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Introduction Grizzly bears in the Yellowstone ecosystem have long been viewed as
charismatic symbols of the wilderness. Unfortunately, several grizzly bear (Ursus
arctos horribilis) researchers have recently expressed alarm about the future status
of grizzly bear populations in the Greater Yellowstone ecosystem because of the potential
sharp decline of several important bear food sources. One of the most important fall
foods, the seeds of whitebark pine trees (Pinus albicaulis), will likely be
negatively affected in the near future by the spread of white pine blister rust (Cronartium
ribicola), a non-native fungus that was introduced to North America from Europe in
the early 1900's and often kills whitebark pine trees. Several authors have suggested that
availability of whitebark pine seeds may be the single most important food-related factor
likely to affect long-term health and survival of the Yellowstone grizzly bear population.
Therefore, it is essential that bear use of this food resource be explored, and that
measures are taken to protect whitebark pine trees
Grizzly Bear Use of Whitebark Pine Seeds and Their
Nutritive Qualities
Grizzly bears, especially grizzly bear females, in
Yellowstone rely heavily on the energy-rich seeds of whitebark pine to increase their fat
reserves before hibernation. The seeds weigh an average of 180 milligrams, and are about
the size of a small kernel of corn. They are comprised of 21% carbohydrates, 21% protein,
and 52% fat, which is significantly more fat than most other wild bear foods. Whitebark
pine cone maturation occurs from August to October, during which time the seeds are
heavily harvested by red squirrels and cached, or stored, for later use. This period of
seed maturation and squirrel caching coincides with a period of intensive feeding, or
"hyperphagia", by grizzly bears before hibernation. During years of high seed
availability, bears have been reported to eat nothing but whitebark pine seeds in the late
fall.
The Importance of Red Squirrels
Several factors influence whether or not bears will use
whitebark pine in any given year. The first among these is whether or not the whitebark
pine cone crop is substantial. The second factor is the abundance of red squirrels,
because bears acquire seeds by raiding red squirrel caches, or "middens".
Several researchers argue that the density of red squirrels may be one of the most
important factors affecting bear use of whitebark pine seeds. One study found that between
1986 and 1991, 86% of the sites where grizzly bears were known to have foraged on
whitebark pine seeds involved the excavation of red squirrel middens. In an additional
study, bears made exclusive use of squirrel middens while foraging on whitebark pine
seeds.
White Pine Blister Rust
Unfortunately, whitebark pine populations in the
inter-mountain west are seriously threatened by white pine blister rust. White pine
blister rust was accidentally introduced to western North America in 1910 and has since
been a major source of whitebark pine mortality. The disease kills the upper, cone-bearing
branches of whitebark pine long before the tree dies so that cone production is greatly
diminished and subsequent tree regeneration is impossible. Blister rust has had the most
devastating effects in climates with coastal influences but has the potential to affect
all whitebark pine stands, including those in Yellowstone where the climate is drier and
colder. Though whitebark pines in Yellowstone have currently evaded significant damage by
blister rust, stands in the higher elevations will likely be infected in the future.
Grizzly Bear Reproductive Success and Whitebark
Pine Seed Availability
As previously stated, whitebark pine seeds provide
substantial fat reserves for the period of winter dormancy when females give birth.
Preliminary examination by several authors of recent Yellowstone bear data suggests that
female grizzly bears use proportionately more pine seeds than do males. Additionally,
female grizzly bears that frequently made use of whitebark pine seeds reproduced at an
earlier age and exhibited higher reproductive rates than females who consumed few pine
seeds.
Increases in Bear/human Conflicts During Years of
Low Whitebark Pine Seed Availability
Due to the high-elevation and remote habitats occupied by
whitebark pine, whitebark pine stands provide refuges for bears away from sources of
human-induced mortality such as roads and settlements. Therefore, during years of
whitebark pine seed-crop failure, bears not only lack in a significant food resource, but
they are also more detrimentally affected by human activity. Studies have found that
during years of whitebark pine crop failure, grizzly bears tended to utilize areas for
foraging that were much closer to human habitation, and were therefore more likely to be
killed by humans. When sex and age-specific mortalities were examined, researchers
determined that 2.3 times as many adult females and 3.3 times as many sub-adult males
suffered human-caused mortalities during years when they did not use whitebark pine seeds,
and that total human-caused mortalities increased 1.9 times during these years.
How Do Bears Learn to Forage on Whitebark Pine
Seeds?
Because whitebark pine seeds are important to grizzly bear
long-term survival, how grizzly bears learn to utilize this food resource needs to be
understood. Bears tend to be solitary animals throughout most of their lives; therefore,
the most important period of social learning occurs during the three years that grizzly
bears are with their mothers. This prolonged period of mother-cub association leads to the
cub's acquisition of a myriad of critical information, such as skills and information
pertaining to foraging opportunities, predators, and other bears. One especially critical
piece of information gained by bear cubs from their mothers involves where, when, and how
to forage.
Bears are able to integrate important pieces of information
learned from their mothers with those learned from individual experience to discover and
later re-locate important foraging areas. Grizzly bears that discover whitebark pine seeds
will likely have a positive experience with that food resource, and they will therefore
seek that habitat type again. Individuals that learn where the highest concentrations of
food resources are, such as whitebark pine seeds, and how to exploit them efficiently will
experience the highest rate of energy return, and will therefore experience the highest
fitness.
Management Solutions and Conclusions
In response to concerns over the effects of whitebark pine
seed crop fluctuations and the potential loss of whitebark pine forests on Yellowstone
grizzly bears, managers have proposed several management possibilities for alleviating
these concerns. Several solutions are associated with forest health, while others involve
management of human activity during years of low whitebark pine seed availability. The
slow-growing and slow-to-respond nature of whitebark pine trees precludes immediate action
to forestall blister rust invasion; however, deliberate or natural selection of
blister-rust resistant trees will be beneficial in the future. An urgent need exists to
implement programs which facilitate blister-rust resistance in the wild, or to propagate
blister-rust resistant trees in nurseries, before whitebark pine tree populations are
further compromised.
In addition to changes in forest management practices, the negative consequences of low
whitebark pine seed availability on grizzly bears may be temporarily mitigated through
people-management strategies in Yellowstone National Park. During years of poor whitebark
pine availability, the potential for increased bear/human conflicts can be predicted and
managers can respond accordingly. Increased food-storage patrols, increased public
education, implementation of travel restrictions, and closures of areas known to be used
by bears during years of low seed availability should all be considered as viable options.
Important Literature
Schmidt, W.C., and K.J. McDonald, editors. 2000. Proceedings of the symposium on whitebark
pine ecosystems: ecology and management of a high-mountain resource. U.S. Forest Service
General Technical Report INT-270.
Gilbert, B.K. 1995. Opportunities for social learning in bears. Pages 225-235 in Mammalian
social learning. Edited by H.O. Box, and K.R. Gibson. Symposia of the Zoological Society
of London (72).
Tomback, D.F., S.F. Arno, and R.E. Keane, editors. 2001. Whitebark pine communities:
ecology and restoration. Island Press, Washington, D.C.
Mattson, D.J., B.M. Blanchard, and R.R. Knight. 1992. Yellowstone grizzly bear mortality,
human habituation, and whitebark pine seed crops. Journal of Wildlife Management
56:432-442.
Reinhart, D.P., M.A. Haroldson, D.J. Mattson, and K.A. Gunther. 2001. Effects of exotic
species on Yellowstone's grizzly bears. Western North American Naturalist 61:277-288.
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