U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version



Pepin KM, Kay SL, Davis AJ. 2017 Comment on: ‘Blood does not buy goodwill: allowing culling increases poaching of a large carnivore’. Proc. R. Soc. B 284: 20161459. http://dx.doi.org/10.1098/rspb.2016.1459


Copyright 2017 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License.


Chapron & Treves [1] present a framework for examining effects of wolf culling policies on wolf population growth rate. They develop a population growth model that estimates an effect of the amount of time per year legal culling is allowed (‘policy effect’) on wolf population growth rates, separate from an effect of culling. They infer that there is substantial evidence for a negative relationship between the proportion of the year that the culling policy is in effect and the population growth rate because 83% of the posterior distribution for the policy effect parameter was negative. They conclude that when it is legal to cull wolves, their population growth rate is slower than it would be when it is not legal to kill wolves, even after accounting for effects of culling on population growth rates. By considering additional analyses showing that the levels of legal culling are not causing negative density-dependence, they argue that wolf culling policies devalue wolves in the public’s eye such that poaching activity increases. We have several major issues with the conclusions drawn from this work.

First, the magnitude of the policy effect is biologically weak, but the biological significance (impact to the wolf population) was not presented or discussed in [1]. To show the biological significance, we plotted predictions from the model [1] with and without the policy effect included (figure 1). If the policy effect is biologically meaningful, there should be substantially fewer wolves in the model that includes the policy effect relative to one that does not. However, when the policy effect is included, predicted abundance from the two models did not appear to be biologically meaningful to wolf population growth rate, with an average of -7.8 wolves different per year that the policy was in place ([-28.2, 5.5] range for 95% credible intervals), which is on average 1.5% of the population ([-5.8%, 1.1%] range for 95% credible intervals).

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