Deer Options Enterprise
White-Tailed Deer Biology and Ecology
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Impact of Wolves on Deer in Wisconsin.
by Ronald N. Schultz, Keith R. McCaffery, and Adrian P. Wydeven
Wisconsin Department of Natural Resources
Many hunters continue to be concerned about the impact wolves may have on deer populations. During fall 1997 hunters became aware of the lower deer numbers across northern Wisconsin, and some blamed the deer decline on the increasing wolf population. The severe winters of 1995-1996 and 1996-1997 were the main factor that caused the deer decline across northern Wisconsin. Because such deer declines do create concerns over the impact of wolf predation, careful monitoring of wolf and deer populations will continue to be important aspects of management for both species.
Winter mortality is the main factor affecting deer numbers in northern Wisconsin. (Figure E1) During winter 1995-96 as many as 170,000 deer died in northern Wisconsin due to harsh winter weather. In the 1996-97 winter another 70,000 may have died. Winter Severity Indices correspond to severe winters and declines in the deer population.
There have been a few cases where wolves have limited ungulates (hooved mammals) to low population densities (Mech and Karns 1977; Gasaway et al. 1992). Generally such wolf impact would occur when ungulate populations are also stressed by severe winters, habitat deterioration, and/or overharvest. Fuller (1990) monitored a deer herd decline in Minnesota wolf range that went from 28 to 10 deer per square mile, but wolves accounted for only 10% of the deer mortality. Mech (1984) indicated that wolves rarely limit deer populations. Deer populations would normally need to be reduced to fewer than 3 deer/mi2 for wolves to limit growth of the deer population (Mech 1984). Generally wolf predation is not a major mortality factor to deer populations until deer densities drop to fewer than 10 deer/mi2 (Wydeven 1995). Deer densities of fewer than 10 deer/mi2 occur infrequently in Wisconsin.
Wolves in the Great Lakes region normally consume 15-18 deer per wolf per year (Fuller 1995). At a rate of 18 deer per wolf pack per year an average Wisconsin wolf pack of four wolves on a 70-square mile territory would consume about 72 deer or about 1 deer per square mile. Wisconsin's wolf population in 1999 consisting of about 200 wolves probably consumed 3,000 -3,600 deer. The total 1998 harvest within the central and northern forest zones where wolves occur was 112,936 by firearm hunters, 29,266 by bow hunters and another 10,000 by motor vehicles.
Mortality due to wolves occurs year round which is much different than hunting mortality which is compressed into one season and has less effect on herd dynamics and hunter opportunity, because some wolf predation is compensatory.
The projected potential wolf population in Wisconsin could be 300-500 wolves (Appendix C). At a rate of 18 deer per wolf year, wolves would annually remove 5,400-9,000 deer. This rate of wolf predation would occur across 6000+ square miles, therefore would consist of 0.9 to 1.5 deer per square mile. Deer population density over winter across this region would generally range from 10 to 25 deer per square mile
The overall deer population and deer density were compared for 4 deer management units with wolves and 4 deer management units without wolves across northern Wisconsin (Table E1). Population fluctuations were relatively similar across deer management units with or without wolves. Deer density was slightly more in units without wolves than units with wolves, but the results were not statistically different (t-test P>0.10). The over winter management goals for the units with wolves is 18.7 deer per square mile. The management goals for the units without wolves is 21.3 deer per square mile. These goal differences reflect habitat and climatic effects unrelated to wolves. It appears that habitat and climatic effects have greater impacts on deer population trends than wolf predation.
Furthermore, the average rate of herd increase from post-harvest to subsequent pre-harvest (1981-1997) was 1.33 for units without wolves and 1.31 for units with wolves which shows similar recruitment (net increase in herd size) in both sets of management units.
Overall it does not appear that wolves are likely to be a major mortality factor to deer in northern Wisconsin under current conditions or in the near future. Even with a population of 500 wolves, annual predation of 9000 deer would represent only 2.6% of the overwinter population of 343,000 deer in the Northern Forest and Central Forest. The area has an average fall population of about 450,000. Much of the predation by wolves would probably compensate for other natural mortality because it occurs year-round. A large proportion of northern Wisconsin deer die from natural causes, which can vary drastically depending on severity of winter (Creed et al. 1984). Wolves would probably remove some of these animals that would die from other causes. A deer killed by wolves won't be killed by winter stress or other mortallities.
Wolves may also displace other predators such as coyotes (Peterson 1995); under some circumstances coyote predation may have more of an impact on deer populations than wolves (Mech 1984). The current deer management system in Wisconsin adjusts antlerless deer harvest in individual deer management units by limiting the number of hunter choice permits per unit (VanderZowen and Warnke 1995). This system should be able to adequately adjust for the impacts of wolf predation in deer management units. Generally, wolf predation would have very limited impact on the number of hunter-choice permits issued, or the overall deer harvest within specific management units.