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Community Ecology

Community Ecology

Community ecology is the study of the organization and functioning of communities of organisms. As populations of species interact with one another, they form biological communities. A community of organisms consists of all the interacting populations of the species living within a particular area or within a particular habitat. Community ecology also studies the relationships of the members of a community to their environment. Community ecology is usually subdivided according to habitat or biome . Typical habitats include forest, grassland, desert, and stream or lake environments.

The Trophic Pyramid

All biological communities have a similar structure called a trophic pyramid. Each pyramid contains four or five levels. Food energy is passed from one level to the next along a food chain. Since energy is lost to heat at each level in the pyramid, it takes many organisms at a given trophic level to support those in the next level up. The base of the pyramid in every biological community is composed of species called autotrophs, organisms that harvest sunlight (or in rare cases, heat) directly through photosynthesis (or chemosynthesis). All other organisms in the pyramid are called heterotrophs.

A food chain typically contains four or five links, from autotrophs, through grazers and other herbivores, then culminating with a carnivore as top predator. Many animals, however, eat more than one species. Also, animals may eat different foods at different stages of their growth. Many animals eat both plants and other animals and therefore feed at more than one trophic level. Consequently, food chains are usually interconnected into highly complex food webs.

In addition to eating one another, species also compete for resources and interact in other ways within a community. Nontrophic relationships between species are as important as food chains and food webs in shaping the organization of biological communities.

Ecological Succession

Through the process of ecological succession, communities are constantly changing. Disturbances to communities may be local, such as a tree falling and opening the canopy to allow more sunlight, or widespread, such as fires and storms. Whether local or general, each disturbance creates an opportunity for a new species to colonize that region. These new species can alter the biological structure of the community and create an environment that is suitable to other new species. By this process, the community evolves over time.

In some environments, succession eventually produces a stable community dominated by a small number of species. This is called a climax community. The web of biological interactions has become so intricate and interconnected that no other species can successfully compete for food resources. In other environments, small disturbances produce communities that are a diverse mix of species. Some tropical forests contain hundreds of thousands of species within a square kilometer. When a tree dies and falls, the dense canopy is opened and new space is available for different species to take root. Some coral reefs contain thousands of different species, and whichever species is able to rapidly colonize a new disturbance patch will be successful.

Ecological Niches

The way of life of an organism is shaped by its environment and by its interactions with other organisms through the processes of evolution. The role an organism plays in its relation to other species and its environment is known as its ecological niche. The niche of an organism includes what it eats, how it obtains food, where in the environment it lives, what temperature it prefers, how much light it can tolerate, and many other factors.

Guilds

Some similar species have evolved strategies that allow them to allocate resources in a way that avoids competition. For example, different species of warblers that prey on the same species of insects may forage at different levels in the same trees. A group of organisms that share a common food resource is called a guild. Guild members may have strong interactions with each other but only weak interactions with other members of the community. In the American Southwest, birds, rodents, and ants constitute a guild that competes for the same seeds. Whereas birds exploit temporary patches of seeds, rodents and ants are permanent residents. Ants generally take smaller seeds than rodents. In East Africa, communities of animals form a guild of grazers. First, elephants and buffalo eat the tall, coarse grasses and then move on. They are able to consume large quantities of this low-nutrition food source. Zebras follow along behind the elephants, reducing the plant biomass even more. The zebras are followed by a still smaller animal, the wildebeest, which selects among the lower growing plants that remain after the zebras have fed. Finally, the smallest grazers, such as Thompson's gazelles, are able to reach the young, protein-rich sprouts of grass missed by the wildebeest.

Interactive Relationships

The interactive relationships that arise between populations of different species form the interactive web of communities. These interactions range from antagonistic to cooperative and have positive, negative, or neutral effects on the species involved. In antagonistic relationships the interaction is detrimental to individuals of either one or both species; in commensal relationships (commensalism) one species benefits while the other remains unaffected; and in mutualistic relationships (mutualism) both species benefit. The organization and stability of biological communities results from the mix of these different kinds of interaction.

There are many different kinds of interspecific interactions within an ecological community. These relationships between species are not static; they evolve as natural selection continually shapes and reshapes them. The complex relationships between prey and predators, for example, are snapshots of one instant during the evolution of interactions. As interactions between species evolve, the nature of relationships may shift. Nineteenth-century British naturalist Charles Darwin called this ever-changing mix of species and their interactions the "entangled bank" and stressed its importance in the evolutionary process. While antagonistic relationships, such as predator-prey or parasite-host, are the most dramatic kinds of relationships, other forms of interaction such as mutualism or commensalism are just as important.

Mutualism.

This is a relationship where both participants in the interaction receive benefit. For example, plants are hosts for insects that pollinate them or eat their fruit and for microorganisms that attach themselves to their roots. Mutualistic associations between animals and microorganisms are an important part of the structure of communities. Most animals rely on the microorganisms in their gut to properly digest and metabolize food.

Parasitism.

This is possibly the most common way of life in nature. Parasitic organisms may account for half of all living species. The majority of species of wasps are tiny parasites that lay their eggs on a specific host organism. Some wasps are parasitic on plants, some are parasitic on insects, some even parasitize other wasps! The larvae hatch and burrow into the host species. As it is consumed from the inside out, the host species survives long enough to allow the larvae to mature.

Antagonism.

Antagonism is a form of relationship where one species benefits and the other is harmed. Grazing, parasitism, and predation are examples of antagonistic relationships. While we generally think of grazers as large herbivores, a grazer is defined as any species that moves from one organism to another, feeding on part of each without actually killing it outright. Grasshoppers are grazers that jump from plant to plant, chewing a portion of the leaves of each one they visit. Some caterpillars are grazers that crawl from one plant to another during development rather than remain as parasites on an individual plant. The grazing lifestyle differs from the parasitic lifestyle in a few important ways. Individuals can vary their diets with different foods. Also, because grazers do not remain attached to a single individual for long periods, their victims do not have time to develop induced specialized defenses, such as an immune response that a host can develop against a parasite.

Predation.

This form of relationship differs from both parasitism and grazing. In predation, the victims are killed and often consumed immediately. Predators therefore differ from parasites and grazers in their effects on the dynamics of populations and the organization of communities. As with parasitism and grazing, predation is an interaction that has arisen many times in many taxonomic groups worldwide.

Competition is an important form of interaction in communities in which neither species benefits. In competitive interactions, species evolve either to avoid each other, to tolerate the presence of the other, or to aggressively exclude the other.

Species compete for almost every conceivable kind of resource. Birds compete for nesting sites. Male birds compete for preferred sites to defend as territories for attracting females. While species compete for many resources at the same time, there is often a single resource, called the limiting resource, that is in scarce supply. This resource restricts the growth of each species. In deserts, water is often the limiting resource.

Commensalism.

In this kind of interaction, one species benefits and the other is unaffected. For example, cattle egrets (Bubulcus ibis) forage around the feet of cattle. The grazing behavior of the cattle stirs up many small insects and other arthropods that the cattle egrets eat. The cattle egrets receive a benefit, but there is no indication that the cattle are affected in any way.

The richness and ubiquity of interactions among populations of organisms demonstrate that the characteristics of all species have been influenced by the interaction with other species. Species have coevolved with each other. Predators have evolved along with their prey. Parasites evolve with their hosts. Rarely is only one interaction responsible for the evolution of a species, however. More common is a sort of diffuse coevolution where the traits of a single species are influenced by interactions with many other species. Such diffuse coevolution may prevent a sort of evolutionary "arms race," where predator and prey become ever faster or stronger.

see also Coevolution; Ecology; Ecosystem; Habitat.

Elliot Richmond

Bibliography

Billington, Elizabeth T. Understanding Ecology. New York: F. Warne, 1971.

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.

Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth, 1990.

Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer, 1987.

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community ecology

community ecology An approach to ecological study which emphasizes the living components of an ecosystem (the community). Typically it involves description and analysis of patterns within the community, employing methods of classification and ordination, and examines the interactions of community members, e.g. in the partitioning of resources and in succession. See also synecology.

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"community ecology." A Dictionary of Ecology. . Encyclopedia.com. 24 May. 2017 <http://www.encyclopedia.com>.

"community ecology." A Dictionary of Ecology. . Encyclopedia.com. (May 24, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/community-ecology

"community ecology." A Dictionary of Ecology. . Retrieved May 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/community-ecology

community ecology

community ecology An approach to ecological study which emphasizes the living components of an ecosystem (the community). Typically it involves description and analysis of patterns within the community, employing methods of classification and ordination, and examines the interactions of community members, e.g. in the partitioning of resources and in succession. See also SYNECOLOGY.

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"community ecology." A Dictionary of Plant Sciences. . Encyclopedia.com. 24 May. 2017 <http://www.encyclopedia.com>.

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"community ecology." A Dictionary of Plant Sciences. . Retrieved May 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/community-ecology-0

community ecology

community ecology An approach to ecological study which emphasizes the living components of an ecosystem (the community). Typically, it involves description and analysis of patterns within the community, employing methods of classification and ordination, and examines the interactions of community members (e.g. in the partitioning of resources).

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"community ecology." A Dictionary of Zoology. . Encyclopedia.com. 24 May. 2017 <http://www.encyclopedia.com>.

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"community ecology." A Dictionary of Zoology. . Retrieved May 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/community-ecology-1