The Future of Cutworm moths & Bears

There is no longer any doubt that we are in the midst of a rapidly warming climate, with much more yet to come. Even if humans had the will and where-with-all to engage in a massive transition to non-polluting energy sources and undertake equally massive mitigation measures, the thermostat has already been figuratively set. Warming will continue to levels eclipsing that of the brutally hot and dry Altithermal period. The trends are not only evident globally--and unignorably stark in the far north--but also well-documented at mid-latitudes in places such as the northern US Rocky Mountains.

The figure at right shows the results of recent efforts by Tony Chang and Andy Hansen of Montana State University to scale down general circulation (climate) models to derive a long-term projection for climate of the the Greater Yellowstone Ecosystem. For many years regional-scale projections such as these were notoriously fickle, but recent advances in methods coupled with additional data have allowed researchers greater confidence in the fruits of their labors. The top figure shows temperature projections and, the bottom, precipitation projections. All are based on a consensus of climate models produced by the International Panel on Climate Change (IPCC). The different trend lines represent essentially three different futures employing different assumptions about economic, social, demographic, and physical changes. Notice that even the most optimistic (i.e., coolest) still shows major warming for the Yellowstone ecoregion.

The bottom line? The northern US Rockies will be much warmer in the future compared to now. Although precipitation is projected to increase, it will not be enough to offset the drying effects of proportionally greater increases in temperature, with a future full of more severe and chronic drought. All of this will have major effects on vegetation, along with direct and indirect effects on animals, including army cutworm moths and grizzly bears.

The Fate of Alpine Environments

The prognosis for alpine tundra and associated cold environments is obviously highly relevant to judging prospects for army cutworm moth aggregations and associated feeding opportunities for bears. Alpine environments are defined by a very specific climatic envelope, as is the corresponding domain of plants that comprise alpine tundra. It is therefore not surprising that every scientific projection for alpine environments under different plausible scenarios of climate change projects catastrophic losses.

 

Projections specific to the alpine zone in the western United States are no exception. I lifted and modified the emblematic maps at left from a 2012 monograph on which Gerry Rehfeldt was lead author. The map at far left shows the current extent of alpine tundra (in red), the map at right the extent projected for 2090--which is to say, none. In fact, alpine climates are projected to be gone by 2050.

This projection and others like it are consistent with the climatic and physiological factors that sustain alpine plants in the face of competition from other species better adapted to less harsh lower-elevation environments (see the rich scientific literature dating back to Tranquillini [1964] and Billings & Mooney [1968]).

The Future of Moths and Bears

Given this prospectively bleak prognosis for alpine tundra in the western US, it is significant that every instance where bears have been documented to feed on aggregations of army cutworm moths in the northern Rocky Mountains has been restricted to alpine fellfields near alpine tundra (see the Introduction). As a corollary, studies of summer cutworm moth ecology have consistently shown that moths subsist on nectar of tundra flowers. Amplifying this dependency, cutworm moths are remarkably sensitive to warm temperatures, even in alpine environments, evident in the extent to which they seek out optimally cool micro-environments within their daytime talus refugia (see Temporal patterns). It is thus not surprising that the odds of finding a site where bears feed on moths increases with elevation, theoretically off the tops of available mountains, and that a model developed by Hilary Robison predicted the that all prospective, as yet undocumented, sites would be found in alpine environments.

 

Given these facts, it is highly unlikely that grizzly bears will shift to feeding on moths in environments where feeding on moths has never before been documented and where moths have never before been documented to thrive. Although it is true that no one can say for sure how over-summering army cutworm will respond to loss of alpine environments, this does not in any way substantiate concluding that alpine environments will somehow be unaffected by climate warming, that moths will somehow adapt, and that grizzly bear foraging will be minimally affected. Such claims, in fact, defy not only the weight of evidence, but also everything we do know about grizzly bears, cutworm moths, and prospective effects of climate change on the habitats that over-summering moths currently depend on.