By James Cash, TAMU WFSC ‘17
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Google Earth satellite imagery of the Bryan-College Station area shows habitat fragmentation from agriculture
and urban land use. Photo credit: Google Earth
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Habitat fragmentation involves
large tracts of suitable wildlife habitat being divided into smaller parcels,
leading to degradation of habitat as a whole. As suitable habitat patches are converted to
other uses, those that remain become more isolated and no longer hold the same
value for wildlife (Ricklefs 2008). Habitat
fragmentation affects each species differently, and this will be examined in
this two-part series. This article will
focus on how patch size and connectivity influences population declines. Part 2 will examine how edge affects different
wildlife species.
Patch Size
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| Large patches of habitat such as this section of the Sam Houston National Forest are generally better than many small habitat patches. Photo credit: Google Earth |
The
concept of patch size is important when considering habitat fragmentation. Many species of animals require a minimum population
size in order to be self-sustaining. If
a population is too small, it is vulnerable to local extinction. Local extinction can happen when a natural or
manmade event, including extreme weather, wildfires, overharvest, disease, and
others occur. If that habitat is too isolated for immigration to occur from
other habitat patches, then that species is effectively extinct in that area. Today, many wildlife species exist only in
small habitat patches that are becoming increasingly isolated from each other
and some local populations are lost one-by-one to the eventual downfall of the
species as a whole.
A prime
example of the importance of patch size relates to the northern bobwhite (Colinus virginianus). Research suggests that in order for a local
population of bobwhite to have a good chance of persisting for 100 years or
more, it must consist of at least 800 individuals (Guthery et al. 2000).
With fewer individuals the population is at risk for local extinction due
to natural or manmade causes. In order to support 800 individuals, a habitat
patch needs to be about 2,000 to 4,000 acres (Guthery et
al. 2000).
Management Implications
Upon
learning about minimum patch size for viable bobwhite populations, some
landowners might despair over the size their property and the recommended
minimum. The average landholding in Texas is about 500 acres, which is much
less than the minimum habitat size for many species of wildlife (Texas Land
Trends 2014). That is where wildlife
cooperatives come in to play. Working
with neighbors through similar land management practices can ensure that
suitable wildlife habitat extends past your property line. For more information on forming wildlife
cooperatives, see our blog article Land
Owner Cooperatives Get a Boost From Friends.
Patch
size is also an important consideration for local, state, or federal agencies trying
to conserve wildlife habitat. Wildlife
management areas should be designed to maximize patch size. A 5,000-acre property is usually better than five
1,000-acre properties. The Natural
Resources Conservation Service has a great article on designing
wildlife reserves.
Connectivity Among
Patches
Promoting connectivity among
habitat patches can help smaller habitat patches resist local extinction
events. Habitat patches can be connected
in 3 different ways.
Habitat corridors are narrow strips
of suitable habitat connecting larger habitat patches. These corridors can be highly effective at
allowing dispersal among patches. Rivers
and their accompanying riparian habitats are examples of common habitat
corridors. Riparian habitats are made up
of the many terrestrial and semi-aquatic plants that grow along the bank of a
river or stream. They can provide
corridors for movement for terrestrial and avian species, including quail. Riparian areas can also support small
populations of quail due to their high productivity. In addition to the wildlife benefits, these
buffer zones also serve to filter the run-off entering the stream and can
result in improved water quality (Cunningham et al.).
A second possibility for some
species is stepping stones where habitat patches are interspersed among larger
habitat blocks. These smaller patches
may not be large enough to support individuals for long periods of time, but
can serve as rest stops as individuals travel in-between larger areas. Stepping stones are especially useful for
birds because they can fly from stop-to-stop. Terrestrial species, especially those with
narrow tolerance ranges and short dispersal abilities, may not be able to
utilize stepping stone patches as effectively.
A third way that that wildlife
species can disperse among patches is through a hospitable matrix. The matrix is the intervening land between suitable
habitat patches. A simple approach is to
consider the matrix as an inhospitable sea that wildlife must cross to get to
habitat patches. However, the reality is
that the matrix is much more complex and dynamic than that. Usually the matrix is a dynamic mix of habitat
that ranges from nearly suitable enough to be considered a habitat patch to so
inhospitable that it is almost impossible to disperse across. An example of this is the difference between a
multi-lane freeway and an agricultural field. Neither can be considered quality wildlife
habitat, however an agricultural field can be crossed and even utilized to some
extent by many species of wildlife, while a freeway is much harder for
non-avian wildlife to cross.
Management Implications
Both private landowners and
government agencies can help promote connectivity between habitat patches. Most large scale projects will obviously be
conducted by government agencies, but private citizens can still influence
these projects by asking agencies to implement measures that promote
connectivity. Examples include bridge/
tunnel crossing to help wildlife cross obstacles such as freeways that would
otherwise be a barrier to dispersal and migration. If private property lies within potential
travel corridors between important habitat patches, then land owners can manage
their property to promote dispersal between patches. As discussed previously, it is common for
rivers and their associated riparian habitats to form contiguous corridors
between major habitat patches. If you
have control over land that includes one of these corridors, then protecting
the riparian habitats around the river should be a top priority. The Texas A&M AgriLife Extension publication
“Riparian
Restoration on Farms and Ranches in Texas” can help you get started on
restoring and protecting riparian areas on your property. According to the Virginia
Department of Game and Inland Fisheries (VGDIF), 25 to 50 foot wide vegetation
buffer zones can be planted to serve as habitat and travel corridors for quail
if natural riparian areas are insufficient (VDGIF 2015). Buffer strips should contain a mixture of
bunch grasses, heavy seeded forbs, and protective shrubs to benefit quail. Land
owners should work with their neighbors upstream and downstream to make sure
that these corridors stay intact.
Landowner
cooperatives can be used to create habitat corridors. A great example of this
is the Wildlife Habitat Federation. This group of landowners came together to
create a wildlife habitat corridor that is 7 miles long and up to a mile wide.
This corridor connects their properties to the Attwater Prairie Chicken
National Wildlife Refuge near Sealy, Texas. Dispersion along this corridor has
led to 31 different grassland bird species being detected on those properties. For
more information on the Wildlife Habitat Federation and other land owner
cooperatives, watch the AgriLife Extension video “Reversing the Quail Decline
in Texas: Wildlife Habitat Federation Model.”
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| This Google Earth image shows how the riparian area around a river can serve as a corridor for dispersal between habitat patches |
If your property does not already have a
contiguous corridor running through it, then your best option may be to manage
your property to serve as a stepping stone for species on the move. This is an especially good option for people
who own property that is too small to support a permanent population of a
certain species. Even a suburban
backyard could be set up to be used as a stepping stone for birds moving
between large habitat patches. The
AgriLife Extension publication “Native
Habitat” describes the Texas Parks and Wildlife “Wildscapes”
program, which promotes the use of native plants to create backyard wildlife
sanctuaries.
Habitat Fragmentation Part 2: The Edge describes the edge effect of habitat fragmentation and the benefits of habitat mosaics.
Works Cited
Cunningham, K., H. Liechty, C. Stuhlinger.
Riparian buffers: functions and values. University of Arkansas, Division of
Agriculture. Publication FSA5026. Little
Rock, Arkansas. USA
Guthery, F. S., M. J. Peterson, R. R.
George. 2000. Viability of northern bobwhite populations. The Journal of
Wildlife Management. 64(3): 646-662
Ricklefs, R.E. 2008.
The economy of nature. Sixth edition.
W. H. Freeman and Company, New York, New York, USA.
Texas Land
Trends. 2014. Status update and trends of Texas rural working lands. Texas
A&M Institute of Renewable Natural Resources, College Station, Texas.
Volume 1, Issue 1. 13 pages.
Virginia Department of Game and Inland Fisheries
Small Game Committee. 2015. Forest land
quail habitat management. <http://www.dgif.virginia.gov/quail/forest-land-habitat-management.asp>.
Accessed 13 April 2015