University of Nebraska - Lincoln University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln
USDA National Wildlife Research Center - Staff
Publications
U.S. Department of Agriculture: Animal and
Plant Health Inspection Service
February 2005
Food Habits of Wolves in Relation to Livestock Depredations in Food Habits of Wolves in Relation to Livestock Depredations in
Northwestern Minnesota Northwestern Minnesota
Andreas S. Chavez
Utah State University, Logan
, chavez.102@osu.edu
Eric M. Gese
Utah State University
, eric.gese@usu.edu
Follow this and additional works at: https://digitalcommons.unl.edu/icwdm_usdanwrc
Part of the Environmental Sciences Commons
Chavez, Andreas S. and Gese, Eric M., "Food Habits of Wolves in Relation to Livestock Depredations in
Northwestern Minnesota" (2005).
USDA National Wildlife Research Center - Staff Publications
. 505.
https://digitalcommons.unl.edu/icwdm_usdanwrc/505
This Article is brought to you for free and open access by the U.S. Department of Agriculture: Animal and Plant Health
Inspection Service at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USDA
National Wildlife Research Center - Staff Publications by an authorized administrator of DigitalCommons@University
of Nebraska - Lincoln.
Food Habits of Wolves in Relation to Livestock
Depredations in Northwestern Minnesota
ANDREAS S. CHAVEZ
1
Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan 84322
AND
ERIC M. GESE
U.S. Department of Agriculture, Wildlife Services, National Wildlife Research Center,
Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan 84322
A
BSTRACT.—Wolves (Canis lupus) have recolonized many areas of the Midwestern United
States, prompting concern over the possible risk wolves may pose to livestock producers. To
better understand the risks wolves may pose to livestock, we initiated a 3-y study examining the
food habits of wolves in an agricultural area of northwestern Minnesota and their relation to
depredation records of livestock losses in the same area. We collected 533 wolf feces during the
non-winter seasons from 1997–1999. White-tailed deer (Odocoileus virginianus) was the most
abundant food item (39.2%) of all prey items for all 3 y combined. The deer component
comprised both adult deer (26.9%) and fawns (12.3%). Muskrats (Ondatra zibethicus) were the
second highest food item at 16.6%. Moose (Alces alces), both adults (13.0%) and calves (0.6%),
comprised 13.6% of the diet of wolves, followed by cattle (10.3%), domestic pig (4.4%),
lagomorphs (3.6%) and beaver (Castor canadensis; 2.1%). During our 3-y study, eight head of
livestock were officially reported as wolf depredations in the agricultural lands within the study
area. The confirmed losses included one sheep, one injured cow, one blind cow and five calves.
Even with very low deer and moose densities in the study area and a high preponderance of
cattle in the area (.1000 head), the wolves in the area preyed mostly on native prey species.
INTRODUCTION
The historical range of the gray wolf (Canis lupus) included most of North America where
wild ungulates were abundant (Carbyn, 1987). European settlement along with the
introduction of domestic livestock into these areas altered the relationship between wolves
and their native prey. Across the contiguous United States, domestic livestock replaced or
coexisted with native ungulates. Wherever wolves and livestock coexisted, wolf depredations
on livestock occurred (Young and Goldman, 1944; Gunson, 1983; Tompa, 1983; Fritts et al.,
1992). As a result, the threat of wolf predation on domestic livestock became one of the
leading reasons for humans to eradicate wolves throughout the contiguous United States. By
the middle of the 20
th
Century, humans had successfully eradicated wolves from most of the
contiguous United States, except for a single population in northeastern Minnesota.
The distribution of wolves in the contiguous United States began to increase during the
latter half of the 20
th
Century due to increasing public empathy (Mech, 1995). This shift in
public attitude prompted the U.S. Fish and Wildlife Service to protect the wolf in 1974
under the Endangered Species Act. In 1978 the Eastern Timber Wolf Recovery Team set
forth guidelines for the recovery of wolves into parts of their former range in the upper
Midwest. Wolves were able to rapidly recolonize most of the northern forests in the upper
Midwest because these areas supported high deer populations that wolves used as their
1
Present address: U.S. Fish and Wildlife Service, 6010 Hidden Valley Road, Carlsbad, California 92009
253
Am. Midl. Nat. 154:253–263
primary prey (Fritts and Mech, 1981; Fuller, 1989). As most of the northern forest in
Minnesota became saturated with wolf packs, wolves began establishing territories in peri-
pheral semi-agricultural areas containing abundant livestock and native prey (Fuller et al.,
1992; Berg and Benson, 1999). This population expansion of wolves into semi-agricultural
areas culminated in increasing concerns from farmers about wolves killing livestock (Mech
et al., 1988; Mech, 1995).
Little is known about the feeding ecology of wolves in a semi-agricultural area in the
upper Midwestern U.S. Studying the food habits of wolves in a semi-agricultural system is
necessary because it permits a more objective understanding of wolf behavior and ecology in
relation to their prey (e.g., Mills, 1996). We examined the food habits of wolves in a semi-
agricultural area of northwestern Minnesota during the non-winter seasons. Our objective
was to document the food habits of wolves during the time of year that livestock were grazing
in pastures (non-winter seasons), document the number of livestock killed by wolves and
investigate factors influencing wolf diet in an area with both native and domestic ungulates.
M
ETHODS
Study area.—The study was conducted in the Red River Valley floodplain of northwestern
Minnesota, along the northwest periphery of the gray wolf range in the Great Lakes region
(Berg and Benson, 1999). Euro-American settlement has converted many of the native
vegetation communities into agricultural lands; now the region is dominated by agricultural
settlements mixed with intermittent woodlands, grasslands and marshes. The 1200 km
2
study area, (hereafter referred to as Agassiz, 488199N, 958599W) was comprised of an ‘‘island’’
of natural habitat, consisting of Agassiz National Wildlife Refuge and three adjoining state
wildlife management areas (Elm Lake, Eckvoll and Mudlac Wildlife Management Areas) and
its surrounding agricultural lands (Fig. 1). The study area was comprised of 53% cultivated
lands, 20% wetlands, 10% brushlands, 10% forests (deciduous and coniferous) and 7%
pasture and grasslands. The climate was characterized by short warm summers and long
cold winters. Snow-cover was generally continuous from late November until April,
restricting livestock grazing in pastures from May to early November. Moose (Alces alces),
white-tailed deer (Odocoileus virginianus), beaver (Castor canadensis), muskrats (Ondatra
zibethicus) and eastern cottontail rabbits (Sylvilagus floridanus) represented the main native
prey available to wolves.
Food habit analysis.—Wolf food habits were determined by fecal analysis. Wolf feces were
collected every 2 wk on standardized routes within our study area during spring, summer
and autumn of 1997–1999. Feces were identified based on their shape, size and odor in
order to distinguish them from non-wolf feces (Weaver and Fritts, 1979). Feces were placed
in nylon stockings (mesh size ,0.25 mm
2
) and washed in a washing machine with bleach to
reduce exposure to hydatid tapeworms (Echinococcus granulosus). Feces were then air-dried
and examined macroscopically for prey remains. Hairs and solid fragments (i.e., bones,
teeth) were segregated by species. In instances where macroscopic inspection was not
possible for prey identification, negative impressions of hair cuticular scale patterns were
examined under a compound microscope (Adjoran and Kolenosky, 1969). In addition,
a reference collection of hairs and their cuticular scale impressions of all potential prey
items were made to help in microscopic prey identification. A visual estimate, to the nearest
FIG. 1.—Location of Agassiz NWR and State WMA’s, wolf territory boundaries and livestock pastures
during (A) 1998 and (B) 1999, northwestern Minnesota
!
254 THE AMERICAN MIDLAND NATURALIST 154(1)
255CHAVEZ & GESE:WOLF FOOD HABITS2005
5%, of the percent volume of each prey species was made for each feces. We did not use
frequency of occurrence of each prey species because small prey is often over-represented by
this technique (Weaver and Hoffman, 1979).
Prey estimates.—We used different methods for estimating native ungulate and domestic
livestock abundance. We used data from the U.S. Fish and Wildlife Service’s mid-winter aerial
surveys for deer and moose. The survey area included Agassiz NWR and the adjacent state
wildlife management areas; the surrounding agricultural lands were not surveyed. Surveys
were flown at 75 m above ground. A sightability correction factor was assigned for population
estimates for each species. The size of the cattle population was determined by requesting
numbers from each cattle producer (n¼ 27) within 4.8 km of the boundary of the refuge and
its adjacent state lands. We did not estimate sheep or domestic pig densities because they
occurred in very small numbers (two sheep operations and one domestic pig operation) and
because domestic pigs were not vulnerable to wolf predation (they were all enclosed within
a barn). However, wolves did scavenge dead pigs at the farm’s open-carcass dump.
Livestock depredations.—During the 3-y of our study, wolf depredation events in the
agricultural areas within our study area were documented by reports from U.S. Department
of Agriculture-Animal and Plant Health Inspection Service-Wildlife Services (WS) wolf
specialists. In Minnesota, farmers may contact either a Minnesota Department of Natural
Resources (MDNR) conservation officer or a WS wolf specialist for verification of claims of
potential wolf depredations on livestock (Fritts et al., 1992). This action determines if
farmers receive compensation for animals killed by wolves.
R
ESULTS
A total of 533 wolf feces were collected during the non-winter seasons from 1997–1999.
White-tailed deer was the most abundant food item for all 3 y combined (Table 1). The deer
component in the feces comprised both adult deer (26.9%) and fawns (12.3%). Muskrats
were the second highest food item by volume for wolves, followed by adult moose (13.0%)
and moose calves (0.6%), cattle, domestic pig, lagomorphs and beaver. Various birds, rodents,
insects and vegetation were minor components of the diet, totaling 10.6% by volume of all
other prey items identified. There was some minor annual variation in the role of different
prey species in the wolves during 1997–1999 (Table 1). In 1997 the percent volumes of
muskrat and deer remains in the wolves diet were very similar. During the following 2 y, deer
was the main prey item in the wolves diet. Furthermore, the percent volumes of muskrat,
moose and cattle remains in the diet of wolves were similar during 1998 and 1999. The role
that domestic pig played in the diet of wolves declined during the 3 y (Table 1).
There were fluctuations in the percent volume of deer, moose and muskrat remains in the
diet of wolves at 2-wk intervals during the non-winter months (Fig. 2). Decreases of deer in
the diet of wolves were often complemented by increases in muskrats, but less by moose.
Similarly, deer and muskrat were represented more consistently than moose in the diet of
wolves at 2-wk intervals during spring, summer and autumn. Also, small sample sizes of scats
across the 2-week time periods likely contributed to the variation observed.
During the period of our study, the deer population at Agassiz was at its lowest levels in the
past 30 y but slightly increasing (G. Huschle, unpubl. data). Winter surveys of 1996–1997
and 1997–1998 estimated the deer population to be 455 6 165 deer (90% confidence
interval) and 432 6 226 deer, respectively. In 1999 the deer population increased slightly to
504 6 190 deer. These numbers were considered dramatically low compared to the past
30 years when deer population estimates were never lower than 1172 deer (confidence in-
tervals were not calculated before 1995); in the winter of 1993–1994 the population was
estimated to be 2670 deer. Similarly, the moose population during the period of our study
256 T
HE AMERICAN MIDLAND NATURALIST 154(1)
