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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

Whitebark Pine Seeds, Red Squirrels, and Grizzly Bears:
An Interconnected Relationship

By Kim Sager


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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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/