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Grizzly Bear Recovery Estimates Flawed, Population May Be In Decline

June 25, 2013
Image Credit: Thinkstock.com

Lawrence LeBlond for redOrbit.com – Your Universe Online

Yellowstone grizzly bears were first given endangered species status in 1973 after extensive hunting and killing had decimated the wild populations of this apex predator. Through the years since, the populations of these grizzlies have been closely monitored by the US Fish and Wildlife Service (FWS), with numbers reportedly rebounding from a mere 200 animals in the 1970s to more than 700 today.

The steady increase in grizzly population has led the FWS to once remove the Yellowstone bears from the Endangered Species Act list in 2007. However, a federal judge reversed that decision two years later and the bears went back on the list. That reversal came in light of a finding that one of the bears’ main food sources – white bark pine tree nuts – were being wiped out by damaging insects.

Removal in 2007 from the Endangered Species list would have allowed hunters to once again trap and kill limited numbers of bears in the region. However, after the FWS has once again considered a proposal to lift the endangered status off the Yellowstone grizzly, new research indicates that a perceived population growth may be based on flawed methodology.

Daniel Doak, a professor of environmental studies at the University of Colorado, along with study coauthor Kerry Cutler, published a new study in the journal Conservation Letters describing between the years of 1983-2010 the FWS greatly increased observations of Yellowstone grizzlies. While the effort may have been viewed with optimism, the researchers believe more bears were counted not because there were necessarily more bears to be counted, but because FWS members spent more time looking for them.

Sylvia Fallon, a senior scientist with the Natural Resources Defense Council, which partially funded the new study, wrote in a blog changes to the landscape and the loss of two major food sources for Yellowstone grizzlies – cutthroat trout and white bark pine – have led the bears to move to more open areas for feeding, leading to increased “sightability.”

An increase in sightability means FWS members had more chances to see bears; at times the same bears, over and over again.

Fallon also wrote that the FWS “has been relying on two fundamentally flawed assumptions about grizzly bears to calculate their population estimates:  1. that bears categorically live to the age of 30 and 2. that they have a constant reproductive rate for that entire time. In actuality bears, like almost all organisms, “senesce,” or age, meaning that some don’t live past the age of 20 and their reproductive rate decreases with age.”

Doak and Cutler are in agreement that the region’s bear populations may have grown very little or perhaps have even headed back in decline. For sure, the researchers argue the bear populations have not been increasing at the rates declared by the Fish and Wildlife Service.

The two researchers noted little is known “about the past trends of this population, and hence about their likely future fate, especially with rapid declines in multiple food resources and increases in opportunities for human conflicts.”

Even with previous errors leading the FWS to estimate more population growth in the region, it has since shown growth has slowed, perhaps meaning the population has met its “carrying capacity” – the maximum number of bears the current ecosystem can support.

According to Fallon, the NRDC has maintained that instead of leveling off, the populations are in decline due to food scarcity. The study authors concur.

Still, the FWS is considering lifting the federal protection status for the bears, possibly as early as next year. Biologists with the government said there is no evidence of a decline and maintained that newly-revised population data shows that roughly 718 grizzlies live in and around Yellowstone National Park. That number is up from a previous estimate of around 600.

FWS researchers attribute rising numbers to an increase in bear-human conflicts – one of the latest attacks occurring last week when a man was taken to the hospital with severe facial lacerations due to a dangerous encounter. An increase in attacks has lent urgency to allowing limited hunting to resume.

While Doak and Cutler argue that an increase in conflicts doesn’t necessarily mean an increase in population, their study did not offer an alternative population size, nor did they outright say the current estimate was wrong.

One bear researcher, Frank van Manen, of the USGS, noted that the newest revisions to the bear population estimate are based on adult male death rates that were lower than previously thought. He said more effort has gone into bear counting because the grizzlies have spread out across a much wider landscape.

Manen himself runs an interagency grizzly bear study team for the Yellowstone region. He said his studies, which have included aerial surveys and trapping programs, have consistently identified new bears, as well as new areas of habitat.

FWS grizzly bear coordinator Chris Servheen said the methods used to count bear populations have been reviewed by outside scientists and other government agencies.

“We’re certainly interested in what they did,” Servheen said of Doak’s work in an interview with the Associated Press. “But we’ve done a lot of work on this. We’ve given very careful consideration and critiques to everything we’ve done multiple times.”

However, David Mattson, another USGS bear researcher, said the study findings are in line with his own conclusions that the FWS estimation methods are flawed.

“There is this belief that somehow, through some sort of statistical magic, you can compensate for bias in your field methods,” Mattson said. “My conclusion is that’s just not simply possible.”

According to Fox News affiliate KLIX, The FWS has received nearly 160,000 public comments on proposed changes to the recovery criteria for grizzlies in the Yellowstone region. Proposed revisions would change how wildlife managers count bears to ensure the current population does not fall below 500 bears total with at least 48 females with cubs. A revision would also restrict reliance on bear mortality counts.


Source: Lawrence LeBlond for redOrbit.com - Your Universe Online



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