Opportunistic Species

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.