Northern Prairie Wildlife Research Center
To maintain the same size census area each year while individual wolf packs shifted their use of the area somewhat, we only counted the number of wolves proportionate to the percent of the census area that a given pack used that winter, based on radiotracking data. We subtracted the number of wolves killing primarily moose from our total wolf census to derive the number of wolves dependent on deer (Mech 1986; L. D. Mech, U.S. Geological Survey, unpublished data).
Information on buck harvest was obtained from the mandatory registration of bucks with the Minnesota Department of Natural Resources by hunters in various "kill blocks" in and adjacent to our wolf-census area (Lenarz 1997 and M.S. Lenarz, Minnesota Department of Natural Resources, personal communication). These kill blocks included an area east of Ely, an area around Isabella, and an area south of Ely (Fig. 1). There was a good relationship between the trends of the harvests in the latter 2 areas (r2 = 0.60, P < 0.001), but not between the harvest in the first area and in either of the other two. We did not use numbers of antlerless deer harvested because those numbers fluctuated with the number of permits granted.
No measure of hunting effort was available for our study area to test whether variable hunting pressure obscured effects of wolves. Nevertheless, we hypothesized that if wolves had a strong negative effect on number of bucks harvested, we should find an inverse relationship between wolf numbers in the deer-killing packs one winter and the buck harvest the following fall. Thus, we used simple linear regression (Statistix 4.1 1994) to compare harvest statistics to wolf numbers.
So as not to overlook possible relationships that might support our hypothesis, we deliberately ran regressions on whatever combinations of our 2 variables we thought logical. This approach would assure that if we did not find significant relationships, that negative finding would tend to indicate either that wolves were having little effect or that variable hunting effort might be masking any wolf effect. We analyzed data from each pack and from our entire census area against harvest statistics in 2 kill blocks partly in the wolf-census area and harvest data from the zones immediately west of our wolf-census area (Fig. 1). We assumed that annual changes in estimates of wolf density represented changes in the surrounding area as well. We also examined relationships between buck harvest in each kill block and wolf populations in and near each of those blocks.
With our largest data set, we also examined the individual annual changes in wolf and deer numbers (Table 1) and examined plots for any lag between wolf numbers and buck harvest that might confound regression analyses (Fig. 2). Annual numbers of deer hunters fluctuated widely in our study area before 1988 and after 1995, but remained reasonably constant from 1988 to 1995 (M. S. Lenarz, Minnesota Department of Natural Resources, personal communication). Therefore, we conducted separate analyses for 1988 to 1995.
|Fig. 2. Buck harvest (solid line) in the Isabella area and population trend of wolves that were dependent on deer in the census area (Fig. 1).|