Yellowstone grizzly population jumps with count revision
April 15, 2021
JACKSON — Federal scientists who track grizzly bears in the Greater Yellowstone Ecosystem are revising a key criterion that underlies how they count bears, with the effect being a 34% to 43% jump in the population estimate.
The change brings the estimated population from 727 grizzlies up to more than 1000 animals. The protocol used to count grizzlies — called the Chao 2 method — is staying the same, continuing a method that’s been used since 2007. What’s changing is a filter that determines how many female grizzlies with cubs get plugged into that model.
Historically, sows and cubs spotted from the air within 19 miles of each other were only tallied once to avoid double-counting bears. But an Interagency Grizzly Bear Study Team analysis found that this buffer was too broad. So going forward only sows with cubs detected within 10 miles of each other will be excluded to avoid duplication.
“There are, of course, implications for the estimates,” Study Team leader Frank van Manen told a committee of grizzly bear managers on Thursday. “First of all, they are more accurate and less biased.”
The Chao 2 model, he said, has long been known to be “biased low,” and so an upward revision was expected.
“An important point here is that, on the ground, nothing has changed,” van Manen said.
The correction to the Yellowstone grizzly population count uses the “best available science,” he said. An 85-page report was produced supporting the change, which will be used this year and is not subject to public comment or review. (The report is attached to the online version of this story at JHNewsAndGuide.com.)
A graph van Manen virtually displayed at a Yellowstone Ecosystem Subcommittee meeting showed how the revision would have influenced population estimates going back in time. In 2002, for example, there were fewer than 600 grizzlies in the official population estimate, but this would have exceeded 700 using the 10-mile criteria for counting females with cubs. By 2019 the revision would have increased the estimate from the low-700s up to more than 1,000 bears.
Grizzlies were protected by the Endangered Species Act in 1975, an era when the population was largely confined to Yellowstone National Park and there were as few as 136 bears remaining. The slow-reproducing large carnivores have gradually gained ground and recovered ever since, and now occupy nearly all 19,300 square miles in the ecosystem that managers deem suitable for grizzlies.
The U.S. Fish and Wildlife Service has delisted Yellowstone-region grizzly bears and turned management over to the states twice — in 2007 and 2017 — but both times the decision was overturned by litigation.
Among the issues that a U.S. District Court judge faulted the federal agency for during the last case was its lack of planning for what happens if the population is recalibrated and revised upward — like what’s happening now. Hunting seasons that the states of Wyoming, Montana and Idaho agreed to were predicated on 700-something grizzlies, not more than 1,000, which could theoretically allow for much higher hunting quotas unless state agreements were also revised.
“If they’re going to change the counting criteria, they also need to change all the criteria in the conservation strategy and all the criteria in any potential future delisting rule,” Center for Biological Diversity Senior Attorney Andrea Zaccardi said. “This was an issue they lost over in court.”
The subcommittee is considering revising the “conservation strategy” for Yellowstone grizzlies. If that happens, there would likely be an opportunity for the public to comment on the changes, committee chair Tricia O’Connor said.
Although females with cubs are the foundation of the estimate, the Chao 2 method also uses data on survival and reproductive rates and sex and age ratios to estimate the Yellowstone population. As soon as late this year, the Interagency Grizzly Bear Study Team is planning to move to a much more robust “integrated population model” — which uses the Chao 2 estimate as one data point among many.
“We’re really encouraged by the preliminary findings from this work,” van Manen said. “We’re really moving into the next century with this type of monitoring.”