From the new report on National Parks in Peril
YELLOWSTONE CASE STUDY:
Global Warming, Bark Beetles, Whitebark Pine, and Grizzly Bears
By Dr. Jesse A. Logan, contributing author to National Parks In Peril
I consider the large-scale bark beetle mortality occurring in lodgepole pine forests across the West
interesting and unusual-- but I have no doubt that lodgepole forests will remain on the landscape for
generations. The current mortality in whitebark pines, though, breaks my heart. We are witnessing the
catastrophic collapse of high mountain ecosystems as a result of human-caused climate change, and
grizzly bears could pay the price.
The grizzly bear is the most emblematic symbol of America’s remaining wildlands. Unfortunately, in
one of its last strongholds, the greater Yellowstone ecosystem, its very existence is in peril. The most
challenging of its many threats there is a loss of critical food resources. Most important in the grizzly
diet are the large and nutrient-rich seeds of whitebark pine, as the bears depend on them in the fall to
prepare for hibernation. Nutritionally stressed bears in years with poor whitebark nut supplies have a
lowered over-winter survival rate, and, more importantly, lower cub birth rates as embryos will be
reabsorbed if pregnant females lack sufficient fat entering hibernation. Without enough whitebark pine
nuts, grizzly bears are also more likely to get into human conflicts as they search for other foods.
In recent years, a new threat has erupted to this critical element in the grizzly diet: the expansion into
high-elevation forests of a small, native bark beetle in response to a warming climate.
The mountain pine beetle is a native insect that has co-evolved with some pine forests. Trees killed by
the beetles (and fire) open up the forests to new growth; otherwise, some types of trees, especially
lodgepole pine, would be replaced by shade-tolerant spruce and fir. But whitebark pines are different
from lodgepoles. Whitebarks live for centuries, not decades, and are restricted to high elevations (with
one of their adaptations being their large, highly nutritious seeds). Whitebark pines do not depend on
catastrophic forest disturbances to survive; instead, they are threatened by them. One hypothesized
reason for the restriction of whitebark pines to high elevations is that they are poorly defended against
the insect pests and pathogens of lower elevations. Mountain pine beetles have not before been a
major threat to whitebark pine survival; their defense has been the high-elevation climate, historically
too cold for long-term survival of large beetle populations.
Unfortunately, things have dramatically changed in response to climate warming since the mid 1970s.
Computer simulations had predicted mountain pine beetle outbreaks into high-elevation systems, but
even the modelers were surprised by how quickly and how far beetles have now spread into whitebark
pines. Significant mortality is occurring across the entire American distribution of whitebark pine, with
no sign of it diminishing. When added to another stress—from a pathogen, white pine blister rust—the
spread of bark beetles into higher elevations puts in question the continued existence of these
ecosystems and of Yellowstone’s grizzly bears.
Given the likelihood of continued warming, what, if anything can be done to protect whitebark pines
and the grizzlies that depend on them? First, we need to better understand mountain pine beetle
infestations of whitebark pine, which differ from the host/insect interactions of other pine species.
Understanding the unique aspects of mountain pine beetle in whitebark pines may let us tip the scale to
favor the host. Second, we need better tools to evaluate the extent of mortality. Whitebark pine habitats
are in the most remote and wild places (often designated wilderness areas) in the Rocky Mountains,
where mortality goes almost undetected. Advanced technology, such as satellite imagery combined
with traditional aerial photography and ground surveying, is needed. Third, management tools (e.g.,
pheromone strategies) need to be fine-tuned for high-elevation environments. All of these approaches
need to be integrated across large, remote, and inhospitable landscapes.
Dr. Logan, an entomologist, retired in 2006 from the U.S. Forest Service.