By James Cash, TAMU WFSC ‘17
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
Brittingham, M.
C., S. A. Temple. 1983. Have cowbirds caused forest songbirds to
decline? Bioscience 33(1): 31-35
Natural Resource
Conservation Service (NRCS). 1999. Wild Turkey (Meleagris gallopavo). Fish and Wildlife Habitat Management Leaflet,
Madison, Mississippi, USA.
Ricklefs, R.E. 2008.
The economy of nature. Sixth edition.
W. H. Freeman and Company, New York, New York, USA.