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Assessing Factors That May Predispose
|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.
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|
|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 and barnyard||0||3|
|Pasture and barn||1||0|
|Pasture and barnyard||1||1|
|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|
|More than twice/day||0||2|
|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|
|Carcass dump and burn||0||1|
|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|
|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|
|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).
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.
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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