Whitebark Pine and Grizzly Bears

The last remaining grizzly bears in the contiguous United States reside in two major populations—one in northwestern Montana and the other in the GYE. All in all, fewer than 1000 grizzly bears now exist where 100,000 once roamed the entire western portion of the United States and Mexico (Craighead et al. 1995). Because of their small size, the existing remnant populations are vulnerable to extirpation and are currently the focus of recovery efforts under the Endangered Species Act (Shaffer 1980). Because the GYE is bounded on all sides by human development and the recreational and economic activities that surround and isolate the region, continued grizzly bear occupancy is reliant on the quality of the habitat within the immediate region, and the biological and behavioral requirements of grizzly bears (Pease and Mattson 1999).

Several factors combine to make P. albicaulis seeds of critical importance to grizzly bears. First, whitebark pine seeds are much larger and more durable than the seeds of other available conifers, requiring less energy expenditure to obtain them. Second, they are highly nutritious (Mattson and Jonkel 1990, Lanner and Gilbert 1992, Mattson et al. 2001); the high fat content of P. albicaulis seeds allows bears to acquire significant adipose reserves (Mattson et al. 2001).Third, whitebark pine seeds are usually ripe by mid-August, at which time nutcrackers, red squirrels, and other creatures are harvesting the cones en masse (Mattson and Reinhart 1992). This temporal availability of seed coincides with hyperphagia, a physiological state in which bears become almost wholly consumed with the search for food, ensuring they will survive the long winter months of hibernation. Fourth, when whitebark pine seeds are abundant, they are the highly preferred food source. At these times, bears eat whitebark pine seeds almost exclusively, especially adult females whose ability to store fat is linked to reproductive success and better overall fitness (Mattson et al. 2001).

In reference to their location, the remote subalpine distribution of whitebark pine trees makes whitebark pine a refuge for bears and reduces the risk of higher mortality at lower elevations where human densities are higher (Mattson and Reinhart 1997). Overwhelmingly, mortality of grizzly bears can be attributed to conflicts with humans. A higher frequency of encounters with humans occurs when remote food sources become unavailable, forcing grizzly bears to seek out foods distributed close to human settlements (Knight et al. 1988, Mattson 1990, Mattson et al. 1995, Mattson 1997). The relationship between grizzly bears, human encounters, and white-bark pine crop size variability has been established empirically, and is highly correlated (Fig. 8.2). Studies have revealed that the frequency of grizzly bear captures in the GYE increases sixfold during years of low whitebark pine seed production, and bear mortality rates are two to three times higher (Mattson and Reinhart 1997).

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Year

Figure 8.2. (A) Management trappings of grizzly bears. (B) Whitebark pine seed crop size (represented by percent of pine seeds in grizzly bear scat in the months of September and October). Here, the close association between grizzly bear distribution in years of good and poor seed availability is revealed. In years when mast is low, bears roam more widely, requiring greater management efforts to avoid conflicts with humans. In high mast years, bears are more often aggregated in the high elevations, remote from centers of human concentration, requiring less management effort and incurring less mortality. (Data provided by David J. Mattson, U.S. Geological Survey, Biological Resources Division, pers. comm. Used here with permission.)

Moreover, greater nonlethal contact with humans encourages bears to become habituated to humans. Once bears become habituated, they are 3.1 times as likely to die as a result of conflict with humans than bears that are more wary of humans (Mattson et al. 1992).

Seed crop production by whitebark pine trees is highly variable, however (Morgan and Bunting 1992). Reflecting seed crop variability, years of whitebark pine seed use by bears occur with almost equal frequency to nonuse years. Below a certain threshold value, estimated at 20 cones per tree, bears shift their foraging activities to alternate food sources and different locations. This behavior implies that pine seeds may actually be unavailable to bears long before albicaulis disappears from the Yellowstone landscape. As whitebark pine and overall stand productivity declines, grizzly bears may desert the whitebark pine zone, deeming the energetic profitability of remaining in those stands too low.

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