed in the intestines of humans ans other animals. MUTUALISM Mutualism is a type of species interaction in which both participating species generally benefit. The honeybee and certain flowers hace a mutualistic relationship. Research indicates that mutualism increases when resources become scares. COMMENSALISM In another type of species interaction, called commensalism, one species benefitswile the other is neither helped not harmed to any great degree. An example is the relationship between warious trees and epiphytes or “air plants” that attach themselves to tree branches. The epiphytes benefit by obtaining water and nutrients from air or bark surfaces without penetrating or harming their hosts. INTERDEPENDENCE AND COMMECTEDNESS The essential features of the living and nonliving parts of individual terrestrial and aquatic ecosystems, and of the ecosphere, are interdependence and connectedness.
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