alien species See nonnative species.

climax community See mature community.

commensalism 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. Compare mutualism.

community development See ecological succession.

competition 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. See ecological niche, fundamental niche, realized niche.

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

disturbance 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.

ecotone 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.

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

exotic species See nonnative species.

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.

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

host Plant or animal on which a parasite feeds.

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.

immigrant species See nonnative species.

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

inertia 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.

mutualism Type of species interaction in which both participating species generally benefit. Compare commensalism.

native species Species that normally live and thrive in a particular ecosystem. Compare indicator species, keystone species, nonnative species.

natural rate of extinction See background extinction.

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

parasite Consumer organism that lives on or in and feeds on a living plant or animal, known as the host, over an extended period of time. The parasite draws nourishment from and gradually weakens its host; it may or may not kill the host. See parasitism.

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

persistence 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. See immature community, mature community.

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

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. See pollution prevention.

predation Situation in which an organism of one species (the predator) captures and feeds on parts or all of an organism of another species (the prey).

predator Organism that captures and feeds on parts or all of an organism of another species (the prey).

predator-prey relationship Interaction between two organisms of different species in which one organism, called the predator, captures and feeds on parts or all of another organism, called the prey.

prey Organism that is captured and serves as a source of food for an organism of another species (the predator).

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

realized niche Parts of the fundamental niche of a species that are actually used by that species. See ecological niche, fundamental niche.

resilience 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. See ecological niche, fundamental niche, realized niche.

secondary succession Ecological succession in an area in which natural vegetation has been removed or destroyed but the soil is not destroyed. See ecological succession. Compare primary succession.

soil 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.

stability Ability of a living system to withstand or recover from externally imposed changes or stresses. See constancy, inertia, resilience.

succession See ecological succession, primary succession, secondary succession.

symbiosis Any intimate relationship or association between members of two or more species. See symbiotic relationship.

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.

territoriality Process in which organisms patrol or mark an area around their home, nesting, or major feeding site and defend it against members of their own species.

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. See law of tolerance.