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The Grizzly Moth-Eating Gig
David Mattson
Back in 1955, a year after I was born, John Chapman published a paper in the journal Ecology describing a peculiar feeding activity by bears in the Mission Mountains of western Montana. Grizzlies and black bears were both rummaging through alpine talus fields—eating something. As it turned out, they were slurping up both ladybird beetles and army cutworm moths that had concentrated there during the summer to either aestivate or to feed on nectar of high-elevation flowers. Twenty plus years later John Craighead and his colleagues described grizzly bears gobbling up cutworm moths from under overturned talus rock in alpine reaches of the Scapegoat Mountains roughly 50 miles east of the Missions. Barring a few published descriptions by early mountaineers, these were the first and—for a while—the only written descriptions of bears feeding on cutworm moths.
In the late 1970s and early 1980s, when I first started working for the Interagency Grizzly Bear Study Team (IGBST), the Missions, Scapegoats, and their resident grizzlies seemed pretty remote and exotic, albeit only about 250 miles away as the crow flies from where I was working in the Yellowstone ecosystem. The idea that grizzlies in Yellowstone might feed on cutworm moths seemed equally strange and exotic. In fact, if anyone had asked me in 1985 if bears in Yellowstone ate moths I would have said “nope, no evidence of it at all,” and this after having covered over 1000 miles by shanks mare each of the previous six years in this ecosystem. But one year later I would be proven wrong.
In July of 1986 the IGBST’s veteran pilot, Dave Stradley, located a radio-collared grizzly camped on a talus slope straddling an alpine divide in the Absaroka Mountains east of Yellowstone Park. It seemed odd. Next year the same bear camped in the same location. Moreover other radio-collared bears were found camped on yet other remote talus slopes in the same area. This anomaly catalyzed an expedition to find out what the heck was going on, consisting of myself, Bart Schleyer, Carrie Hunt, and Kurt Inberg. Carrie and Kurt were employees of Wyoming Game & Fish at the time. What we found were multiple grizzlies on the site where our collared bear had first been located during 1986, all rummaging through alpine talus, feeding on slithering masses of cutworm moths. The same phenomenon was documented on three more sites during 1987 and 1988 thanks to strenuous efforts by IGBST field crews that included Gerry Green, Jamie Jonkel, Dan Reinhart, and Doug Dunbar.
Since then, ever more grizzlies have been found on ever more alpine sites gobbling up cutworm moths—a total of 31 now. All of the sites are above 10,000’ elevation, all in the Absaroka Mountains east and southeast of Yellowstone Park (see the map immediately below). Anymore, it is probably not too much of a stretch to claim that the majority of the bears in this part of the ecosystem spend the majority of their time between mid-July and mid-September on these exceedingly remote moth sites eating moths gathered to feed on nectar of alpine flowers. The moths feed primarily during nighttime, dawn, and dusk, and spend the remainder of the day in the chilly cracks and crevices of angular rocks accumulated on talus slopes, which is where the bears find them. Why congregate like this in a cold microenvironment if you are moth? Who knows, but I suspect it has something to do with avoidance of predators and parasites—barring bears.
The moth sites in this map are encompassed by the yellowish-green blobs, with reference to the boundary of Yellowstone National Park (the gray line) and the Primary Conservation Area (PCA) for Yellowstone’s grizzly bears (yellow line). There are three take-away points: first, all of the moth sites are in the Absaroka Mountains on the east side of the ecosystem; second, all of the sites are outside of the Park; and, third, some moth sites are outside of the PCA.
Since the early 1990s the phenomenon of grizzly bears eating moths in Yellowstone has been pretty thoroughly documented thanks to an initial research publication in 1991, successive papers by Steve and Marilyn French (in 1994) and Sean O’Brien (in 1998), and annual updates in the IGBST’s Annual Reports. Of even greater import, bears eating moths in the remote and startlingly beautiful alpine haunts of Yellowstone has captured the public imagination, aided and abetted by the efforts of several enterprising and sometimes intrusive film crews, most notably from BBC. Their footage has probably been seen by millions of people worldwide thanks to being aired as part of the BBC series Planet Earth.
I could wrap things up here and leave this simply as an interesting bit of history. But I can’t help think of larger ramifications for Yellowstone’s grizzly bears, our current approaches to research and management, and a pending move by the US Fish & Wildlife Service (USFWS) to remove Endangered Species Act (ESA) protections for this bear population. Making these kinds of connections seems to be my plight.
As it turns out, the burgeoning use of moth sites by Yellowstone’s grizzly bears probably explains much of the major increase in their distribution to the east and southeast that occurred between the mid-1980s and 2000. The match between where we find moth sites and where the greatest expansion occurred is uncanny. Remember, too, that this period was on the heels of the 1988 wildfires that killed nearly 30% of cone-producing whitebark pine in the core of the ecosystem (whitebark pine is another key source of bear food). The map below shows details of all this.
Here the moth sites are superimposed on the distribution of Yellowstone’s grizzly bears, represented by the green shading, with the beige arrows denoting areas of major increase in distribution between the 1980s and 1990s. The red represents areas burned during the epic wildfires of 1988. There are two takeaway points: first, the largest increase in distribution was in the direction of the newly-discovered moth sites; and, two, these increases also occurred after the 1988 fires had taken out roughly 30% of the whitebark pine in the core of the ecosystem.
Coincidentally, dramatic increases in grizzly bear activity on moth sites have also contributed to inflated estimates of growth for the Yellowstone grizzly bear population. Bears on moth sites are almost certain to be seen by airborne researchers and managers out looking for females with cubs-of-the-year (COY) at their side. By contrast, bears engaged in virtually any other kind of feeding activity are likely to be seen only 1%…at most only 40% of the time… when somebody flies over. Which is to say, grizzlies have, in the net, become one heck of a lot easier to see in the last couple of decades, at the same time that managers and researchers have quadrupled their efforts to find bears. Given that sightings of females with COY are the foundation of all estimates of population trend, these estimates have correspondingly been inflated upward—because of increased search effort, but also by the increased ease of sighting females with COY on moth sites. Perhaps an unintended consequence? Maybe not.
Less positively for Yellowstone grizzlies, their expansion into Wyoming in apparent pursuit of moths has taken them deeper into cow country. Not surprisingly, a considerable portion of the increasing number of conflicts between grizzlies and ranchers over cattle (another nutritious bear food) in the Yellowstone ecosystem is concentrated not too far downslope from a number of moth sites. So, perhaps paradoxically, increasing exploitation by grizzlies of a food in some of the most remote parts of the ecosystem (that is, moths) has probably contributed to a substantial increase in the numbers of bears dying downslope and down-elevation in retaliation for predation on livestock.
And having expanded well inside the frontier of regressive Wyoming politics and attitudes, the arrangements proposed by the US Fish & Wildlife Service as part of a package for removing ESA protections from Yellowstone’s grizzly bears would leave some of the moth-eating grizzlies high and dry outside the zone of meaningful protections, and the rest exposed to the potential excesses of Wyoming Game & Fish Department’s post-ESA management. Wyoming is frothing at the mouth to institute a sport hunt, and there is going to be no easier bear for a hunter to find (albeit a little difficult to reach) than one camped on a moth site. The Department also seems dedicated to the proposition of reducing grizzly bear densities on the ecosystem periphery, which coincides with the areas containing all of the moth sites—this as part of a putative strategy for reducing levels of livestock depredation. The net prospects for moth-eating grizzly bears are not good. And, as I pointed out a little earlier, these moth-eaters comprise a substantial portion of the total Yellowstone grizzly bear population.
In light of all this, it only makes sense to expand the Primary Conservation Area out to include all of the known moth sites, and then protect these sites and a substantial surrounding area from any sport hunting. Better yet, don’t hunt delisted grizzlies. And, even better yet, don’t remove ESA protections and, instead, prioritize the protection of these grizzly bears that are otherwise at the edge of all protections, and the most exposed of all to thuggish enclaves of Wyoming citizenry.
But, I have one more thought that arises from the grizzly bear x moth phenomenon. As I noted before, the emergence of moth-eating by Yellowstone’s grizzly bears was a complete surprise for me. And I’ve continued to be surprised by all sorts of things that I never could have imagined: the 1988 Yellowstone wildfires; the near-extirpation of cutthroat trout in Yellowstone Lake by climate change and predation by an illegally introduced predator; the widespread losses of whitebark pine to an unprecedented bark beetle outbreak driven by a warming climate; massive declines in virtually all of the ecosystem’s elk herds, also driven in part by climate change; the emergence of Chronic Wasting Disease as a threat to larger mammals in the ecosystem; and more… The theme here is surprise and, barring cutworm moths, all of the surprises have so far been (more or less) really unpleasant.
The take away? Perhaps humility is in order for our federal and state wildlife managers—humility and caution. As is, I see little evidence of either in the USFWS’s rush to remove ESA protections, Wyoming Game & Fish’s eager embrace of lethal grizzly bear management, or the cocky attitudes of the current crop of IGBST researchers. What to do? I don’t know exactly, other than sure as Hell don’t turn the keys to the car over to Wyoming any time in the near future.