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Living on the Edge

By Liz N. Clift

In ecology, edge effect refers to changes in a population or community along the boundary of a habitat. A clear example of this is when an agricultural field meets a forest. Perhaps a less well-defined example is a fragmented habitat (such as those that occur because of selective logging or in areas impacted by human development (e.g. urban greenways or small areas of clear-cutting for ranching). Edge effect impacts of fragmented habitats may extend further into target habitat.

Think about it like this:

Assume, for the image above, that each example of a specific target habitat (green) has an area of 100m2 and that the edge effect for the target habitat is 10m. Habitat A has a relatively large area that has the least amount of impact due to edge effect (represented by the black outline). The interior of the habitat is undisturbed.

Habitat B, is discontinuous due to a meandering divided highway. This creates an edge effect (in black) that extends 10 meters on either side of the highway (represented by the dashed white lines) leaving the habitat fragmented and vulnerable to edge effects at each curve in the road as well as at the perimeter of the target habitat.

Habitat C is encroached upon (or is encroaching upon) a different habitat type (yellow)). Habitat C demonstrates the “peninsula” effect in varying degrees, which means that certain areas are fully impacted by the edge effect and other areas are less impacted. This habitat has greatest amount edge exposure.

Edges are sometimes thought to create areas of higher biodiversity, which can be true for soft edges, like ecotones. Ecotones (e.g. – the border between the High Plains and the Southern Rocky Mountains that makes up portions of Colorado’s Front Range or the banks of a pond) represent a gentler transition between two environments. Soft edges can also be designed, and in the ecological restoration field these are often referred to as “buffer zones.” In soft edges, the edge effect can become the transitional zone, which allows an intermixing of species that can move readily between both environments. For example, frogs begin their life in water and, as adults, split their time between land and water. A new hole in the canopy of a forest, because of selective logging or a tree falling because of natural causes, creates opportunities for other species to take hold.

Some birds of prey use the edge agricultural fields, parks, and roads  as a fruitful hunting ground (not to mention the raptors that have adapted to urban living!). Not only is there no where for their prey to hide, they may also benefit from killed or injured animals that didn’t make it across unscathed.

Of course, ecology has no easy answers. The above examples can also lead to colonization of a habitat by an invasive or noxious species (e.g. – bull frogs along a pond edge, in areas where bull frogs are not native; English ivy in American forests). And in fact, edges can be detrimental for certain species.

The extent to which a species is impacted by edge effect is sometimes referred to its sensitivity to habitat edges.  Sensitive species may be dependent on the state of interior conditions for their survival. In the example of a new hole in the forest canopy, shade-loving plants that survived due to the protection of that tree may fail to thrive (sub-lethal implications) or die back (which could provide the perfect place for an invasive species to take hold!). Trees along the (abrupt)edge of an agricultural field will experience more wind pressure, which could lead to die-back or stunted growth, even if they are established

In Braiding Sweetgrass, Robin Wall Kimmerer shares an example of how a fragmented habitat (which is divided by a highway) impacts a yellow-spotted salamander population:

“[Ambystoma maculata] come from under logs and across streams all pointed in the same direction: the [vernal] pool where they were born. Their route is circuitous because they don’t have the ability to climb over obstacles. They follow along the edges of any log or rock until it ends and they are free to go forward, on to the pond. The natal pond may be as much as half a mile away from their wintering spot, and yet they locate it unerringly…Though many other ponds and vernal pools lie along the route, they will not stop until they arrive at the birthplace…”

These migrating salamanders, Kimmerer goes on to describe, may face no greater danger than cars. Unlike frogs and other more ambulatory creatures that must cross a road during an annual migration, salamanders move slowly. They have no way to get out of the way of cars. This is where program’s like Burlington, Ontario’s (closing the road during salamander migration season) and amphibian passage ways (like this one in New Jersey) can help reduce edge effect—even if only temporarily.

Unfortunately, corridors are not always the straightforward answer—because these areas too, are impacted by edges. This should be planned for during the design and construction of such corridors, to whatever degree possible. Monitoring for invasive species or antagonist species (like predators) should also be part of corridor planning and management, since these species may also benefit from corridors connecting habitat areas.

Edge effects can differ by target habitat or population—and so it’s critical to clearly identify which specie(s) are of concern and their habitat requirements (including, potentially, abiotic conditions such as soil temperature or wind pressure). It may also be useful to identify corollary information such as:

  • Is there good pollinator habitat, which will help attract pollinators that pollinate a variety of species including the target species?
  • Does the target species need to pass through the digestive track of a particular bird or mammal in order to germinate? If so, is the habitat for that animal present and accessible?
  • Is the habitat geographically isolated from the target population?
  • Is there a known relationship between human presence and rates of target-species reproduction?
  • Is the habitat size (or other factors) adequate to sustain a reproductive population?

Keeping edges in mind can help assess the impact of certain projects and help the public understand the benefits of a particular restoration project. Understanding edge effect can also guide management plans, which supports the long-term success of a restoration project or species conservation plan.

Featured image by: US Fish and Wildlife Service Mountain Prairie

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