The Ecological Relationship of Gray Wolves and White-tailed Deer in Minnesota
Prepared by Glenn D. DelGiudice, Ph.D., Forest Wildlife Populations Research Group, Minnesota Department of Natural Resources, as an unpublished “white paper” prepared for the Minnesota Wolf Management Roundtable.
© 1998 Minnesota Department of Natural Resources, State of Minnesota – 10 June 1998
To best understand and appreciate the relationship of gray wolves (Canis lupus) and white-tailed deer (Odocoileus virginianus) in Minnesota today, information concerning their evolutionary backgrounds is helpful. Modern white-tailed deer have been evolving for about 1 million years (since the Pleistocene epoch). However, artiodactyls (even-toed ungulates or hooved animals) from which they evolved have existed for at least 58 million years (Kulp 1961). Similarly, wolves have existed in their present form for 1-2 million years, but evolved from flesh-eating ancestors that occurred some 45-55 million year ago (Matthew 1930). Because wolves have ranged over most of the Northern Hemisphere, a greater area than inhabited by any of their prey, primary species in the wolf’s diet, has and still, depends on geographic location (Mech 1970). Today in the Great Lakes region, and in Minnesota specifically, white-tailed deer are the primary prey of wolves, with moose (Alces alces) typically being of secondary importance where they both exist (Stenlund 1955, Mech and Frenzel 1971, Van Ballenberghe et al. 1975, Fritts and Mech 1981, Fuller 1989). Clearly, white-tailed deer and gray wolves have helped hone morphological and behavioral adaptations in each other for at least 1 million years of coevolution (Nelson and Mech 1981).
Herein, the objectives of the Minnesota Department of Natural Resources (MNDNR) are to (1) provide a general historical perspective for gray wolves and white-tailed deer in Minnesota and (2) highlight research findings specifically concerning wolf-deer interactions and aspects of each species’ ecology pertinent to those interactions.
Wolf and Deer Populations
White-tailed deer.–White-tailed deer range throughout Minnesota and are considered the state’s most valued big game animal. Recent estimates of the state’s pre-fawning (spring) population have exceeded 600,000 deer, with fall estimates reaching as high as 1 million deer (MNDNR 1990). White-tailed deer were not always so abundant. Prior to European settlement (before 1860), deer were most common in the hardwood forests of central and southeastern Minnesota and were relatively rare in the pine (Pinus spp.) forests east and north of the Mississippi River (MNDNR 1990). Two principal factors positively impacting the growth of the deer population have been (1) alterations of their habitat by timber harvesting, fire, and agriculture; and (2) increasingly intense management directly by manipulation of the population’s sex and age composition (through hunting) and indirectly via habitat management programs.
The first big game law affecting deer in Minnesota was established in 1858, although enforcement was likely difficult (MNDNR 1974). Initial management attempts at estimating the annual deer harvest and written records date back to 1918. The methodology for estimating annual harvests and hunter success rate gradually improved, so that by 1956 a statistically valid sampling method was adopted (MNDNR 1974).
Over the years, deer population trends have varied around the state. By 1920, deer had become fairly common in northern Minnesota, but they were nearly eliminated from the prairies due to farming and subsistence hunting (MNDNR 1990). Since the 1950s, documented fluctuations in the state’s deer populations have been attributable to changes in winter severity, habitat, hunting, and predation primarily by wolves. A pronounced decline in deer numbers and the harvest in northern Minnesota occurred in response to a series of severe winters, over-harvests (either sex seasons), and overly mature habitat during the late 1960s and early 1970s (MNDNR 1974, 1990; Mech and Karns 1977). Deer populations across eastern North America similarly experienced dramatic decreases, irrespective of whether the deer were impacted by wolf predation or hunting (Voigt 1990). Minnesota’s 1971 deer hunting season was closed, a practice which had been exercised intermittently by the state in the past.
Minnesota was divided into Deer Management Units and Permit Areas (subunits) following the closed season of 1971. The Permit Areas allow a more refined level of local deer management, specifically with respect to setting goals and managing the annual harvest. Further, the MNDNR changed from earlier either-sex hunting to a limited harvest of females. From 1976 to 1995, the state’s deer population increased rather steadily with record harvests being documented (MNDNR 1974, 1997). Relatively high losses of deer were observed in the state’s northern forests during the historically severe winter of 1995-96, and evidence indicated low reproductive success during the subsequent spring (DelGiudice 1996, 1998; Nelson, personal communication). Mortality rate was markedly lower during winter 1996-97, despite weather conditions nearly as severe (G. D. DelGiudice, unpublished data; M. E. Nelson, personal communication). Followed by an historically mild winter (1997-98) and an expect ed relatively high reproductive success during the 1998 fawning season, initiation of recovery of the northern deer population is imminent.
Timber Wolves.–Today, wolves do not range statewide, and their numbers have followed a trend different from that of deer. Further, the factors directly responsible for the changes in their numbers are somewhat different from those affecting deer. Historically, wolves occupied all habitats of Minnesota, ranging from prairies to forests (Young and Goldman 1944, Mech and Frenzel 1971). As early as 1849, the state began paying bounties for wolves, and by 1900, wolves were rare in southern and western Minnesota (Herrick 1892, Surber 1932, Fuller et al. 1992). As both forests and wolves were harvested aggressively, wolf numbers and range continued to diminish, despite increases in some of the local deer populations. By 1941, the highest wolf densities (estimated 39 wolves/1,000 km 2 [386 mi 2], Olson 1938) occurred in the northern third of Minnesota (Surber 1932, Fuller et al. 1992). Based on the field research of Stenlund (1955) in northeastern Minnesota and MNDNR records of bounty rates in northwestern Minnesota, Fuller et al. (1992) estimated that during 1950-1952, the wolf population was roughly 430-636 for the 31,080 km 2 (12,000 mi 2) of “major wolf range and 450-700 wolves when areas of occasional wolf observations were included. The average number of wolves submitted annually for bounty was 253, not including individuals collected by MNDNR personnel (MNDNR files, Fuller et al. 1992). The average annual number of wolves submitted for bounty decreased to 186-189 during 1953-1965, and a gross estimate of the population was 350-700 wolves, restricted primarily to the northeastern portion of the state (MNDNR files, Cahalane 1964).
Bounties for wolves and all predator species were discontinued in 1965, and in 1966, wolves were classified by the federal government as an “endangered species, but legal harvests continued through 1973. Harvests included wolves trapped by a MNDNR Directed Predator Control Program which concentrated efforts in areas of livestock depredation (Fritts 1982, Fuller et al. 1992). By 1970, the estimate of wolves was 750, changing little since the 1953-1965 estimate (Leirfallom 1970, Nelson 1971, Fuller et al. 1992), and they were assigned full protection in the Superior National Forest (Van Ballenberghe 1974).
In 1974, wolves were fully protected statewide by the Endangered Species Act of 1973. Subsequent estimates of wolf numbers were 500-1,000; 1,000-1,200; and 1,235 in 1973, 1976, and 1979, respectively (Mech and Rausch 1975, Bailey 1978, Berg and Kuehn 1982). The latter estimate was associated with a major or “primary wolf range of 36,500 km 2 (14,093 mi 2) and a “peripheral range of 55,600 km 2 (21,467 mi 2) (Berg and Kuehn 1982).
Wolves in Minnesota were reclassified as “threatened in 1978 (U.S. Fish and Wildlife Service 1978 from Fuller et al. 1992), and occupied wolf range had increased to 57,050 km 2 (22,027 mi 2), nearly double the estimate for 1950-1952 (Fuller et al. 1992). Most wolves occurred in areas where there was less than 0.70 km roads/km 2 and less than 4 humans/km 2 (Mech et al. 1988 b). An extensive survey during winter 1988-89, estimated an occupied wolf range of at least 53,100 km 2 (20,502 mi 2), excluding approximately 8,000 km 2 (3,089 mi 2) within this contiguous range (Fuller et al. 1992). These authors identified an additional 11,500 km 2 (4,440 mi 2) of potential range. Using two different approaches, Fuller et al. (1992) estimated the population of winter 1988-89 at 1,521 and 1,710 wolves. The survey of 1988-89 was repeated during winter 1997-98 with far more observers and technology, and although the data are not yet analyzed, preliminary evidence indicates that wolf numbers have increased and range expanded (Paul 1997).
Wolf Territories, Deer Movements and Distribution, and Predation Risk
Olson (1938), Murie (1944), and Young and Goldman (1944) pioneered ecological study of wolves and their prey. The earliest studies of the relationship between wolves and white-tailed deer in Minnesota and Wisconsin were conducted by Olson (1938), Thompson (1952), and Stenlund (1955). Including and since that time, most of the research information concerning wolf-deer interactions has been generated from studies conducted in what is considered primary wolf range of northern Minnesota. The most long-term and diverse ecological data have been generated from research conducted in northeastern Minnesota (Mech and Frenzel 1971; Hoskinson and Mech 1976; Mech 1977 a,b,c, 1994; Mech and Karns 1977; Rogers et al. 1980; Nelson and Mech 1981, 1986a,b, 1991; DelGiudice et al. 1991; Kunkel and Mech 1994), followed by northcentral (Berg and Kuehn 1980, 1982; Fuller and Snow 1988; Fuller 1989, 1990, 1991; DelGiudice 1995, 1996, 1998; DelGiudice and Riggs 1996) and northwestern Minnesota (Fritts and Mech 1981). Overall, these regional studies have illuminated both strong similarities, as well as quantitative variation of different aspects of wolf and white-tailed deer ecology and the interactions of these 2 species.
The territorial nature of wolves has been thoroughly documented. In Minnesota, their territories range from 52 to 555 km 2 (20-214 mi 2) and remain relatively stable throughout the year with no distinctly different winter and summer ranges (Mech 1972, 1973, 1974, 1977 a; Van Ballenberghe et al. 1975; Fritts and Mech 1981; Fuller 1989). The wide variation in territory sizes is in part linked to prey density. Wolf pack territories are smaller as deer become more numerous (Fritts and Mech 1981, Fuller 1989). Further, territory size of a newly formed pack may be large, but decreases have been observed once such packs become established in an area (Fritts and Mech 1981). Wolf packs tend to be discriminatory in the use of their territory, using some portions more than others; this appears to be related to differences in physiography and deer densities, as well as proximity of neighboring wolf pack territories (Ho skinson and Mech 1976; Mech 1977 a,b,c; 1994; Fritts and Mech 1981). With respect to the latter, evidence has indicated that boundaries of territories of neighboring packs commonly overlap by an estimated 6.4 km (4.0 mi) (3.2 km inside and outside estimated territorial borders, Mech 1994). This area, referred to as a “ buffer zone, may be contested by neighboring packs, and consequently receives less use by the wolves of these packs (Peters and Mech 1975; Hoskinson and Mech 1976; Mech 1977 a,c, 1994; Fritts and Mech 1981; Nelson and Mech 1981). Indeed, the inter-pack strife associated with buffer zones may be so serious that a wolf’s “risk of a fatal encounter with neighboring packs may be notably increased (Mech 1994).
In contrast to wolves, most white-tailed deer in northern Minnesota, up to 85%, are seasonal migrators with a high fidelity for distinctly different winter and spring-summer-fall home ranges (Hoskinson and Mech 1976; Nelson and Mech 1981,1987; Fuller 1990; DelGiudice 1993). Observed spring and fall migration distances of deer have ranged from 3 to 37 km (2-23 mi), with most being 8-16 km (5-10 mi) (Hoskinson and Mech 1976, Nelson and Mech 1981, DelGiudice 1993). Long-term research collected in northeastern Minnesota has shown that deer are particularly vulnerable to predation by wolves during fall migration to winter yards (Nelson and Mech 1991). Duration of this migration is typically brief (average of 4.5 days), but daily mortality of deer has been disproportionately high. Specifically, study deer spent approximately 1% of the year migrating, but deer killed by wolves during this time comprised 21% of all kills. The reasons for this increased vulnerability are n ot fully understood, but changing weather conditions, less familiar terrain, and other factors have been proposed as contributing factors that warrant further study. Aggregations of deer and extensive trail networks once in the winter yard assist them in their defense against wolf predation (Nelson and Mech 1981, 1991). Higher winter mortality for male and female yearlings and adult females that did not yard compared to those that did has been reported (Fritts and Mech 1981, Nelson and Mech 1981). Further, several studies have observed a preponderence of deer-kills by wolves along the edges of wintering yards and more adult male than female deer killed by wolves during winter, which may be related to differences in the spatial distribution of the adults of the sexes in the yards (Stenlund 1955, Pimlott et al. 1969, Mech and Frenzel 1971, Mech and Karns 1977, Fritts and Mech 1981, Nelson and Mech 1986 a). All of this suggests that yarding is, at least in part, an antipredatory behavior not simply a strategy strictly linked to nutritional and thermal benefits.
Size of seasonal home ranges of deer can vary markedly (Kohn and Mooty 1971, Hoskinson and Mech 1976, Nelson and Mech 1981). Winter home ranges are the smallest with reported average sizes ranging between 18 and 36 ha (44-89 ac) for all sex and age classes (Nelson and Mech 1981; G. D. DelGiudice, unpublished data). Following spring migration, widely dispersed summer home ranges have averaged 83, 109, and 319 ha (205, 269, and 788 ac) for adult females, yearling bucks, and adult bucks, respectively (Nelson and Mech 1981). Does with fawns reduce their home range size by as much as 65% after giving birth (e.g., from 108 to 38 ha [267 to 94 ac], Nelson and Mech 1981). The wide dispersion of does prior to fawning and the reduced home range sizes during the fawning season appear to be antipredatory strategies—newborn fawns attempt to avoid predation by hiding not fleeing. In northeastern Minnesota, fall home ranges of bucks increased significantly to 225 and 749 ha (556 and 1,850 ac) for yearlings and adults; ranges of adult females tended to increase, but differences were not significant (Nelson and Mech 1981).
Interestingly, preliminary evidence has suggested that the territorial buffer zones of wolf packs may have real significance for the long-term persistence of deer populations in Minnesota’s forests. Studies of radio-collared deer in primary wolf range have shown that most of their seasonal home ranges were located at the boundaries of juxtaposed territories of wolf packs, where, as discussed above, inter-pack strife is greatest and wolf activity tends to be less (Peters and Mech 1975; Hoskinson and Mech 1976; Mech 1977 a,b, 1994; Fritts and Mech 1981; Nelson and Mech 1981). Deer home ranges in these buffer zones may serve as “reservoirs of deer when the combined influence of several factors (e.g., severe winters, wolf predation, hunting, maturing habitat–Mech and Karns 1977) causes the decline of a local deer population.
Wolf Predation of Deer and Food Habits of Wolves
Across northern Minnesota, hunter harvest and wolf predation are the primary causes of mortality of white-tailed deer (Hoskinson and Mech 1976; Nelson and Mech 1981, 1986 a,b; Fuller 1990; DelGiudice 1994, 1998; DelGiudice and Riggs 1996). However, it is important to note that the relative importance of these 2 causes of mortality vary among years and local deer populations, dependent upon differences in hunter pressure, winter severity, wolf and deer densities, and sex and age composition of the deer population. It has been well documented in studies across northern Minnesota that wolves primarily kill young of the year and older deer in the population, and these individuals are often compromised by deterioration of their overall physical condition or a specific abnormality (Mech and Frenzel 1971, Fritts and Mech 1981, DelGiudice 1996, DelGiudice and Riggs 1996). In northeastern Minnesota, the average age of deer (both sexes) killed by hunters was 2.6 years compared to 4.7 years for deer killed by wolves during December-March (1966-1969) (Mech and Frenzel 1971). Similarly, in northwestern Minnesota, the average ages of deer (both sexes) killed by hunters and wolves were 3.3 and 7.6 years, respectively (Fritts and Mech 1981). In north-central Minnesota, the median ages (half the cause-specific mortality occurred by this age) of female deer killed by hunters and wolves were 2.5 and 6.5 years, respectively (DelGiudice 1996).
Wolf predation on deer is greatest during mid-late winter, coinciding with the period of poorest condition and deepest snow (Mech and Frenzel 1971, Mech 1977 b, Moen and Severinghaus 1981, DelGiudice et al. 1992, DelGiudice 1998), then again during the fawning period of early summer when neonates are vulnerable prey to wolves (Pimlott et al. 1969; Mech et al. 1971; Fritts and Mech 1981; Nelson and Mech 1981, 1986 a,b; Fuller 1989; Kunkel and Mech 1994) and other predators (see Fuller 1990; Dickson 1992; L. L. Rogers, unpublished data). For the period 1973-1984, Nelson and Mech (1986 a) reported an annual mortality rate of radio-collared deer (both sexes) attributable to wolves of 0.16-0.19 (i.e., 16-19%), except for yearling females which experienced a rate of 0.05. In a study of wolves and deer in north-central Minnesota (Bearville study area north of Grand Rapids), Fuller (1990) documented an annual wolf-caused mortality rate of 0.04 for radio-collared deer >/=1.0 year old (both sexes) during the period 1981-1986. From 1991 to 1997, DelGiudice (1996) observed annual, wolf-caused mortality rates of 0.09-0.26 for radio-collared, female deer (primarily >/=1 yr old). In eastern Ontario, Kolenosky (1972) documented a wolf-caused mortality rate of 0.09-0.11 when snow depths were fairly deep. The variation of mortality rates of deer from area to area and year to year are primarily attributable to variations in deer:wolf ratios, winter severity, presence of alternate prey (e.g., moose), and the sex and age composition of the deer populations. Of note, wolf predation rates on deer appear to be higher in areas with lower deer:wolf ratios (Kolenosky 1972, Nelson and Mech 1981, Fuller 1990). Moreover, there is good evidence that pairs of wolves in a given area have higher winter kill rates of deer per wolf than packs of wolves (>/=3 members), suggesting a low optimum pack size for maximizing energy to individual members (Fritts and Mech 1981, Schmidt and Mech 1997).
Severe winter weather negatively impacts survival of northern white-tailed deer either through nutritional restriction when predators are scarce or by predation where wolves or other predators are common (Severinghaus 1947; Erickson et al. 1961; Mech et al. 1971; Verme and Ozoga 1971; Nelson and Mech 1986 b; DelGiudice 1996, 1998). Several studies have demonstrated that snow depth (and density) specifically has a strong, direct influence on wolf predation of deer in Minnesota and elsewhere (Pimlott et al. 1969; Mech et al. 1971; Nelson and Mech 1986 b; DelGiudice 1996, 1998). In the extreme case, during severe winters of exceptionally deep snow (>/=70 cm), excessive or surplus killing of deer by wolves, characterized as multiple kills made over short distances and brief periods of time with less than complete or no consumption, has been documented (Mech et al. 1971, DelGiudice 1998). Deteriorating condition of deer as winter progresses and impedence of deer movements by deep snow that is less supportive of deer than wolves have been implicated as primary factors contributing to this phenomenon (Mech et al. 1971, DelGiudice 1998). Surplus killing of prey under certain environmental conditions has been documented for a wide variety of predators (Kruuk 1972).
Scat analyses have revealed much about the year-round food habits of wolves, including that deer are their single most important food, except during winter in the Boundary Waters Canoe Area where they rely primarily on moose (Frenzel 1974, Van Ballenberghe et al. 1975, Fritts and Mech 1981, Fuller 1989). Both in northwestern (i.e., Beltrami State Forest) and northeastern Minnesota, deer occurred in the scats of wolves more in winter than in the summer (Thompson 1952, Pimlott et al. 1969, Van Ballenberghe et al. 1975, Theberge et al. 1978, Fritts and Mech 1981); this is congruent with the evidence from monitoring cause-specific mortality of radio-collared deer. Fawns are an important food item in the summer diet of wolves in the Lake Superior region (Thompson et al. 1952, Pimlott et al. 1969, Frenzel 1974, Van Ballenberghe et al. 1975, Fritts and Mech 1981, Nelson and Mech 1986 a, Kunkel and Mech 1994). Indeed, evidence indicates that fawns probably account for most of the individual deer consumed by wolves during June-July, as well as a higher proportion of the overall biomass consumed (Fritts and Mech 1981). Vulnerability of fawns to predation by wolves and black bears (Ursus americanus) is at least partly related to their nutritional status, which may be directly influenced by the severity of the previous winter and its effect on prenatal nutrition (Verme 1962, Mech and Karns 1977, Mech et al. 1987, Kunkel and Mech 1994). Annual percentages of young fawns taken by wolves (and other predators) are still relatively unknown, and additional study is warranted.
In some areas such as northwestern Minnesota, where deer are largely the most important year-round food for wolves, there are brief periods (April-May) when wolves rely more on moose. Certainly, moose are the second most important food item in this area during summer and winter, with wolves consuming more individuals and biomass during summer than winter (Fritts and Mech 1981). The relative importance of moose in the diet of wolves that depend primarily on deer year-round may vary, dependent upon deer densities and the degree with which factors such as winter severity and disease or parasites (e.g., winter tick infestation, cerebrospinal nematodiasis) predispose moose to predation or scavenging by wolves.
Snowshoe hares (Lepus americanus) are consumed by wolves year-round, and although the number of individuals consumed may be second only to deer, the biomass contributed to the diet is rather insignificant (Fritts and Mech 1981, Fuller 1989). Beaver (Castor canadensis) have been found in the diet of wolves in spring and summer (Byman, unpublished thesis, Frenzel 1974, Fritts and Mech 1981, Fuller 1989); data indicate that its importance may be greater in northeastern than northwestern Minnesota. An increased consumption of beaver accompanied a decline in white-tailed deer populations in southern Ontario (Voigt et al. 1976, Theberge et al. 1978). Other mammals documented in the diet of wolves include bog lemming (Synaptomys sp.), fox squirrel ( Sciurus niger), jumping mouse (Zapus hudsonius), meadow vole (Microtus pennsylvanicus), white-footed mouse (Peromyscus sp.), and woodchuck (Marmota monax) (Young and Goldman 1944, Fritts and Mech 1981). Additional miscellaneous foods have included black bear, striped skunk (Mephitis mephitis), wolf, unidentified Canis, various bird species, duck and duck egg shells, and insects. Wolves have also consumed fruits such as blueberries (Vaccinium sp.) and strawberries (Fragaria sp.) during summer and early fall (Van Ballenberghe et al. 1975, Fritts and Mech 1981).
Livestock Depredation by Wolves
Where they coexist, wolves will occasionally prey on livestock, including cattle (primarily calves), sheep, turkeys, swine, and goats, but generally, they occur in the wolf’s diet in low amounts, both with respect to the number of individuals and biomass consumed (Fritts and Mech 1981, Fuller 1989, Fritts et al. 1992). There is substantial temporal and spatial variation concerning livestock depredation by wolves, with wolves in some areas relying on domestic prey more heavily than others in certain years (Fritts et al. 1992). Most depredations occur in north-central and northwestern counties where farm and livestock densities are highest within wolf range (Fritts et al. 1992). Research has indicated an inverse relationship between the severity of the previous winter, and thus the vulnerability of newborn fawns, and the amount of depredation on livestock (and pets) (Mech et al. 1988 a, Fritts et al. 1992). Livestock depredation by wolves tends to be higher in some areas following a less severe winter when the nutritional status of deer fawns is better and they are less vulnerable.
During the period 1975-1986, the wolf depredation control program verified an average 30 complaints per year involving an average 21 farms. However, for the shorter, more recent interval 1979-1986, these figures were 33 verified complaints and 24 farms (0.33% of farms in wolf range). During 1979-1986, average annual, verified losses of 23 cattle, 49 sheep, and 173 turkeys were reported; most complaints occurred during May-September. Numbers of animals wounded or killed peaked in May, July-August, and August-September for cattle, sheep, and turkeys, respectively (Fritts et al. 1992). The area affected by verified depredations expanded 48.7% from 38,228 km 2 (14,760 mi 2) in 1975-1980 to 56,827 km 2 (21,941 mi 2) in 1981-1986; the expansion occurred in all directions, but north (Fritts et al. 1992). When adult and yearling wolves were removed by the control program, additional losses did not occur in 55% of the cases, 22% of the cases when young of the year were removed. Removal of breeding versus nonbreeding members of packs did not have any greater impact on subsequent losses of livestock.
From 1987 to 1997, verified complaints of wolf depredation on livestock and the number of farms involved has tripled (Paul 1997). Specifically, in 1997, there were 109 verified complaints at 93 farms. Also during this period, the number of wolves captured and removed from the population has increased from 30-35 per year to just over 200 per year; the average has been 158 wolves captured per year during the past 5 years (Paul 1997). A state program which compensates farmers for livestock lost to wolves, paid an average $37,299 per year over the past 5 years. Greater detail of wolf depredation of livestock and the depredation control program in Minnesota can be obtained elsewhere (Fritts et al.1992, Paul 1997).
Wolves and Deer—Influences at the Population Level
As is evident from the above, wolf and white-tailed deer populations are intricately linked, each having a potentially strong influence on the other’s population performance (i.e., survival rates, reproductive success). The specific type and strength of the influences or interactions between wolf and deer populations are a function of factors such as individual densities, sex and age structures, human-related activities (e.g., hunting, poaching, dogs, timber harvesting, road development, supplemental feeding), winter severity, presence of alternate prey, and habitat quality (Mech 1986, Mech et al. 1988 b). Clearly, nutrition is the basis of this link in both directions. Wolves have a direct impact on white-tailed deer populations in being the primary cause of natural mortality, again, with the most vulnerable individuals being newborn fawns and fawns (>/=0.6 yr old) and older adults during winter (Mech and Frenzel 1971; Nelson and Mech 1986 a,b; DelGiudice and Riggs 1996; DelGiudice 1996). Thus far, there is little evidence to show that wolves are a limiting factor on deer, or alone, have caused population declines (Voigt 1990). However, in combination with severe winter conditions, overly mature habitat, and/or human exploitation, wolf predation has contributed to locally declining deer populations such as occurred in the central Superior National Forest in the late 1960s-early 1970s and elsewhere during winter 1995-96 (Mech and Karns 1977; DelGiudice 1996, 1998). In the central Superior National Forest and areas further north and northeast, wolves may be affecting the recovery of the deer population (Nelson and Mech 1981). Wolf predation may represent compensatory or additive mortality. For example, if wolves kill deer during winter that ultimately would have died of starvation, the predation is compensatory mortality. However, if in very severe winters, wolves are able to kill some relatively healthy deer that would have survived t he winter nutritional stress, then that predation is additive. Generally, based upon ages of deer killed by hunters and wolves (discussed earlier), it is believed that most deer deaths from wolves in winter are a combination of compensatory and additive mortality, whereas, the hunter-kill tends to be more additive (i.e., prime age classes). A recent study reports that during 1975-1997, numbers of wolves in the east-central Superior National Forest accounted for only 23-38% of the inter-year variation in buck harvest for areas in and near the 2,060 km 2 (795 mi 2) wolf census area (Mech and Nelson 1998). Year-to-year analysis of annual changes in numbers of wolves and harvested bucks revealed inconsistent associations. Increases in annual deer harvests in much of Minnesota wolf range, even as it expanded, indicates that generally, the wolves’ effect on deer harvests has been minimal. Presently, too little is known about t he mortality of deer neonates to speculate whether its effects are compensatory or additive; however, preliminary evidence has indicated that some newborns killed by wolves and bears may be compromised nutritionally (Kunkel and Mech 1994).
Because deer are the wolf’s primary food across most of their range in Minnesota, it is easy to understand the eventual, nutritional consequences for wolves of a local deer decline. The estimated minimum maintenance requirement of food for free-ranging wolves is 1.7 kg per day of deer-equivalent biomass (assuming an average 54.5 kg of food provided from an adult deer [portions of this can be from beaver, snowshoe hares, moose, etc…]) (Mech 1970). Estimated daily consumption rates per wolf have ranged from 0.2 to 7.3 kg (Mech 1966, Mech and Frenzel 1971, Kolenosky 1972, Kuyt 1972, Fritts and Mech 1981, Fuller 1989). It has been estimated that a wolf requires the equivalent of 15-20 adult-sized deer per year (Mech 1971, Kolenosky 1972, Fuller 1989). However, low deer numbers; fewer old, vulnerable deer (i.e., a young age structure); or a mild winter in a given area will influence the number of deer or biomass each wolf actually consumes. Consequence s of submaintenance consumption and nutritional stress for wolves during the breeding (February-March) and denning seasons (late April-May) can be increased adult mortality (undernutrition, more trespassing and inter-pack strife), small litters, and decreased pup survival. In the past in the central Superior National Forest, the collective effect of such dynamics involved a local wolf population that decreased 50% in 4 years (Seal et al. 1975; Van Ballenberghe and Mech 1975; Mech 1977a,b, 1986). This type of predator-prey relationship—a predator population contributing to the decline of its prey population, followed by a lag period and subsequent decline in the predator population—is classical, but it is not simple. As mentioned above, many other factors will interact, influence the specific dynamics of the relationship (what changes occur and the magnitude of those changes) and determine the temporal scale over which they occur.
I gratefully acknowledge the reviews and comments of L. David Mech, Michael E. Nelson, Mark S.Lenarz, and W. E. Berg.
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