Yellowstone's grizzly bears face rapid changes: what are the cumulative impacts?

After the recent meeting of Yellowstone’s grizzly bear managers in Jackson, Wyoming, a manager asked me: “so what are the cumulative effects of the loss of Yellowstone cutthroat trout, whitebark pine, as well as the effects of climate change and wolves on Yellowstone grizzly bears?  This was an astute question, especially regarding an animal with the lowest reproductive rate of any mammal in North America.  The idea of “cumulative effects” is also important for grizzlies because of the large territory they need to survive—200 to 400 square miles in the Greater Yellowstone Ecosystem.  In the world of bears, it is never one small insult —one timber sale, one subdivision, one poor year for berries —that ultimately sinks a population: it is the combination of many factors, perhaps best described as a “death from a thousand cuts”.  These “cuts” take the form of excessive loss of wildland habitat and foods that bears need, and/or too much killing.

The issue of “cumulative effects” was a hot topic for Yellowstone bears 15 years ago, especially in the context of massive logging and roading that occurred on National Forests like the Targhee and Gallatin. The great swaths of clearcuts and associated road building resulted in major negative impacts on bears, including the extirpation of female bears from some areas. It didn’t take a complex “cumulative effects” computer model to know that there was a problem resulting from these industrial-scale activities that occurred for more than a decade—although bear managers did develop such a model. From an airplane, you could see a line of clearcuts 30 miles long that defined Yellowstone Park’s western border.  

 

The 30 mile long clearcut line that defines Yellowstone Park’s border: cutting stopped and restoration began in this area following successful litigation brought by conservation groups.

In the early 1990s, I was involved in several lawsuits under the Endangered Species Act that prompted the Forest Service to establish new restrictions on road-building, and forced the restoration of areas where habitat had been significantly degraded. These measures proved successful: female grizzlies have since recolonized areas on the Targhee Forest where they had been eliminated. Since grizzly bears serve as an umbrella species that are a barometer of the health of the whole ecosystem, other wildlife, such as elk and fish, benefited from these new roads restrictions and restoration efforts  as well.  

Today the discussion about “cumulative effects” has gone beyond clearcutting, roadbuilding and energy development, as scientists worry increasingly about the overall impact on bears from a combination of major changes in bear foods, drought and climate. To put this issue into context for grizzly bears in the Yellowstone Ecosystem, it’s helpful to look at how food conditions for bears have changed during the last four decades.  Several food-related factors gave Yellowstone bears a significant boost after their listing under the Endangered Species Act in 1975.  There were three major positive trends related to bear foods, starting in the mid 1970s and continuing through the late 1980s. 

First was a move by Yellowstone National Park in the late 1960s to stop its big-game killing programs.  (Yes, the Park Service killed hundreds of animals on the Northern Range of Yellowstone for decades, until public controversy shut it down.)  This program significantly reduced the numbers of elk, a key bear food,  in the northern part of Yellowstone Park.  After the killing program stopped, elk numbers rebounded, providing grizzly bears a more abundant source of winter-killed carrion during spring and newborn elk calves during early summer, when high-quality bear foods are otherwise scarce. 

Second, in the mid-1970s, Yellowstone Park imposed greater restrictions on the types of cutthroat trout that could be taken by fisherman from Yellowstone Lake.  The Park Service’s revised rules allowed the numbers of trout spawning in Yellowstone Lake’s shallow tributaries to rebound, providing grizzly bears with another readily available food source during the early summer. 

Third, in the early 1980s, grizzly bears discovered a new food source in the east and southeast parts of the ecosystem, in the form of army cutworm moths that the bears feed on obsessively during mid-late summer.  These moth “fat bombs” are concentrated in alpine cirques, in some of the most remote wilderness areas in the ecosystem, which keep the bears that feed on them safely far away from people—and conflicts.

So what’s been happening more recently with Yellowstone’s grizzly bear foods, and what can we say about the cumulative effects of recent changes on the status and trends of the population?  First, Yellowstone cutthroat tanked as a result of drought and predation by Lake trout, an introduced non-native fish. And, unlike cutthroat trout, Lake trout spawn in deep water, where they are unavailable to bears and other scavengers.  Today, cutthroat trout are virtually gone from the grizzly bear’s diet. 

                                     cutthroat trout in the mouth of lake trout

Second, as I’ve described earlier, whitebark pine crashed.  There is overwhelming scientific evidence that whitebark pine seeds are  a critical driver of the health of the Yellowstone grizzly bear population—and that during and after bad whitebark pine seed crops grizzly bears die in much large numbers and produce fewer cubs. And third, although army cutworm moths are still abundant, there is growing concern that climate warming will greatly shrink the amount of moth habitat, including the abundance of alpine flowers they depend on.

So what are the long-term consequences of losing whitebark pine and cutthroat trout—a fact of life now in the Greater Yellowstone Ecosystem?  And what will happen if cutworm moths decline due to warming temperatures and the loss of alpine habitat? Time will tell, but the loss of whitebark pine will more likely be more problematic for the grizzly bear population than the loss of trout. We know too that females will be much more adversely affected than males by the lack of whitebark pine seeds, because historically females ate nearly twice as many of the high-fat pine seeds compared to males.  Compounding this problem, females will also probably be more adversely affected than will males by wolves on the landscape.  As discussed in my last blog wolves help feed male bears, which aggressively seek out wolf-killed elk and bison.  But they don’t help female bears with cubs much, because females tend to avoid carcasses that are likely to be dominated by males. 

So tying these treads together, can we say anything about the future trajectory of grizzly bears in the Yellowstone ecosystem?  Although no one has a crystal ball,  it’s a fact that the formerly robust growth rate in the population has ground to a halt in recent years (see last  year’s report of the Interagency Grizzly Bear Study Team, which is the official keeper of records for Yellowstone’s bears).  Changes in foods probably have something to do with it, as well as the higher levels of conflicts and grizzly bear mortality resulting from the collapse of whitebark pine.  The weight of evidence strongly suggests that the future does not look auspicious.

Whitebark pine, formerly a key food for Yellowstone’s grizzy bears, has been functionally eliminated from much of the Greater Yellowstone Ecosystem 

More clarity on the cumulative effects of recent and projected changes in bear foods is central to gauging the current health and future prospects of Yellowstone’s grizzly bear population.  Interestingly, at the Jackson grizzly bear managers’ meeting, among the long, highly technical list of research projects being undertaken by the Interagency Grizzly Bear Study Team, the question of cumulative effects from major declines in key foods and climate change was not among them.  For anyone who cares about the Great Bear’s fate, it should be.

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