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4. Estimating Carrying Capacity

Timothy E. Fulbright Texas A&M University Press ePub

4

Estimating Carrying Capacity

KEY CONCEPTS

▼ Carrying capacity is the number of animals per unit area that a habitat can support without degrading forage and other resources.

▼ The number of animals the habitat can support changes continually in time and space depending on availability of food, water, cover, and usable space.

▼ Nutritional-based estimates of carrying capacity should incorporate nutrient needs of free-ranging animals and production needs such as lactation, account for effects of antinutrition factors such as tannins, and include adjustments for habitat preferences.

▼ Forage- or nutritional-based models to estimate carrying capacity may provide values useful as a guideline for management, but values should be regarded as ballpark estimates and may be inaccurate. Management decisions regarding whether deer numbers exceed carrying capacity of the habitat are best made by monitoring level of utilization of important, or key, deer forages.

Carrying Capacity Defined

White-tailed deer population densities vary geographically. The Edwards Plateau region of Texas supports higher densities of white-tailed deer than any other rangeland area in the United States, with greater than 45 deer/km2 (see fig. 1.2; Quality Deer Management Association 2008). In contrast, much of the Great Plains region from Canada south to the Texas Rolling Plains supports fewer than 15 deer/km2. Deer densities in the Great Plains may be locally greater along riparian corridors and other wooded areas such as shelterbelts. Much of the Cross Timbers and Prairies and South Texas Plains supports 15 to 30 deer/km2. Although white-tailed deer were almost extinct by the 1970s because of poaching, habitat destruction, and screwworm infestation, an estimated white-tailed deer population density of 10 to 20 deer/km2 exists in northeastern Mexico (Villarreal G. 1999). These regional differences in deer densities result in part from regional differences in hunting pressure, geographic variation in human population densities, and numerous other factors, many of which may be beyond the control of wildlife managers. Carrying capacity can be enhanced and maintained by habitat management. Proper wildlife habitat management is based on the concept of carrying capacity and an understanding of the limitations of the concept. One of the limitations is that carrying capacity is conceptual rather than an absolute value.

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Appendix 4. Planting Summary for Selected Forages

Timothy E. Fulbright Texas A&M University Press ePub

Appendix 4

Planting Summary for Selected Forages

Sources of Information

The follow planting recommendations for selected plant species were compiled from several sources, including Heath, Metcalfe, and Barnes (1973); Vallentine (1980); Koerth and Kroll (1994); Fulbright (1999a); Redmon, Caddel, and Enis (n.d.); Texas AgriLife Research and Extension Center at Stephenville (http://stephenville.tamu.edu/topics/forages/forage-species/); Natural Resources Conservation Service, Conservation Plant Characteristics (http://plants.usda.gov/); Pogue Agri Partners Web site (www.pogueagri.com); and Turner Seed Company Web site (www.turnerseed.com). Information for individual traits was selected from a single source when recommendations varied among sources; local extension specialists should be consulted for recommendations in specific locations. Many different varieties are available for plants such as hairy vetch, oats, wheat, rye, triticale, cowpeas, and soybeans. Extension specialists or seed dealers should be consulted to determine the variety adapted to the locality where food plots will be planted.

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3. Ecological Principles Underlying Habitat Management

Timothy E. Fulbright Texas A&M University Press ePub

3

Ecological Principles Underlying Habitat Management

KEY CONCEPTS

▼ Current ecological theory should be the basis of predictions about the outcome of managerial actions.

▼ Vegetation and white-tailed deer response to management practices in rangeland environments may differ radically from those expected in humid environments where many management practices originated.

▼ Disturbance is a natural factor in ecosystems, and moderate levels of disturbance may be optimal for maintaining deer habitat. Past and contemporary land use practices have altered natural patch dynamics, resulting in the need for human-imposed disturbances such as prescribed fire.

▼ Plant species diversity results in greater ecosystem stability and resilience to disturbances such as drought and may provide for greater diet and nutritional stability for deer.

Importance of Theory in Habitat Management

All management practices are based on theory. This is an important fact for wildlife managers to appreciate and understand. For example, managers often plant food plots based on the theory that diet quality of deer feeding in the plots will improve. The anticipated outcome of most management practices is a prediction based on theory. A management plan is a model based on predictions about the outcome of management practices. Wildlife managers should have in mind a model to guide them in developing management plans. Ecological theory is the most important body of theory in wildlife habitat management. A basic understanding of ecological theory and its application is critical for habitat managers in developing management plans and in making decisions about implementing management practices.

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Appendix 3. Determining Adequate Sample Sizes

Timothy E. Fulbright Texas A&M University Press ePub

Appendix 3

Determining Adequate Sample Sizes

Sample size can be determined by iteration using the following equation (Zar 1996, 107):

n = (t2 × S2) ÷ d2 = (t × S ÷ d)2

In this equation, n is the estimated sample size; t is Student’s t with n – 1 degrees of freedom for a particular alpha; S is an estimated standard deviation (may be the sample standard deviation of an initial sample); and d is the half width of the desired (1 – alpha) 100 percent confidence interval.

If you are estimating biomass of vegetation and desire to estimate the true population mean with a 95 percent confidence interval no wider than 200 kg/ha, then d in the equation would be 100 kg/ha, and the t-value for α = 0.05 would be used for t. If the objective is to obtain an adequate sample that will detect a 10 percent change in vegetation parameters, such as biomass or cover, from one sampling period to the next, ()2 can be used for d, where x k = 0.10 and is the mean of the presample values (Bonham 1989).

Example: A researcher wants to determine carrying capacity of a ranch for white-tailed deer. The researcher obtains an initial sample consisting of twenty 1 m2 sampling frames in which the standing crop of forbs and browse are clipped, oven dried, and weighed. The mean weight is 1,000 kg/ha, and the sample standard deviation is 800 kg/ha. The researcher desires a 90 percent confidence interval with width of 200 kg/ha to estimate the population mean. The following is the initial equation to obtain an estimate of the adequate sample size:

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8. The Gun: Harvest and Management Planning

Timothy E. Fulbright Texas A&M University Press ePub

8

The Gun: Harvest and Management Planning

KEY CONCEPTS

▼ Maintaining deer populations within the carrying capacity of the habitat should be the primary goal of harvest management.

▼ Management decisions regarding whether deer numbers exceed or are below carrying capacity of the habitat are best made by monitoring utilization of key deer forages and monitoring trends in deer body mass, antler development, and fawn survival.

▼ Establishment of a management goal is important, whether managing for trophy males or for maximum-sustainable-yield harvest.

▼ Developing a sound management plan and keeping records of the number, age, sex, body mass, and antler dimensions of harvested deer are important aids in meeting management objectives.

Harvest as a Habitat Management Tool

Aldo Leopold (1933) asserted that game can be “restored” by hunting. Our primary emphasis in this chapter is use of hunting as a tool to maintain deer densities within carrying capacity of the habitat. Maintaining deer populations within carrying capacity allows the most preferred plant species in the habitat to reproduce, affords maximum protection to other resources, and benefits all organisms in an ecosystem. We recommend managing populations based on our forage-based definition of carrying capacity, which results in densities lower than K-carrying capacity, commonly used as the basis for modeling deer population growth and harvest management. Much of the theory underlying harvest management, including the concepts of density dependence, density independence, and compensatory mortality, is based on K-carrying capacity.

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