New math could reveal more grizzly bears

Posted 4/13/21

The Interagency Grizzly Bear Committee is moving toward changing the way it counts grizzlies in the Greater Yellowstone Ecosystem. The changes could result in the official count of the large …

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New math could reveal more grizzly bears


The Interagency Grizzly Bear Committee is moving toward changing the way it counts grizzlies in the Greater Yellowstone Ecosystem. The changes could result in the official count of the large carnivores rising by as much as 43%.

Currently the IGBC reports about 737 bears inside the borders of what is considered suitable habitat in the Yellowstone region. That number, derived by using a math equation called Chao 2, could actually be well over 1,000, said Frank van Manen, leader of the committee’s Grizzly Bear Study Team.

“Chao 2 estimates suffer from an underestimation bias,” van Manen told the Yellowstone Ecosystem Subcommittee Thursday.

The equation was adopted several years ago and was intentionally developed to be a conservative estimate while the team monitored a recovering species. But now the grizzly bear is fully recovered in the ecosystem, van Manen said, and it’s time to look for a more accurate population estimate.

“Population status has changed dramatically,” he said. “We have a biologically recovered population and so there is now a need to really address that underestimation bias and design an approach that is a more accurate reflection of the population as a whole.”

The study team has been working on changes to the equation for years. Currently, if researchers spot a female with cubs of the year, they assume that bear will travel 30 kilometers. However, the study team wants to decrease that distance criterion in the equation to 16 kilometers, believing that would more accurately portray current population density. The equation would remain a conservative count, the team says, but would increase the population of grizzly bears in the area deemed as suitable habitat by the committee by more than 300 individuals to about 1,050.

The estimate only includes individuals inside the invisible borders of suitable habitat, known as the Demographic Monitoring Area (DMA). As the population of grizzly bears has increased, the bruins have expanded their range outside the DMA by an area larger than the state of New Jersey, according to an official with the Wyoming Game and Fish Department. However, the committee doesn’t have adequate resources to count bears outside the borders, van Manen said.

Getting an accurate estimate of grizzly bears outside the DMA is impossible, but Gov. Mark Gordon and other state officials claim there are upwards of 1,400 individuals in the ecosystem — nearly twice the number of the species reported by the committee charged with conservation efforts.

The news of the planned revisions to the counting methods comes just days after the U.S. Fish and Wildlife Service recommended keeping the species listed for protections under the Endangered Species Act in the lower 48 states in a required five-year status review. But that doesn’t mean grizzlies in the Yellowstone are in need of continued protection, said Hilary Cooley, grizzly bear recovery coordinator for Fish and Wildlife.

“Within the lower 48 we have six different recovery zones. And two of those, including the [Greater Yellowstone Ecosystem], we recognize that they have achieved biological recovery,” Cooley said during Thursday’s meeting.

The recommendation to keep the species listed was made due to two of the six habitats in the lower 48 — the Bitterroot and Cascade ecosystems — not having any breeding pairs and two other habitats where the grizzly has yet to be deemed recovered, she said.

Changes to the Chao 2 method would require an amendment to the 2016 Conservation Strategy. Critics have accused the committee of attempting to change the population estimates to rush the species out of federal protections and into control of states, which hope to open hunting opportunities for the carnivore.

“I think we’re talking about a really profound change to how we estimate the population … potentially a change of 43%,” said Bonnie Rice, senior representative for the Sierra Club’s Greater Yellowstone/Northern Rockies campaign.

“I think that this is a big change, and it’s directly relevant to the conservation strategy,” Rice said. “And so as a change to the strategy, we believe that there should be an opportunity for the public to comment, you know, and possibly others in the scientific community.” The subcommittee plans to revisit the subject in their fall meeting.