Wolverines on the Brink

I attended the Fish and Wildlife Service’s recent wolverine listing hearings in Helena, Montana.  Opponents, including a number of Montana state legislators as well as MDFWP, argued that wolverine populations were “stable” or even “increasing” and therefore did not warrant listing under the Endangered Species Act.

While the ill-informed state legislators who testified could be forgiven for not understanding basic biology, MDFWP does not have that excuse. Their opposition to listing of the wolverine and their insistence on continued trapping when the state’s wolverine population is so tiny is both ecologically irresponsible and a breach of the public trust to protect Montana’s wildlife heritage.

The most obvious long-term threat to the wolverine is climate change. That will become a greater threat to wolverine in the future. However, what is ignored by many opponents of listing is that with a population of only 250-300 animals in the entire lower 48 states, and perhaps no more than 175 in Montana, wolverine are already well under what most conservation biologists believe is a minimum number for long-term survival. In other words from a purely biological perspective they are clearly endangered with extinction now, regardless of what happens with global climate change in the future.

Wolverine have one of the lowest reproductive rates of any of the West’s mammals. Even in the best of circumstances, wolverine can barely balance their population increases against losses. Any increase in mortality, for whatever reason, could cause such a small population to spiral downward to extinction.

In addition, its breeding behavior makes the wolverine more prone to extirpation than other animals. Wolverine males will stake out a territory that over lapses with a number of females. This automatically ensures lower genetic diversity because the one male may breed several females.

With only 250-300 animals, due to this behavioral trait the “effective breeding population” is only 30-40 animals. This tiny reproductive effort almost guarantees a loss in genetic diversity and indeed, recent genetic studies have confirmed that wolverine in the lower 48 states suffer from low genetic diversity compared to wolverine in more robust populations in Canada.

Wolverine. Photo: US Fish and Wildlife Service

Wolverine. Photo: US Fish and Wildlife Service

Loss of genetic diversity can lead to all kinds of genetic disorders that can threaten the species survival, and also limits its ability to cope with random stochastic events that individuals with greater genetic diversity might be able to cope with. As a rule of thumb, most geneticists believe that an effective breeding population of at least 500 individuals is needed to ensure the long term survival of carnivore species. With only 35-40 breeding animals in any one year, wolverine are well below this safe threshold.

Worse for the wolverine’s long term survival, the remaining wolverine populations are frequently found in small isolated groups in widely scattered mountain ranges. The survival of the larger population is dependent on the health of these small populations. Trapping or any other increase in mortality could easily wipe out the animals from mountain range to mountain range by removal of just one or two individuals.

As the FWS notes: “Human-caused mortality of wolverines is likely additive to natural mortality due to the low reproductive rate and relatively long life expectancy of wolverines… This means that trapped populations likely live at densities that are lower than carrying capacity, and may need to be reinforced by recruits from untrapped populations to maintain population viability and persistence.”

Even though Montana’s wolverine trapping regulations are conservative, it can still be additive to overall wolverine losses. For instance, the trapping of one male may result in immigration of a new male into the territory. Male wolverine will often attempt to kill the young sired by other males, so it’s entirely possible that the loss of one male to a trapper, may in effect result in the loss of many more wolverine.

Similarly the trapping of a pregnant female or a female with young may result in the starvation of her kits, again creating a loss to the population. If this trapping mortality were to occur in one of the smaller sub populations, it could result in the local extirpation of that population.

As trustees responsible for preserving Montana’s wildlife heritage, MDFWP needs to use biology in formulating policies. And biology clearly warrants endangered listing status under the Endangered Species Act.

George Wuerthner is the Ecological Projects Director for the Foundation for Deep Ecology and has published 35 books, including soon to be released Energy: Overdevelopment and the Delusion of Endless Growth



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George Wuerthner has published 36 books including Wildfire: A Century of Failed Forest Policy. He serves on the board of the Western Watersheds Project.

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