The edge effect in ecology is the effect of the juxtaposition or placing side by side of contrasting environments on an ecosystem.
This term is commonly used in conjunction with the boundary between natural habitats, especially forests, and disturbed or developed land. Edge effects are especially pronounced in small habitat fragments where they may extend throughout the patch. Increasing edge effects allows more habitat structure to increase biodiversity within the area.
When edges are expanded into any natural ecosystem, and the area outside the boundary is a disturbed or unnatural system, the natural ecosystem can be seriously affected for some distance in from the edge. "Eugene P. Odum, professor of zoology, 1971: 'The tendency for increased variety and diversity at community junctions is known as the edge effect.... It is common knowledge that the density of songbirds is greater on estates, campuses and similar settings...as compared with tracts of uniform forest.'" quoted in William T. Vollmann, "Another Roadside Attraction," New York Times Book Review, at 9, February 21, 2010. In the case of a forest where the adjacent land has been cut, creating an open/forest boundary, sunlight and wind penetrate to a much greater extent, drying out the interior of the forest close to the edge and encouraging growth of opportunistic species at the edge. Air temperature, vapor pressure deficit, soil moisture, light intensity and levels of photosynthetically active radiation (PAR) all change at edges.
It has been estimated that the amount of Amazonian area modified by edge effects exceeded the area that had been cleared. Forest fires are more common close to edges as a consequence of increased desiccation at edges and increased understory growth present because of increased light availability. Increased understory biomass provides fuel that allows pasture fires to spread into the forests. Increased fire frequency since the 1990s are among the edge effects which are slowly transforming Amazonian forests.
The amount of forest edge is also orders of magnitude greater now in the United States than when the Europeans first began settling North America. Some species have benefited from this fact, for example the Brown-headed Cowbird, which is a brood parasite that lays its eggs in the nests of songbirds nesting in forest near the forest boundary. Thus, the more edge in relation to the forest interior, the more cowbirds and the fewer songbirds as a result.
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