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07/24/2013

Is Greater Yellowstone’s grizzly population declining?

Grizzly-bear-in-yellowstone_USFWSPhoto courtesy U.S. Fish and Wildlife Service

new study raises serious questions about the health of the Greater Yellowstone grizzly bear population, and consequently, whether federal Endangered Species Act protections should be removed.

Federal scientists with U.S. Fish and Wildlife (USFWS) studying the bears have long held that the population grew at a rate of 4-7% from 1983-2001. More recently, USFWS scientists have acknowledged that the growth rate has slowed, but assert that it is because the population has met its “carry capacity” i.e. the maximum number of bears that the ecosystem can support. From a low of approximately 150-200 bears when they were placed under ESA protection in 1973, the USFWS now estimates that there are around 700 bears in the ecosystem.

However, new research by Dan Doak of the University of Colorado and Kerry Cutler of UC Berkeley  point out significant flaws in the methods used by federal scientists to estimate the Yellowstone population, and that these errors have led to inflated estimates of the overall growth rate in the past and  how many bears there are on the landscape.  Since the 1980s when USFWS began aerial flights to monitor the bears, the agency has greatly increased ‘observer effort’ i.e. the amount of time they spent looking for bears. If you spend more time looking for bears, chances are you’ll see more bears.  Also, as a result of landscape changes affecting bears’ major food sources like whitebark pine seeds and cutthroat trout – both of which have steeply declined in the region in the past decade  - bears are now foraging for food in more open spaces, such as moth sites on talus mountain slopes, where they are much more visible. The authors assert that neither of these ‘sightability’ factors was adequately taken into account in USFWS’ past estimates of the population.

Secondly, Doak and Cutler’s research highlight the fact that the USFWS has relied on two flawed assumptions in estimating the number of bears: that all bears live to the age of 30, and that they reproduce at a constant rate during that entire time.  In actuality, many bears don’t live for 30 years, and a 7-year-old bear’s reproductive rate can be very different than a 25 or 30-year-old bear. As the authors note, “there is clear evidence for both reproductive and survival senescence in grizzlies, including the GYE population,” however, inexplicably, senescence has not been taken into account in past and current estimates of the bear population.  

When these factors are taken into account, the new study shows that the population may have grown very little over the past decades, and “it is quite likely that the population is now, in fact, declining.”

Based on an apparently inflated growth rate and current estimate of more than 700 bears in the ecosystem, the USFWS is again gearing up to delist the bear in 2014. In light of Doak and Cutler’s research, however, the agency should take a big step back and re-evaluate its flawed methodology – particularly in the face of declining food sources in the ecosystem and a lack of understanding on how those shifts in major food sources will impact the bears’ future. The Yellowstone grizzly is too treasured and plays too important of a role in this ecosystem for guesswork and flawed science.

-- by Bonnie Rice, Sierra Club Greater Yellowstone 

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