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Posted by davemech on 31 Oct 2011 at 16:23 GMT
Readers interested in this subject should also see Webb, N. F., J. R. Allen, and E. H. Merrill. 2011. Demography of a harvested population of wolves (Canis lupus) in west-central Alberta, Canada, Can. J. Zool. 89:744-752. That paper shows (1) why recruitment needs to be explicitly considered in assessing harvest rates and population change (lambda) as per Gude et al. (2011); (2) that wolf populations can sustain 34% annual harvest, contrary to Creel and Rotella; and (3) that natural mortality is reduced with human-caused mortality as per Fuller et al. 2003.
Webb et al's study... some important points
scott_creel replied to davemech on 31 Oct 2011 at 20:46 GMT
Webb et al's 2011 study is well executed, and their paper is an important contribution to our understanding of harvesting's effect on wolf dynamics.
Nonetheless, an unusual by critical feature of their data is important to note, when evaluating Dr. Mech's inferences:
1. Webb's study examined a population in which two thirds of the human offtake was through trapping (rather than shooting). Trapping and shooting yield very different patterns of mortality across ages.
2. As a consequence, 71% of the wolves killed in this study were immature individuals, not yet contributing to population growth, with low reproductive value.
3. Furthermore, loss of immature wolves has not been shown to disrupt pack stability and the probability of subsequent reproduction, as has been shown for loss of adults, particularly alphas.
4. For these reasons, it is not surprising that the maximum sustainable offtake rate estimated by Webb's study (34%) is higher than the rate of offtake that yields lambda =1 with more typical harvests. Indeed, this very point is the final sentence of Webb et al.'s abstract:
"We suggest that a high proportion of juveniles harvested and the spatial structure of the registered trapline system contributed to the sustainability of harvests."
5. The 34% maximum sustainable harvest rate reported in Webb's study falls right at the edge of the 95% confidence limits from Creel & Rotella (2010)'s analysis. In contrast, it is less than half of the maximum sustainable harvest rate (77%) suggested by Gude & Mech in their 2011 paper.
Reply to Creel's "important points"
davemech replied to scott_creel on 03 Nov 2011 at 16:25 GMT
In reply to Creel’s response to my 31 October 2011 comment, I offer the following:
1. There are few data available indicating that trapping and shooting result in harvesting different proportions of breeding wolves.
2. The human take under fair-chase rules almost always consists primarily of non-breeders, simply because those form the highest percent of any wolf population and because those are the most vulnerable.
3. It is correct that loss of non-breeders tends not to cause pack disruption, whereas loss of breeders does. However, I know of no data that indicate that the proportion of breeders taken by public harvest in the NRM is any different from that elsewhere.
4. For the above reasons, this is why the Webb et al. study's conclusion typifies those of other harvested populations, contrary to Creel.
5. The Webb et al. estimate of 34% sustainable yield not only falls at the extreme edge of the Creel and Rotella 95% confidence limit, but it falls well within the Gude et al. model estimates. The Gude et al. model correctly identified every wolf population increase in the NRM during 1999-2009.