New study questions how Greater Yellowstone bears are counted

Uncertainty over the health of grizzly population arises as officials consider removing it from the Endangered Species list.

  • The health of the Greater Yellowstone grizzly population is up for debate, with some calling for the bears to be taken off the endangered species list, while a few scientists claim their numbers may actually be declining.

    Flickr user Shane Lin (CC)
 

How do you count grizzly bears in a region like the Greater Yellowstone, with its 20 million acres of forest and mountains centered on the national park? Most wild animals can't be individually tallied, so biologists use complex statistical approaches that collect wildlife observations for a given year and extrapolate that into a total.

Accuracy is crucial, particularly now, because for endangered or threatened animals like the grizzly, population levels and trends are key to continued protection under the Endangered Species Act. The U.S. Fish and Wildlife Service is considering whether to propose delisting the population in 2014. Wyoming, Montana and Idaho are eager to take over grizzly management and perhaps set hunting seasons, much as they have for gray wolves, which were returned to state control in 2011.

But are the grizzly counts accurate? A new study strongly suggests that the agency's calculations may be badly flawed -- so much so, say the paper's authors, that no one knows how many bears are really out there. The population may even be dropping: Whitebark pine nuts, a key food source, are rapidly disappearing, and the impact on bears is in dispute.

"The Yellowstone grizzly is at greater risk of decline," says Dave Mattson, a wildlife biologist who worked on the interagency grizzly bear team from 1974 to 1993, "because we know less than we thought we did about population trends and the major deterioration of habitat and food sources."

The Fish and Wildlife Service currently says there are 718 bears in the Greater Yellowstone area, well above recent estimates of 600. Since the recovery threshold is 500, this could be a green light for delisting.

The Greater Yellowstone grizzlies were first delisted in 2007, only to be relisted in 2009 after environmental groups sued. The agency now has until October to finish a court-ordered study of how a sharp decrease in whitebark pines, caused by pine beetles and blister rust, affects the Yellowstone bears before it can consider delisting them again. All critical food sources for the bears are either in decline or at serious risk, Mattson says. Yellowstone Lake's cutthroat trout, for example, have been overrun by invasive lake trout.

The vanishing food sources and a resulting change in grizzly habits may be leading researchers to assume that more bears are out there than there really are, according to Daniel Doak of the University of Colorado-Boulder and Kerry Cutler of the University of California-Berkeley. Their peer-reviewed scientific paper, from a study that was funded in part by the Natural Resources Defense Council, was published in Conservation Letters in July.

Doak and Cutler note that Yellowstone grizzlies have begun seeking high-calorie replacements for pine nuts and cutthroat trout, such as army cutworm moths, which are found on open, rocky slopes. A bear eating moths above treeline is more easily seen -- and counted -- than one eating nuts or fish deep in a forest.

The researchers also note that the number of federal and state aerial survey flights has more than tripled since 1995. "If you fish one hour a week," Doak says, "you get a rough idea of how many fish are in the pond."  If you double the time spent fishing, you may catch more -- but that doesn't mean there are actually more fish in the pond. Similarly, the "additional" bears seen might well have been already counted.

It's too soon to tell whether the increased flights and changes in food sources have skewed population counts, says U.S. Geological Survey bear researcher Frank van Manen, the new head of the Interagency Grizzly Bear Study Team. His team is reviewing data about observation flights and the size of aerial search areas, he says: "My job is to provide the best possible information to the policy-makers." Federal biologists and Doak and Cutler disagree over whether the feds' approach adequately accounts for more frequent searches and increased visibility of bears, though.

Despite the uncertainties, the federal government seems to favor delisting. In a July 2012 letter to Wyoming Gov. Matt Mead, then-Interior Secretary Ken Salazar optimistically summarized the findings of a February 2012 grizzly workshop: "All participants agreed that the Yellowstone grizzly bear was recovered and that declines in whitebark pine do not threaten (its) future."

In contrast, a March 2012 email assessment by Gary Frazer, Fish and Wildlife's assistant director for endangered species, stated that "the Yellowstone grizzly population (may not be) just stabilizing but declining, which would make a delisting decision even more difficult to defend."

Today, Frazer says, federal biologists increasingly believe that the Yellowstone grizzly has exceeded the region's carrying capacity. That means the population will probably decline before it reaches a new equilibrium, he added, and any decline makes it harder to justify delisting.

For his part, van Manen believes that the population may have reached a plateau, neither growing nor declining.

The controversy remains unresolved. But in the end, the bear's fate may depend less on science than on Interior's internal discussions about how to balance that science with the politics and the law involved, before it decides what to present in court next year.

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