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An ecotone is a zone of transition between distinct ecological communities or habitats. Usually, the word is used to refer to relatively sharp, local transitions, also known as edges.

Because many physical and chemical changes in the environment tend to be continuous, ecological transitions are often similarly gradual. For example, climate and precipitation change steadily across continents and up the slopes of mountains. Because these environmental changes are gradual, communities of plants and animals often intergrade through wide, continuous transitions.

Frequently, however, there are relatively sharp environmental interfaces associated with rapid changes occurring naturally at the edges of major geological or soil discontinuities along the interface of aquatic and terrestrial habitats or associated with the boundaries of disturbances such as landslides and wildfires. These are the sorts of environmental contexts in which ecotones occur naturally. Human activities also favor the occurrence of many ecotones, for example, along the edges of clearcuts, agricultural fields, highways, and residential areas.

Disturbance-related ecotones exist in space, but often they eventually become indistinct as time passes because of the ecological process known as succession. For example, in the absence of an intervening disturbance, an ecotone between a forest and a field will eventually disappear if the field is abandoned and succession allows a mature forest to develop over the entire area.

The sharp ecological discontinuities at ecotones provide habitat for so-called "edge" species of plants and animals. These have a relatively broad ecological tolerance and within limits can utilize habitat on both sides of the ecotone. Examples of edge plants include many shrubs and vines that are abundant along the boundaries of forests in many parts of the world. Some animals are also relatively abundant in edges and in habitat mosaics with a large ratio of edge to area. Some North American examples of edge animals include white-tailed and mule deer, snowshoe hare, cottontail rabbit, blue jay, and robin.

Because human activities have created an unnatural proliferation of ecotonal habitats in many regions, many edge animals are much more abundant than they used to be. In some cases this has resulted in important ecological problems. For example, the extensive range expansion of the brown-headed cowbird, a prairie-forest edge species, has caused large reductions in the breeding success of many small species of native birds, contributing to large declines in some of their populations. This has happened because the cowbird is a very effective social parasite which lays its eggs in the nests of other species who then rear the cowbird chick to the severe detriment of their own young.

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