Definition

In ecology, edge effects refer to changes in the number of animals or plants and how they group together at the edges where two or more habitats meet. These changes happen because the environment near the edges is different from the environment inside the habitats. Edges of habitats often have more wind, more sunlight, and bigger changes in temperature compared to the inside. In places where habitats are broken into small pieces, edge effects are more noticeable and can cover a larger area. As edge effects become stronger, the areas where habitats meet can support more different types of plants and animals.

Biodiversity

Environmental conditions allow certain plants and animals to live in the areas where different habitats meet. Plants that grow along the edges of forests are usually those that do not need much shade and can survive in dry conditions, such as shrubs and vines. Animals that live in these edge areas often need to use more than one type of habitat, like white-tailed and mule deer, elk, cottontail rabbits, blue jays, and robins. Some animals move between habitats, while others stay only near the edges. Larger areas of habitat support more types of plants and animals compared to smaller areas. The width of the habitat also affects diversity: for edges to have strong effects, the boundary must be more noticeable than just a simple line between two areas.

When animals move between different areas, they can create paths along the edges. These paths let more sunlight reach the plants growing along them, which helps the plants grow better. As plants grow more, they support more insects that eat plants, which in turn support more birds that eat insects, and so on up the food chain.

In some cases, very wide or overgrown edges can cause certain species to live only on one side of the edge, even if they could live on both sides. Edge areas can change both non-living and living conditions in ways that reduce natural variety and harm ecosystems. Non-living changes, called abiotic effects, include things like higher temperatures and stronger winds. Living changes, called biotic effects, include more predators, competition between species, and changes in how animals interact. Harmful edge effects can also come from physical or chemical changes, such as fertilizer from farms leaking into nearby forests and polluting the habitat. The three main factors that affect edges are:

• Abiotic effects—Changes in non-living conditions caused by being near a different type of environment
• Direct biological effects—Changes in how many animals or plants live in an area because of conditions near the edge
• Indirect biological effects—Changes in how animals and plants interact, such as more predation, competition, or changes in how seeds are spread.

Types

• Inherent – Natural features keep the border location stable.
• Induced – Short-term natural events (e.g., fire or flood) or human actions cause the border to change gradually over time.
• Narrow – One habitat ends suddenly, and another begins (e.g., an agricultural field).
• Wide (ecotone) – Two clearly defined habitats are far apart, and a large area with a gradual change in environment separates them.
• Convoluted – The border has an irregular or crooked shape.
• Perforated – The border has openings where other habitats are present.

Height differences can also create borders between different areas.

Urbanization

Urbanization causes humans to break landscapes into smaller parts, which increases edge effects. These changes in landscape ecology are having major effects on ecosystems. Animals and plants that can live in many different places, especially those that are not native to an area, often benefit from these changes. However, animals and plants that need specific living conditions are harmed. For example, the variety of bird species that cannot tolerate edges in the Lacandona rainforest in Mexico is decreasing as edge effects become more common.

Effects on succession

Edge effects also influence how plants grow over time during ecological succession. Some plant species are better adapted to live near the edges of an area, while others thrive in the middle sections, creating different patterns of where plants grow. The amount of sunlight at edges can vary depending on their direction: edges facing north or south may receive more or less sunlight than the opposite side. Other factors, such as how much light is available, the type of soil, and how long plants can grow each year, can change the types and arrangement of plants, causing areas where edge effects are more noticeable.

Human effects

Human activities like building roads, cutting down trees, and expanding cities break up habitats into smaller areas. These changes often harm the size of habitats and the animals that live there. Examples of how humans affect habitats include:

• Introducing non-native plants or animals
• Farming
• More severe and frequent fires
• Pets acting as predators or competitors for wildlife
• Creating trails
• Pollution and soil erosion
• Loss of areas where animals search for food
• Breaking up habitats into smaller parts
• Cutting down forests and changing how land is used

Examples

When edges separate a natural ecosystem from an area that is damaged or changed, the natural ecosystem can be harmed for some distance near the edge. In 1971, Odum wrote, "The increase in variety and diversity where two communities meet is called the edge effect. It is well known that songbirds are more common in areas like estates and campuses compared to large, uniform forests." In a forest next to land that has been cut down, creating a boundary between open land and forest, sunlight and wind reach deeper into the forest. This dries the forest near the edge and encourages the growth of plants that take advantage of these conditions. Factors like air temperature, moisture in the soil, light levels, and the amount of light that plants use for growth all change near edges.

A study found that the area of the Amazon Basin affected by edge effects was larger than the area that was actually cleared. In research on forest fragments in the Amazon, changes in temperature, humidity, and light were found up to 100 meters (330 feet) into the forest. Smaller forest fragments are more likely to catch fire from nearby farmland. Forest fires are more common near edges because more light reaches the ground, drying it out and increasing the growth of plants near the surface. This extra plant material provides fuel for fires to spread from pastures into forests. More frequent fires since the 1990s are one example of edge effects changing Amazonian forests. Changes in temperature, humidity, and light encourage non-forest plants, including invasive species, to grow. Over time, these changes reduce the number of native plant and animal species in forest fragments, depending on the size, shape, and isolation of the fragment.

Today, the amount of forest edge in the United States is much greater than when Europeans first settled in North America. Some species benefit from this, like the brown-headed cowbird, which lays its eggs in the nests of songbirds living near forest edges. Poison ivy also grows better in edge areas. However, dragonflies, which eat mosquitoes, struggle to survive near human settlements. Because of this, areas near human homes often have more mosquitoes than deep forest areas. Plants like grasses, huckleberries, flowering currants, and trees such as Douglas-fir grow well in edge habitats.

When developed areas are next to wild areas, invasive plants often cause problems. Examples include kudzu, Japanese honeysuckle, and multiflora rose, which harm natural ecosystems. However, open areas and edges also support species that need sunlight and low-growing plants. Deer and elk benefit because they eat grasses and shrubs, which are found only near the edges of forests.

Alternative definitions of edge effect

The increase in the number of different plants and animals at the edge area between two habitats, called an ecotone, is known as the edge effect. This happens because the edge area often has a wider range of living conditions that support more types of life.

In biological experiments, the edge effect refers to errors in data that occur because of the location of test wells on a plate, not because of actual biological causes.

In scanning electron microscopy, the edge effect happens when more electrons escape from a sample near the edge of the material than from the middle. This is because electrons generated deep inside the sample can escape through the side of the material near the edge, even though they usually cannot escape from deeper areas. The depth from which electrons can escape is typically about 10 nanometers, but this depends on the material being studied.