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The Edge Effect

By Jo-Anne Nina Sewlal BSc., MPhil., FLS., AMSB Dept of Life Sciences, University of the West Indies

In order to better understand the concept of edge effects think about when you enter an air conditioned building from a hot street.

In that situation the edge is the change in temperature and humidity that we feel. While the edge effect is our reaction to this change, for example, we might put on a sweater to feel warmer. Contrasting habitats can occur naturally, by human activities or intentionally. However, regardless of how these edges are created they exhibit characteristic conditions that allow certain species to call these edges home.

In this article I will go into more detail as to how edges both natural and human influenced affect the environment and the organisms that live in these neighbouring habitats.

The edge effect occurs when contrasting habitat types are situated side by side (known as an ecotone) in an ecosystem. However this term has become more associated with the boundary between natural habitats and those that have undergone disturbance or development. However, there are cases where the edge effect occurs naturally, for instance in our own Aripo Savanna in Northeast Trinidad, where the grassy savanna is bordered by marsh forest. But, some ecotones are intentionally made, for example, with chinampas, which is a system of parallel banks and canals used to help establish irrigation systems for crops, or used to herd fish into specially designed ponds.

The area influenced by the edge effect is not limited to the boundary between these two habitats but extends for quite some distance from the actual physical boundary of the neighbouring habitats. As a result there are many unique conditions that occur as a result of the edge effect, such as the effect on the microclimate of the area. Ecotones affect the microclimates found in ecotones, by altering air temperature, wind speed, humidity and solar radiation.

In clear-cut areas or where there are roads, the increase in sunlight and areas or where there are roads, the increase in sunlight and air temperature means an increase in soil temperature and a decrease in the moisture the soil contains.

These conditions mean that plants that are tolerant to shady conditions as well as cooler and moister environments will not do well or survive in ecotones, leading to an increase in plants that thrive in intense sunlight and the microclimatic conditions it brings. Also the understory vegetation from forested habitats that is exposed in the ecotone can become stressed due to attack from insects, parasites and fungi and as a result "die-back" and are not found in the ecotones.

Forest fires also occur at forest edges, because of the increased desiccation. At forest edges there is no canopy to shelter the understory, therefore, leaf litter will be drier and as it exposed to the direct sunlight. Also in forests many plants in the understory remain "stunted" because of the limited amount of light that manages to filter down through the canopy.

Ecotones go through three stages where they are inhabited by different categories of organisms. In the first stage, the true habitat species leave, that is, the inhabitants that would be found there if the area was not disturbed. In the second phase animals with a wider tolerance for a range of conditions occupy the ecotone. In the final phase more aggressive "edge-dwelling" species better adapted to life in ecotones would displace the occupants in the second phase. The edge effect occurs when there is an increase in the diversity of the species present at the junctions between these different habitats. The explosion in plant diversity seen at an ecotone occurs when their seeds are blown or carried into the ecotone from both habitat types.

Animals like stray dogs, cats and foxes also tend to move around habitat edges, along tracks and roads because of easy access to the forest habitat and its species. This also occurs in the ecotone between aquatic and terrestrial habitats for example, fox predation along bush tracts besides rivers resulted in a decrease in platypus populations in Australia. Some animals also prefer to live in around these edges, because they can find food in the clear area and retreat to the forested section when they feel threatened.

In ecotones one will also find an increase in pest species. In terms of plants, edges provide ideal habitat for weeds. The area of disturbed land is open so that seeds of weed species which are usually dispersed by wind can settle there.

Also there are no other or not many established plants to compete with them for resources like sunlight, water and nutrients.

The noise from adjacent habitats, especially if it is an area under construction or in continuous use, like a highway can result in species abandoning their nests. Besides noise, ecotones are prone to other disturbances such as run off containing pesticides and fertilizers from adjacent farmland. If the area is used to rear livestock, the animals can trample the soil in the ecotone thus compacting it and making it difficult for water to enter and for plant roots to extend.

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