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Computer Simulation of Wolf-removal Strategies for Animal Damage Control

Introduction


As a result of human tolerance, reintroduction, and natural repopulation, gray wolves (Canis lupus) have now recolonized parts of Europe and the United States (Promberger and Schroeder 1993, Fritts and Carbyn 1995, Mech 1995). As wolf populations increase and expand their ranges, local decision makers must choose management strategies that balance competing demands for wolf protection and animal damage control (Mech 2001). Wolf management planners in the western Great Lakes states (i.e., Mich., Minn., Wis.) face these conflicting demands. Since the gray wolf received legal protection in 1974; the Minnesota population grew from <1,000 wolves to 2,450 wolves in 1997-1998 and expanded its range from <40,000 km² in the northeast to 90,000 km² in the northern and central parts of the state (Fuller et al. 1992, Berg and Benson 1999). Wolves recolonized Wisconsin and upper Michigan in the late 1970s, and populations in each state exceeded 200 wolves in 2000 (United States Fish and Wildlife Service [USFWS] 2000). As a result, in 1999 wolf numbers and distribution exceeded the goals identified in the recovery plan for the western Great Lakes population (USFWS 2000). In addition, each state adopted a wolf management plan with the primary goal of ensuring the long-term survival of the wolf (Michigan Department of Natural Resources [MDNR]) 1997, 2001; Wisconsin Department of Natural Resources [WDNR] 1999). In July 2000 the USFWS proposed that the gray wolf be reclassified from endangered to threatened throughout the western Great Lakes region and considered proposing its removal from the federal list of endangered and threatened species (USFWS 2001a). Such delisting would give most legal responsibility for wolf management to state and tribal authorities.

Concurrent with increasing wolf numbers in the western Great Lakes states, wolf range expanded into areas with farms and wolf depredations on livestock, poultry, and pets increased. For example, annual cases of depredation increased from 29 farms in Minnesota in the 1980s to 71 farms in the 1990s (Michigan Department of Natural Resources 1997; Minnesota Department of Natural Resources [MDNR] 2001; Wisconsin Department of Natural Resources [WDNR] 1999). To address wolf depredation, the Minnesota and Wisconsin management plans proposed detailed animal damage control programs that added to or enhanced current federal regulations (WDNR 1999, MDNR 2001). Each program divided the state into management zones and defined wolf-control guidelines that depended on habitat and the potential for conflicts with humans.

The relative performances of prescriptions alternative to Minnesota's current animal damage control program (Mech 1998) have not been evaluated. As a result, here we compare 3 types of wolf-removal strategies. Reactive management (wolves removed in summer from farms immediately after depredation occurs) and preventive management (wolves removed in winter from firms in which depredation had occurred at least once in the previous 5 years) are similar to the depredation control measures proposed in management plans, whereas population-size management (wolves removed in winter from all territories surrounding farms, regardless of depredation activity) operates at the population level. We analyzed these removal strategies, applied alone and in combination, using a stochastic, spatially structured, and individually based simulation model of a hypothetical wolf population composed of up to 64 packs in a region with farm and wild areas. Simulations compared removal strategies in terms of occurrence of depredation, wolves removed, population size, and costs. Simulations also included sensitivity analyses with respect to assumptions about wolf immigration, trapping success, and probability that packs become prone to depredation.


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