Spatial Relations of Cutworm moths & Bears

All of the locations where grizzly bears are known to consume army cutworm moths are located in the northern Rocky Mountains of the US, in the states of Wyoming and Montana, between 43.5 and 49 degrees N latitude. The only other site with documented moth-feeding, involving black bears, is in the Jemez Mountains of New Mexico, roughly 570 miles south from the next nearest site to the north in the Absaroka Mountains of Wyoming (see the map at the bottom of Introduction to army cutworm moths). All of the northern sites are located in alpine talus slopes near areas of alpine tundra where over-summering cutworm moths can forage on concentrated nectar resources.

The map at right focuses on the US northern Rockies, from the Yellowstone ecosystem in the south to the US-Canada border in the north. Alpine areas are shown in red and areas containing sites where bears eat moths are shown dusky-green. The Primary Conservation Areas for the Threatened Greater Yellowstone (south) and Northern Continental Divide (north) grizzly bear populations are delineated by a black line.

There are several take-aways from this map. First, most of the moth sites are located in the Greater Yellowstone Ecosystem in the southern Absaroka Mountains. A secondary concentration is located to the north in Glacier National Park. The only other known moth sites are in the Mission Mountains (the site farthest west) and the Scapegoat Mountains directly east. Barring the Mission Mountains, all of the sites are located on the comparatively drier eastern side of the Rocky Mountains, which begs the question: Why?. Another noteworthy question pertains to the absence of moth-feeding in areas with ample alpine terrain such as the Beartooth Plateau and Crazy Mountains.

Crazy Mountains

Mission Mountains

Beartooth Plateau

The northern US Rocky Mountains

The Yellowstone Ecosystem

There is no easy or definitive answer to why there are no moth sites in alpine areas such as occur in the Crazy Mountains, where one might expect to find aggregations of cutworm moths and associated feeding by bears. Part of the answer might involve lack of effort by researchers to find either moth aggregations or bear activity. However, a closer look at the Yellowstone Ecosystem offers a clue why there are no known moth-feeding sites on the west side or in the Beartooth Mountains of this region, in areas that have been closely surveyed for bear activity.

The map at left shows most of the Yellowstone region, with Yellowstone National Park delineated in the middle, alpine areas shown in red, and areas encompassing where bears feed on moths in dusky green. The patterns here reiterate those shown in the larger regional map above, but with the added element of heavy snowpack shown in blue. These blue-shaded areas consistently experience the heaviest winter snow accumulations and, correspondingly, the latest snowmelt.

The pattern is fairly obvious. Moth aggregation sites and associated feeding by bears are concentrated in areas with an alpine climate that is comparatively dry during winter. Why? The answer can only be speculative, but it may have to do with the earlier exposure of alpine talus, which is critical daytime loafing habitat for moths, as well as earlier exposure and phenology of the tundra that provides moth food in the form of nectar.

Support for this interpretation is evident in exceptions to the apparent rule. There are no sites in the Wind River Mountains at the far southern margin of the map, above, yet there is ample alpine habitat here that receives comparatively little winter snow. So there should be evidence of bears feeding on moths if the explanation holds true. As it turns out, a preliminary survey by Jonathan Ratner revealed numerous sites with moth aggregations. So, the moths are there. The absence of bear feeding is probably a simple function of low densities of newly-arrived grizzly bears. Grizzlies have been colonizing the Wind River Range from the north and west only since the late 1990s.

All of the moth sites that I visited are located in glacial cirques on scree slopes immediately below steeper headwalls or cliffs. These slopes are active, with high rates of debris accumulation as a result of frost-heaving and other erosive forces. Virtually none of the scree interstitia is filled with finer debris, which reflects the active rockfall. Individual rocks are slightly rounded to angular and intermediate in size between gravel and boulder, roughly 8 to 40 cm in size. These scree fields are also deep enough to allow moths scope for finding optimal microenvironments from day to day and hour to hour. Combined, all of these features create ideal niches for moths to loiter during the comparatively warm summer days.

In fact, conditions beneath the veneer of rocks are much like that of a refrigerator--cold enough to reduce moths to a sluggish lethargy that makes them all the more vulnerable to bear predation. So, why hide out in a refrigerator where you might be gobbled up by a bear?

 

Army cutworm moths are, in fact, remarkably cold tolerant, which is a corollary to their intolerance of heat. They can abide the chill of alpine rock piles. But, at the same time, they are consumed in their thousands by bears. The usual explanation for such a paradox is through understanding the threats that shaped evolution, in the case of cutworm moths comprised of parasitoid wasps and flies more than predatory bears. In fact, wasp and fly parasites are the primary cause of mortality for moths while in the lowlands. Moths plausibly seek the chill of alpine rock fields primarily to escape insect parasites, simultaneously reaping the benefits of alpine nectar. The big challenge ends up being the transport of their frail bodies hundreds of miles west and then east, at least for moth denizens of the Great Plains. 

Scaling up a bit, the figure above right summarizes the results of an analysis I did for Yellowstone moth sites along with a similar analysis done by Don White for sites in Glacier National Park, roughly 5 degrees in latitude and 350 miles north. Both analyses focused on features that distinguished sites with bear feeding on cutworm moths from a random selection of surrounding sites--in essence an analysis of joint habitat selection by army cutworm moths and bears.

The results of both analyses were remarkably similar. Bear feeding on moths was disproportionately concentrated on West to Southwest-facing slopes (figure A) that were inclined 35 to 40 degrees from horizontal (B)--which is to say, moderately steep wind-exposed slopes. Prevailing winter winds in both areas are westerly, which translates into relatively snow-free conditions on the talus slopes selected for cover by moths and, later, for feeding by bears, which is consistent with my speculation, above, that cold but snow-free conditions are a key feature of moth-aggregation sites.

 

As might be expected, moth-feeding sites tend to be concentrated at considerably higher elevations in Yellowstone, above 10,000 feet, compared to in Glacier, above 8,000 feet, although the odds of bear feeding increases with elevation in both areas (figure C). Cold wind-exposed but comparatively snow-free alpine conditions were apparently a critical prerequisite.

 

Hilary Robinson undertook an analysis during the 2000s that was more spatially extensive than the one that either Don or I undertook, and benefiting from a larger sample of identified moth feeding-sites (see Temporal patterns). She also investigated the effects of surficial geology and heat load. As might be expected by the consistent presence of talus at known moth aggregation sites, the presence of such features was a strong positive correlate. Her analysis was plagued by confounding correlations among explanatory variables, but ended up broadly consistent with previous results. Bear feeding on moths was more likely to occur at the highest elevations, on steep westerly slopes, subject to higher heat loading. Consumption of moths was also positively correlated with an abstract metric called "wetness" obtained from satellite-based remote sensors, but with little obvious meaning to the occurrence of either moths or bear foraging.

Landscape features

The map at left shows the distribution of moth sites relative to areas in the Yellowstone ecosystem relevant to conservation of Yellowstone's grizzly bears. These bears recently lost Endangered Species Act protections and are now subject to the authority of state wildlife managers intent on starting a grizzly bear trophy hunt. Prior to institution of ESA protections for grizzly bears in 1975, grizzlies were aggressively hunted, especially in Wyoming. And, of particular relevance here, a number of grizzlies were killed on or near moth sites, often over baits. In fact, it is not unusual to find piles of decades-old bleached horse and mule bones near moth sites--all that remain of old broken down animals led into the mountains and shot to lure bears in to hunters.  It is not clear yet whether hunters will be allowed to use baits to hunt grizzlies, but the vulnerability of bears on moth sites to hunting of any sort is indisputable. The only place where hunting will not be allowed is in Yellowstone and Grand Teton National Parks.

With all this in mind, note that none of Yellowstone's moth sites occur inside National Parks. All sites will potentially be open to hunting that targets bears still present at the end of the moth-eating season. Prior to removal of ESA protections it mattered whether moth sites occurred inside or out of the grizzly bear Primary Conservation Area (PCA; delineated by the yellow line at left). Bears that ranged inside this Area were given greater protection and deference, which meant that vulnerabilities attached to using moth sites mattered only for a limited number of sites outside the PCA boundary...but no longer.