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Trends and Management of Wolf-Livestock Conflicts in Minnesota

Data Analysis


Regression analysis was used to determine the relation between time and severity of the winter before a depredation season on the number of verified complaints, number of farms with verified complaints, and number of wolves captured and killed. Winter severity index (WSI) values were based on a combination of the cooling (temperature, wind, and relative humidity as an integrated measure), the supporting capability of snow, and total snow depth Werme 1968). Lower WSI values represent mild winter weather; higher values represent a severe winter. Deer fawns, the primary wild prey of wolves, are more vulnerable to predation after severe winters because they are more poorly nourished in utero. The WSI values we used were the average of values from two reporting stations at opposite ends of the region-International Falls and Agassiz National Wildlife Refuge. The hypotheses that the regression slopes did not differ from zero (no trend) versus the one-sided alternative that slopes were greater than zero (positive slope) were tested (Sokal and Rohlf 1969). When multiple regressions were used, partial sums of squares were used to compute test statistics. Equality of the variance in yearly depredation indices among prey types was tested using F-ratios (Sokal and Rohlf 1969).

We used the minimum convex polygon method to calculate the total land area that the depredation sites covered during the different periods. Shifts in these polygons were useful in assessing trends in distribution of depredations. We also calculated geometric means of these sites for different periods as indicators of distribution shifts.

The relation between trapping success and subsequent depredations at the same location was examined with 2 × 2 contingency tables and chi-square tests. This method was also used to test for relations between the removal of specific age classes of wolves and subsequent depredations (Snedecor and Cochran 1973). Unless stated otherwise, P < 0.05 was considered statistically significant.

Because several data from 1975 through 1978 were incomplete, comparison with more recent data was not always appropriate. Many of the analyses and discussion in this paper refer only to 1979-86. Several farms experienced depredations in more than 1 year. Therefore, figures for the number of farms with depredations during multiple years are cumulative totals derived by summing the numbers from each year.


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