Is climate change “melting wolverines” as well as snowpack?

BBC released an article asserting that “climate change causes wolverine decline across Canada.” The article references a study by Jedediah Brodie of the University of Montana; the study, according to the BBC article and the study abstract, compares snowpack records and wolverine harvest records between 1968 and 2004, and finds a decline in both, except in the Northwest Territories, where wolverine populations are steady despite declining snowpack. The study concludes that “In provinces where winter snowpack levels are declining fastest, wolverine populations tend to be declining most rapidly,” and suggests that there is a causal link. From the abstract:

‘Fur returns have declined in many areas; our models show that snowpack has strong, nonlinear effects on wolverine population dynamics. Importantly, wolverine harvests dropped the fastest in areas where snowpack declined most rapidly and also where snowpack had the greatest effect on population dynamics. Moreover, declining snow cover appears to drive trends in wolverine population synchrony, with important implications for overall persistence.’

Aside from the fact that I’m not sure exactly what “population synchrony” means, and fully admitting that I haven’t actually read the study (because non-profits, due to miniscule budgets, don’t have access to academic journals), this research raises a couple of red flags. First, the reliability of trapping records as indicators of population trends seems questionable. Variables such as trapping intensity and effort, and wolverine movements across the landscape, could influence trapper take in ways that don’t necessarily have to do with wolverine population trends. Maybe the researchers accounted for this in their methodology, but it seems like it would be difficult to do. Second, the scientific mantra of “correlation is not causation” should be invoked here. The study suggests that there is a causal relationship, and the BBC story makes the leap from suggestion to certainty: the headline of the article states that climate change is, in fact, responsible for declining wolverine populations. Third, I’m not aware of any study that definitively shows that wolverine populations are actually declining – in the US Rockies, at least, this is a question that we are still seeking to answer. This mix of claims and questions illustrates one of the biggest challenges in communicating science to the general public: the fact that science is an endeavor of nuance and suggestion, while reporting is an endeavor of linear storytelling. In environmental narrative, it’s particularly important to try to raise the bar on what we claim as certain, causal relationships, and on guiding our readers through a more sophisticated understanding of the issues. I’m not going to write more about this until I’ve actually read the study, but it doesn’t sit well with me.

I want to be clear that I don’t doubt that there could be a connection between climate change and the wolverine’s prospects, particularly at the southern edge of the species’ range. Female wolverines den in snow and rely on deep snowpack through at least late April to keep their kits safe. Wolverine appear highly physiologically adapted to snowy conditions, and once, in early August at 11,000 feet, I watched a wolverine run from snow patch to snow patch, appearing to zig-zag out of his way just to feel the snow under his paws. But in the scientific realm, the wolverine’s relationship to snowpack has yet to be definitively elucidated. Another study, out of The Rocky Mountain Research Station in Missoula, Montana, is currently in press at the Canadian Journal of Zoology. “The bioclimatic envelope of the wolverine (Gulo gulo): do climatic constraints limit its geographic distribution?,” by Jeff Copeland et al, will hopefully help clarify some aspects of the complicated relationship between wolverines and different levels of snowpack, and could provide a sound basis for suggesting that decline in spring snowpack will have a negative impact on wolverine population through a decline in reproductive success. But making this leap based on trapping and snowpack records leaves out several steps in the chain of evidence. If we’re building a case for the claim that wolverine are endangered by climate change, let’s be sure we don’t leave ourselves open to criticisms of our methods.

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