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Competition as an ecological and evolutionary factor

Competition is a biological interaction among organisms of the same or different species associated with the need for a common resource that occurs in a limited supply relative to demand. In other words, competition occurs when the capability of the environment to supply resources is smaller than the potential biological requirement so that organisms interfere with each other. Plants, for example, often compete for access to a limited supply of nutrients, water, sunlight, and space.

Intraspecific competition occurs when individuals of the same species vie for access to essential resources, while interspecific competition occurs between different species. Stresses associated with competition are said to be symmetric if they involve organisms of similar size and/or abilities to utilize resources. Competition is asymmetric when there are substantial differences in these abilities, as occurs in the case of large trees interacting with plants of a forest understory.

Individuals of the same species have virtually identical resource requirements. Therefore, whenever populations of a species are crowded, intraspecific competition is intense. Intraspecific competition in dense populations results in a process known as self-thinning, which is characterized by mortality of less-capable individuals and relative success by more-competitive individuals. In such situations, intraspecific competition is an important regulator of population size. Moreover, because individual organisms vary in their reproductive success, intraspecific competition can be a selective factor in evolution.

Interspecific competition can also be intense if individuals of the various species are crowded and have similar requirements of resources. One ecological theory, known as the competitive exclusion principle, states that species with ecologically identical life styles and resource needs cannot coexist over the longer term; the competitively less-fit species will be displaced by the better fit species. Although it is debatable that different species could have identical ecological requirements, it is not difficult to comprehend that intense competition must occur among similar species living in the same, resource-limited habitat. In such situations, interspecific competition must be important in structuring ecological communities and as an agent of natural selection.

The term competitive release refers to a situation in which an organism or species is relieved of the stresses associated with competition allowing it to become more successful and dominant in its habitat. For example, by the early 1950s the American chestnut ( Castanea dentata) had been eliminated as a dominant canopy species in deciduous forests of eastern North America by the accidental introduction of a fungal pathogen known as chestnut blight (Endothia parasitica). Other tree species took advantage of their sudden release from competition with the chestnut by opportunistically filling in the canopy gaps that were left by the demise of mature chestnut trees. Similarly, competitively suppressed plants may be released when a mature forest is disturbed, for example, by wildfire, a windstorm, or harvesting by humans. If the disturbance kills many of the trees that formed the forest canopy but previously suppressed plants survive, then these understory plants will gain access to an abundance of environmental resources such as light, moisture, and nutrients, and they will be able to grow relatively freely.

Competitive displacement is said to occur when a more competitive species causes another to utilize a distinctly sub-optimal habitat. A number of interesting cases of competitive displacement have been described by ecologists, many involving interactions of plant species. In eastern North America, for example, the natural habitat utilized by the silver maple tree (Acer saccharinum) is almost entirely restricted to forested wetlands, or swamps. However, the silver maple is more productive of biomass and fruits if it grows on well-drained, upland sites, and for this reason it is commonly cultivated in cities and towns. In spite of this habitat preference, the silver maple does not occur in the natural forest community of well drained sites. It appears that the silver maple is not sufficiently competitive to cooccur in well-drained sites with more vigorous tree species such as the sugar maple (Acer saccharum), basswood (Tilia americana), or the red oak (Quercus rubra). Consequently, the silver maple is displaced to swamps, a distinctly sub-optimal habitat in which there is frequent physiological stress associated with flooding.

Over long periods of time, competitive displacement may lead to evolutionary changes. This happens as species displaced to marginal environments evolve to become better adapted to those conditions, and they may eventually become new species. Competitive displacement is believed to be the primary force leading to the evolution of species swarms on isolated islands such as those of fruit flies (Drosophila spp.) and honeycreepers (Drepaniidae) on the Hawaiian Islands and Darwin's finches (Geospizinae) on the Galapagos Islands.

In the cases of the honeycreepers and Darwin's finches, the islands are believed to have been colonized by a few individuals of a species of finch. These founders then developed a large population which saturated the carrying capacity of the common habitats so that intraspecific competition became intense. Some individuals that were less competitive in the usual means of habitat exploitation were relegated to marginal habitats or to unusual means of exploiting resources within a common habitat. Natural selection would have favored genetically based adaptations that allowed a more efficient exploitation of the marginal habitats or lifestyles of the populations of displaced birds, leading to evolutionary changes. Eventually, a condition of reproductive isolation would have developed, and a new species would have evolved from the founder population. Competitive displacements among species of finches could then have further elaborated the species swarms. The various species of Darwin's finches and Hawaiian honeycreepers are mostly distinguished on the basis of differences in the size and shape of their bills and on behavioral differences associated with feeding styles.

It must be understood that not all environments are resource limited, and in such situations competition is not a very important process. There are two generic types of non-competitive environments—recently disturbed and environmentally stressed. In habitats that have recently been subjected to a catastrophic disturbance, the populations and biomass of organisms is relatively small, and the biological demand for resources is correspondingly not very intense. Species that are specialized to take advantage of the resource-rich and competition-free conditions of recent disturbances are known as ruderals. These species are adapted to rapidly colonizing disturbed sites where they can grow freely and are highly fecund. However, within several years the ruderals are usually reduced in abundance or eliminated from the community by slower growing, but more competitive species that eventually take over the site and its resources and dominate later successional stages.

Some habitats are subject to intense environmental stress such as physical stress associated with climate or toxic stress associated with nutrient deficiency or pollution. Because of the severe intensity of environmental stress in such habitats, the productivity of organisms is highly constrained, and there is little competition for resources. The arctic tundra, for example, is an ecosystem that is highly stressed by climate. If the density of individual plants of the tundra is experimentally decreased by thinning, the residual plants do not grow better because their productivity was not constrained by competition. However, the intensity of environmental stress can be experimentally alleviated by enclosing an area of tundra in a greenhouse and by fertilizing with nutrients. In such a situation, competition among arctic plants can become a significant ecological interaction, and this change can be experimentally demonstrated.

Because the effects of competition can be profound and are nearly always measurable in at least some parameter, the processes surrounding and affecting competition, as well as the environmental forces affected or shaped by competition, are an active area for research by ecologists. Competition is believed to have a strong result on, for example, the process of speciation. Speciation is the formation of two distinct species from a single one over time. Therefore, ecologists might compare the divergence of genetic characteristics between organisms in an area with high levels of intraspecific competition for a limiting resource versus those that are not.

Similarly, competition as a major force that structures communities of organisms within ecosystems is a major area or research. The relative abundances of different organisms in a community, for example, is determined in part on the levels of competition for resources found in their habitat. Diversity, another very popular topic of active research in ecology, also deals with competition. Competitive interactions are believed to increase the amount of diversity in an environment. In other words, the number of species present in a given ecosystem increases in areas with increased competition. The current global biodiversity project, which is attempting to catalog all of the species found on Earth, has helped to establish the link between diversity and competition.

See also Stress, ecological.



Begon, M., J. L. Harper, and C. R. Townsend. Ecology: Individuals, Populations and Communities. 2nd ed. London: Blackwell Sci. Pub., 1990.

Ricklefs, R. E. Ecology. New York: W. H. Freeman, 1990.

Bill Freedman

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Science EncyclopediaScience & Philosophy: Cluster compound to Concupiscence