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Assessing Factors That May Predispose
Minnesota
Farms
To
Wolf Depredation on Cattle
L. David Mech, Elizabeth K. Harper, Thomas J. Meier,
and William J. Paul
Abstract:
Wolf (Canis lupus)
depredations on livestock cause considerable conflict and expense in Minnesota.
Furthermore, claims are made that such depredations are fostered by the type of
animal husbandry practiced. Thus, we tried to detect factors that might
predispose farms in Minnesota to wolf depredations. We compared results of
interviews with 41 cattle farmers experiencing chronic cattle losses to wolves
(chronic farms) with results from 41 nearby "matched" farms with no
wolf losses to determine farm characteristics or husbandry practices that
differed and that therefore might have affected wolf depredations. We also used
a Geographic Information System (GIS) to detect any habitat differences between
the 2 types of farms. We found no differences between chronic and matched farms
in the 11 farm characteristics and management practices that we surveyed, except
that farms with chronic losses were larger, had more cattle, and had herds
farther from human dwellings. Habitat types were the same around farms with and
without losses. The role of proper carcass disposal as a possible factor
predisposing farms to wolf depredations remains unclear.
Introduction
Wolf (Canis lupus) depredations on livestock are a serious concern to
Minnesota farmers, resource managers, agricultural officials, environmentalists,
and state legislators. The wolf in Minnesota is currently on the federal
endangered species list in the "threatened" category. However, because
wolf numbers there have exceeded recovery levels (B. Berg and S. Benson,
unpublished report, 1999), the federal government will soon propose removing the
wolf in Minnesota from the endangered species list. Minnesota will then be
responsible for wolf management, and continued control of wolves preying on
livestock will be one of the greatest management needs (Mech 1998).
Although the total proportion of farms in wolf range that suffer verified
wolf depredations is only about 1% per year (W. J. Paul, unpublished report,
1998), several factors must be considered to provide a more complete
understanding of the importance of wolf depredations: 1) because it is difficult
to verify wolf depredations, far more livestock may be lost to wolves than are
verified (Roy and Dorrance 1976, Fritts 1982); 2) to farmers who do suffer
damage, the loss is real and significant economically, even though partially
offset by state compensation payments for verified losses; 3) over a period of
years, livestock from hundreds of farms have been preyed upon; 4) number of
farms sustaining such damage is increasing at an accelerating rate (Mech 1998);
5) wolf range is currently expanding into some of Minnesota's greatest densities
of livestock (Minnesota Agriculture Statistics 1997); and 6) the wolf population
has reached a level at which standard hunting and trapping techniques may be
unable to prevent increases (Mech 1998).
Since 1978, when the wolf in Minnesota was downlisted from federally endangered
to threatened, the United States Fish and Wildlife Service and then the United
States Department of Agriculture's Wildlife Services (WS) have conducted lethal
control of wolves around farms where depredations have been verified (Fritts
1982, Fritts et al. 1992), a program costing $300,000 in 1998. In addition, the
Minnesota Department of Agriculture currently pays about $67,000/year in
compensation for livestock confirmed lost to wolves. Conservative projections of
these 2 costs exceed $400,000/year for the next few years (Mech 1998).
Concurrent with the increase in wolves and wolf range, the number of wolves
killed for depredation control has increased dramatically from 6 in 1979 to 216
in 1997. Projections show that a conservative estimate of the number of wolves
that may need to be killed for depredation control by 2005 might exceed 400/year
(Mech 1998), a serious concern to wolf advocates and environmentalists (Anderson
1999).
There has long been a belief that wolves prey on livestock because of poor
husbandry practices by farmers. This could be a misinterpretation of the claim
that "many instances of wolf depredation on livestock in Minnesota seem to
be related to animal husbandry practices" (Fritts 1982:7), a statement
which implicates poor husbandry practices but does not place sole blame on them
for wolf depredations. Acknowledging that "data collection on these issues
was not extended beyond that taken for the earlier report," Fritts et al.
(1992:14) indicated that "any further conclusions are subjective" and
that "research is needed to . . . determine the causes of the onset of
stock-killing behavior." Fritts (1982) and Fritts et al. (1992) identified
3 factors as potentially predisposing livestock to wolf depredations: 1)
pasturing in wooded-brushy areas, 2) calving in wooded-brushy areas or in remote
open range rather than in or near barns, and 3) improper disposal of carcasses,
which can attract carnivores; this practice could affect the farm involved or
even neighboring farms. Similarly, livestock depredations in western Canada seem
to be related to the forest-agricultural edge (Gunson 1983, Bjorge and Gunson
1985), livestock production in forested areas (Gunson 1983), and improper
carcass disposal (Tompa 1983). Gunson (1983) also stated that livestock
depredations at these edges are influenced by the number of livestock present,
animal husbandry practices, and potentially relative abundance of natural prey.
We sought to assess the role of suspected major factors that may predispose
cattle to wolf depredations and to attempt to elucidate any unknown factors. We
did not intend to examine such basic husbandry practices as maintaining herds in
good health and nutrition and taking reasonable care of them.
Methods
To attempt to identify factors that predispose some cattle farms in Minnesota to
wolf depredations, we interviewed farmers who had recurring depredation problems
between 1989 and 1998 (chronic farms). We also interviewed neighboring farmers
who had no wolf depredations during the same period (matching farms, Figure 1).
We hypothesized that if 2 farms were close enough to be within a reasonable
range of the same wolves, but one farm had depredation problems and the other
did not, there could be discernible differences between the 2 farms that would
lend insight into why wolves preyed on livestock at some farms but not others.
| Figure 1. Locations of
Minnesota farms suffering chronic depredations by wolves during
1989-1998 and farms not suffering losses, chosen as a matching sample. |
We chose to study chronic farms rather than those experiencing only
occasional loss because chronic farms are more likely to have some
characteristic that predisposes them to depredations. Farms that experience only
occasional losses are more apt to be affected by random events, such as presence
of a dispersing wolf passing through the area (Fritts 1982).
To assess which farms suffered chronic losses, we created a database from WS
records of all verified wolf depredation complaints from 1989 to 1998. We ranked
farms according to number of calendar years when they suffered verified losses.
We defined chronic farms as those where WS personnel had verified at least one
wolf depredation in each of 3 or more years during the 10-year period (Fritts et
al. 1992).
We deemed 51 farms (4 sheep, 4 turkey, and 43 cattle) in 15 counties as
chronic during 1989-1998. Because of the low number of sheep and turkey farms
and the difficulty of finding a match for them, we considered only cattle farms.
We used 41 of 43 cattle farms in the analysis as we were unable to interview
owners of one chronic farm and unable to find a match for another. Though all
had wolf losses during at least 3 years in 10, the history of wolf depredations
on these farms varied considerably. Individual farms experienced up to 18
episodes of depredation during the study and had depredations during 8 years of
the 10-year period.
Around each chronic farm, we attempted to locate other farms raising the same
type of livestock (beef cattle or dairy cattle) where wolf depredation had not
occurred (matching farms). To randomize our matching sample, we chose a cardinal
direction from the depredated farm by throw of a die and first searched for
matching farms in that direction within 8 km of the chronic farm. The principal
method of locating matching farms was driving in the random cardinal direction
looking for livestock, pasture areas, and hay storage. WS personnel, county
extension agents, cattlemen's associations, and other farmers also were
questioned as to the locations of potential matching farms.
If we did not find a farm without claimed wolf losses within 8 km in the
initial compass quadrant, we extended the search in other directions, working
clockwise from the initial random direction. In some cases, we needed to go up
to 15 km from the chronic farm to locate non-problem farms to survey. Several
farms were usually surveyed near each chronic problem farm until a suitable
matching farm was found.
We avoided using as matching farms those with verified wolf problems that did
not reach the level of chronic farms. If an operator claimed to have suffered
wolf depredation, even if no losses had been verified in the last 10 years, we
rejected that farm as a matching (non-depredated) farm and chose other matching
farms.
We visited each of the chronic and matching cattle farms one to 4 times
between July 1998 and January 1999 to survey the owner or manager in person.
When this could not be done, we conducted telephone interviews (n = 15).
Interviews covered location and size of the livestock operation, history of
livestock raising and depredation problems, farm size, number of cattle, number
of years raising cattle, amount of pasture bordered by brush or forests, longest
distance of livestock from house, pasture characteristics, calving locations,
number of times stock were checked each week, presence of carcass dump, and
carcass disposal methods.
Besides the 41 farmers at chronic farms, we interviewed 145 farmers at
matching farms and chose 41 matches that fit the criteria stated above. We then
summarized the answers to the survey questions that might provide insight into
factors predisposing livestock to wolf depredations.
A factor identified as possibly being important in predisposing certain farms
to wolf depredations was leaving livestock carcasses where scavengers could use
them (Fritts 1982, Fritts et al. 1992). As part of a separate survey involving
use of rendering plants for carcass disposal, we requestioned farmers in our
sample of matching farms about their carcass disposal methods. We attempted to
phone each matching farm during 15 April to 2 May 1999.
The group of chronic farms we surveyed was essentially an entire population
rather than a random sample. Therefore, to determine significant differences
between measures derived for chronic farms versus measures for our sample of
matching farms, we used the following approaches. We considered any average
measure of the chronic population to differ significantly from that of the
matching sample if the average for the chronic farms fell outside the 95%
confidence limits of the average of the matching sample. To compare
distributions of characteristics between our 2 types of farms, we used the
chi-square test.
We created a GIS coverage of chronic farms and another of matching farms.
Using ArcView (Environmental Systems Research Institute, Redlands, California,
USA) GIS software, we created zones with radii of 1.6 km and 4.8 km around farms
to examine surrounding habitat. We dropped one chronic farm and its match from
the analysis of the 4.8-km radius because the radius extended out of Minnesota,
where we had no habitat data.
For our habitat analysis we used a coverage assembled by the Minnesota
Department of Natural Resources, Grand Rapids, which mapped 8 cover types (urban
and rural development, cultivated land, hay-pasture-grassland, brush, forest,
water, bog-marsh-fen, and mining). The source data were collected between 1987
and 1996 and were originally captured in 30-m (13 counties) and 90-m (2
counties) cells and then converted into a feature data source.
We then used ArcView to estimate percentage of each habitat type for chronic
and matching farms within the 1.6-km and 4.8-km radii. The data for each kind of
farm were pooled to give a single set of percentages of habitat for each kind.
We hypothesized that if farms with chronic losses were surrounded by some
specific cover or land-use type that predisposed them to wolf depredations, then
the pooled data should differ from those for the matching farms in proportions
of habitat types.
Because of the possible importance of carcass disposal as a predisposing
factor and because improper carcass disposal is illegal, we attempted to
cross-check reporting about this subject. For chronic farms, we asked WS
personnel about their personal knowledge of carcass disposal at these farms and
compared their replies with those obtained from direct interviews.
WS personnel had no personal knowledge of conditions on matching farms,
however. Thus, as a cross-check for those farms, we compared replies about
carcass disposal at matching farms with replies to a similar question asked of
the same farms during the special telephone survey about rendering plants.
Results
All but 3 of the 11 farm characteristics and management practices we assessed
were similar for chronic and matching farms, with one factor being equivocal
(Tables 1-4). The 3 factors that differed were size of farm, number of
livestock, and distance of livestock from human dwelling and these factors were
correlated (r2 = 0.09-0.37, P =
0.001-0.05). The chronic farms were larger (491 vs. 292 ± 71 ha), had more
cattle (158 vs. 82 ± 18), and had herds farther (mean maximum distance = 2.8 km
vs. 1.8 ± 0.5 km) from human dwellings (Table 1).
| Table 1. Mean (±95%
confidence limits) values of Minnesota farm characteristics for 41 farms
suffering chronic wolf depredations on cattle and 41 nearby matching
farms that experienced no such losses, 1989-98. |
| |
Chronic a |
Match a |
| Farm size (ha) |
491 |
292 ± 71 |
| Number of cattle |
158 |
82 ± 18 |
| Number of years raising cattle |
38 |
35 ± 8 |
| Amount (arc°) of pasture bordered by brush-forest |
213 |
205 ± 38 |
| Longest distance livestock is from house (km) |
2.8 |
1.8 ± 0.5 |
| a Chronic farms represented a
complete population, except for 2 farms, whereas matching farms were a
sample. |
| Table 2. Types of pasture
where cattle were located at 41 Minnesota farms suffering from chronic
wolf depredations on cattle and 41 nearby matching farms experiencing no
such losses, 1989-98. |
| Type of pasture |
Total chronic a |
Total matching a |
| Brushy |
0 |
2 |
| Open |
13 |
14 |
| Wooded |
0 |
0 |
| Open-brushy |
4 |
4 |
| Open-brushy-wooded |
11 |
7 |
| Open-wooded |
13 |
14 |
| Total |
41 |
41 |
a Chronic farms represented a
complete population, except for 2 farms, whereas matching farms were a
sample. ( = 2.96, P = 0.56). |
| Table 3.
Calving locations for 41 Minnesota farms suffering chronic wolf
depredations on cattle and 41 nearby matching farms that experienced no
such losses, 1989-98. |
| Location of calving |
Total chronic a |
Total matching a |
| Barn |
3 |
4 |
| Barnyard |
29 |
25 |
| Barn and barnyard |
0 |
3 |
| Pasture |
7 |
6 |
| Pasture and barn |
1 |
0 |
| Pasture and barnyard |
1 |
1 |
| No calves |
0 |
2 |
| Total |
41 |
41 |
a Chronic farms
represented a complete population, except for 2 farms, whereas matching
farms were a sample. ( = 6.52, P
= 0.37). |
| Table 4. Number
of times/week Minnesota farmers checked cattle at 41 farms suffering
chronic wolf depredations on cattle and 41 matching farms that
experienced no such losses, 1989-98. |
| Times stock checked |
Chronic farms a |
Matching farms a |
| 0 |
1 |
0 |
| 1 |
1 |
3 |
| 2 |
5 |
5 |
| 3 |
3 |
1 |
| 4 |
2 |
6 |
| 7 |
21 |
20 |
| 14 |
7 |
4 |
| Almost daily |
1 |
0 |
| More than twice/day |
0 |
2 |
| Total |
41 |
41 |
a Chronic farms
represented a complete population, except for 2 farms, whereas matching
farms were a sample. ( = 8.84, P
= 0.36). |
The equivocal factor was method of carcass disposal. Contrary to
expectations, more farms with chronic losses reported properly disposing of
carcasses than did matching farms not suffering cattle depredations (Table 5).
However, WS personnel indicated that they had observed evidence of at least an
intermittent carcass dump on all except 2 of the 41 farms with chronic losses.
Number of carcasses that matching farms disposed of varied from 2 to 10/year.
| Table 5. Carcass disposal
methods for 41 Minnesota farms suffering chronic wolf depredations on
cattle and 41 nearby matching farms that experienced no such losses,
1989-98. |
| Carcass disposal method |
Total chronic farms a |
Total matching farms a |
| Bury |
24 |
13 |
| Burn |
3 |
4 |
| Carcass dump |
1 |
3 |
| Carcass dump and burn |
0 |
1 |
| Rendering plant |
3 |
2 |
| Leave in pasture |
2 |
10 |
| Bury and lime |
1 |
0 |
| Bury and burn |
2 |
3 |
| Leave in pasture and burn |
1 |
0 |
| Leave in pasture and feed to dogs |
1 |
1 |
| Leave in pasture and bury |
1 |
1 |
| Rendering plant and bury |
1 |
1 |
| Rendering plant and feed to dogs |
1 |
0 |
| Rendering plant and pasture |
0 |
1 |
| Unknown |
0 |
1 |
| Total |
41 |
41 |
a Chronic farms represented a
complete population, except for 2 farms, whereas matching farms were a
sample. ( = 6.15, P = 0.30). |
Habitat-land-use characteristics for chronic farms and their matches were
similar in all respects that we could measure for the 1.6-km and 4.8-km radii
(Table 6). In other words, neither habitat nor land-use proportions within 1.6
or 4.8 km around farms differed between chronic and matched farms.
| Table 6. Percentage of
habitat types within circles of 1.6-km and 4.8-km radii around the farms
summed for 41 Minnesota farms suffering chronic wolf depredation on
cattle and 41 nearby matching farms that experienced no such losses,
1989-98. |
| Habitat Type |
1.6-km radius |
4.8-km radius a |
| Chronic farms b |
Matching farms b |
Chronic farms b |
Matching farms b |
| Bog-marsh-fen |
12 |
9 |
17 |
14 |
| Brushland |
10 |
10 |
9 |
9 |
| Cultivated |
19 |
22 |
15 |
16 |
| Forested |
35 |
34 |
43 |
43 |
| Hay-pasture-grassland |
23 |
22 |
13 |
14 |
| Mining |
0 |
0 |
0 |
0 |
| Urban-rural development |
1 |
1 |
1 |
1 |
| Water |
1 |
1 |
2 |
3 |
a Chronic farm without a
matching farm not included in analysis.
b One chronic farm and its match removed as the 4.8-km buffer
extended outside of habitat coverage. Because results were so obviously
similar, they were not tested statistically, in keeping with Chatfield
(1995) and Cherry (1998). |
Discussion
The only definite and significant differences we found between farms suffering
chronic losses to wolves and their nearby matching farms that experienced no
losses were a suite of related size characteristics: size of farm, number
of cattle, and longest distance of stock from human dwellings. Our findings
regarding carcass disposal were unclear.
A number of possibilities may explain why larger farms with more cattle
pastured farther from human dwellings suffered more wolf depredations. Larger
operations may have had greater exposure to wolf depredations simply because of
their size and perhaps because wolves were attracted to larger herds. Maximum
distance that stock was pastured from human dwellings, due to the larger farm
size, would not seem to be relevant because wolves often kill stock near houses
and buildings. Furthermore, we know of no reason the difference between the
2.8-km mean maximum distance for chronic farms and the 1.1 (± 0.5)-km distance
for the matching farms would be meaningful to wolves, and the difference between
the mean distances of cattle from the houses in the 2 groups would be even less.
Larger farms and herds also may have had less human presence. Conceivably, farm
size itself was a neutral factor, but some unknown factor related to farm size
was causative.
There are several possible explanations for the counter-intuitive and
equivocal nature of the findings about carcass disposal. Eighty-five percent of
chronic farms reported properly disposing of carcasses, whereas only 56% of
matching farms reported proper disposal during the same survey. Conceivably, at
least some farms with chronic losses, having been visited so frequently by
government personnel and advised to dispose properly of carcasses, actually did
so, an interpretation at least partly supported (Fritts et al. 1992).
Other possible explanations are:
1.) farmers with chronic losses may be making
a sincere effort to alleviate their problem by properly disposing of carcasses;
2.) larger operations may have more need for systematic carcass disposal and
therefore more efficient methods —for example, preparing a large pit for
frequent use; or
3.) larger farms may be more likely to own heavy equipment to
bury carcasses.
On the other hand, false reporting about livestock carcass disposal also may
have been a problem with chronic farms. This interpretation is supported by the
disparity between interview results from farmers suffering chronic losses and
the recollections of WS personnel. This disparity may be due to the different
periods covered by the 2 types of data collection. Our survey covered only 1998,
whereas the recollection of WS personnel spanned a decade or more. Perhaps some
chronic farms had carcass dumps prior to 1998 but no longer have them.
Potentially all these factors were operating.
Although these confounds prevent any firm conclusion, some interesting
insights into responses to questions about carcass disposal can be extracted
from the matching sample results. Of 18 matching farms that answered the basic
survey and the rendering plant survey, 44% replied similarly in both surveys
that they burned or buried carcasses or sent them to rendering plants (proper
disposal), 28% replied similarly in both surveys that they left carcasses above
ground (improper disposal), and 28% replied dissimilarly on the 2 surveys. Thus,
56% of farmers who had not suffered wolf depredations admitted on either or both
surveys that they improperly disposed of carcasses. Nevertheless, all these
farms are within 15.2 km (mean of these 56% farms = 6.2 km, range 1.6-15.2 km)
of farms that experienced chronic depredations by wolves. If improper carcass
disposal were of prime importance in predisposing farms to wolf depredations,
one wonders why matching farms did not suffer such depredations.
One possible explanation is that because matching farms held fewer cattle,
they may have sustained fewer general losses and thus had fewer carcasses
available. Larger farms would generally have more natural losses and thus might
have provided a more reliable food source at carcass dumps, thus attracting
wolves more often. This interpretation could even be the explanation for why
larger farms with more cattle tended to experience wolf depredations. However,
the whole subject of carcass disposal as a factor predisposing cattle to wolf
depredations remains open.
Acknowledgments
This study was funded by a grant from the Minnesota Department of Agriculture to
the University of Minnesota's Department of Fisheries and Wildlife through an
appropriation by the Minnesota Legislature; the Biological Resources Division of
the United States Geological Survey; and the United States Department of
Agriculture's North Central Research Station. We thank the North Star chapter of
the Sierra Club for proposing the study and promoting its funding. We also thank
the following personnel of the Wildlife Services Division of the United States
Department of Agriculture for general advice, suggestions, and background
information, critiquing the interview questions, and interviewing some of the
farmers: J. P. Hart, D. P. Sahr, J. P. Grabarkewitz, and G. J. Halvorson.
Glen A. Sargeant provided statistical advice, and Steven H. Fritts critiqued
this manuscript and made many helpful suggestions for its improvement. We
especially thank all the farmers who participated in this study and gave freely
of the information we requested.
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