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.
The 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 of species.
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 of species.
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 ecological understanding.
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 height.
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 Vireo
- Comparison of Expected Results with Actual Sightings
- The Report
Since the Eckhardt tract is located in two counties, you will need to:
- Download two county.dgn files 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 % canopy cover.
- 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 canopy cover".
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 10 July 1997. 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