parasitism

parasitism An interaction of species populations in which one (typically small) organism (the parasite) lives in or on another (the host), from which it obtains food (when the parasite may be called a biotroph), shelter, or other requirements. Whereas a predator kills its host (i.e. lives on the capital of its food resource) a parasite does not (i.e. lives on the income). Parasitism usually implies that some harm is done to the host, but this interpretation must be qualified. Effects on the host range from almost none to severe illness and eventual death, but even where such obvious immediate harm accrues to the individual host it does not follow that the relationship is harmful to the host species in the long term or in an evolutionary context (e.g. it might favour beneficial adaptation in the host species population). Obligate parasites can live only parasitically. Facultative parasites may live as parasites or as independent saprotrophs. Partial parasites are facultative parasites that live more successfully as parasites than they do independently. Ectoparasites live externally on the host. Endoparasites live inside the body of the host. Compare COMMENSALISM; MUTUALISM; NEUTRALISM. See also HYPERPARASITE; NECROTROPHIC.

parasitism An interaction of species populations in which one (typically small) organism (the parasite) lives in or on another (the host), from which it obtains food (when the parasite may be called a biotroph), shelter, or other requirements. Whereas a predator kills its host (i.e. it lives on the capital of its food resource) a parasite does not (i.e. it lives on the income). Parasitism usually implies that some harm is done to the host, but this interpretation must be qualified. Effects on the host range from almost none to severe illness and eventual death, but even where such obvious immediate harm accrues to the individual host, it does not follow that the relationship is harmful to the host species in the long term or in an evolutionary context (e.g. it might favour beneficial adaptation in the host species population). Obligate parasites (holoparasites) can live only parasitically. Facultative parasites may live as parasites or as independent saprotrophs. Partial parasites (semiparasites) are facultative parasites that live more successfully as parasites than they do independently. Ectoparasites live externally on the host. Endoparasites live inside the body of the host. Compare commensalism; mutualism; and neutralism; see also hemiparasite; hyperparasite; and necrotrophic.