Land Stewardship
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Land Stewardship

Habitat Fragmentation Part 2: The Edge

Habitat loss and fragmentation are leading causes in the decline of many wildlife species. A previous blog post, Habitat Fragmentation Part 1: Patch Size and Connectivity introduced the concept of habitat fragmentation and then described how the size and location of habitat fragments affects the wildlife that depends on them. This blog post will focus on how edge habitat affects wildlife in different ways.

Edge habitat is the transition between different habitat types or suitable habitat and land that is not suitable for wildlife such as urban areas. Edge increases as habitat becomes more fragmented. There is a distinction between natural edge that exists between different habitat types and anthropogenic edge that is created when humans fragment the landscape with roads, powerline right-of-ways and urban areas. Certain species of animals are more suited to edge habitat than others, so increases in edge habitat can change the species composition of a region.

Negative Effects of Edge

A common problem with edge habitat is that those are areas of focus by predators. Some wildlife species rely on the interior habitat that is deeper in the habitat patch to escape from the relative chaos and high predation that occurs in the edge. Edge effects have been known to extend hundreds of yards into habitat patches (Ricklefs 2008). This means that species that rely on the safety and stability of interior habitats may not be able to utilize the smaller habitat patches because they are effectively entirely composed of edge habitat. Both natural and anthropogenic edge can present problems for interior species, but anthropogenic edges are generally worse for wildlife. Typically, natural edge is more of a gradient, which provides a transition area that certain species prefer. Anthropogenic edges usually have a sharper transition that does not provide much wildlife habitat.

A classic example of the edge effect on wildlife populations is the brown-headed cowbird (Molothrus ater). This species is a nest parasite, meaning it lays its eggs in the nests of other bird species. Doing so decreases the nesting success of the host bird species. The brown-headed cowbird utilizes open fields and grasslands for feeding and focuses on forest edge habitats to find existing nests to lay its eggs in. Increases in habitat fragmentation have led to increases in edge habitat in many areas of the United States. This increase in edge habitat has allowed the brown-headed cowbird to become a more efficient nest parasite, which is likely a contributing factor in the decline of song bird populations in the eastern United States (Brittingham and Temple 1983).

This transition from “improved” pasture to forest allows brown-headed cowbirds access to vulnerable songbird nests. Photo credit: Texas A&M AgriLife Extension

Management Implications

The key to success when trying to reduce edge is geometry. The best shape possible to reduce edge habitat is a circle. The more corners and other bends/curves a parameter has, the more edge it has. This means that when wildlife habitat patches are designed, whether private or public management areas or preserves, they should be designed to reduce corners. Obviously a perfect circle is hard to develop, but the closer you can get to one the better. Try to avoid as many corners, peninsulas, and inlets as possible in your habitat patch if your goal is to reduce edge. Also, remember that as patches are divided into smaller, more numerable patches the edge increases rapidly even if the total habitat area is not affected. For instance, if you have a large patch of forest that is crossed by many roads, the total area of the forest might be nearly the same as before the roads were built, but each road creates new edge habitat so there is now exponentially more edge than before. This can allow predators and parasites to invade deeper into the forest. For this reason, keeping the number of roads and other intrusions into a habitat at the bare minimum is an important consideration.

Habitat Mosaics

There are many species that rely on a mosaic of different habitat types to meet their life history needs. For these species, one large patch of homogenous forest or grassland is not sufficient. For instance, wild turkeys (Meleagris gallopavo) rely on a mosaic of interspersed grassland and forest to meet both their daily requirements and their life-cycle requirements (NRCS 1999). The less distance a turkey must travel to use both these habitats the more likely it is to survive and thrive. It is common for turkeys to be found in or near the edges that separate these two habitat types. White-tailed deer (Odocoileus virginianus) and northern bobwhite (Colinus virginianus) are examples of other species that do well in areas with a good interspersion of habitat types. Even though species like these prefer a mosaic of habitat types, they are still negatively affected by habitat fragmentation. Habitat fragmentation in this sense doesn’t refer to the fragmentation of a certain habitat type (i.e. forest) but rather the fragmentation of the usable space for a species. Usable habitat can be fragmented by roads, urban areas, and large expanses of agricultural lands.

This modified Google Earth image shows a portion of the Texas Parks and Wildlife Department’s Gus Engeling Wildlife Management Area (WMA) near Tennessee Colony, Texas. The yellow lines are roughly the west/ east boundaries. The WMA extends north and south. The light blue circles show areas of open grassy areas that are interspersed in the forest. This type of arrangement provides good habitat for wild turkey and deer.

Management Implications

When managing for species that need a habitat mosaic or that are edge specialists, there are several important considerations. For species that need a mosaic of several different habitat types it is important to make sure that there is good interspersion between the different habitat types. For instance, when managing for quail or turkey it is important to have nesting habitat and brood-rearing habitat close together so that the newly hatched chicks/ poults do not have to travel far to get from their nest to feeding grounds. The Texas A&M AgriLife Extension Eastern Wild Turkey video playlist has several videos that describe the habitat requirements of eastern wild turkey, including habitat interspersion. You can manipulate the landscape with different techniques to create good interspersion. For instance, you may have a field of native bunch grasses which provide great quail nesting habitat, but there are no areas with good forb production nearby that could serve as brood-rearing habitat once the chicks have hatched. To remedy this, you may consider disking strips in the field with a tractor to promote areas of fresh forb growth, which will provide good brood-rearing habitat that is easy for chicks to get to. The benefits of disking are described in further detail in the Texas A&M AgriLife Extension video “Disking for Quail Habitat in the Rolling Plains of Texas.” On other properties there may be too much brush, so brush sculpting through mechanical or chemical techniques can create the necessary interspersion of open areas and cover.

Before implementing any intensive wildlife management plan, especially one that involves manipulating the habitat in a significant way, it is a good idea to consult a local biologist. To find a Texas Parks and Wildlife Department biologist click here, to find a Natural Resource Conservation Service office click here, and to find your County Extension Agent click here. These resources can help you decide on how to balance the needs of edge specialists with the needs of species that utilize interior habitat on your property.

Literature Cited

  1. Brittingham, M. C., S. A. Temple. 1983. Have cowbirds caused forest songbirds to decline? Bioscience 33(1): 31–35
  2. Natural Resource Conservation Service (NRCS). 1999. Wild Turkey (Meleagris gallopavo). Fish and Wildlife Habitat Management Leaflet, Madison, Mississippi, USA.
  3. Ricklefs, R.E. 2008. The economy of nature. Sixth edition. W. H. Freeman and Company, New York, New York, USA.

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Texas A&M Natural Resources Institute

Texas A&M Natural Resources Institute

At the Texas A&M Natural Resources Institute, our work improves the conservation and management of natural resources through applied research. nri.tamu.edu