The new way biologists count Yellowstone-region grizzly bears, explained

Since the grizzly bear population hit rock bottom in the Greater Yellowstone Ecosystem some four decades ago, biologists have kept close track of the population and generated an annual estimate.

For the last 20 years, the Interagency Grizzly Bear Study Team has produced a yearly population estimate using a relatively simple calculation centered on the minimum known number of females with cubs at their side. Essentially, the team counted family groups from the ground and sky and then adjusted the numbers based on estimates of missed bears. In a separate step, they’d use survival and reproduction rates to estimate numbers of male bears and solo females. The computation would ultimately churn out an estimated headcount.

But recently, federal and state biologists who keep watch over the isolated, southernmost grizzly population in the United States adopted a whole new system. Peer-reviewed in the journal Global Ecology and Conservation, the new method is an “integrated population model” that essentially mirrors how a demographer would make projections about human populations.

“In principle, that’s exactly what we’re doing,” Interagency Grizzly Bear Study Team leader Frank van Manen told WyoFile. “Demographers use things like census data — how many people are in a country — reproductive rates, longevity, that sort of thing. That’s more or less what we’re doing with this grizzly bear population.”

The integrated population model relies on computer calculations made with many more categories of data to generate an estimate. In development since 2018 and launched in 2023, the new model initially predicted 965 grizzlies in the 19,278-square-mile “demographic monitoring area” in the core of the Yellowstone ecosystem. (On the outskirts of the region — about 40% of the occupied range — grizzly numbers are unmonitored.)

The model requires that different grizzly population indicators jibe with each other. It wouldn’t allow, for example, for an increasing population projection and a declining reproductive trend to exist simultaneously.

“The model self-reconciles all the numbers against one another,” van Manen said. “By doing so, it improves the accuracy of all the numbers — and the accuracy of the population estimate.”

Improved accuracy

The improved accuracy comes, in part, from taking human error and subjectivity out of the equation. That’s according to Josh Nowak, who presides over Speedgoat, the wildlife consulting business that handled the coding for wildlife managers’ new grizzly-counting technique.

“When we do [estimates] mentally, we don’t have to make everything agree,” Nowak said. “We’re free to take all sorts of liberties. The model takes us to task, and makes sure that everything agrees.”

The finished product that Nowak left grizzly bear biologists with is basically computer software. Flaws in the model can easily be rectified, he said.

“By building software and testing it and implementing it, the results are repeatable,” Nowak said. “If a bug were found, it could be fixed. And that fix would propagate everywhere — through all years, space and time.”

The newly adopted integrated population model  estimates that there were around 250 grizzly bears in 1983 . That’s the first year of data that went into the model. The estimate more than quadrupled to 1,030 grizzlies in the Greater Yellowstone Ecosystem’s core most recently. There’s “no doubt,” van Manen said, that it’s a more accurate population estimate than before.

Until a few years ago, grizzly bear scientists knew they were underestimating numbers. The old Chao2 method — named after a researcher — estimated that there were around 700 grizzlies for much of this century. But in 2021, they updated the criteria for qualifying unique females with cubs — the effect being a 34% to 43% on-paper jump in the population, though the number of flesh-and-blood bruins roaming the landscape was likely largely unchanged.

Adaptable

Wildlife managers will continue to hard count unique females and use the old estimates as feedstock for the new integrated population model. Some types of data are harder and costlier to collect than others, and the new population model, van Manen said, will help bear biologists identify what’s most critical to judging the numbers.

“We all have limited budgets,” he said. “This would also help us identify data sources in the future that we could maybe let go.”

Yellowstone-region grizzly managers also have access to a new tool that will allow them to assess grizzly population dynamics based on geography. Currently, metrics like survival rates are evaluated across the entire tri-state ecosystem — but going forward they could “partition” different areas, discerning differences in how grizzlies are faring, van Manen said.

It’d take a lot more data collection and funding, but theoretically the new population model could even open the door for generating an ecosystem-wide population that includes areas outside of the demographic monitoring area.

“There would be some challenges in doing that,” van Manen said, “but in principle it’s possible.”

Grizzly bears are still protected by the Endangered Species Act throughout the Lower 48 states, though Wyoming is petitioning to again gain jurisdiction . If that happens — and a decision is overdue, to the frustration of state officials — the new integrated population model will factor into how decisions are made during expected future grizzly bear hunting seasons administered by the Wyoming Game and Fish Department.

In general, the population model will allow the state agency to have a better grip on what’s going on with the Yellowstone region’s grizzly bear population.

“It allows us to make some projections,” said Game and Fish large carnivore supervisor Dan Thompson, a co-author on the peer-reviewed study . “How quick can the population grow or stabilize?”

The plan, he said, is to stick with the new population modeling tool into the next era of grizzly bear management.

“We’re committed to using this unless something better comes along,” Thompson said.

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