Opportunistic Species

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 phenotypically plastic responses to variations of resource 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.