Edge effects

In ecology, edge effects refer to the changes in the population or community structure that occur at the point where two habitat types meet. Edge effects are especially pronounced in small habitat fragments where the edge effects may extend throughout the patch. Increasing edge effects allows more habitat structure to increase biodiversity within the area.

Edges arise where two habitat types come into contact.

Types

• Inherent- long term natural features underline adjoining vegetation, they are stable and permanent
• Induced- from natural disturbances or human related activities, they are subject to successional changes over time
• Narrow- straight, sharp, abrupt (forest and agricultural field)
• Wide (ecotone)- distance between border and point where physical conditions and vegetation do not differ from interior of patch
• Convoluted- has curves, not a straight line
• Perforated- not a solid border, has gaps

Height can create borders between patches as well.

Edge species (biodiversity)

Environmental conditions enable certain species of plants and animals to colonize on the borders. Plants that colonize tend to be shade-intolerant and tolerable of dry conditions, such as shrubs and vines. Animals that colonize tend to be those that require two or more habitats such as, white-tailed and mule deer, elk, cottontail rabbits, blue jays, and robins. Some animals may travel between habitats, while those that are restricted only to the edge are known as edge species. Larger patches include more individuals and therefore have increased biodiversity. The wideness of the patch influences diversity, a patch must be deeper than its border in order to develop interior conditions.

Advantages

They offer unique habitats with easy access to adjacent communities and therefore can support more plants and animals from these adjacent communities. These species can adapt and increase the areas biodiversity. The easy flow of animals to adjacent areas creates travel lanes along borders. There is an increased availability of light to plants along the borders that promotes primary production. For example, the increased availability of light can allow more plants to be supported, which increases herbivorous insects, and then nesting birds, and nest predators are attracted.

Disadvantages

The narrow borders act as travel lanes for predators and increase predation along the edges. Species can be restricted to one area if the border is too wide or overgrown. Edge effect can cause changes in abiotic and biotic conditions which can cause the natural variation to be lost and make the habitat unsuitable for the original ecosystem. Edge effect can also affect the physical and chemical conditions of the species on the borders. For example, fertilizer from an agricultural field can run off into a bordering forest and contaminate that habitat. The three factors affecting edges can be summarized:

1. Abiotic effect – involving changes in the environmental conditions that result from the proximity to a structurally dissimilar matrix
2. Direct biological effects – involves changes in the abundance and distribution of species caused directly by the physical conditions near the edge
3. Indirect biological effects which involve changes in species interactions such as predation, brood parasitism, competition, herbivory, and biotic pollination and seed dispersal

Human effects on edges

Humans have created borders and fragmentation with development and agriculture. This causes habitat loss restricting species to certain areas and thus creates lower biodiversity in the ecosystem. A few examples of human impacts are:

• Introduction of invasive exotic vegetation
• Higher severity and frequency of fires
• Companion animals acting as predators and competitors
• Use of and creating trails
• Introduction of exotic animals
• Pollution, erosion
• Loss of foraging habitats

Cooperation between landowners and environmental agencies is needed to maintain habitats and avoid fragmentation and edge effect

Examples

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.

Amazon rainforest

It has been estimated that the amount of Amazonian area modified by edge effects exceeded the area that had been cleared. "In studies of Amazon forest fragments, microclimate effects were evident up to 100m (300) into the forest interior." The smaller the fragment, the more susceptible they are to fires spreading from nearby cultivated fields. Forest fires are more common close to edges as a consequence of increased desiccation at edges and increased understory growth present due to 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 changes in temperature, humidity, and light levels promote invasion of non-forest species, including invasive species. The overall effect of these fragment processes is that all forest fragments tend to lose native biodiversity depending on the fragment size and shape, isolation from other forest areas, and the nature of the forest matrix.

North America

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.

Another example of a species benefiting from the proliferation of forest edge is poison ivy. Dragonflies eat mosquitoes, but have a more difficult time than mosquitoes do at surviving around the edges of human habitation. Thus, trails and hiking areas near human settlements often have more mosquitoes than do deep forest habitats. Grasses, huckleberries, flowering currants and shade-intolerant trees such as the Douglas-fir all do well in edge habitats.

In the case of developed lands juxtaposed to wild lands, problems with invasive exotics often result. Species such as Kudzu, Japanese Honeysuckle and Multiflora Rose have done damage to localized natural ecosystems. Beneficially, the open spots and edges provide places for species that thrive where there is more light and vegetation that is close to the ground. Deer and elk benefit particularly^{[citation needed]} as their principal diet is that of grass and shrubs which are only found on the edges of forested areas.

Effects on succession

Edge effects also apply to succession, which is where vegetation is spreading outwards rather than being encroached upon. Here different species will be more suited to the edges or central sections of the vegetation, resulting in a varied distribution. Edges themselves also vary with orientation - for example edges on the north or south will receive less or more sun than the opposite side (depending on hemisphere), resulting in differing vegetation patterns.

Other usage

The phenomenon of increased variety of plants as well as animals at the community junction (ecotone) is also called the edge effect and is essentially due to a locally broader range of suitable environmental conditions or ecological niches.

Edge effects in biological assays refer to artifacts appearing in data which are caused by the position of the wells on a screening plate rather than a biological effect.

Edge effect in scanning electron microscopy is a phenomenon where the number of secondary and/or backscattered electrons which are able to escape the sample and reach the detector is higher at an edge than at a surface. The interaction volume spreads far below the surface, but secondary electrons can only escape if they are close to the surface (generally about 10 nm, although this may differ depending on the material). However, when the electron beam impacts an area close to the edge, electrons that are generated below an impact point that is close to an edge but that is far below the surface may be able to escape through the vertical surface instead.

See also

• Ecotone
• Habitat fragmentation
• Landscape ecology
• Ruderal species
• Spatial ecology

References

Reducing Edge Effects in biological assays