Comments on Brodie and Post: Climate-driven declines in wolverine populations: causal connection or spurious correlation?
- 713 Downloads
The recent paper by Brodie and Post (“Nonlinear responses of wolverine populations to declining winter snowpack”, Popul Ecol 52:279–287, 2010) reports conclusions that are unsupportable, in our opinion, due to both mis-interpretations of current knowledge regarding the wolverine’s (Gulo gulo) association with snow, and the uncritical use of harvest data to index wolverine populations. The authors argue that, because the wolverine is a snow-dependent species, average annual provincial snowfall, based on weather station data, can be expected to correlate strongly and positively with wolverine population numbers, which in turn can be accurately indexed by trapper harvests. Thus, correlations between declines in wolverine harvests and declining average snowpack are interpreted to reflect a climate-driven decrease in wolverine populations. This conclusion overstates the nature of the wolverine’s association with snow, and makes unsupportable assumptions about the reliability of harvest data as a proxy for population size.
KeywordsClimate change Gulo gulo Harvest data Trapping
Snow has long been recognized as a key component of wolverine (Gulo gulo) habitat (Hatler 1989; Banci 1994). Snow dens provide offspring with protection from predators and an insulation advantage (Pulliainen 1968; Magoun and Copeland 1998). Recently, we showed that wolverines appear to be restricted to cold areas represented by the availability and distribution of spring (mid-April to mid-May) snow cover (Copeland et al. 2010). Not only do wolverines require spring snow cover for reproductive dens, but year-round habitat use (Copeland et al. 2010) and travel routes associated with successful dispersals (Schwartz et al. 2009) are also confined almost exclusively to this climatic zone. Moreover, both historical occurrence records (Aubry et al. 2007) and genetic patterns (Schwartz et al. 2007) indicate that observed habitat-use patterns in the southern portion of their range in North America are not recent anthropogenic artifacts. There is no historical or current evidence that wolverines can persist outside climatically cold areas with spring snowpack. All the findings to date indicate that a warming climate would likely alter the nature and/or geographic extent of wolverine habitat on a world-wide scale. Although we agree that global warming can be expected to adversely affect wolverine populations, we contend that the data Brodie and Post (2010) used in their analyses cannot be used to investigate this hypothesis.
Climate change is a gradual process, and its effects on habitat suitability for wolverines and other organisms will vary geographically depending on a broad array of covariates occurring at multiple spatial scales. Additionally, it co-occurs with rapid anthropogenic changes which, in turn, may also be correlated with changing climates. Consequently, the kinds of correlative relationships described in Brodie and Post (2010) need to be explored in detail to ascertain the likelihood that such observations are causal in nature. However, Brodie and Post (2010) make no attempt to determine whether regional snow averages accurately describe the snow conditions where wolverines live, no attempt is made to confirm that the trapping data used provides a meaningful population index, and no attempt is made to explore whether the observed trends could be due to causes other than climate.
Additionally, there is no corroborative evidence to support their claim that wolverines are declining. Wolverine populations are generally described as stable or increasing across Canada (Lofroth and Krebs 2007; Slough 2007). Krebs et al. (2004) performed a vital rate meta-analysis of 12 North American wolverine studies conducted between 1972 and 2001, and concluded that unharvested populations were growing (λ ~ 1.06). In the contiguous US, wolverine populations appear to have increased during the last 50 years, as indicated by recolonization of areas from which they were previously extirpated (Aubry et al. 2007). Recently, dispersing males have been documented in California (McKelvey et al. 2008; Moriarty et al. 2009) and Colorado (Inman et al. 2010). These are the first verifiable wolverine records in these states since 1924 and 1919, respectively (Aubry et al. 2007), and are likely associated with current population expansions such as the recent observed recolonization of the northern Cascade Range in Washington (K. Aubry, unpublished data).
Brodie and Post (2010) do document a correlation—both the number of wolverines trapped annually in Canada and average snowfall have been declining in recent years. But the conclusion that this correlation reflects a causal relationship cannot be inferred from the data presented. Populations of wolverines may or may not have declined during this period. Population changes, if they occurred, may or may not have been due to shifting climate. Thus, the authors’ claim that they have detected a climate-driven decline in wolverine populations using these data is, in our opinion, unsupportable.
We thank Michael Schwartz, John Squires, and two anonymous reviewers for helpful comments on previous drafts of this manuscript.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
- Banci VA (1994) Wolverine. In: Ruggiero LF, Aubry KB, Buskirk SW, Lyon LJ, Zielinski WJ (eds) The scientific basis for conserving forest carnivores: American marten, fisher, lynx and wolverine in the western United States. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, General Technical Report RM-254. Fort Collins, Colorado, pp 99–127Google Scholar
- Copeland JP, McKelvey KS, Aubry KB, Landa A, Persson J, Inman RM, Krebs J, Lofroth E, Golden H, Squires JR, Magoun A, Schwartz MK, Wilmot J, Copeland CL, Yates RE, Kojola I, May R (2010) The bioclimatic envelope of the wolverine (Gulo gulo): do climatic constraints limit its geographic distribution? Can J Zool 88:233–246CrossRefGoogle Scholar
- Gese E (2001) Monitoring of terrestrial carnivore populations. In: Gittleman JL, Funk SM, MacDonald DW, Wayne RK (eds) Carnivore conservation. Cambridge University Press, Cambridge, pp 372–396Google Scholar
- Hatler DF (1989) A wolverine management strategy for British Columbia. Wildlife Bulletin B-60. Ministry of Environment, Wildlife Branch, VictoriaGoogle Scholar
- Inman RM, Packila M, Inman K, Aber B, Spence R, McCauley D (2010) Greater Yellowstone wolverine program. Unpublished report, Wildlife Conservation Society. Ennis, MontanaGoogle Scholar
- Lambin X, Krebs CJ, Moss R, Yoccuz NG (2002) Population cycles: inferences from experimental, modelling, and time series approaches. In: Berryman A (ed) Population cycles: the case for trophic interactions. Oxford University Press, New York, pp 155–176Google Scholar
- Novak M, Obbard ME, James JG, Newman R, Booth A, Satterthwaite AJ, Linscombe G (1987) Furbearer harvests in North America, 1600–1984. Ministry of Natural Resources, OntarioGoogle Scholar
- Pulliainen E (1968) Breeding biology of the wolverine (Gulo gulo L.) in Finland. Ann Zool Fenn 5:338–344Google Scholar