Fire suppression and illegal marijuana cultivation threaten rare Pacific fishers


The Pacific fisher, a small, carnivorous forest-dwelling mammal, is a candidate for listing under the federal Endangered Species Act this year, and big wildfire could be to blame – or rather, the lack of it.

Ecologist Chad Hanson’s recent research on the fisher population of the southern Sierra Nevada shows that the animals – aptly described as “the love child of a ferret and a wolverine” – actually seek out post-fire habitat, especially areas that have burned at higher severity, where most of the trees are killed. In a 2013 study, the first to ever examine the relationship between fishers and fire, Hanson used dogs trained to detect Pacific fisher scat, tracking where they eat, sleep, raise their young and otherwise use forest habitat. He has yet to decipher exactly why fishers need post-fire habitat, but he suspects that the combination of downed logs, standing burned trees and natural regrowth create an ideal environment for the small mammals that fishers prey on.

A fisher, which has been proposed for the endangered species list. Image courtesy the USFS Region 5 Flickr.

The idea that there might be a wildfire deficit might seem odd in a time when the consensus among fire managers appears to be that we are experiencing increasingly frequent and more intense wildfire in the West. But a study released this month, co-authored by Hanson, says that contrary to commonly held belief, the majority of Western forests actually had more high-severity fires before fire suppression began 100 years ago than they do now. It seems ironic, then, that current Western forest management practice is based on the belief that only low- to moderate-severity fire was common before fire suppression, and that high-severity fire is a product of that suppression, exacerbated by climate change, and is largely damaging to wildlife and habitat. So high-severity fire – which burns 70 to 100 percent of woody vegetation and climbs from the ground to the treetops – is prevented as a matter of policy.

The means of suppression can include tree thinning designed to prevent high-severity fire, which Hanson said often means intensive commercial logging that removes up to 80 and even 90 percent of the trees in a given area. Managers think of this type of suppression, Hanson said, as “a sort of lesser-of-two-evils approach” for the fisher.

This mechanical thinning project was conducted in previously suitable Pacific fisher habitat in California's Sierra National Forest. Ironically this commercial logging project was promoted as a project designed to restore fisher habitat, but Hanson said the homogenized conditions it created are exactly the opposite of what scientific studies are concluding Pacific fishers need. Photo courtesy Chad Hanson.

“There’s acknowledgement that the logging projects are not good for fisher habitat – that it actually degrades it,” he said. But forest managers typically think that thinning is necessary to save the species from the greater harm of fire.

Before Hanson, nobody had tested the assumption that high-severity fire is harmful to fishers, and his research shows that the post-fire forest is in fact a significant part of the fisher’s home range. This has to do with what ecologists call the “bed and breakfast” phenomenon. 

“(Fishers) get certain habitat requirements from the dense old forests,” like denning and nesting, Hanson said. “But they also get certain habitat requirements (like food) from the complex early seral forest – what we call snag forest habitat – created by high-severity fire. It’s the juxtaposition of those two things that really gives them that range of what they need.”

The McNally fire area, where Hanson's fisher scat surveys were conducted, shows the complex post-fire forest habitat that fishers use. The many standing snags, downed logs, and montane chaparral patches and natural conifer regeneration provide the biomass and structural that make for good habitat for the fisher's small mammal prey. Photo courtesy Chad Hanson.

But the dense forests of Humboldt, Mendocino and Trinity counties in California are posing a second major threat to the fisher. Illegal marijuana growers in what's come to be known as the “Emerald Triangle,” the largest cannabis-producing region in the U.S., replace patches of dense forest with marijuana plantations, and use an anticoagulant rat poison to keep rodents away from their weed. (Rats are probably drawn to camp trash and food, but will also chew on the pot plants themselves.) Research by wildlife pathologist Mourad Gabriel shows that the rat poison is also killing fishers, who eat dying poisoned rats and possibly even eat the poison directly. Gabriel has spent years tracking fishers deep in the remote forests of California, and a recent Mother Jones story reported that at least 48 of the 60 fishers collected by Gabriel tested positive for the rodenticide. Death by this poison is unmistakable: the anticoagulant properties cause blood to pool in the fisher’s stomach, and the poison is detectable in its blood. The illegal pot plantations are the only nearby source of the poison, which is banned for agricultural use. Gabriel has said that on a single plant, he’s seen growers use up to 50 times more poison than would kill a 500-pound lion.

The historical range of the Pacific fisher in California has been significantly reduced over time. Dark green shows its current region and white patches indicate where it was once much more common. Courtesy California Department of Fish & Game. Click for larger image.

It’s possible that the absence of post-fire snag forest habitat may even be pushing fishers to seek food around the illegal pot plantations – where there are plenty of rats to eat – hidden in the type of dense forests fishers typically use for their dens and nests. But despite the acute danger from rodenticide, Hanson said that the greater threat is still the ongoing lack of high-severity fire. Indeed, past petitions from environmental groups to list the fisher as an endangered species emphasize historic habitat loss due to logging and fire suppression as the underlying threat.

It’s still unclear whether the combined impact of too much rodenticide and too little high-severity fire will be more than the species can handle. Limited relocation efforts have been underway for several years. California’s Northern Sierra Fisher Reintroduction Project, for instance, relocates fishers to habitat in the southern Cascades and northern Sierra Nevada, where an agreement between a private timber company and U.S. Fish and Wildlife offers them protection. But these fragmented efforts are not an ideal solution. Manson said the stress of relocation can be traumatic, and it could take years to determine whether a relocated population will survive.

“All species are adaptable in some sense, but I don’t think we can call the fisher a habitat generalist,” Hanson said. “Based upon the data, it’s actually quite specific in its habitat.”

In the meantime Hanson wants to see the fisher make the federal endangered species list. And such a listing might just carry enough weight to start to tip wildfire management policy in a new direction across the West, by adding to a growing group of species known to depend on snag forest habitat, like the threatened northern spotted owl and imperiled black-backed woodpecker. Hanson’s fire-severity study recommends a policy shift toward reintroducing high-severity fire on public forested land, while creating a safe buffer between public forests and private housing.

Fishers are already listed as endangered in the state of Washington and as “sensitive” in Oregon, but are not listed at all in California. A federal endangered species listing decision probably won’t be made until September of this year. Hopefully the Pacific fisher won’t be left high and dry, between wildfire and weed.

Christi Turner is an editorial intern at High Country News. She tweets @christi_mada.

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