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Interaction between organisms of different species in which one type of organism benefits and the other type is neither helped nor harmed to any great degree.


Two or more individual organisms of a single species (intraspecific competition) or two or more individuals of different species (interspecific competition) attempting to use the same scarce resources in the same ecosystem.

competitive exclusion principle

No two species can occupy exactly the same fundamental niche indefinitely in a habitat where there is not enough of a particular resource to meet the needs of both species.


Ability of a living system, such as a population, to maintain a certain size. See homeostasis.


A discrete event that disrupts an ecosystem or community. Examples of natural disturbances include fires, hurricanes, tornadoes, droughts, and floods. Examples of human-caused disturbances include deforestation, overgrazing, and plowing.

early successional plant species

Plant species found in the early stages of succession that (1) grow close to the ground, (2) can establish large populations quickly under harsh conditions, and (3) have short lives. Compare late successional plant species, midsuccessional plant species.

ecological succession

Process in which communities of plant and animal species in a particular area are replaced over time by a series of different and often more complex communities. See primary succession, secondary succession.


Transitional zone in which one type of ecosystem tends to merge with another ecosystem. See edge effect.

edge effect

Existence of a greater number of species and a higher population density in a transition zone (ecotone) between two ecosystems than in either adjacent ecosystem. See ecotone.


Plant that uses its roots to attach itself to branches high in trees, especially in tropical forests.

exploitation competition

Situation in which two competing species have equal access to a specific resource but differ in how quickly or efficiently they exploit it. See interference competition, interspecific competition.

fundamental niche

The full potential range of the physical, chemical, and biological factors a species can use if there is no competition from other species. See ecological niche. Compare realized niche.


Maintenance of favorable internal conditions in a system despite fluctuations in external conditions. See constancy, inertia, resilience.

immature community

Community at an early stage of ecological succession. It usually has a low number of species and ecological niches and cannot capture and use energy and cycle critical nutrients as efficiently as more complex, mature communities. Compare mature community.

indicator species

Species that serve as early warnings that a community or ecosystem is being degraded. Compare keystone species, native species, nonnative species.


Ability of a living system to resist being disturbed or altered. Compare constancy, resilience.

interference competition

Situation in which one species limits access of another species to a resource, regardless of whether the resource is abundant or scarce. See exploitation competition, interspecific competition.

interspecific competition

Members of two or more species trying to use the same limited resources in an ecosystem. See competition, competitive exclusion principle, intraspecific competition.

intraspecific competition

Two or more organisms of a single species trying to use the same limited resources in an ecosystem. See competition, interspecific competition.

keystone species

Species that play roles affecting many other organisms in an ecosystem. Compare indicator species, native species, nonnative species.

late successional plant species

Mostly trees that can tolerate shade and form a fairly stable complex forest community. Compare early successional plant species, midsuccessional plant species.

mature community

Fairly stable, self-sustaining community in an advanced stage of ecological succession; usually has a diverse array of species and ecological niches; captures and uses energy and cycles critical chemicals more efficiently than simpler, immature communities. Compare immature community.

midsuccessional plant species

Grasses and low shrubs that are less hardy than early successional plant species. Compare early successional plant species, late successional plant species.


Type of species interaction in which both participating species generally benefit.

native species

Species that normally live and thrive in a particular ecosystem.

natural rate of extinction

Normal extinction of various species as a result of changes in local environmental conditions

nonnative species

Species that migrate into an ecosystem or are deliberately or accidentally introduced into an ecosystem by humans. Compare native species.


Interaction between species in which one organism, called the parasite, preys on another organism, called the host, by living on or in the host.


How long a pollutant stays in the air, water, soil, or body. See also inertia.

pioneer community

First integrated set of plants, animals, and decomposers found in an area undergoing primary ecological succession.

pioneer species

First hardy species, often microbes, mosses, and lichens that begin colonizing a site as the first stage of ecological succession.

precautionary principle

When there is scientific uncertainty about potentially serious harm from chemicals or technologies, decision makers should act to prevent harm to humans and the environment.

primary succession

Ecological succession in a bare area that has never been occupied by a community of organisms. See ecological succession..

realized niche

Parts of the fundamental niche of a species that are actually used by that species.


Ability of a living system to restore itself to original condition after being exposed to an outside disturbance that is not too drastic. See constancy, inertia.

resource partitioning

Process of dividing up resources in an ecosystem so species with similar needs (overlapping ecological niches) use the same scarce resources at different times, in different ways, or in different places.

secondary succession

Ecological succession in an area in which natural vegetation has been removed or destroyed but the soil is not destroyed.


Complex mixture of inorganic minerals (clay, silt, pebbles, and sand), decaying organic matter, water, air, and living organisms.

species diversity

Number of different species and their relative abundances in a given area. See biodiversity. Compare ecological diversity, genetic diversity.

species equilibrium model

See theory of island biogeography.

symbiotic relationship

Species interaction in which two kinds of organisms live together in an intimate association. Members of the participating species may be harmed by, benefit from, or be unaffected by the interaction. See commensalism, interspecific competition, mutualism, parasitism, predation.

theory of island biogeography

The number of species found on an island is determined by a balance between two factors: the (1) immigration rate (of species new to the island) from other inhabited areas and (2) extinction rate (of species established on the island). The model predicts that at some point the rates of immigration and extinction will reach an equilibrium point that determines the island's average number of different species (species diversity).

tolerance limits

Minimum and maximum limits for physical conditions (such as temperature) and concentrations of chemical substances beyond which no members of a particular species can survive.

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