 WolvesWolf History, Conservation, Ecology and Behavior
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Wolfology Item # 368
Source:
n105 (October 1989)
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Population Dynamics of Wolves in North-Central Minnesota
Todd K. Fuller
1989
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Abstract
During September 1980 - December 1986, 81 radio-collared wolves (Canis lupus) were monitored in and near the 839-km2 Bearville Study Area (BSA) in north-central Minnesota. Each year winter-territory size averaged 78-153 km2; no territories had road densities >0.72 km/km2. From zero to 30% of radio-marked pup, yearling, or adult wolves left their territories each month. Pups left natal packs during January-March and older wolves left frequently during September-April. Wolves temporarily leaving territories moved 5-105 km away and were absent 3-118 days; up to 6 exploratory moves were made prior to dispersal. Dispersing wolves traveled 5-100 km away during periods of 1-265 days. One disperser joined an established pack, but 16 others formed new packs. Annual dispersal rates were about 0.17 for adults, 0.49 for yearlings, and 0.10 for pups.
Each year mean pack size ranged from 5-9 in November-December to 4-6 in March. Annual wolf density (including 16% lone wolves) ranged from 39 to 59 wolves/1,000 km2 in November-December to 29-40 wolves/1,000 km2 in March....Litters averaged 6.6 pups at birth and 3.2 by mid-November, at which time pups made up 46% of pack members. Annual survival rate of radio-marked wolves .5 months old was 0.64. Despite legal protection, 80% of identified wolf mortality was human caused (30% shot, 12% snared, 11% hit by vehicles, 6% killed by government trappers, and 21% killed by humans in some undetermined manner); 10% of wolves that died were killed by other wolves.
During sample periods in 2 winters, wolves were located twice daily to estimate predation rate on white-tailed deer (Odocoileus virginianus)....Winter consumption averaged 2.0 kg deer/wolf day....Scat analysis indicated deer were the primary prey in winter and spring, but beaver (Castor canadensis) were an important secondary prey (20-47% of items in scats) during April-May....Overall, deer provided 79-98% of biomass consumed each month. Adult wolves consumed an estimated 19 deer/year of which 11 were fawns.
A review of North American studies indicates that wolf numbers are directly related to ungulate biomass. Where deer are primary prey, territory size is related to deer density. Per capita biomass availability likely affects pup survival, the major factor in wolf population growth. Annual rates of increase of exploited populations vary directly with mortality rates, and harvests exceeding 28% of the winter population often result in declines. Management decisions concerning wolf and ungulate densities and ungulate harvests by humans can be made using equations that incorporate estimates of wolf density, annual ungulate kill per wolf, ungulate densities, potential rates of increase for ungulates, and harvest.
 Introduction
Minnesota is the only state of the contiguous 48 states within the U.S. with a large...self-sustaining wolf population, and, consequently, the species is legally protected and classified as "threatened" under the federal Endangered Species Act. Since repeal of the wolf bounty in 1965 and complete federal protection in 1974, controversy over wolves in Minnesota has abounded. Some Minnesota residents believe wolves seriously compete with hunters for white-tailed deer and moose (Alces alces) or cause major livestock depredations; such individuals thus feel that wolf numbers should be reduced. Others view wolves as a symbol of wilderness, having only incidental impact on ungulate populations or livestock and often believe that wolves should be rigorously protected. Although aspects of these opposing views are sometimes both correct, responsible management of wolves in Minnesota and elsewhere continues to rely heavily on sound knowledge of wolf population dynamics, prey relationships, and interactions with humans.
In the Great Lakes region, the primary ungulate prey of wolves is white-tailed deer....Impacts of wolf predation on livestock in Minnesota and subsequent efforts to minimize such depredations also have been reported. These and studies conducted elsewhere in North America suggest that (aside from human-caused mortality) food resources of varying size, availability, and distribution are the ultimate factors and social behavior is the major proximate factor that cause changes in wolf populations.
Studies of wolf population dynamics were reviewed and summarized by Keith (1983) who found that 4 factors dominate wolf population dynamics: wolf density, ungulate density, human exploitation, and ungulate vulnerability. Yet many of the relationships between rates of wolf population change and human-caused mortality, food supply, and wolf behavior (e.g., dispersal) are poorly quantified, particularly where numbers of deer and hunters are relatively high....
For this study, field work on wolves was carried out during September 1980 - December 1986. Specific objectives were to relate wolf density and annual rates of population increase to wolf reproduction, survival, dispersal, and feeding ecology. Results of this study were combined with those of other wolf studies throughout North America to provide summary models that predict the impact of various management strategies on wolf and ungulate population dynamics....
Study Area
Wolf research centered on the 839-km2 Bearville Study Area (BSA...), located 50 km north-northeast of Grand Rapids, Minnesota. Most data were collected on 5-8 contiguous wolf packs whose territories encompassed part of the BSA, but dispersing wolves that settled up to 70 km away were monitored on an intermittent basis. The BSA is adjacent to a relatively uninhabited, forested area to the north and on the developed, populated Iron Range to the south. It lies within primary wolf range in Minnesota, but also is on the northern edge of an area designated Zone 4 by the Eastern Timber Wolf Recovery Team. The Team considered that, within wolf range, this zone had the highest potential for human-wolf conflicts and thus was the area where wolves should be harvested if a change in legal status allowed such activity.
....Approximately 66% of the land in the BSA is in public ownership...; the remaining private land is owned by paper companies (20%) or private citizens (14%). Densities of roads and trails in the study area averaged 0.44 km/km2....Estimated hunter density in the BSA during opening day of the 16-day deer-hunting season in November was about 2.5/km2.
....Midwinter density of white-tailed deer declined about 50% during 1982-86, but averaged about 6/km2. Moose density was about 1/50 km2. Density of beaver colonies increased from 0.58 to 0.86/km2 during 1981-85, and numbers of snowshoe hares (Lepus americanus) peaked during 1977-80 and declined precipitously during 1981-84.
....Territory Size and Movements
Estimated wolf pack territory boundaries sometimes shifted between winters, and therefore territory sizes were calculated for a "summer" (15 Apr - 14 Sep) and a "winter" (15 Sep - 14 Apr) period each year. The beginning and ending dates correspond to documented changes in pack cohesiveness related to denning and travel as a pack....
Monthly rates at which resident wolves left packs temporarily (temporary excursion) or permanently (dispersal), or non-resident wolves established new packs or joined packs already present (i.e., settled) were extrapolated from estimates of daily rates, similar to mortality rate calculations....
Pack Size, Density, and Population Change
Pack size in "early" (Nov - Dec) or late (Mar) winter was the maximum number of wolves that were observed together or that could be accounted for during telemetry flights....Early winter counts provided a maximum estimate of the number of adult-size wolves present each year, and late winter counts (just before denning) provided an estimate at the population nadir....
Annual wolf density during early and late winter was estimated by first delineating a census area that encompassed all adjacent winter territories in the BSA and the area between them. The summed maximum pack sizes plus the estimated proportion of lone wolves were divided by this census area to give an overall density. Not all wolf packs in the study area contained marked wolves each winter and limited aerial searches were conducted to identify pack sizes in these territories. The territories of these packs was assumed to be similar to their territory in previous or subsequent years when 1 or more pack members were marked....
....Immigration rates of pack wolves were estimated from numbers of wolves outside the census area that established new packs in areas where other radio-marked packs in the census area had disappeared (these are maximum rates because some unmarked resident wolves may have established packs in the census area). In addition, some wolves from outside the census area likely joined already-established packs. To calculate their numbers, I assumed that the ratio of wolves joining packs to those starting new packs was similar for both marked and unmarked wolves. Thus, the proportion of immigrants in the study area was the proportion of new wolves (probably not born in the study area) in the census area in late winter.
Reproduction and Survival
Mean litter size was estimated from counts of placental scars of 4 radio-collared wolves that died and observations of presumably complete litters of pups in 4 consecutive years at 1 natal den....
Pup survival from birth to about 7 months of age was estimated from the difference between numbers born and numbers observed in packs the following October-December. During autumn and early winter, pups were identified by their smaller size and behavioral characteristics. When such identification was tenuous, reduction in pack size due to known mortality and dispersal of yearlings and adults also was taken into account....
Age-specific survival rates of radio-collared wolves were estimated by extrapolation of daily survival rates; the relative importance of various mortality factors was similarly derived....Wolves were classified as residents when inside or within 5 km of their normal territory and nonresidents when they were alone and traveling outside their territory boundaries.
When radio signals of wolves could not be located by routine monitoring, searches were conducted up to 100 km from the study area within the next several weeks, and their frequencies continued to be monitored in the study area for the next several months to determine if dispersing wolves had returned. Resident wolves whose radio signals disappeared during the 16-day...deer-hunting season in November were presumed illegally shot and their transmitters destroyed. Similarly, socially dominant wolves (not likely to disperse) that disappeared during the legal trapping and snaring season for other large furbearers (Dec - Jan) were presumed captured and illegally killed. If radio contact was lost immediately after a wolf had dispersed from its territory, or soon after it had made [one or more] exploratory movement out of its home territory, I assumed that the animal was alive at last contact and had moved beyond range of our monitoring equipment. Expected life of radio transmitters was 3 years; signals not received after that time span were assumed to be the result of battery failure. Using all these criteria, I could not ascertain probable fates for only 5 animals, all of whose signals disappeared during March-May. For purposes of calculation, these animals were considered alive on the last day of radio contact, and the resulting estimates of survival are thus maximal. Overall, this assumption had little effect on my estimates as pooled annual survival rates were not significantly different...between the maximum estimate (0.64) and the resulting minimum estimate if all 5 wolves had died (0.60).
Feeding Ecology
....Wolf predation rates on ungulates during winter are most accurately determined by locating wolves on consecutive days and following their entire path of travel. Wolf trails were not followed in this study because tracking conditions were never suitable due to adverse snow and vegetation characteristics. Therefore preliminary winter kill rates were estimated from the frequency wolves were found near recently killed deer. For example, if a pack was found at kill sites on 2 of 14 days they were located, the estimated kill rate would be 1 kill/7 days....
....Because a pack of wolves can consume a white-tailed deer in <24 hours, unadjusted kill rates for wolves killing deer could be low. To help correct for this potential bias, radio-collared wolves in packs of 2-9 were located in early morning and in late afternoon for multiple periods of 2-7 consecutive days during portions of January and February 1984 and 1985....
Relative biomass and relative numbers of prey consumed were estimated from scat-analysis data. Similar to Ballard et al. (1987), the total number of deer consumed during April-October, when only 4 aerial observations of kills were made, was estimated from (1) the consumable weight of both adult and fawn deer, accounting monthly for changes in fawn size, (2) the relative biomass provided by both adults and fawns in the diet each month, and (3) mean monthly consumption rates of deer by adult wolves. Consumption rates (kg/day) by adults in summer were assumed equivalent to winter rates. Even though wolf metabolic rate may be somewhat lower in summer than in winter, wolves had the additional burden of providing food for pups. In addition, summer predation rates on both caribou (Rangifer tarandus) and moose have been found to be similar to those in winter....
RESULTS AND DISCUSSION
Radiotelemetry
During September 1980 to August 1985, 81 wolves were radio-collared in the BSA. The proportion of wolves in each sex and age class within the monitored sample did not differ among years; average numbers monitored were 7 adult males, 9 adult females, 2 yearling males, 2 yearling females, 2 male pups, and 3 female pups. Sex ratios of 48 adults and yearlings (46% male) and 33 pups (42% male) did not differ from parity.
Radio-marked wolves were located, on average, every 6.8 days....Marked wolves or associated pack members were observed most often (76% of attempts) during January-February when snow cover was complete and least often (16%) during May-October when leaves were on trees. Overall observability of solitary marked wolves was lower 31 vs. 46%) than for those founds in groups....
Marked pups and 41 older wolves initially were members of 12 packs; 7 others were solitary....During November-December each year, 23-54% of wolves in packs whose territories were included wholly or partly within the BSA were radio-collared.
....Thirteen packs with all or part of their territories in the BSA were monitored for 1-6 years; 5 of these were formed during the study period and reoccupied vacant territories. An additional 10 pairs or packs monitored 1-3 years had territories wholly outside of the BSA; 9 of these territories were formed by wolves that dispersed from the BSA.
....Movements
Territory Size and Location. -- Mean minimum winter territory sizes of packs in part or all of the BSA ranged from 78 to 153 km2 over the years of the study, and individual territory sizes ranged from 50 to 223 km2....Deer densities within each territory were not known, but a review of studies where wolves prey primarily on white-tailed deer, indicates wolf territories are smaller and territory size per wolf is lower where deer are more numerous. These regression analyses exclude the large mean winter-territory size in a newly protected population in northwest Minnesota; however, as wolf numbers increased there, territory size decreased 35-45%.
Within any winter, little overlap of territorial boundaries occurred among adjacent marked packs in the BSA. Wolves may be more territorial in Minnesota where territories are smaller and thus boundaries are easier to maintain, but...apparent overlap in other studies may be the result of the method used to depict boundaries and may not reflect temporal separation of packs.
Relatively stable, long-established territorial boundaries in the BSA likely fluctuate little with short-term changes in pack size....Pack size and territory size have been correlated in other areas where wolf densities were lower, territories larger, and exploitation was higher than in the BSA. In some areas, territories have expanded, contracted, disappeared, or been established when wolf mortality was high and pret availability or distribution changed.
Most territories in the BSA encompassed relatively roadless tracts of land....Road density within all pack territories ranged from 0.15 to 0.72 km/km2, but was not related to the rate of human-caused mortality of radio-collared wolves....However, these results corroborate work elsewhere that suggests road densities >0.6-0.8 km/km2 provide enough access for humans to limit wolf numbers through legal or illegal trapping or shooting.
Extraterritorial Movements. -- Each month, up to 30% of wolves of any 1 age class in the BSA left their territories...; some wolves returned (temporary excursions), but other wolves never returned and died or settled elsewhere (dispersals). Monthly rates of departure did not differ between sexes within an age class, but yearlings did differ in both rate and pattern of departure. Most departures of pups occurred during January-March when they were 9-11 months old, and departure rates of new yearlings (12 months old) were very high in April when new pups were born. Older yearlings left their territories most often during September-January and in March. Monthly rates of departure for adults generally were low throughout the year, but, as with pups and yearlings, were highest in months preceding or following breeding in February and during April when parturition occurred.
....In Quebec, most departures of pups occurred after February, those of yearlings during December-March, and those of adults during December-August. Overall rates of departure were slightly higher for adults than for yearlings in Quebec, contrary to the findings in the BSA. In Alaska...extraterritorial movements of pups were negligible.
Of the temporary excursions made by 25 wolves in the BSA, 79% were made alone. All 5 excursions by yearlings were solitary, but 2 of 6 by pups and 10 of 45 by adults were with other pack members. Yearling wolves in the BSA that left their territories only made 1 temporary excursion each and pups sometimes made 2, but adults made up to 10. Maximum distance...and duration...of temporary excursions from the BSA did not differ between wolves traveling alone or with other pack members, or by sex or age.
....In contrast to the BSA wolves, yearlings in Quebec initiated an average of 3 extraterritorial movements per year and adults only 1. Pack excursions were more frequent in northeast Minnesota when prey was scarce, but none on the Kenai Peninsula seemed associated with food shortage. Distance and duration of excursions for wolves in Quebec were similar to the BSA where ungulate biomass per wolf was similar, but excursion distance and duration were somewhat greater for packs living in areas of lower prey abundance.
Twenty-eight radio-collared wolves from the BSA dispersed from their home territories; 2 of these settled and subsequently dispersed again. At least 12 of 28 dispersing wolves had previously made at least 1, and up to 6, extraterritorial excursions....
One BSA wolf left its home territory with 2 other unmarked pack members; another left with its mate (the only other pack member) after <1 year in a newly formed territory. The remaining 28 dispersers (93%) were solitary....
The proportion of wolves that dispersed from BSA packs each month ranged from 0-11% for pups [5 months or older] and 0-7% for adults, but 0-20% for yearlings. Six of 17 dispersing "adult" wolves were known to be >2.0 years old, although others could have been misclassified yearlings (1.0-2.0 years old) because of the method used to assign ages. Of 18 dispersers of known age on the Kenai Peninsula, Alaska, 1 was a pup, 11 were yearlings and 2-year-olds, and 6 were [three years old or older]. Similarly, 40% of female and 50% of male dispersers in south-central Alaska were [two years old or younger]....
Most of the wolves that dispersed from the BSA did so during November-January and March-May. Wolves in other areas dispersed during autumn when pack members begin traveling together, prior to the breeding season, and/or in spring after pups are born.
There were no sex-related differences in dispersal rates of wolves in the BSA, nor did maximum distance and duration of movements differ by sex. Ballard et al. (1987) reported higher dispersal rates for males. Peterson et al. (1984) also reported a preponderance of male dispersers during the first 4 years of their study, but in the fifth year more females dispersed and overall there was no significant sex-related difference.
Average dispersal distances of males and females are not consistently different: Peterson et al. (1984) reported females tended to disperse a shorter distance than males; Ballard et al. (1987) documented that...females dispersed farther than males; and Mech (1987) found no clear difference in dispersal distance. However, males predominate in reports of farthest dispersing wolves.
Annual age-specific dispersal rates of resident wolves in the BSA were 17% for adults, 49% for yearlings, and 10% for pups >5 months old....Overall, dispersal rates were higher for pups and yearlings in the BSA than in Quebec or Alaska, but differences cannot clearly be attributed to prey density..., pack size..., annual survival..., or percentage pups in packs....
Survival rates sometimes are lower for wolves traveling outside home territories, so extraterritorial excursions might be more common when opportunity to establish new packs is high, food is scarce within territories, or chances to breed in natal packs are low. Fritts and Mech (1981) found "a high rate of dispersal of young wolves" in an expanding, newly protected population, and Ballard et al. (1987) reported apparently "higher rates of dispersal from areas of relatively low ungulate densities," although no quantitative data concerning these rates were presented in either study. On Isle Royale, more wolves left packs when food supplies decreased.
Five of 28 wolves that dispersed from packs in the BSA died while traveling alone, 10 likely dispersed beyond the range of our receiver. [One] was still alone at the end of the study, and 12 others settled...after traveling for 4-210 days and moving 5-70 km from home territories. Four lone wolves that settled established territories 3-76 km...after traveling 142-267 days. Fifteen of these 17 settlers found mates and established territories as a pair, 1 yearling male wolf joined 2 radio-marked females that were the only remaining members of an already-established pack, and 1 wolf established a pack with 2 others that likely were pack mates....Four wolves from the BSA established territories immediately adjacent to their home territories, and 1 wolf marked as a pup joined an adjacent pack. The 8 other dispersers established territories 8-70 km away from home territories.
Whether dispersing wolves pair and settle in a vacant area or join already-established packs...success likely depends on a combination of relative prey abundance, the availability of vacant territories, and survival rates of breeding pack wolves. In Quebec, where prey was relatively scarce, a pair of dispersers acquired another associate and formed a pack, and 2 others were accepted by adjacent packs. In south-central Alaska, where pack sizes were reduced by high human harvest, 8 of 23 dispersing wolves later found with other wolves were in pairs and at least 8 more were apparently accepted into existing packs. Of the 5 settlers on the Kenai Peninsula, where prey was abundant but mortality high, 3 paired with another wolf and 2 joined 2 other wolves. In northwest Minnesota where the wolf population was rapidly expanding, all 9 radio-marked lone wolves paired rather than join a pack, but at least 1, and possibly 3, nonradioed wolves were thought to have joined packs....
Demography
Pack Size and Lone Wolves. -- Mean number of wolves per pack monitored in the BSA each year ranged from 4.7 to 9.3 in early winter versus 3.9 to 6.3 in late winter. Individual pack size was as large as 13 in early winter and 9 in late winter....[L]arger packs lost more wolves through mortality and dispersal each winter than did small packs....
....Lone wolves and pairs without territories have been ignored in some population estimates, but have composed 2-29% of winter populations elsewhere. Lone wolves are difficult to census because (1) the chances of observing them incidentally is less likely than for larger groups and (2) capture rates may be biased if trapping is concentrated inside territories where lone wolves are less likely to travel....
Density and Population Change. -- During winters 1980-81 to 1985-86, density of wolves increased from 39 to 50/1,000 km2 in early winter and from 31 to 35/1,000 km2 in late winter; thus, average midwinter density was about 39/1,000/km2. The number of packs in the census area did not increase, but the mean number of wolves per pack increased from 6.2 to 8.0 in early winter and 4.6 to 5.0 in late winter; also, territory size was somewhat smaller.
From 1 year to the next, some packs disappeared and others reoccupied vacant territories or settled in other unoccupied areas. Each March, 6-8 packs composed of 27-37 wolves had territories that encompassed all or part of the BSA, and 1 or 2 of these packs, each consisting of 2-4 wolves, had reoccupied vacant areas. Most of these "new" packs were composed of wolves that probably were not originally members of BSA packs, i.e., were immigrants. In addition to these immigrants, a wolf or 2 may have joined already-existing packs; 1 of 17 radio-marked wolves that dispersed and settled joined a pack rather than establishing its own.
....Pimlott (1967) suggested that an intrinsic control of wolf numbers limited density to a maximum of about 39 wolves/1,000 km2 in most areas. Mech (1973) concurred, but noted exceptions where prey densities were extremely high. Packard and Mech (1980) submitted that food supply critically affects wolf numbers over the long term. Subsequent research has demonstrated that if a limit occurs, densities are much higher than previously suggested....Peterson and Page (1988) concluded that the only natural limits to wolf density are those ultimately imposed by food.
Changes in wolf density due to varying ungulate density have been documented by long-term studies in northeast Minnesota and Isle Royale and in areas of varying moose density in southwest Quebec. A compilation of additional studies throughout North America confirms that, regardless of prey type or stability of wolf populations, average wolf densities are clearly correlated with biomass of ungulates....In the BSA, where both wolf and white-tailed deer densities were high and deer were heavily harvested, the ungulate biomass index per wolf (161) was well below the mean (249) and median (255).
Reproduction. -- Mean litter size for 5 females from the BSA whose reproductive tracts were examined (one who had 2 sets of placental scars) and one whose pups were seen outside of the natal den in 4 consecutive years was 6.1; mean size of all 9 litters was 6.6. Litter size was small for unexploited populations in Alaska and northeast Minnesota, leading Van Ballenberghe et al. (1975) and Keith (1983) to suggest that litter size may increase with greater ungulate biomass per wolf. A summary of 8 studies does not support this contention.
No packs in the BSA were known to have produced >1 litter each year. Most wolf packs produce only 1 litter of pups per year, although 2 litters per pack (by different females) have been reported. Consequently, if there are >2 female wolves [two years old or older] per pack, some likely do not breed, and populations with larger packs have a lower proportion of breeders. Peterson et al. (1984) found that increased harvest on the Kenai Peninsula resulted in smaller packs and territories and in establishment of new packs in vacated areas. As a result, breeders made up a higher proportion of the population, and the rate of pup production increased.
One female and 1 male wolf in the BSA, both captured and marked as pups, were 22 months old when they first bred; each, with its mate, successfully raised pups. Mech and Seal (1987) reported that the youngest wild female wolf known to have produced pups was 3.0 years old, although 1 known 2.0-year-old wolf in Alaska produced pups, as have several 1-year-old and 2-year-old captive wolves. Thus, all wolves except pups probably are capable of producing young, and the proportion that do breed is a function of age structure, pack size, and number of packs versus lone wolves in the population. It is not surprising that some radio-marked 2.0-year-olds in the BSA produced pups, as the proportion of pups (and subsequently yearlings) and dispersal rates of yearlings were relatively high and pack sizes were relatively small.
By November each year, an average of 3.2 pups remained in each pack, composing about 46% of pack members. The greatest number of pups that survived in a pack until late autumn was 8....
Keith (1974) noted that the proximate and ultimate factors affecting wolf pup mortality during the first 5 months were almost wholly unknown and, because both summer food abundance and pup mortality are difficult to measure, represented "the single greatest enigma in wolf biology today."....
Survival. -- A pup survival rate of 0.48 from birth through mid-November was estimated from mean numbers of pups per pack at both times...and is lower than the rate of 0.76 on the Kenai Peninsula or 0.89 in south-central Alaska. This may be related to ungulate availability....
Forty-six radio-collared wolves died or were assumed to have died during this study. There was no significant difference related to sex or age in annual survival rates...and survival of resident wolves [5 months old or older] was not statistically greater than that of nonresidents. Mean annual survival rate for all radio-collared wolves was 0.64....
In the BSA, no mortality of marked wolves occurred during April-May, but some mortality occurred throughotu the summer and increased significantly during the November deer-hunting season. Mortality also was high in December (the major fur-taking season), then decreased through the winter. Despite legal protection, at least 80% of wolf mortality was human caused....Many wolves (26% of mortality) were shot by deer hunters during the 16-day season in November, and others (27%) were shot, snared, or hit by a vehicle in other months. Two wolves also were killed by federal depredation trappers in July and August at a farm where pack members had killed livestock. Intraspecific aggression (10%) occurred only during October-December. One wolf died of pneumonia in February and perhaps was predisposed by a previous infection of canine distemper....
Potential rates of wolf population increase should be higher after human exploitation because increased per capita food availability results in increased pup production and survival. However, reproduction may not fully compensate for exploitation rates that are too high. Keith (1983) estimated that maximum sustainable exploitation was probably <30% of the early winter population. The human-caused mortality rate in the BSA was at least 29%, and the population was stable or slightly increasing. Peterson et al. (1984) reported that wolf density on the Kenai Peninsula declined following 2 annual kills of >40%, but increased after harvests of <35%. Gasaway et al. (1983) reported stable populations following harvests of 16-24% of the early-winter population, but found numbers declined 20-52% after harvests of 42-61% of the population....For the unexploited wolf population on Isle Royale, annual mortality of adult-sized wolves was 13-16% when numbers were increasing or stable at high numbers, 51% during a population crash, and 33-34% when the population subsequently stabilized.
Summary analyses...suggest that population size would stabilize with an overall mortality rate of 0.35, or a human0caused rate of 0.28. However, these values may vary with the age and sex structure of the population. For example, a population with a high proportion of pups may be able to withstand somewhat higher overall mortality because pup...may be more vulnerable to some harvest techniques and make up a disproportionate part of the harvest. Also, net immigration or emigration may mitigate effects of harvest.
Feeding Ecology
Winter Predation. -- Wolves located from aircraft were observed at ungulate carcasses on 176 occasions. Of these, 145 were fresh kills of which 74 were identified during ground investigations, 61 were identified only from the air by signs of a chase or fresh blood, and 10 were assumed to be kills because packs stayed in areas of dense conifer cover on successive days and 3-15 ravens (Corvus corax) were present....
....The proportion of days packs were at kill sites varied with pack size....However, no packs, regardless of size, stayed at kill sites an average of more than 12 hours, and thus unadjusted kill were underestimated by at least 50%. The corrected mean kill rate in 21.2 days/kill/wolf, and the minimum number of deer killed per wolf per winter (1 Nov-31 Mar) was 7.1.
Kill rates vary with pack size in other areas where moose are the primary prey, but wolves stay at moose kills 1-8 days and daily locations of packs are adequate to document kill rates of packs of different sizes. Deer, however, are about one-sixth the size of moose, and it is not surprising that no packs in the BSA stayed at deer kill sites >24 hours. Larger packs in the BSA likely remained at kills <12 hours (resulting in an apparently lower kill rate per wolf than for smaller packs), and thus overall kill rates likely were underestimated.
Fawns composed 54% of wolf-killed deer examined in situ for which sex and/or age could be determined, and females composed 60% of deer [1 year old or older]....Given a mean edible weight of about 42 kg for all wolf-killed deer, the total deer biomass eaten per wolf per winter would then be 298 kg. The average daily consumption would be 2.0 kg/wolf or 0.06 kg/kg wolf/day, based on an average wolf weight of 34 kg.
This estimate assumes complete consumption of edible parts and does not include any small prey such as snowshoe hares or food scavenged from deer or moose dying of other causes. Only 6 of 78 (8%) deer killed by wolves in winter and examined in situ were not totally consumed (all meat and many bones)....Also, hares likely composed [2% or less] of food biomass in winter. Wolves were seen at 24 (14% of 169) carcasses of deer that had died previously and were being scavenged, but at least 7 of these were kills made earlier by the wolves. No wolf-killed moose were observed, but carcasses of 7 adult moose...were scavenged.
....Pimlott et al. (1969) and Mech et al. (1971) reported wolves abandoned partially eaten deer kills during winters with deep snow, and Carbyn (1983) found that wolves consumed only about half the biomass of elk they killed in a winter with relatively deep snow. Such weather-related influences cannot be incorporated into this analysis....
Scat Analyses. -- Deer remains composed 45-91% of identified items in scats each month....During winter, deer and snowshoe hares composed the majority of prey, but, in April and May, beaver occurred in 20-47% of items. In June and July, occurrence of beaver diminished and deer fawns became most common (25-60% of all occurrences, 32-69% of deer occurrences). Occurrence of beaver increased during August-October (3-22%), but not to the high levels of spring. Vegetation (mainly berries) increased in occurrence during July-September (4-52%).
Extrapolation of scat data indicate that deer provided 79-98% of animal biomass consumed by wolves each month; beavers provided >10% of biomass consumed only in April and May....Deer fawns were eaten 4 times more often than adults in June, but by July were eaten only as often as adults....Snowshoe hares were apparently killed and consumed about as often as deer in most months....
....[A] majority of studies indicates that even though alternate foods are most available in summer (and ungulate prey are less important than in winter), ungulates usually compose >75% of biomass consumed by wolves at that time. Although wolves likely consume 1 to 9 times more newborn ungulates than adult ungulates, no studies have identified the factors influencing rates of predation on juveniles.
....Numbers of Deer Killed. -- ....[T]otal annual per capita consumption in the BSA was 11.0 fawns and 7.8 adults, or 18.8 deer/wolf/year.
Mech (1971) guessed that wolves killed 15 deer/wolf/year, and Keith (1983) extrapolated an average winter kill rate...resulting in an annual average of 16.6 deer/wolf. Pimlott (1967) estimated an annual kill of 36.7 deer/wolf, but assumed daily consumption was 3.6 kg/wolf/day, almost twice that in the BSA..., but lower than for areas where ungulates other than deer are the primary prey of wolves.
Management
Rates of wolf population increase are most affected by relative ungulate biomass availability...and human-caused mortality. Fortunately, the most direct manipulative tool in managing these 2 parameters is legal regulation of both wolf and ungulate harvests. A prerequisite for responsible management, however, is accurate monitoring of population density and harvest.
Ungulate biomass availability depends on ungulate size, density, and vulnerability, but, in most calculations, ungulate density is the major component of availability because size is known and factors affecting vulnerability can rarely be used in a predictive way. Relative "condition" of a population is seldom known, age structures are not highly correlated with vulnerability, and effects of winter weather on food availability are imperfectly known. On the other hand, ungulate densities can be estimated by a number of methods, and regulation of ungulate harvest is a major concern for all government agencies in areas where wolves occur.
....In areas where wolves are legally harvested, the total number killed often is monitored closely through registration. Thus, changes in wolf numbers are predictable, given a reliable estimate of wolf population density and relatively constant natural mortality due to disease and intraspecific strife. In Minnesota, however, wolves are legally killed only through depredation control, but many are killed illegally and accidentally; this mortality cannot be enumerated except through intensive radiotelemetry studies. Nevertheless, human-related wolf mortality is correlated with distribution and density of roads; therefore, regulating road acess (especially during hunting seasons) likely can control human-caused mortality of wolves.
It is apparent that under certain circumstances wolves can regulate ungulate numbers. Ungulate populations that declined precipitously in several areas due to severe winter weather, habitat change, overhunting, and/or predation remained low because of low ungulate:wolf ratios. In some areas, hunting of ungulates was restricted and/or wolf numbers were reduced; this increased ungulate:wolf ratios and sometimes was followed by immediate increase in ungulate numbers. In areas where providing a huntable surplus of ungulates is a major management concern, restricted hunting combined with wolf reductions may, over a short time, result in increased ungulate numbers, greater ungulate harvest, and, eventually, higher wolf densities. In south-central Alaska where wolf numbers were reduced 58%, they recovered to 81% of former levels within 1 denning season after control was terminated and exceeded precontrol numbers after 3 seasons.
....The deer:wolf ratio was 100:1 in an unhunted deer population in Ontario, and Pimlott (1967) estimated that a finite rate of increase of deer of [1.37 or greater] was required to support an annual kill by wolves of 36.7 deer/wolf....The annual potential rate of increase for radio-collared deer in the BSA was about 1.21 when excluding wolf predation, hunting, and wounding loss (but including car kills and poaching)....Given an annual kill rate of about 19 deer/wolf/year in the BSA, the deer:wolf ratio required to maintain a stable deer population (without hunting) would be 90:1....
Conclusions
1. Small territory size and high wolf density (39/1,000 km2 in mid-winter) in north-central Minnesota likely results from relatively high white-tailed deer numbers in January (6.2/km2).
2. Extraterritorial forays, usually made alone, were common for wolves and many resulted in permanent dispersal. Yearlings dispersed at a higher rate than did adults or pups. Overall decreases in pack size from early to late winter (including dispersal and mortality) were greatest for large packs.
3. Wolf numbers were stable to slowly increasing because pup recruitment was moderate (3.2 pups/pack in autumn) and mortality of wolves [5 years old or older] was not excessive (annual survival = 0.64). Annual rates of population change directly reflected the proportion of pups in the population the previous autumn.
4. Despite the fact that wolves were legally protected, 80% of mortality was human caused. Road density was higher where wolves died of human-related causes. No wolf pack territories had road densities >0.72 km/km2.
5. Wolves killed mostly fawn deer in both summer and winter, but in terms of biomass, older deer still were of primary importance. Winter consumption rates were low (2.0 kg/wolf/day), but may have reflected the inaccuracy, in general, of techniques used to estimate predation on deer.
6. A majority of studies in North America, including this one, indicate that wolf numbers in North America are directly related to ungulate biomass, and potential wolf density (with no human-caused mortality) ultimately is limited only by ungulate availabiltiy.
7. Consumption rates of wolves in North America likely increase with higher per capita food availability; this likely results in the higher pup survival observed where ungulate biomass is higher.
8. Annual rates of wolf population increase vary in direct response to mortality rates, and, where wolves are killed by humans, harvests exceeding about 28% of the autumn or early winter population often result in population declines.
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