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Computer Simulation of Wolf-removal Strategies for Animal Damage Control

Discussion


Where recovering wolf populations have expanded their range into areas near farms, wolf management goals may include maintaining wolves and reducing wolf depredation on livestock and pets (WDNR 1999, MDNR 2001, Mech 2001). Programs for reducing wolf depredation usually include prescriptions for wolf removal. Because the relative performance of removal strategies has not been evaluated, we developed a simulation model to evaluate and compare alternative prescriptions. Those prescriptions included reactive management, in which wolves were removed in summer from territories immediately after depredation occurred (similar to the existing program in Minnesota); preventive management, in which wolves were removed in winter from territories in which depredation had occurred at least once in the previous 5 years (similar to the proposed program in Minnesota); and population-size management, in which wolves were removed in winter from all territories surrounding farms regardless of current or previous depredation activity.

Four results emerged from the simulations that were largely robust to changes in assumptions about immigration, trapping success, and likelihood of packs engaging in depredation. First, by focusing wolf removal in territories near farms, each strategy substantially reduced depredation. Compared with a no-action strategy, single strategies reduced depredation by at least 40%, while combined strategies reduced depredation by at least 70%. Second, strategies that included preventive removal or population-size management removed fewer wolves than reactive management, primarily because removal occurred in winter before birth. Third, strategies that included population-size management were least expensive (in terms of compensation for lost animals and cost of wolf removal) because repeated annual application kept most of the territories around farms free of wolves. Finally, because wolf removal took place near farms and not in wild areas, none of the strategies threatened to extirpate populations unless populations were isolated (no immigration). In that case, population-size management caused a steady decline.

Although the wolf model accounted for some compensatory behavior between natural mortality and wolf removal, it likely underestimated the capacity of wolf populations to respond to exploitation. For example, the model predicted a sustainable yield of 20-25% of the wolves in foam territories under the population size control strategy, but maximum sustainable harvest rates of 30-50% have been estimated for free-ranging populations (Mech 1970, Gasaway et al. 1983, Peterson et al. 1984, Bollard et al. 1987, Larivière et al. 2000). The model likely overestimated natural mortality, which decreases when a wolf population is harvested (Peterson et al. 1984, Bollard et al. 1987, Mech 2001). Further, the model likely underestimated the number of breeding pairs in farm territories because the rate of adult capture was too high or the likelihood of surviving adults finding mates and colonizing territories was too low: As a result, none of the removal strategies may be as effective as the model suggested. Altering the model to increase population productivity would change the magnitude of the performance measures; however, the relative performance of the management strategies probably would not be affected.

Keeping this caveat in mind, the simulation results suggested strengths and weaknesses of each removal strategy. Reactive management, which has been used for animal damage control in Minnesota since 1978, could be relatively expensive because of the high cost of targeted removal and the large numbers of wolves removed in summer after pups are, born. However, because reactive management removes wolves only after depredation is confirmed in summer, more wolves can live near farms, and populations are more likely sustainable, especially when isolated.

Although our analysis included a strategy for population-size control, none of the state management plans have proposed such a strategy. Strategies involving population-size control, including public hunting or trapping seasons, will be considered after wolves are removed from the federal endangered species list. The population-size strategy we considered would be similar to implementing a public trapping season if trapping took place in winter and was limited to areas near farms. The simulation results suggested that such a strategy would be relatively inexpensive after repeated annual application because fewer wolves would live near farms and engage in depredation. However, we might have underestimated the administrative costs of public trapping, which could include law enforcement, public relations, and compensation. The simulation results suggested that population-size control in farm territories would not threaten a population that received a small number of immigrants, which are critical to the maintenance of exploited wolf populations (Fritts and Carbyn 1995, Larivière et al. 2000). Although the simulation results suggested that population-size management was not sustainable in isolated populations, it is well known that wolf populations can recover rapidly following cessation of intensive wolf removal (Fritts and Mech 1981, Peterson et al. 1984, Hayes and Harestad 2000). Furthermore, few if any wolf populations in the United States are isolated.

Preventive removal was a mild version of population-size control because wolves were removed near farms only when there had been a history of depredation. As a result, mole wolves could live near farms with more chances for depredation and higher cost. On the other hand, because fewer wolves were removed, preventive removal was less likely than population-size control to threaten the sustainability of isolated populations. It should be noted that the preventive removal strategy in our model was more specific about the timing of wolf removal than the preventive strategies described in the wolf management plans, which did not specify time of year when removals could take place. Our simulations suggested that removing wolves in winter before pups are born could reduce the number of wolves removed as well as reduce depredations.

The simulation results have implications for management plans that include the use of agricultural practices to reduce or prevent depredation. In both Wisconsin and Minnesota, management plans proposed depredation prevention activities as well as wolf removal. If effective prevention activities were discovered that could reduce the likelihood that packs near farms engaged in depredation, the simulation results suggested that any one of the removal strategies would nearly eliminate wolves with tendencies for depredation. Further, strategies involving preventive and reactive removal would allow relatively large populations to live near farms without removing many wolves.

Our simulation model represented a wolf population much smaller than the wolf population in the western Great Lakes region. The landscape in the model was bounded by the assumption that it could support a maximum of 64 pack territories in a landscape including farm and wild range. This scale of analysis was consistent with a small portion of the Minnesota wolf population on the frontier of its range or the smaller populations in Wisconsin and Michigan. Therefore, the simulation results should be viewed as predictions of the relative performance of alternative wolf-removal strategies applied to a small subset of the wolves in the western Great Lakes region.


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