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By Zoë Hanley
Photo 1. A view of mountainous, forested terrain from a public grazing allotment in the Cascade Mountains of northern Washington. Photo credit: Zoë Hanley
Livestock depredation (predation on domestic animals) by carnivores is one of the primary causes of human-carnivore conflict worldwide. Many carnivores have large home ranges and specialize in ungulate predation; consequently, some carnivores kill domestic ungulates when the opportunity arises. Human encroachment into wild areas, with activities such as livestock grazing, increases the occurrence of human-carnivore conflicts and retaliatory killing of carnivores. Considering many carnivore populations are in global decline (Ripple et al. 2014), minimizing conflict is important to preserving biodiversity and healthy ecosystems.
The gray wolf is a wide-ranging carnivore with a considerable history of conflict with humans, primarily over livestock. Bounties and a federally-funded extermination program – largely fueled by the livestock industry – led to wolf extirpation in the contiguous United States by the 1930s. However, a surge of public support for gray wolf recovery and the Endangered Species Act protection of 1974 opened the door to wolf reintroductions and recolonization in the northwest, southwest, and Great Lakes regions of the United States. Gray wolves recolonized parts of their historic range in the north-western United States through natural dispersal from Canada and reintroductions (Yellowstone National Park and central Idaho) in the mid-1990s. The first gray wolf pack since the 1930s was confirmed in Washington in 2008, and wolves are currently classified as endangered under Washington state law (Washington Department of Fish and Wildlife et al., 2018).
On public lands in the western United States, conflict between humans and wolves can occur when livestock are in “grazing allotments.” Grazing livestock on allotments began in 1905, when the federal government began issuing grazing permits to limit the number of cattle and sheep on federal lands, and with the Taylor Grazing Act of 1934, which established a federal grazing allotment system that is still in use today (Pub.L. 73e482; Bureau of Land Management, 2011). Livestock is placed or “turned out” on grazing allotments generally from May through October, increasing opportunities for wolf-livestock interaction during the wolf denning and pup-rearing periods, which last from April through September in the north-western United States. Identifying grazing allotments at risk to livestock depredation by wolves can support the placement of proactive measures to minimize conflict, benefiting both livestock interests and wolf recovery efforts.
Prior to this study, no regional or state-wide studies in the western United States had attempted to predict or map cattle depredation risk by wolves on grazing allotments. Our primary goal was to assess depredation risk for cattle grazing allotments in Washington. However, to date, there have been too few wolf packs and depredation events in the state to model risk for Washington alone. Therefore, we incorporated data from adjacent states which had larger wolf populations over a longer period of time. Our final risk models represented wolf-livestock depredation on grazing allotments in Idaho (2004 – 2008), Montana (1991 – 2008), and Washington (2008 – 2016, Figure 1).
Figure 1. Study area including livestock grazing allotments in Idaho, Montana, and Washington, USA used in wolf-cattle depredation risk models and prediction maps for Washington. Image republished from open-access article with permission from Science Direct: https://doi.org/10.1016/j.gecco.2018.e00453
We asked, “Which factors are most commonly associated with cattle depredation risk by wolves on grazing allotments?” The list of potential candidates broadly included wolf demographics, cattle abundance, dates of use, terrain characteristics, and land cover types. We found that depredation risk and the predicted number of cattle killed on a grazing allotment increased for grazing allotments with higher cattle and wolf abundance, indicating that more individuals of either species in a grazing allotment amplified the probability of depredation occurring.
Now that we understood which factors increased risk, we used the model to map depredation risk for grazing allotments in Washington. Since wolf packs do not currently overlap with all grazing allotments in the state, we assumed pack sizes of five or ten wolves to forecast what the depredation risk would be if wolf packs’ territories overlapped with all cattle grazing allotments. Figure 2 shows that, for pack sizes of five or ten wolves, 10% or 15% of cattle grazing allotments in Washington were forecasted as cattle depredation hotspots (>60% depredation risk). At the time of this study, the average wolf pack size in Washington was five wolves.
Figure 2. Predicted cattle depredation risk for cattle grazing allotments in >40% probability of wolf habitat occurrence (Maletzke et al., 2016) in Washington, USA. Colors categorize risk in five equally-sized bins for (A) a pack size of five wolves and (B) a pack size of ten wolves. Gray allotments were either vacant or data were unavailable to calculate cattle depredation risk. Image republished from open-access article with permission from Science Direct: https://doi.org/10.1016/j.gecco.2018.e00453
Areas identified as cattle depredation hotspots were primarily in the Northern Cascades and Eastern Washington Wolf Recovery Regions (Figure 2). To date, most livestock depredations by wolves in Washington have occurred in the Eastern Washington Recovery Region (Washington Department of Fish and Wildlife et al., 2018). However, our top model only identified two (22%) of the grazing allotments in Washington with verified cattle depredations from 2008 to 2016 as depredation hot spots, suggesting that important factors to answering this question were missing from our analysis. Information such as where wolves were denning, livestock husbandry practices, and ungulate prey abundance would help improve these risk models and produce a more accurate map of cattle depredation risk by wolves on grazing allotments. These data points are rarely collected consistently, and many times are not documented at all. Therefore, it is important for wildlife and range management agencies to record this information consistently over time in an effort to improve our understanding of the factors that influence depredation risk and identify areas in which preventative measures would be most effective.
We also conducted a similar analysis of livestock depredation risk in wolf pack territories. The results of that study, Forecasting cattle depredation risk by recolonizing gray wolves, were published in Wildlife Biology in July 2018.
These findings are described in the article entitled Cattle depredation risk by gray wolves on grazing allotments in Washington, recently published in the journal Global Ecology and Conservation.
Bureau of Land Management, 2011. The Taylor Grazing Act
http://www.blm.gov/wy/st/en/field_offices/Casper/range/taylor.1.html (2011), Accessed 2nd Feb 2017
Ripple, W.J., J.A. Estes, R.L. Beschta, C.C. Wilmers, et al. 2017. Status and ecological effects of the world’s largest carnivores. Science 2014. 343(6167): 1241484.
Washington Department of Fish and Wildlife, Confederated Colville Tribes, Spokane Tribe of Indians, USDA-APHIS Wildlife Services, and U.S. Fish and Wildlife Service. 2018. Washington Gray Wolf Conservation and Management 2017 Annual Report. Washington Department of Fish and Wildlife, Wenatchee, WA, USA.
2014), minimizing conflict is important to preserving biodiversity and healthy ecosystems.