Parasites
Organisms that live in or upon the body of a host organism and are metabolically dependent on the host for completion of their life cycle. Parasites may be plants, animals, viruses, bacteria, or fungi. Parasites feed either on their host directly or upon its surplus fluids. Some parasites, known as endoparasites, live inside their host, while ectoparasites live on the outside of their host. Organisms in which parasites reach maturity are called definitive hosts, and hosts harboring parasite stages are called intermediate hosts. Organisms which spread parasite stages between hosts are known as vectors. Full-time, or obligatory, parasites have an absolute dependence on their hosts. Examples of this type are viruses, which can only live and multiply inside living cells, and tapeworms, which can only live and multiply inside other species. Part-time, or facultative, parasites, such as wood ticks, have parasitic and free-living stages in their life cycle and are only temporary residents of their hosts.
The effects of parasites on their hosts depend on the health of the host, as well as the severity of the infestation. In diseased, old, or poorly-fed individuals, parasite infestations can be fatal, but parasites do not typically kill their hosts, though they can slow growth and cause weight loss. Some plant parasites do kill their host and then live on its decomposing remains, and certain species of hymenopteran insect are parasitoids, whose larva feeds within the living body of the host, eventually killing it. Some parasites, such as Sacculina, castrate their host by infecting its reproductive organs. Cuckoos are brood parasites, and they lay their eggs in the nest of another bird species, evicting the eggs that were there and leaving the young cuckoos to be raised by the parents of the host species.
In community ecology, parasites are often grouped together with predators, since both feed directly upon other organisms, either harming them or killing them. Ecologically, small disease-causing organisms (viruses, bacteria, protozoans) are regarded as microparasites, while larger parasites (flatworms, roundworms, lice, fleas, ticks, rusts, mistletoe) are macroparasites. Together with predators and diseases, parasites are one of the natural components of environmental resistance which serves to limit population growth. Microparasites that are transmitted directly between infected hosts, such as rabies or distemper, target herd animals with a high host density and can significantly reduce population levels. Macroparasites that employ one or more intermediate hosts, such as flukes, tapeworms, or roundworms, have highly effective transmission stages but usually have only a limited effect on the population of the host.
Parasites can play a larger role in altered ecosystems. In the eastern United States parasitic infections have held populations of cottontail rabbits well below the carrying capacity of the habitat. Parasitic insects have been used to control populations of the olive scale insect, a serious pest of olive trees in California. Accidentally introduced parasites have negatively impacted populations of commercial species by altering the balance of the ecosystem. For example, a protozoan parasite infecting California oysters was introduced to French oyster beds, wiping out the native European oysters and seriously damaging commercial fishing there.
Population Biology; Predator-Prey Interactions
Resources
Books
Bullock, W. L. People, Parasites, and Pestilence: An Introduction to the Natural History of Infectious Disease. Minnesota: Burgess Publishing Company, 1982.
Crew, W., and D. R. W. Haddock. Parasites and Human Disease. London: Edward Arnold, 1985.
Despommier, D. D., and J. W. Karapelou. Parasite Life Cycles. New York: Springer-Verlag, 1987.
Noble, E. R., and G. A. Noble. Parasitology: The Biology of Animal Parasites. 6th ed. Philadelphia: Lea & Febiger, 1989.
Schmidt, G. D., and L. S. Roberts. Foundations of Parasitology. 4th ed. St. Louis: Times Mirror/Mosby College Pub., 1989.
Smith, R. L. Elements of Ecology. 3rd ed. New York: Harper Collins, 1991.
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