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Competition is a negative interaction that occurs among organisms whenever two or more organisms require the same limited resource. All organisms require resources to grow, reproduce, and survive. For example, animals require food (such as other organisms) and water, whereas plants require soil nutrients (for example, nitrogen), light, and water. Organisms, however, cannot acquire a resource when other organisms consume or defend that resource. Therefore, competitors reduce each other's growth, reproduction, or survival.

Interference and Exploitation

Biologists typically recognize two types of competition: interference and exploitative competition. During interference competition, organisms interact directly by fighting for scarce resources. For example, large aphids (insects) defend feeding sites on cottonwood leaves by kicking and shoving smaller aphids from better sites. In contrast, during exploitative competition, organisms interact indirectly by consuming scarce resources. For example, plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants. Plants that produce many roots typically reduce soil nitrogen to very low levels, eventually killing neighboring plants.

Within Species and Between Species

Competition can occur between individuals of the same species, called intraspecific competition, or between different species, called interspecific competition. Studies show that intraspecific competition can regulate population dynamics (changes in population size over time). This occurs because individuals become crowded as a population grows. Since individuals within a population require the same resources, crowding causes resources to become more limited. Some individuals (typically small juveniles) eventually do not acquire enough resources and die or do not reproduce. This reduces population size and slows population growth.

Species also interact with other species that require the same resources. Consequently, interspecific competition can alter the sizes of many species' populations at the same time. Experiments demonstrate that when species compete for a limited resource, one species eventually drives the populations of other species extinct. These experiments suggest that competing species cannot coexist (they cannot live together in the same area) because the best competitor will exclude all other competing species. Why then do communities seem to have many competing species that coexist in the same area?

The Competitive Exclusion Principle

To explain how species coexist, in 1934 G. F. Gause proposed the competitive exclusion principle: species cannot coexist if they have the same niche. The word "niche" refers to a species' requirements for survival and reproduction. These requirements include both resources (like food) and proper habitat conditions (like temperature, pH ). Gause reasoned that if two species had identical niches (required identical resources and habitats) they would attempt to live in the exact same area and would compete for the exact same resources. If this happened, the species that was the best competitor would always exclude its competitors from that area. Therefore, species must at least have slightly different niches in order to coexist.

Peter Grant and colleagues tested Gause's principle by studying seed-eating finches (birds) that live on the Galapagos Islands in the Pacific Ocean. They found that different finch species can coexist if they have traits that allow them to specialize on particular resources. For example, two finch species, Geospiza fuliginosa and Geospiza fortis, vary in a key trait: beak size. Beak size is a critical trait because it determines the size of a seed that a finch can eat: Individuals with small beaks eat small seeds, individuals with intermediate sized beaks can eat intermediate size seeds and individuals with large beaks can eat large seeds. G. fuliginosa and G. fortis do compete for intermediate sized seeds because each species has some individuals with intermediate sized beaks. However, G. fuliginosa specializes upon smaller seeds because it has more individuals with small beaks. Conversely, G. fortis specializes upon larger seeds because it has more individuals with large beaks. Thus, these species niches differ slightly because a specific trait, beak size, allows them to specialize upon a particular seed size.

Joe Connell also tested Gause's principle by studying barnacles (shelled marine organisms) that live on rocks along European coastlines. In 1961, Connell found that two barnacle species, Balanus and Chthamalus, can coexist because they differ in two traits: growth rate and vulnerability to desiccation . Balanus 's growth is rapid, which allows it to smother and crush the slower-growing Chthamalus. Balanus, however, dies close to shore because it gets too dry during low tide. In contrast, Chthamalus tolerates these dry conditions. Consequently, even though Balanus is a better competitor for space, these barnacles coexist because Chthamalus can survive in areas that Balanus cannot survive. These and many other examples support the competitive exclusion principle: Species can only coexist if they have different niches.

Character Displacement

Competition can cause species to evolve differences in traits. This occurs because the individuals of a species with traits similar to competing species always experience strong interspecific competition. These individuals have less reproduction and survival than individuals with traits that differ from their competitors. Consequently, they will not contribute many offspring to future generations. For example, the finches previously discussed can be found alone or together on the Galapagos Islands. Both species' populations actually have more individuals with intermediate-sized beaks when they live on islands without the other species present. However, when both species are present on the same island, competition is intense between individuals that have intermediate-sized beaks of both species because they all require intermediate sized seeds. Consequently, individuals with small and large beaks have greater survival and reproduction on these islands than individuals with intermediate-sized beaks.

Studies show that when G. fortis and G. fuliginosa are present on the same island, G. fuliginosa tends to evolve a small beak and G. fortis tends to evolve a large beak. The observation that competing species' traits are more different when they live in the same area than when competing species live in different areas is called character displacement. For the two finch species, beak size was displaced: Beaks became smaller in one species and larger in the other species. Studies of character displacement are important because they provide evidence that competition plays a very important role in determining ecological and evolutionary patterns in nature.

see also Adaptation; Community; Evolution; Extinction; Natural Selection; Population Dynamics; Symbiosis

J. P. Cronin and Walter P. Carson


Connell, Joseph. "The Influence of Interspecific Competition and Other Factors on the Distribution of the Barnacle Chthamalus stellatus. " Ecology 42, no. 4 (1961): 710723.

Gause, G. F. The Struggle for Existence. Baltimore: Williams & Wilkins, 1934.

Grant, Peter R. Ecology and Evolution of Darwin's Finches. Princeton, NJ: Princeton University Press, 1986.

Wedin, David, and David Tilman. "Competition Among Grasses Along a Nitrogen Gradient: Initial Conditions and Mechanisms of Competition." Ecological Monographs 63, no. 2 (1993): 199229.

Whitham, Thomas G. "Costs and Benefits of Territoriality: Behavioral and Reproductive Release by Competing Aphids." Ecology 67, no. 1 (1986): 139147.


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Populations of animals are controlled by many factors. Natural selection is a broad term that describes one effect of these controls on population. For example, one form of population control that can result in natural selection is competition.

There are a number of essential resources upon which animals' lives depend. Whenever these resources are limited, animals are forced to compete for survival. Competition can be intraspecific , between the same species, or interspecific, between different species. Since resources are rarely abundant in any given environment, competition can be fierce. Three resources that animals are likely to complete for are space, water, and food.

Competing for Space

The availability of space is a primary consideration in any habitat. The actual territory in which an animal lives is vitally important since an animal's environment must be able to support the needs of each species. All animals must have enough room for feeding, reproducing, and exercise in order to live comfortably.

Along with the amount of space, some animals require a particular terrain, such as the prairie dog. A prairie dog colony must have enough flat and fairly soft earth in which the animals can build elaborate tunnel systems. If the soil is too rocky, the prairie dogs cannot build their brooding dens and escape tunnels.

Similarly, some birds require lots of open sky in which to perform their courtship flights. Without these elaborate flights the birds do not become stimulated to reproduce. If the terrain does not permit the birds to move in wide-open spaces, the birds' chances of successful reproduction in that habitat are reduced.

Competing for Water

Another important resource that animals compete for is access to water. Most animals require a particular amount of water everyday, which they may use as drinking water, or to contribute to their overall health. In general, animals must maintain a certain moisture balance in order for their tissues to work properly. When animals do not have enough to drink their bodily tissues become dehydrated and cannot function properly. Organs, such as kidneys, stop working, resulting in death.

Using larger bodies of water for bathing purposes is another common use of water as a resource. For instance, overheating in elephants is a problem usually countered by cool baths in rivers or mud. When water becomes scarce in the dry season, competition between individual elephants or between elephants and other species can reach a dangerous peak.

Finally, aquatic animals are especially vulnerable when the water level of their environment drops. Fish living in the shrinking rivers and streams are easy prey for birds when the waters become so shallow that they are easily seen. The fish will compete for choice hiding spots from hungry predators, with the unlucky losers being eaten, therefore removing their genetic material from the gene pool.

Competing for Food

Food is one of the most basic resources over which individuals compete. In extreme cases where animals have gone for long periods without enough food to sustain them, individuals will compete, sometimes to the death, for mere scraps of food. Animals that practice a high level of parental care have to carefully balance finding enough food for both themselves and their offspring. When food is scarce, the parent may go for months without eating but will rarely risk their own death for the sake of their young. Usually, by the time the parent is in danger of starvation the resources for the young will have already been exhausted and the young will have already died.

The reason the parent allows its young to die is motivated by the parent's instinct to survive. Once the offspring dies the parent is free to read-opt food-finding methods that might lead it farther from the nest or den where its young were kept. The parent is driven to preserve its genetic information by remaining healthy enough so that it can reproduce again, hopefully when there is more food available.

Sometimes species will compete for more than one resource at a time. Competitions for territory and food are easily seen in interactions between ants. On one hand, the social structure of the colony is an amazing example of how the members of a species cooperate and specialize for the benefit of all. The queen lays the eggs and produces the offspring. Workers attend to the maintenance and growth of the colony. Soldier ants defend the colony against invaders. The area around an ant colony, however, is often lacking in food after a few seasons of food gathering and resource use.

Some colony members will forage beyond their usual boundaries and encounter ants from another colony. This will immediately set off a warning throughout both colonies. Soldiers and workers rush out of the two colonies to fight each other for the contested territory and food resources. The success of one or both of the colonies may be at risk, and the death toll for each colony can be high. Even colonies of the same species will war against one another. If one species enters the colony area of another, the two colonies may suffer serious battle losses.

Strategies to Avoid Competition

Although competition between animals can be seen in a variety of situations, many species have developed elaborate strategies to avoid competition. It is not the habit of any species to try and obliterate another. Confrontations occur as a last resort. Without this avoidance of direct competition there would be very few stable communities.

More often, the sharing of resources is achieved between species in a habitat. Scientists often refer to the sharing of resources as " niche partitioning." In niche partitioning animals tend to use different parts of a resource without coming into direct competition with one another. For instance, grazing animals in parts of Africa come in all sizes. The smaller Thompson's gazelles eat grass that grows close to the ground. Zebras and wildebeests consume the tall grasses and shrub food. The giraffe has a long neck enabling it to browse far above the ground. While all of these animals use plants as a food resource, none come into direct competition with one another. As a result, they coexist peacefully and reduce confrontation.

Many species of birds and monkeys in rain forests also partition resources. Some are specialized for feeding on fruit from lower branches of trees nearer the forest floor, while others are able to exploit food items found in the top or canopy region of the forest.

It is only when resources are limited that one observes any actual competition. Even then animals will often find a unique solution to the problem. For example, barnacle colonies exist in competition with other animals for limited space on rocks and hard surfaces in the oceans. While the larvae are free-swimming and float as part of the planktonic community throughout their juvenile stage, the adults need a firm surface to anchor and construct hard permanent shells. From within these shells they extend feathery feeding appendages that sweep food particles from the ocean currents.

Barnacles exploit unique measures in order to avoid the fierce competition from ocean-dwelling filter feeders (such as corals, anemones, and limpets) for ocean floor space. Instead of competing, many form colonies on the tough skins of whales. They hitchhike around the sea feeding on food suspended in the water surrounding the whale. It is very common to find whales with large colonies of barnacles on their snouts and jaws. Even ships are suitable landing places for the barnacle larvae. Barnacle colonies can become so large on the bottoms of ships that the ships must be removed from the water and the barnacles scraped off to keep the ship moving smoothly through the water.

Ecological competition is a fascinating and varied topic in biology. Scientists are always discovering new ways in which animals compete with one another. Even more interesting is the way in which they reduce or eliminate competition. Humans can learn a great deal from how animals coexist with one another.

see also Aggression; Populations.

Brook Ellen Hall


Raven, P., and G. Johnson. William C. Brown. Boston: McGraw-Hill, 1999.

Starr, C. Biology: Concepts and Application. Belmont, CA: Wadsworth, 1994.

Internet Resources

University of South Carolina, Department of Biology. <>.

University of Tennessee, The Institute for Environmental Modeling. <>.

University of Texas, Department of Biology. <>.

Examples of competition for water can be found in many nature videos. Parched elephants threaten thirsty lions for limited water in rivers. Baby elephants may get trampled in the rush for water and space in the limited water pools. Even crocodiles find competition as they march to a new water hole only to find resistance from crocodiles already in residence.


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competition The interaction that occurs between two or more organisms, populations, or species that share some environmental resource when this is in short supply. Competition is an important force in evolution: plants, for example, become tall to compete for light, and animals evolve various foraging methods to compete for food. There may be a direct confrontation between competitors, as occurs between barnacles competing for space on a rock, or the numbers or fecundity of the competitors are indirectly reduced through joint dependence on limited resources. Competition occurs both between members of a species (intraspecific competition) and between different species (interspecific competition). Interspecific competition often results in the dominance of one species over another (see dominant). Since competition ultimately results in the displacement by one competitor of the others, it is to the advantage of the competitors to avoid one another wherever possible. Thus in time the competitors become separated from each other geographically or ecologically, which promotes evolutionary change. Competition for mates may lead to sexual selection.


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competition The interaction between individuals of the same species, or between different species populations at the same trophic level, in which the growth and survival of one or all species or individuals is affected adversely. The competitive mechanism may be direct (active), as in allelopathy and mutual inhibition, or indirect, as when a common resource is scarce. Competition leads either to the replacement of one species by another that has a competitive advantage, or to the modification of the interacting species by selective adaptation (whereby competition is minimized by small behavioural differences, e.g. in feeding patterns). Competition thus favours the separation of closely related or otherwise similar species. Separation may be achieved spatially, temporally, or ecologically (i.e. by adaptations in behaviour, morphology, etc.). The tendency of species to separate in this way is known as the competitive exclusion, or Gause, principle.


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An important component of the neo-Darwinian theory of evolution, competition describes the theory that there is a struggle among organisms both of the same species (intraspecific) and between species (interspecific) for food, space, reproduction, and other requirements for existence. Through natural selection organisms develop adaptations to overcome or resist their own destruction in competition with the counter-adaptations developed by other organisms. These adaptations include physiological, chemical, and psychological traits. For example, organisms may evolve to become larger, more poisonous, or more aggressive. Such adaptations are not developed on the short timescale of individual lifetime but on the long evolutionary timescale of the species.

See also Adaptation; Aggression; Evolution; Neo-Darwinism

arn o. gyldenholm