These materials were developed by Alison Baird, Department of Geography, University of Texas at Austin, 1996. These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the author, Alison Baird, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder. These materials may not be copied to or issued from another Web server without the author's express permission. Copyright © 1996. All commercial rights are reserved. If you have comments or suggestions, please contact the author or Kenneth E. Foote at firstname.lastname@example.org.
What is habitat fragmentation and how does it affect endangered species?
In order to answer, we need to consider our role as geographers within
the ultimate goal of species conservation. A subset of the discipline,
biogeography, approaches such questions with an emphasis on the distribution
of diversity on a variety of scales. We will assume the role of biogeographers
for this exercise; as such, we will answer this question by identifying
suitable habitat for endangered species within a study area in Central
Texas, the Balcones Canyonlands National Wildlife Refuge.
More specifically, we will using color infra-red (IR) aerial photos
to identify sites that fit habitat criteria for Golden-cheeked warblers
and Black-capped vireos. It should be noted that while we cannot address
all of the biological considerations for habitat preference with
the given data set, we can identify vegetation types and suitable
canopy cover for either species. In addition, we will compare our habitat
assessments with sightings recorded by field technicians with the US Fish
and Wildlife Service.
Biogeography is a subset of geography that deals with the distribution
of plants and animals over the earth's surface and the processes which
affect this distribution. A biogeographer approaches these processes on
a variety of scales, from global to local. For instance, we can consider
the effects of global climatic regimes (such as temperature, light, moisture
and wind) on the worldwide distribution of vegetation or, alternately,
we can examine these same factors found within a micro-environment, such
as a desert spring. However, biogeographers examine more than just climate
regimes; they also consider the influence of geomorphic factors, soil sciences,
and ecosystem processes. Admittedly, each of these sub-fields is a discipline
within itself. Biogeographers tend to specialize in one or more of these
subjects, for each is rich in uncertainty and complexity. For our exercise,
we will be "specialists" in the realm of ecological processes.
Ecological processes are amazingly complex. These concepts involve
interactions between species, like the familiar "food-chain" theories,
but also incorporate communal, symbiotic, competitive and predator/prey
relationships. These relationships can occur amongst members of the same
species or between groups of different species that interact within biotic
communities. In addition, we can examine the interactions between organisms
and several inorganic factors: water, temperature, altitude, and so on.
While these are all important ecological considerations, they do not even
approach the complications that arise when we think about the influence
of humans on the natural world.
We modify habitat. To some extent, this alteration may not be
harmful to other species. However, if carried out at an unsustainable rate,
habitat modification can jeopardize the well-being of untold numbers of
species and communities. Because human-caused habitat alteration is rooted
in our resource use, biogeographers are often called in to the politcal
and economic realm. The role of a biogeographer in this case is in environmental
management: how best to retain sufficient habitat for species while allowing
adequate use of resources to sustain our own species? This is the question
that faces anyone or any group that attempts to establish reserves or refuges
for various organisms. Indeed, this is the consideration facing the Refuge
in this exercise. We will examine the results of fragmentation on the habitat
for endangered species.
term "habitat fragmentation" indicates an overall decrease of some habitat
type or the partitioning of habitat into smaller, more isolated parcels.
Fragmentation is a complex process that we most often associate with human
settlement patterns: road-building, clearing of land for agriculture and
grazing, and urban encroachment. The following diagram is a typical illustration
of the destruction of forest as a consequence encroaching human settlement.
Notice that, rather than a landscape of total ecological destruction, the
forested habitat is instead reduced to a small patchwork of remaining forest.
The most prevalent cause of habitat fragmentation has more to do with human
consumption of resources than natural disturbance regimes. Our consumption
of resources has risen as human populations have grown. Inversely, our
efficiency of resource use has decreased. The result has been increasing
exploitation of our natural resources. Through our manipulation of ecosystems,
we have fragmented or completely lost vital habitat for untold numbers
It should be noted that habitat fragmentation is not purely anthropomorphic
in origin. Most ecosystems are periodically subject to alteration by fire,
flood, wind and landslides. The effects of these disturbances may seem
disastrous ecologically, but it is important to remember that these ecosystems
evolved in the presence of these disturbances. Thus, they are adapted to
disturbance and, in fact, may even rely on these processes for rejuvenation.
Frequent disturbance is a stabilizing element in many ecosystems.
The effects of anthropogenic habitat loss or degradation on the numbers
and types of species in an ecosystem are still unfolding. Some species
are able to survive in a mosaic of human land uses. Others may colonise
nearby suitable habitat. Certain species may persist in the leftover fragments,
provided that these patches are large enough or that there are "corridors"
of habitat between patches through which species can travel. If the habitat
continues to be degraded or destroyed, the end result could be the extinction
There are two definitions of endangered species: one biological and
one legal. Biologically, an endangered species is one that, under current
conditions, cannot maintain a viable population and will therefore go extinct.
Extinction can occur locally, with other populations of the species
existing elsewhere, or globally as the last surviving members of
a species. Legally, an endangered species is one which is listed under
the Endangered Species
Act (1973) (ESA) as in danger of extinction "as a consequence of economic
growth and development" despite "adequate concern and conservation." The
difference between the two definitions is slight, but significant. Legal
attention has the advantage of legal protection of the species from further
modification of the environment. For more information on the Endangered
Species Act itself or the applications of the ESA, please consult the on-line
texts listed below:
There are 78 state and federally listed endangered species in Texas.
Two of these, the Golden-cheeked warbler and the
Black-capped vireo, will be the subject of this exercise.
For more information on endangered and threatened species in Texas, please
refer to the sources below:
With the introduction of GIS
to the field of biogeography, we have commandeered a powerful key to understanding
the natural world. Given the pertinent field data, we can display the distribution
of biological elements over any given area. By applying the distribution
of species and their habitats to a graphic format, we can easily and quickly
define those areas which can be targeted for preservation, restoration,
limited uses or further study. This technique now is being used nationwide
in the Gap Analysis Project
and by various environmental managers who are attempting to secure the
future of endangered species. Used alone or in conjuction with other quantitative
techniques, such as modeling, GIS is steadily increasing our capacity for
The Balcones Canyonlands is an ecologically diverse belt through Central
Texas distinguished by steep, wooded canyons and streams along the eastern
and southern regions of the Edwards Plateau. The Canyonlands support a
great number and variety of species, many of which are integral members
of unique biological communities. Small, localized habitats play a major
role in this region's diversity; for example, there are 64 species of cave
fauna within a single cave in western Travis County--this is the highest
diversity of cave fauna found within the southwestern United States.
The Balcones Canyonlands National Wildlife Refuge is located in the
Post Oak Ridge section of the Edwards Plateau. The Refuge was established
in 1991 under the authority of the Endangered Species Act (1973), the Fish
and Wildlife Act (1956, as amended) and the Land and Water Conservation
Fund Act (1965, as amended). The proposed boundaries of the Refuge encompass
41,000 acres in Travis, Williamson and Burnet Counties. At present, the
US Fish and Wildlife Service has acquired approximately 14,000 acres in
western Travis County.
The Post Oak Ridge segment of the Edwards Plateau includes at least
1,500 plant species and over 120 native tree species. This number includes
several species of endemic plants, 22 endangered plant species and several
unique vegetation communities. Because this region is at the junction of
the Edwards Plateau and the Lampasas Cut Plain physiographic sections,
there is a diverse arrangement of hydrologic, geologic and climatic features
that "blend", resulting in an unusual mosaic of species. The vegetation
types can range from eastern deciduous trees to both Rocky Mountain species
and Mexican neotropical species. More commonly, the vegetation communities
found here include oak-juniper woodlands, post-oak-grassland savannas,
mesic deciduous forests, stream bottom riparian forests, low shrub communities
and communities of smallish plants (such as mosses) found in and around
springs and seeps.
The Balcones Canyonlands support at least 375 species of birds, one-third
of which nest in this region. 110 of these species are neotropical migrants
(birds that migrate between northern breeding grounds and wintering grounds
in Central and South America). Within this region, there is habitat for
55 species of mammals (including 9 neotropical migratory bat species),
70 species of reptiles and 80 species of fish. At least two of the above
mentioned species are listed with the US Fish and Wildlife Service as "endangered"
species under Endangered Species Act.
Range: The golden-cheeked warbler (Dendroica chrysoparia) is a neotropical
migrant. They arrive in our area sometime in March from their wintering
grounds in Central America and they leave Texas by mid-July. Their only
known nesting habitat occurs exclusively in the oak-juniper woodlands of
Central Texas. The golden-cheeked warbler needs older stands of forest
with a dense canopy cover of at least 70%. The diet of both the adults
and young include insects and caterpillars found in oak and juniper trees.
The main threat for the survival of this species is habitat modification.
Encroaching urbanization and the clearing of land for grazing stock have
decreased the number of viable nesting areas. In addition, the golden-cheeked
warbler can be a victim of nest parasitism by the brown-headed cowbird,
a species that prefers disturbed habitats.
The black-capped vireo (Vireo atricapillus) is also a neotropical
migrant, with wintering grounds on the Pacific coast of Mexico. They used
to nest from northern Mexico up through Kansas, but now the vireo is only
seen as far north as Oklahoma. The black-capped vireo seem to prefer a
semi-open habitat of dense trees; they build their nests at a very low
There are approximately 1500 breeding vireo pairs remaining. Their main
threat to survival is habitat modification. The brushlands they need for
nesting are often cleared for grazing and it is thought that grazing itself
can keep the vegetation from regenerating. In addition, the black-capped
vireo can be a victim of nest parasitism by the brown-headed cowbird, a
species which prefers disturbed habitats.
In this Project you will interpret some general vegetation characteristics
from remotely sensed material. Using this information plus your knowledge
of habitat requirements for the Golden-cheeked warbler and the Black-capped
vireo, you will be able to predict the occurrence of these endangered species
within the Eckhardt tract of the Balcones Canyonlands National Wildlife
Refuge. You will then compare your estimates to records of actual sightings
of these birds. The final product will be a report detailing this process
and your results. There are five main steps in this project that lead up
to your report:
Creating a base map
Determining Vegetation Type
Determining Canopy Cover
Determining Suitable Habitat for the Golden-cheeked Warbler and the Black-capped
Comparison of Expected Results with Actual Sightings
Since the Eckhardt tract is located in two counties, you will need to:
Download two county files in .dgn format from the Texas Natural Resources
Information Systems server.
Merge these files in Microstation
Using the portion of the USGS 1:24 000 topo sheet that is provided, digitize
the boundary of the Eckhardt tract.
There are several general types of vegetation communities within the
Eckhardt tract which are distinguishable from one another in the infra-red
(IR) photographs. You will need to consult both the March and September
photos to check your assessment. The vegetation types you will be using
are given below with a brief explanation of their physical characteristics
and their appearance in remotely sensed material.
Grassland--no trees and, therefore, no shadows. The grasslands will
probably appear bluish/greenish in the winter or spring; yellow during
the summer and early fall.
Savannah--very widely scattered trees in a grassland; very low %
Shinnery--a very dense thicket or shrubbery that is typically not
very tall. The predominant component of these on the Eckhardt tract is
red oak. A shinnery isn't really a "woodland" or a forest; to make your
way through one, you'd need a good machete. The % canopy cover appears
to be very high because the ground is covered with vegetation, but that
is misleading since the vegetation itself does not reach a great height.
Because of this, a shinnery is considered to have a "semi-open" canopy.
Thus, the distinctive mark of a shinnery in aerial photos is that it will
appear very "flat" and will cast no shadow.
Oak/Juniper Woodland--a mixture of juniper and various species of
oaks, including post oak, live oak, red oak and others. In the March 1995
aerial photos, the juniper are evident by their deep red color, which indicates
that they are photosynthesizing at that time of year. The "red" junipers
contrast nicely with the dull greens of the oaks (which are not photosynthesizing
yet). Notice the difference in colors between the September 1993 photos
and the March 1995 photos...can you guess why all of the vegetation is
red in September?
Cactus Patch--a mixture of prickly pear cactus and grassland. In
the March 1995 aerial photos, the prickly pear look like dark speckles
across a relatively open field. HINT: Look in the southern grasslands of
the Eckhardt tract.
To determine the percentage canopy cover of any given area in this exercise,
you will need a clear, plastic sheet that has a "dot grid." Lay this grid
over the aerial photo (this is why it should be clear) and determine what
percentage of each box is covered with trees, or vegetation of significant
height. For example, a grassland will have a value close to 0% , while
a savanna will have slightly more. You'll have the "eye" this percentage;
exact mathematical calculations are not necessary. This figure is the "percentage
In order to display your calculations, you will probably want to lump
the percentages into a category, such as "0%-20%" or "50-75%." Divide the
Eckhardt tract into the resulting categories (use polygons). Try to avoid
being too detailed--this map should only illustrate the general
variation of canopy cover across the tract. HINT: Based on what is written
in the vegetation section above, you might question the % canopy cover
you measure for a shinnery. You may choose to set off these "special" areas
with an asterisk or a footenote explaining why you measurements of canopy
cover might be suspect.
If you were to send a technician into the Eckhardt tract to look for
the golden-cheeked warbler and the black-capped vireo, in which direction
would you send them? Where on the tract would you expect to find these
two species? For this part of the exercise, you will need:
Using the habitat requirements listed earlier in this
exercise plus the information you illustrated in these two maps, identify
those areas where you would expect to find these species.
Your Vegetation Map
Your Map of Canopy Cover
Towards the last week of the exercise (after you have completed your
"expected" results) you will be given the results of your field technician's
observations. Digitize the "actual" results for both the black-capped vireo
and the golden-cheeked warbler into your Microstation file. Compare the
"results" to the "actual." How close were you?
You do not need to hand in a digital copy of your maps, so you will
have to determine your own guidelines for the organization of your file
(colors, levels, etc.) in Microstation.
You will be handing in a report, similar to that which you compiled
for the Texas Election Campaign Project. This report, however, is a professional
assessment of your work in locating two endangered species on a section
of the Refuge. The beginning of this report should contain a brief, to-the-point
executive summary of your work and a summary map. The rest of your
report should be a presentation of your methodology, results and some discussion.
Include whatever maps you feel are necessary to show the different elements
of your project, but please limit these to a reasonable number (up to 3
or 4). You will be graded solely on this report (including the maps) so
make sure it is representative of your effort.
Created 13 November 1995. ALB.
Last updated 25 January 2000. LNC.
* A great deal of the materials for this exercise were provided
by US Fish and Wildlife; many thanks for their generosity and extra special
thanks to Dr. Chuck Sexton. ---alb