Opportunistic Species

Environmental resources and opportunities

Opportunistic species in novel circumstances

Resources

Opportunistic species of animals or plants are adapted to exploit newly available habitats or resources and are typically found in unpredictable, transient, and variable environments. For example, clear-cut forests create well-lit open areas which are colonized rapidly by the windbone seeds of opportunistic species of plants, many of which are regarded as weeds by farmers and gardeners. Besides producing easily dispersed seeds, opportunistic species characteristically have a rapid growth rate, quickly establishing themselves in the new environment. Opportunistic species also have other characteristics: they reproduce early, have a small body size, and produce large numbers of seeds or offspring, a strategy known to ecologists as r-selection. Opportunistic species are most prominent during the early stages of ecological succession, when species that are more competitive in the long run are not very abundant. Opportunistic species have a great ability to alter their growth rate, physiology, or behavior to better suit the environmental conditions with which they are faced. Usually, this opportunistic response is accomplished without changes in the genotype, in which case it is known as phenotypic plasticity.

Environmental resources and opportunities

To be successful in the evolutionary sense, all organisms must grow and reproduce successfully, and to accomplish these functions they have particular requirements for environmental resources. Plants, for example, need access to an appropriate supply of sunlight, water, and inorganic nutrients such as carbon dioxide, nitrate, phosphate, calcium, and about 20 other chemicals. Similarly, animals require a suitable habitat, replete with the appropriate foods to eat and places for shelter to complete their life cycle. The requirements of organisms for resources must be satisfied within an appropriate ecological context, for example, in terms of the temperature regime, or the types of diseases, parasites, or predators that are present.

In some ecological situations, the availability of resources is highly constrained, and this poses severe limitations to the growth and reproduction of organisms. However, some species are genetically adapted to surviving under these sorts of difficult circumstances. Their adaptive syndrome is referred to as competitive if access to resources is limited by the presence of other species with similar needs (an interaction that ecologists refer to as competition). In other cases, the availability of resources may be lacking because of infertile soil, excessively cold or hot temperatures, pollution, or some other type of non-living stressor, in which case the adaptive syndrome is called stress tolerant.

In contrast, certain ecological situations are characterized by a relatively great abundance of resources. This is often the circumstance, for example, after a mature, highly competitive plant community, growing on a fertile site, is subject to a severe disturbance. Because the disturbance kills many mature plants, the biological demand for resources is greatly decreased, so that competition is no longer very important and resources are freely available. The adaptive syndrome exhibited by species that take advantage of this temporary circumstance is referred to as ruderal. Ruderal species are highly opportunistic, in that they are adapted to taking advantage of temporary conditions of a great availability of resources.

The broad characteristics of ruderal species can be illustrated by considering the general features of plants that exhibit this strategy. Ruderal plants are usually herbaceous, small, short-lived, and highly fecund. Ruderal plants devote a large proportion of their productivity to the development of a great number of seeds, which may be long-lived in soil, or are readily dispersed over long distances. Moreover, ruderal plants have a relatively great potential for pheno-typically plastic responses to variations of resource

KEY TERMS

Genotype —The full set of paired genetic elements carried by each individual, representing its genetic blueprint.

Phenotype —The actual, biological expression in an individual organism of its genetically based information, as influenced by environmental conditions.

Phenotypic plasticity —The variable degree of expression of genetically based biological potential, in terms of growth form, biochemistry, behavior, etc., depending on environmental circumstances.

Ruderal —Refers to plants that occur on recently disturbed sites, but only until the intensification of competition related stresses associated with succession eliminates them from the community.

availability. Clearly, the ruderal strategy is highly opportunistic, and has evolved to allow rapid and vigorous colonizations of fertile habitats soon after disturbance, while competition is a weak interaction in the recovering ecosystem.

Opportunistic species in novel circumstances

Sometimes, species that are not particularly prominent in their native habitat become important pests when they are introduced by humans into a new habitat. In such cases, these organisms are opportunistically responding to a novel ecological circumstance that enhances their access to resources. This response is made possible because the species are no longer constrained by the specifically adapted herbivores, predators, or diseases that occur in their native habitat, a situation that is referred to as ecological release. Most of the important species of weeds in agriculture have opportunistically responded to favorable circumstances in the new environments to which they have been introduced, as have invasive animal pests, such as rats and mice.

Of course, there are limitations to the ecological success of opportunistic species. In stable environments, where disturbance is infrequent, there are few circumstances that are favorable to opportunistic species, and they will be rare in the biota. Such conditions are common, for example, in old-growth forests and other climax ecosystems. However, many human activities result in extensive disturbances of natural ecosystems. As a result, opportunistic species are faring much better today than they did prior to the global environmental changes that are being caused by humans.

See also Adaptation; Stress, ecological.

Resources

BOOKS

Ambasht, R.S., and Navin K. Ambasht. Modern Trends in Applied Terrestrial Ecology. New York: Plenum, 2003.

Gibson, David J. Methods in Comparative Plant Population Ecology. Oxford: Oxford University Press, 2002.

Gurevitch, J., Samuel M. Scheiner, and Gordon A. Fox. The Ecology of Plants. Sunderland, MA: Sinauer Associates, 2002.

Bill Freedman