Understanding the Edge Effect With Examples

Edge effects are predominantly harmful, despite the fact that most plants and animals are found on habitat edges.

It has been known for long that clearing a forest for human uses like construction or agriculture has disastrous environmental effects. The soil that was earlier bound to tree roots is blown away by the wind, and its fertility is steadily depleted by successive crop cycles.

However, it can be observed that, when one emerges from within a forest, the climate changes drastically, from the cool, dark interior to the bright and warm barren exterior. Recent research has indicated that this change may be as harmful as the deforestation itself.

But what effects does such a climatic transition have on the plants and animals found within the forest? This is described by a phenomenon called the 'edge effect'. How does the 'edge effect' affect biodiversity? Let's find out.

The term 'edge effect' describes the characteristics seen on the boundary, or 'edge', between two habitats. This effect may be seen between the edge of a forest and a cropland or between a forests and rocky outcrop, although in recent times, it refers to the harmful effects created by tree-felling on a forest boundary.

Abiotic effects are the physical changes observed at a forest edge, such as an increase in sunlight, wind, air temperature, and moisture, which differs from the conditions seen in the interior of the forest.

These effects describe how organisms like plants and animals are directly affected by the physical effects introduced at the edge. These may include the enhanced growth of certain plants because of the extra sunlight, or the drying out of some trees which thrive in cooler conditions seen inside the forest.

Some effects are an indirect result of the changed relationships between different organisms. These include an increase in the number of forest animals killed by predators introduced at the edge, increased wind which transports seeds of new plants, competition between different species, increased plant growth attracting more herbivorous insects, etc.

♦ Plants which cannot tolerate forest shade and need drier conditions thrive at edges.
♦ More organisms are found on the edge between two habitats, rather than in the habitats themselves. This includes shade-intolerant plants, and animals which live in both habitats but need a transitional area for nesting or foraging.

♦ Edges benefit humans, because they reduce the chances of encountering predators, which are mostly restricted to the interior of the forests.

♦ The climate on the edge spreads deeper into the forest, threatening species which are found only in the interiors, and increasing edge species which are already common.
♦ The reduction of the inner climate causes the breakup of the forest into fragments, increasing the edge even further.

♦ It can cause the drying of trees, and increased undergrowth at the edge, which contributes to forest fires.
♦ Powerful winds can uproot and destroy trees or lade them with debris.
♦ Plants which need moderate temperature and light, as is found within the forest, may not survive at the edge.

♦ It hampers the life cycle of some organisms, thus disturbing their role in the forest. For example, the life cycle of dung beetles is hampered at forest edges, which prevents the decomposition of animal waste.
♦ The damage caused by tree-felling at the edge extends far inside the interior.

♦ It can give non-native species and weeds access to the forest, where they may compete with forest flora for survival.
♦ Chemical fertilizers and pesticides used on crops at the forest edge can contaminate the forest.
♦ Human presence near the edges introduces pets, which may prey on wild animals.
♦ Livestock compete with wild animals for grazing sites.

♦ Since small fragments of the forest have a high edge/area ratio, they show more damage. Therefore, two or more fragments can be joined by reforestation between them.
♦ Forests with irregular edges or sharp corners are harmed more. So, if trees are felled for human purposes, a rounded edge can be developed to minimize the impact.

♦ A hedge of light-tolerating bushes can be grown at the forest edge to shield it from windfall, debris, and sunlight.
♦ Creating feathered edges, where the number of trees reduces gradually at the edge, rather than an abrupt clearing, to prevent drastic climate changes.

♦ Fencing the forest edge to prevent livestock from destroying small plants.
♦ Planting a woodlot of trees, like pine, on the edge, which is most vulnerable to wind.

These animals are found in several North-African and South-European countries. Usually, on spotting a sign of trouble, the animal rolls up into a ball to expose its sharp quills, which protect its soft underbelly. While this behavior can protect it from cats, it does little against being run over by vehicles.

An increase in deforestation means that the forest edge is increasing, which has resulted in more cowbirds and lesser songbirds, especially because the former is even known to throw aside the latter's eggs to accommodate its own.

However, this is bad news for us, as the mosquito's natural predator―the dragonfly―is found in higher numbers inside the forest, rather than at its edge. So, an increased edge means more mosquitoes, and lesser dragonflies to control their numbers.

One of the biggest threats is an increase in forest fires caused by the drying of trees, and an increase in undergrowth at the edge which provides a potent fuel source. In the past, the undergrowth size was restricted inside the forest because of the less light that reached its floor.

In the past, the cooler forest climate, coupled with natural parasites of the moth, kept a check on its population. However, increased forest edges due to deforestation has given them access to trees along the edge.

Here, the warmer climate speeds up their life cycle, and slows that of its parasite which hatches on the forest floor. The end result is more damage to forest trees due to more moths.

As can be seen, edge effects have an overwhelmingly negative effect on the natural environment, making one wonder how could they ever be considered beneficial, as in the past. It creates a deadly chain of events that steadily destroy an ecosystem, and requires many years to recover, if it ever will.