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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. We define carrying capacity as the number of animals per unit area that can be supported on a sustained basis without degrading vegetation and other vital resources such as water and usable space. Incorporating the importance of trends in vegetation biomass and forage use in the concept of carrying capacity is important because a decline in vigor and Estimating Carrying Capacity 4 K e y C o n c e p t s ▼ 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. 110 chapter 4 ▼ productivity of deer forages signals that herbivore densities exceed carrying capacity. Deer management should be based on maintaining animal densities at levels that allow continued or increased productivity, reproduction, and diversity of the vegetation, including the most palatable plants. Forage production is one of a variety of factors affecting the number of deer an area can support. Other herbivores such as cattle, jackrabbits, and exotic ungulates may consume the available forage, reducing the number of deer the habitat can support. The amount, availability, and quality of cover, usable space, and water also may strongly influence the number of deer that a habitat supports. In fragmented habitats, carrying capacity is influenced by the size of habitat patches, distance between patches, and patch productivity (Walters 2001). Some definitions of carrying capacity take factors such as behavior, water, and cover into account (Krausman 2002). However, forage-based concepts of carrying capacity provide the most practical conceptual basis for deer management because available forage directly influences population trends. K-carrying capacity is commonly used as the basis for modeling deer population growth and harvest management (Miller and Wentworth 2000). McCullough (1984, 219) defined K-carrying capacity as “the maximum number of animals an environment will support on a sustained basis (i.e. without destruction of the vegetation).” This concept of carrying capacity differs from our forage-based concept because at K-carrying capacity, productivity declines to zero and body condition is poor because forage resources have been depleted. Mysterud (2006) suggested that overgrazing does not necessarily occur if K is exceeded because the plant community may be adapted to intense grazing pressure. We recommend managing for population densities lower than K-carrying capacity, however, because changes in vegetation composition or biomass usually occur before deer populations reach K. Another way of conceptualizing carrying capacity that has possible theoretical application in deer management is based on Mott...

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