Study: Intense Wildfires May Be Good For Rare Mountain Weasel

Pacific fisher in California | Photo: U.S. Fish and Wildlife Service

Good news from the wildfire front: it looks as though high-severity fires might not be as much of a disaster for wildlife as is popularly supposed. That's the takeaway from a new study of the McNally Fire, which burned more than 150,000 acres in the Sequoia National Forest in 2002.

That fire, sparked by an illegal campfire, ended up becoming one of the largest, highest-intensity wildfires to hit the southern Sierra Nevada in its history. It burned for more than a month, and about half the McNally Fire area was burned by "high-severity" or "moderate-severity" fires, often described as devastating for wildlife.

And for that wildlife caught in the blaze in 2002, the McNally Fire was devastating indeed. But a new study suggests that some of the Sierra Nevada's rarest denizens may have been waiting for just such a fire to create their preferred habitat.


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Unlike many large, intense fires in California, the McNally Fire area wasn't subject to post-fire "salvage" or "fuel reduction" logging. In the years since, the forest has made a remarkable recovery, with the U.S. Forest Service describing the area a decade after the fire in glowing terms:

That's a remarkable description, if for no other reason than that it's usually the USFS that advocates the same post-fire logging projects that would have prevented the McNally Fire's recovery. And now, a study of some of the most seriously burned areas in the McNally Fire in 2012 and 2013 indicates that the renewing forest may not be just aesthetically pleasing: it may be fine habitat for one of California's most reclusive predators.

The study looked for evidence of the Pacific fisher, Martes pennanti, a mid-sized member of the weasel family that was once abundant through its range in western coniferous forests stretching from southern California through the Pacific Northwest. Fragmentation of its habitat by logging and development has forced a decline in fisher numbers, and the U.S. Fish and Wildlife Service has agreed that the species deserves protection under the U.S. Endangered Species Act throughout its range in California, Oregon, and Washington, proposing a year ago to list that "West Coast Distinct Population Segment" of the fisher as Threatened. On the state level, the California Fish and Game Commission voted unanimously in August to list fishers under the California Endangered Species Act.

(Sadly, a new threat to the fisher has also arisen in recent years: owners of illegal pot grows using anticoagulant rat poisons to protect their crops. KCET's Char Miller has more on that.)

Where they still exist fishers are omnivores, with a diet ranging from small animals such as fish and frogs to carrion from larger animals, as well as fruit and truffles. Fishers are likely best known for their ability, almost alone among wild predators, to eat porcupines without getting multiply perforated. They accomplish this by making swift and relentless attacks on the large quilled rodents' unarmored underbellies. As porcupines have gotten quite rare in the fisher's California range, the resourceful weasels make do with other prey.

In the southern Sierra Nevada, fishers use a wide range of different habitat types including red fir forest, riparian hardwood forests, ponderosa pine and mixed conifer forests. They prefer to den and nest in habitat with lots of downed trees, dead standing timber ("snags"), and large live trees of great age.

But according to a new study published in the scientific journal Wildlife Society Bulletin, the weasels would also seem quite content to use forests that have burned in recent years, likely relying on them as foraging habitat. In the study, conducted by wildlife biologist Chad Hanson of the Earth Island Institute, dogs specially trained to detect and alert handlers to Pacific fisher scat walked a number of paths through forest burned to varying degrees in the McNally fire, as well as in unburned neighboring patches of forest. The goal was to determine the effect that different intensities of fire had on the animals' post-fire use of forest habitat: the more scat the dogs found, the more fishers could be assumed to be using the area.

For Californians used to hearing about post-fire landscapes as "devastated" or "moonscapes," the results might be surprising.

In general, fishers showed no tendency to avoid burned areas: there was no significant difference in the numbers of scats in high- or moderate-severity burned areas, low-severity areas, or unburned areas. That indicated that overall, fishers don't have a noticeable preference for unburned habitat, and seem to use post-fire forests perfectly readily.

The really interesting results came when the scat went to the lab, where DNA tests determined the gender of the fishers that left each scat. Those tests revealed that female fishers were actually found in greater numbers in areas burned by fire of high severity than in any other category of burn, including unburned forests.

Hanson -- with whom, in the interest of full disclosure, I worked at the Earth Island Institute a decade ago -- speculates that female fishers may find forests hit by high-severity fires to be the best habitat for raising litters. That may be in part due to the greater abundance of small mammals in areas where intense fires have broken up the forest canopy, a phenomenon that's also been offered as a reason why California spotted owls seem to prefer post-fire forest habitat as well.

And fires of sufficient intensity to kill old standing trees are also responsible for many of the snags that eventually offer dens and nest sites for fishers.

"The behavior of the female fishers is significant because it suggests that these post-fire areas may be particularly good places for them to find food and raise a family," said Hansen in a press release announcing the study.

"At a time when there is much fear and misunderstanding about forest fires," Hanson added, "this study is a reminder that intense fires actually create great habitat for wildlife."

And salvage logging would remove that habitat. This study may well prove controversial, as have studies indicating the importance of post-fire habitat in California forests for other species such as the California spotted owl and the black-backed woodpecker. As the drought increases the frequency and intensity of California wildfires, political pressure will likely mount for land management agencies to embark on salvage logging and fuel reduction logging projects.That pressure isn't going unopposed. Environmental activists have opposed salvage logging for decades. In 2013, a letter opposing increased salvage logging in the wake of wildfires, signed by more than 250 scientists, was sent to Obama administration officials and the U.S. Senate. That letter put it simply:

If the Pacific fisher does indeed gain listing as a Threatened species, proponents of such logging projects on public lands will have to make the case that their projects won't harm the fisher in the long run.

And if it turns out that mama fishers prefer to use high-intensity fire areas to find food for their litters, that may turn out to be a hard sell.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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