Opportunistic organism

Opportunistic organisms commonly refer to animals and plants that tolerate variable environmental conditions and food sources. Some opportunistic species can thrive on almost any available nutrient source: omnivorous rats, bears, and raccoons are all opportunistic feeders. Many opportunists flourish under varied environmental conditions: the common house sparrow (Passer domesticus ) can survive both in the warm, humid climate of Florida and in the cold, dry conditions of a Midwestern winter. Aquatic opportunists, often aggressive fish species, fast-spreading plankton , and water plants, frequently tolerate fluctuations in water salinity as well as temperature.

A secondary use of the term "opportunistic" signifies species that can quickly take advantage of favorable conditions when they arise. Such species can postpone reproduction, or even remain dormant, until appropriate temperatures, moisture availability, or food sources make growth and reproduction possible. Some springtime-breeding lizards in Australian deserts, for example, can spend months or years in a juvenile form, but when temperatures are right and a rare rainfall makes food available, no matter what time of year, they quickly mature and produce young while water is still available. More familiar opportunists are viruses and bacteria that reside in the human body. Often such organisms will remain undetected with a healthy host for a long time. But when the host's immune system becomes weak, resident viruses and bacteria seize an opportunity to grow and spread. Thus people suffering from malnutrition, exhaustion, or a prolonged illness are especially vulnerable to common opportunistic diseases such as the common cold or pneumonia.

Adaptable and prolific reproductive strategies usually characterize opportunistic organisms. While some plants can reproduced only when pollinated by a specific, rare insect and many animals can breed only in certain conditions and at a precise time of year, opportunistic species often reproduce at any time of year or under almost any conditions. House mice (Mus musculus ) are extremely opportunistic breeders: they can produce sizeable litters at any time of year. Opportunistic feeding aids their ability to breed year round; these mice can nourish their young with almost any available vegetable matter, fresh or dry.

The common dandelion (Taraxacum officinale ) is also an opportunistic breeder. Producing thousands of seeds per plant from early spring through late fall, the dandelion can reproduce despite competition from fast-growing grass, under heavy applications of chemical herbicides, and even with the violent weekly disturbance of a lawn mower. Once mature, dandelion seeds disperse rapidly and effectively, riding on the wind or on the fur of passing rodents. The common housefly (Musca domestica ) is also an opportunistic feeder and reproducer—it can both feed and lay eggs on almost any organic material as long as it is fairly warm and moist.

Because of their adaptability, opportunistic organisms commonly tolerate severe environmental disturbances. Fire, floods, drought , and pollution disturb or even eliminate plants and animals that require stable conditions and have specialized nutrient sources. Fireweed (Epilobium angustifolium ), an opportunist that readily takes advantage of bare ground and open sunlight, spreads quickly after land is cleared by fire or by human disturbance. Because they tolerate, or even thrive, in disturbed environments, many opportunists flourish around human settlements, actively expanding their ranges as human activity disrupts the habitat of more sensitive animals and plants. Opportunists are especially visible where chemical pollutants contaminate habitat. In such conditions overall species diversity usually declines, but the population of certain opportunistic species may increase as competition from more sensitive or specialized species is eliminated.

Because they are tolerant, prolific, and hardy, many opportunistic organisms, including the house fly, the house mouse, and the dandelion, are considered pests. Where they occur naturally and have natural limits to their spread, however, opportunists play important environmental roles. By quickly colonizing bare ground, fireweed and opportunistic grasses help prevent erosion . Cottonwood trees (Populus spp.), highly opportunistic propagators, are among the few trees able to spread into arid regions, providing shade and nesting places along stream channels in deserts and dry plains. Some opportunists that are highly tolerant of pollution are now considered indicators of otherwise undetected chemical spills . In such hard-to-observe environments as the sea floor, sudden population explosions among certain bottom-dwelling marine mollusks, plankton, and other invertebrates have been used to identify petrochemical spills around drilling platforms and shipping lanes.

See also Adaptation; Contaminated soil; Environmental stress; Flooding; Growth limiting factors; Indicator organism; Parasites; Resilience; Scavenger; Symbiosis

[Mary Ann Cunningham Ph.D. ]

RESOURCES

BOOKS

Foster, H. D. Health, Disease and the Environment. Boca Raton: CRC Press, 1992.

PERIODICALS

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Arcieri, D. T. "The Undesirable Alien—The House Sparrow." The Conservationist 46 (1992): 24–25.

Bradshaw, S. D., H. S. Giron, and F. J. Bradshaw. "Patterns of Breeding in Two Species of Agamid Lizards in the Arid Subtropical Pilbara Region of Western Australia." General and Comparative Endocrinology 82 (1991): 407–24.

Moreno, J. M., and W. C. Oechel. "Fire Intensity Effects on Germination of Shrubs and Herbs in Southern California Chaparral." Ecology 72 (1991): 1993–2004.

Shafir, A. "Dynamics of a Fish Ectoparasite Population: Opportunistic Parasitism in Argulus japonicus." Crustaceana 62 (1992): 50–64.