Mismeasured mortality: correcting estimates of wolf poaching in the United States

Abstract

Measuring rates and causes of mortalities is important in animal ecology and management. Observing the fates of known individuals is a common method of estimating life history variables, including mortality patterns. It has long been assumed that data lost when known animals disappear were unbiased. We test and reject this assumption under conditions common to most, if not all, studies using marked animals. We illustrate the bias for 4 endangered wolf populations in the United States by reanalyzing data and assumptions about the known and unknown fates of marked wolves to calculate the degree to which risks of different causes of death were mismeasured. We find that, when using traditional methods, the relative risk of mortality from legal killing measured as a proportion of all known fates was overestimated by 0.05–0.16 and the relative risk of poaching was underestimated by 0.17–0.44. We show that published government estimates are affected by these biases and, importantly, are underestimating the risk of poaching. The underestimates have obscured the magnitude of poaching as the major threat to endangered wolf populations. We offer methods to correct estimates of mortality risk for marked animals of any taxon and describe the conditions under which traditional methods produce more or less bias. We also show how correcting past and future estimates of mortality parameters can address uncertainty about wildlife populations and increase the predictability and sustainability of wildlife management interventions.

Political populations of large carnivores

“…In another example, Creel et al (2016a) argued that policy governing wolves-perhaps the most politically charged of all carnivores-in the United States does not adequately address hunting threats. Specifically, hunting policies could lead to population declines and do not align with ecological theory or data (Creel et al 2016a;Treves et al 2017a).…”

Predator Control Needs a Standard of Unbiased Randomized Experiments With Cross-Over Design

“…Therefore, interactions between human causes of death are important to our understanding of the intended and unintended effects of predator removal, as are the effects of interventions meant to curb human causes of mortality. For example, poaching (illegal killing by people) was found to be the major cause of mortality in four endangered wolf populations of the USA, and unregulated killing was the major cause in one Alaskan sub-population (Adams et al, 2008;Treves et al, 2017a). Those studies also revealed that poaching was systematically under-estimated by traditional measures of risk and hazard or that mortality of marked animals differed from that of unmarked animals under legal, lethal management regimes (Schmidt et al, 2015;Treves et al, 2017c;Santiago-Ávila, 2019;Treves, 2019a).…”

“…That practice continues today to protect domestic animals and crops from predators, although predators are now recognized for playing major roles in sustaining diversity and improving ecosystem resilience (Estes et al, 2011). Humans are the major cause of mortality of terrestrial carnivores globally, including extirpation, several cases of extinction of species, and protracted risks of extinction despite endangered species protections (Woodroffe and Ginsberg, 1998;Chapron et al, 2014;Treves et al, 2017a). Predator control plays a major role in human-induced mortality.…”

“…We apply the term 'control' (or treatment) to connote the intended management intervention, regardless of whether it proves effective. In particular, removal (usually lethal) as a form of predator control offers an important link to the global problems summarized above because intentional, legitimate, or illegal predator control has been the major component of human-caused mortality (Conradie and Piesse, 2013;Treves et al, 2017a), despite scant evidence worldwide for effectiveness of lethal methods to protect human interests and little of the available evidence provides strong inference (Treves et al, 2016;van Eeden et al, 2018). Removal methods provide an important heuristic for experimental tests of hypotheses about predator control.…”

Differentiating between regulation and hunting as conservation interventions

“…Treves et al. () reviewed 4 other cases in which inaccurate measurement of poaching led governments to downplay the major threat to endangered gray wolves ( Canis spp.). Predators in particular seem to be targets for the idea that hunting itself is a conservation intervention (Fig.…”