Yellowstone's Dispersing Grizzly Bears: Alternative Explanations
In an article in the Jackson Hole News and Guide on July 6, Wyoming officials said that grizzly bears were showing up in unusual places on the periphery of the Greater Yellowstone Ecosystem, in a few cases outside known historical grizzly bear habitat. State officials used these observations of dispersing bears to make the case that there is a growing population of perhaps more than 1000 individuals. But the state failed to offer alternative explanations that are obvious and even more plausible, and that have nothing to do with increasing bear numbers.
Before addressing these alternative, logical, and even compelling explanations, it is important to remember that grizzly bears are the slowest reproducing mammal in North America. Government estimates of the population last year was somewhere around 600 individuals. While Wyoming officials may not agree with these estimates -- and conservationists have concerns about their accuracy too – they do not have any basis for claims that the grizzly bear population may have topped 1000 individuals. Biologically, it is impossible for a grizzly bear population to grow from roughly 600 bears one year to 1000 the next.So, why are bears dispersing to new areas in the Greater Yellowstone? Are there really more bears, or are they dispersing because of other factors?
According to the federal government's official estimates, the Yellowstone grizzly bear population is no longer growing as it once did. While the article claimed that the population has continued to grow at 4% per year since it was listed under the Endangered Species Act in 1975, that claim is flat out wrong. Only when you backcast to the 1980's do you see a growth rate of 4%, averaged over the entire period. The fact is that recently the population growth rate has stalled -- a phenomenon that coincides with the collapse of whitebark pine, the key food driving the growth of the Greater Yellowstone grizzly bear population. Mature whitebark pine trees have been nearly eliminated in this ecosystem by an unprecedented, climate- driven outbreak of mountain pine beetles. High levels of grizzly bear mortality, especially in 2008 and 2010, partially explain the fact that the population is no longer growing as it did over the past two decades.
So if the population is no longer growing as before, what can explain the dispersal of Yellowstone's grizzly bears? To answer this question, all you have to do is look up at the mountains. You don't need to be an expert to know that there is a ton of snow still up there, after a winter of 200-500% above normal snowpack in various ranges in the ecosystem. And you don't need to be an expert to know that bears can't live off of snow. In essence, bears are trying to make a living in the only places where there are edible green things growing, where elk are calving, and/or where they can find carrion – which all happen to be at lower elevations, especially this year.
In some cases, this means that bears are showing up in somewhat unusual places and in areas where more people are. This has been the case this spring in Yellowstone Park, which has witnessed a record-breaking number of early season "bears jams" – traffic blockages due to black and grizzly bears being seen by bear-hungry tourists along roadsides. One park bear ranger recently observed that the reason we are seeing more bears from the roads is because they have been kept out of the high country by the huge snowpack. Bears would ordinarily be scarce in the low country as soon as green-up occurred higher up.
There is another reason too that can help explain the dispersal of bears to the periphery of the ecosystem over the past few years: the loss of whitebark pine. Whitebark pine is a masting tree, which means that it produces large numbers of cones one year, and not so many the next. Until recently, after good whitebark pine cone years, there were typically some residual cones available to bears the next year. So right about now, you would typically expect bears to be seeking any leftover cones from the previous year's crop, at least in the lower areas where whitebark pine grows. But they can't do that any longer because whitebark pine is now functionally gone from most of the ecosystem. (The best whitebark pine is relegated to the highest mountains: the Wind Rivers, Beartooths and Tetons, and especially deep in snow this year). So, now that bears have more limited dining options some, especially males, are prompted to travel long distances to new habitats in order to get the calories they need.
The dispersal of bears in recent years, and especially this year, has much more likely been caused by the recent big snow winter and the loss of whitebark pine, than by an absurdly large increase in the bear population. It is important to note that just because you see more bears does NOT mean there are more out there: they just may be moving around more to make a living, and in places more heavily used by people.
The dispersal of bears to these somewhat unusual places has important implications for we humans who live and recreate in their midst. It means that people and communities on the periphery of the ecosystem must redouble efforts to prevent bears from getting hooked on human foods, which can lead to unnecessary conflicts and premature bear deaths. The long-term health of Yellowstone's grizzly bears, trying to eke out a living in a rapidly changing world, ultimately rests in our hands.
Before addressing these alternative, logical, and even compelling explanations, it is important to remember that grizzly bears are the slowest reproducing mammal in North America. Government estimates of the population last year was somewhere around 600 individuals. While Wyoming officials may not agree with these estimates -- and conservationists have concerns about their accuracy too – they do not have any basis for claims that the grizzly bear population may have topped 1000 individuals. Biologically, it is impossible for a grizzly bear population to grow from roughly 600 bears one year to 1000 the next.So, why are bears dispersing to new areas in the Greater Yellowstone? Are there really more bears, or are they dispersing because of other factors?
According to the federal government's official estimates, the Yellowstone grizzly bear population is no longer growing as it once did. While the article claimed that the population has continued to grow at 4% per year since it was listed under the Endangered Species Act in 1975, that claim is flat out wrong. Only when you backcast to the 1980's do you see a growth rate of 4%, averaged over the entire period. The fact is that recently the population growth rate has stalled -- a phenomenon that coincides with the collapse of whitebark pine, the key food driving the growth of the Greater Yellowstone grizzly bear population. Mature whitebark pine trees have been nearly eliminated in this ecosystem by an unprecedented, climate- driven outbreak of mountain pine beetles. High levels of grizzly bear mortality, especially in 2008 and 2010, partially explain the fact that the population is no longer growing as it did over the past two decades.
So if the population is no longer growing as before, what can explain the dispersal of Yellowstone's grizzly bears? To answer this question, all you have to do is look up at the mountains. You don't need to be an expert to know that there is a ton of snow still up there, after a winter of 200-500% above normal snowpack in various ranges in the ecosystem. And you don't need to be an expert to know that bears can't live off of snow. In essence, bears are trying to make a living in the only places where there are edible green things growing, where elk are calving, and/or where they can find carrion – which all happen to be at lower elevations, especially this year.
In some cases, this means that bears are showing up in somewhat unusual places and in areas where more people are. This has been the case this spring in Yellowstone Park, which has witnessed a record-breaking number of early season "bears jams" – traffic blockages due to black and grizzly bears being seen by bear-hungry tourists along roadsides. One park bear ranger recently observed that the reason we are seeing more bears from the roads is because they have been kept out of the high country by the huge snowpack. Bears would ordinarily be scarce in the low country as soon as green-up occurred higher up.
There is another reason too that can help explain the dispersal of bears to the periphery of the ecosystem over the past few years: the loss of whitebark pine. Whitebark pine is a masting tree, which means that it produces large numbers of cones one year, and not so many the next. Until recently, after good whitebark pine cone years, there were typically some residual cones available to bears the next year. So right about now, you would typically expect bears to be seeking any leftover cones from the previous year's crop, at least in the lower areas where whitebark pine grows. But they can't do that any longer because whitebark pine is now functionally gone from most of the ecosystem. (The best whitebark pine is relegated to the highest mountains: the Wind Rivers, Beartooths and Tetons, and especially deep in snow this year). So, now that bears have more limited dining options some, especially males, are prompted to travel long distances to new habitats in order to get the calories they need.
The dispersal of bears in recent years, and especially this year, has much more likely been caused by the recent big snow winter and the loss of whitebark pine, than by an absurdly large increase in the bear population. It is important to note that just because you see more bears does NOT mean there are more out there: they just may be moving around more to make a living, and in places more heavily used by people.
The dispersal of bears to these somewhat unusual places has important implications for we humans who live and recreate in their midst. It means that people and communities on the periphery of the ecosystem must redouble efforts to prevent bears from getting hooked on human foods, which can lead to unnecessary conflicts and premature bear deaths. The long-term health of Yellowstone's grizzly bears, trying to eke out a living in a rapidly changing world, ultimately rests in our hands.
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