Ecological Competition

views updated

Ecological Competition

Introduction

Ecological competition is the struggle between two organisms for the same resources within an environment. Resources are components of the environment that are required for survival and reproduction such as food, water, shelter, light, territory, and substrate. Members of the same species may also compete for mates. Competition among members of different species is referred to as intraspecific competition, while competition among members of the same species is called inter-specific competition. Because members of the same species occupy the same ecological niche, their needs are nearly identical and so interspecific competition tends to be more intense than intraspecific competition. Ecological competition helps maintain community structure and species diversity within ecosystems. The concept of competitive exclusion is also being used to improve agricultural practices.

Historical Background and Scientific Foundations

Competition in an ecological sense is the struggle between individuals for environmental resources. Resources include anything found in the environment that is necessary for growth and reproduction such as food, shelter, water, light, and substrate or territory. For individuals of the same species, resources can also include mates.

Resources are different from environmental conditions because resources have the potential to become scarce. Environmental conditions include components of the environment that affect the growth and reproduction of organisms but are shared identically among all members of an ecosystem. Such conditions include temperature, salinity, and pH.

A major factor affecting the availability of resources in an ecosystem is the density of individuals, or the number of organisms living in a certain area. If more individuals live in a certain area, resources are depleted more quickly and ecological competition for these limited resources intensifies. On the other hand, environmental conditions are unaffected by density. Regardless of the number of individuals living in an area, the air temperature or salinity of the water will be the same. Resources are therefore referred to as density dependent ecological factors, while environmental conditions are called density independent factors.

The intensity of ecological competition depends greatly on the resources required by individuals in an ecosystem. For example, if two different species of birds eat only a single type of berry, competition for that berry will be intense. However, if the two species of birds eat a certain type of berry but one species also eats local nuts and the other species also eats certain seeds, competition for the berry will not be as great.

The sum of the resource requirements and the suitable environmental conditions of an individual represent its ecological niche. Individuals of the same species have essentially the same resource requirements and therefore occupy identical ecological niches. As a result the competition between members of the same species, also called interspecific competition, is generally more fierce than competition between members of different species, which is called intraspecific competition.

In 1934 Russian ecologist Georgii F. Gause (1910–1986) performed classic experiments that led to the development of the competitive exclusion principle, also known as Gause’s law. Gause grew two species of closely related protozoans, Paramecium caudatum and Paramecium aurelia, supplying them with a controlled food source. When the two species were grown separately, they multiplied exponentially as predicted by theoretical equations of cell division. However, when the two species were grown together on the same food source, only one species survived. The other species was eliminated. Gause’s competitive exclusion principle states that if a limiting resource exists in the environment and two species rely on that resource, only one of the species will survive. The other will either become extinct in the environment or it will develop evolutionary adaptations that shift it toward a different ecological niche.

Ecological competition therefore can act as a mechanism to drive evolutionary adaptation. One of the most famous examples of evolutionary adaptation driven by competition for resources is that of Darwin’s finches. There are at least 13 species of finches on the Galapogos Islands. Scientists believe that all of the species evolved from one ancestral species. Because food resources on the islands are limited, competition between members of the ancestral species drove individuals to consume food that was not optimal. This competitive pressure favored individuals with bill shapes that could eat the food for which the competition was not as intense. These individuals eventually became entirely different species than the original ancestor. Over time, more than a dozen species of finches were established because of ecological competition.

In 1935 British ecologist Arthur Tansley (1871–1955) performed experiments with a plant called Gallium, also known as bedstraw. Tansley planted two species, one that was native to alkaline soil and one found in a more basic soil. When planted alone, both species could grow in either the alkaline or basic soil type. However, when planted together, the plant in its native soil always grew much larger than the plant in non-native soil. Tansley realized that the native plant was able to more effectively obtain resources than the plant grown in non-native conditions. Tansley surmised that competition has broad effects on community structure. In particular, the presence or absence of a competitor can play a large role in determining the size, population size, and health of other organisms in the environment.

Impacts and Issues

Ecological competition affects the community structure in an ecosystem and it places evolutionary pressure on the development of adaptations in a population. As a result changes to the competitive forces in an environment can not only affect the ecosystem as a whole, but also the evolutionary history of a species. As human impact on the environment becomes more and more intense, human activity has created numerous conditions in which existing competitive relationships are disrupted, changing the structure of the ecosystem. One major type of disruption to the competitive relationships in an ecosystem is the immigration of non-native or invasive species. A second disruption results from the removal of

WORDS TO KNOW

INTERSPECIFIC COMPETITION: The competition between individuals of different species for the same limited resource.

INTRASPECIFIC COMPETITION: The competition among members of the same species for the same limited resource.

NICHE: A term describing a species habitat, range of physical and biological conditions, and relationships within its ecosystem.

SPECIES DIVERSITY: The number of different species living in a particular place.

one competitor, as occurs in situations when overhunting or overfishing occurs.

An invasive, or exotic, species is a species that establishes a population in a location in which it is not native. Because of the increase in travel throughout the world, not only humans, but seeds, larvae, plants, and animals are quickly transported long distances. In aquatic systerms, cargo ships carry ballast water in their holds. They fill up in ports of departure and then release the water in destination ports. The ballast water contains many species of microorganisms, clams, crab, and even fish, which are released into new locations. In 1982 a gelatinous animal called a ctenophore, similar to a jellyfish, was released in ballast water from the waters off the United States into the Black Sea. The ctenophore competed for food in the form of microscopic plankton with the native species of fish. Because the ctenophore had no natural predators in the Black Sea, its population exploded. Following Gause’s law of completive exclusion, the fish populations plummeted. This not only disrupted the structure of the ecosystem and the species diversity in the Black Sea, but it also devastated the commercial fisheries in the region.

Invasive species and their ability to put severe competitive pressure on local ecosystems is not an isolated event. The zebra mussel is an exotic species to the Great Lakes. It grows in high densities and causes severe damage to aquatic structures such as pipes, docks, and the bottoms of boats. Because it voraciously filters microscopic algae out of the water, populations of native fish, clams, and mussels that compete for the same food resources have declined severely. In Florida, exotic brown anoles from Cuba replaced native green anoles in a large part of their habitat. The blue water hyacinth, native to South America, was planted in the United States as a decorative flower. It is now a nuisance plant throughout the waterways of Florida, competing for space and nutrients with local flora. There are estimates

that worldwide most ecosystems contain 10 to 30% exotic species. Although not all of these species result in competitive interactions that affect community structure, when these interactions do occur they almost always result in a loss of community stability and decreased biological diversity.

A second way in which human activity can influence competitive relationships within an ecosystem is by the removal of a competitor. For example, in the ocean off of Antarctica, the food chain is based on the small shrim-plike crustacean called krill. Prior to human involvement, baleen whales were the largest consumer of krill in the ecosystem. Beginning in the middle 1850s, whales were regularly hunted on Antarctic waters, decimating the baleen whale populations. During that time, the populations of other competitors for krill, seals, penguins, and smaller whales all grew enormously. In 1996 restrictions on hunting large baleen whales were put in place. However, because of the increased competitive pressure for food, baleen whales have not reestablished previous population sizes.

The idea of ecological competition, specifically the principle of competitive exclusion, has been used to improve agricultural practices. Chicken farmers are concerned about the spread of the pathogen Salmonella. Not only does this pathogen threaten the health of the chickens, it can be passed on to humans and cause disease. Chickens are commonly given significant doses of antibiotics to prevent the growth of pathogentic bacteria in their guts. But this practice has been criticized because humans who eat chicken secondarily ingest the antibiotics. Using the idea of competitive exclusion, farmers have begun introducing a nonpathogenic bacterium into the guts of newborn chickens that attaches to the substrate on which Salmonella would normally grow. By competing for space with the pathogens, the harmless exotic bacteria populations exclude the growth of disease-causing bacteria. Competitive exclusion is also being assessed as a means to prevent the growth of other bacterial pathogens such as Campylobacter and Escherichia coli.

BIBLIOGRAPHY

Books

Odum, Eugene, and Gary W. Barrett. Fundamentals of Ecology, 5th ed. Pacific Grove, CA: Brooks Cole, 2004.

Web Sites

Marietta College. “Competition.” January 11, 2008. http://www.marietta.edu/~biol/biomes/competition.htm (accessed February 11, 2008).

Public Broadcasting Service. “Adaptive Radiation: Darwin’s Finches.” http://www.pbs.org/wgbh/evolution/library/01/6/1_016_02.html (accessed February 11, 2008).

University of Michigan. “Ecological Communities: Networks of Interacting Species.” November 2, 2005. http://www.globalchange.umich.edu/globalchangel/current/lectures/ecol_com/ecol_com.html (accessed February 11, 2008).

Juli Berwald

Environmental Science: In Context