Some populations form intimate associations

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Some Populations Form Intimate Associations Interactions between species can become so intimate that one or both become depen- dent on the other. Such relationships are termed symbioses . Some are extreme. Certain species of fig are totally dependent on certain species of wasp for pollination. These wasps can pollinate no other flower and are totally dependent on the figs for food. Generally, the relationships are not so extreme. Many flowering plants are depen- dent on insects for pollination, but the flowers can be pollinated by various pollinators that can, in turn, pollinate several different kinds of flower. Basically, there are three types of symbiosis: Parasitism, commensalism, and mutualism. Parasitism In parasitism ,as in predation,one species benefits at the expense of another. Major differences between predation and parasitism are matters of degree and intensity. Generally, parasites are much smaller than their hosts (Figure 15-24). Often, parasites are species specific. That is, the tapeworms of dogs cannot infect humans, and the tapeworms of humans cannot infect dogs. There are important exceptions, however. Trichinella spiralis, a roundworm, causes trichinosis in humans, pigs, bears, and walrus. Some species of protozoan that cause malaria infect both humans and monkeys.When influenza epidemics are not ravaging human populations, the viruses apparently take refuge in birds, where they mutate. Usually, parasites do not kill their hosts quickly (as do predators). Using an eco- nomic analogy,whereas predators live off the principal,parasites live off the interest.Still, for most species, parasitism can be a major contributor to mortality, if not directly, then indirectly. Heavy parasite loads can greatly affect survival. Foxes suffering from mange, which is caused by parasitic mites, lose so much fur that they become susceptible to cold, Figure 15-24 Mosquitoes are parasites of humans.
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Exploration 15-4 How Do Populations Interact? 517 wet, and wind. A free-ranging dog may simultaneously support dozens of populations of tapeworms, flukes, roundworms, and protozoa in its brain, heart, lungs, liver, bladder, stomach, and intestines. Parasitism, like predation, generally does not result in extinction of its host species. Numbers may become greatly reduced as a result of parasitism; as they do so, it becomes increasingly more difficult for the parasite to spread. A near exception is the blight infestations of American chestnut trees in the northeastern United States. Commensalism In commensalism , one species benefits while the other is seemingly unaffected (Figure 15-25). An often-cited example is that of sea anemones growing on the backs of crabs.The crab provides its guests with space on which to grow (space is at a premium for many substrate-dwelling creatures in marine environments), moves them from place to place, and may even provide food scraps on which they feed.What possi- ble benefit could the crab derive out of the relationship? Another example might be epiphytes—plants, such as orchids, growing on the trunks or branches of trees. The epiphytes live closer to vital sunlight and do not parasitize the trees.
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