While the official count of grizzly bears in key habitat inside and around Yellowstone National Park remains stable, the leader of the interagency team responsible for conserving the species says …
While the official count of grizzly bears in key habitat inside and around Yellowstone National Park remains stable, the leader of the interagency team responsible for conserving the species says it’s possible there are about 40-45% more bears than reported.
The current official count is 727 grizzlies in the Greater Yellowstone Ecosystem, down about 10 bears from 2019 estimates.
A possible change of the Chao 2 population estimate equation is being studied by a subcommittee of the interagency group responsible for conserving the species and would raise the population estimate to about 1,000 grizzlies inside the federally defined suitable habitat known as the demographic monitoring area (DMA).
In a late October presentation to the Yellowstone Ecosystem Subcommittee of the Interagency Grizzly Bear Committee, study team leader Frank van Manen said the revision would bring population estimates closer to the actual number of bears in the ecosystem than the current conservative estimate.
Scientists can’t count every bear in an ecosystem encompassing nearly 20,000 square miles in the three-state area bordering Yellowstone, van Manen said. And that doesn’t count the 6,500 square miles that the species occupies outside the DMA. Those bears are not included in the official estimates.
“Without someone throwing millions of dollars extra at [monitoring efforts], I don’t see a way of being able to monitor [outside the DMA] into the future,” said van Manen, a research wildlife biologist with the U.S. Geological Survey.
Political leaders pushing to remove Endangered Species Act protections from the area’s grizzlies — including Wyoming Gov. Mark Gordon — have suggested there are many more grizzly bears on the landscape. In recent congressional testimony, given in support of efforts to revise the act, Gordon claimed the number of bears in the region has risen from about 136 bears in the early 1980s to somewhere between 700 and “as many as 1,200 to 1,400 bears.”
The study team’s research might give validity to such claims, but moving away from the Chao 2 method has critics.
“They said we would be using Chao 2 for the foreseeable future,” said Bonnie Rice, senior representative for the Sierra Club’s Greater Yellowstone/Northern Rockies campaign. “It turns out the foreseeable future isn’t very long, because they’re already making changes.”
Rice said the method used to estimate the grizzly population is “critical.”
“A lot of management decisions flow from that estimate,” she added. “If an estimate is going to be hundreds of bears more, then that really needs to be rigorously studied. Are they real bears or are they paper bears?”
The current official count of grizzly bears is derived using the Chao 2 population estimator, which was intentionally set up to underestimate the population. The Interagency Grizzly Bear Committee — consisting of representatives from the U.S. Forest Service, the National Park Service, the U.S. Fish and Wildlife Service, the Bureau of Land Management, the U.S. Geological Survey and representatives of the state wildlife agencies of Idaho, Montana, Washington and Wyoming and Native American tribes — seeks to develop comprehensive recommendations to update monitoring protocols.
Within that mission, van Manen said he is looking at changes to a specific part of the equation to make estimates more closely resemble the actual population: the distance a grizzly sow can reasonably be expected to travel with cubs.
Currently, that distance is set to 30 kilometers (about 18 miles). Van Manen worked a range of distances, as well as other variables, seeking a model that’s conservative, but less conservative than the current one.
The team is currently working on data that’s based on the work of former study team leader Chuck Schwartz, dating back to 2008. Schwartz said he thought the Chao 2 estimator was underestimating populations by about 45%. So the study teams’ suggestion of a 41% undercount “is in line with what [Schwartz] estimated it would be,” van Manen said at an Oct. 28 Yellowstone Ecosystem Subcommittee virtual meeting.
The study team ran about 50,000 replicates of varying criterion to arrive at what they feel is an improved sow travel distance of 16 km (just short of 10 miles). The change in the Chao 2 estimator equation increases the number of sows with cubs by about 40%, and when combined with mortality rates, suggests a lot more bears than the current official population estimate of 727.
“That would put you out somewhere around 1,000 individuals, depending on the current vital rates,” van Manen said.
He is comfortable with 16 km, despite running distances as small as 12 km. In other words, if the actual distance is less than 16 km, there would be more than 1,000 grizzlies inside the DMA.
“When we go to smaller criteria, we’re getting into an arena where we are dealing with overestimation. That’s a place where you want to be real careful because it’s still better to underestimate just a little bit rather than overestimate,” van Manen said.
The subcommittee is not currently proposing changes, rather simply looking for a more accurate count of the species. Yet, the possibility of the subcommittee changing the population estimator is sure to be a hotly debated topic.
When then-Secretary of the Interior Ryan Zinke announced the delisting of Yellowstone area grizzly bears in 2017 — and Wyoming and Idaho created hunting seasons — it immediately resulted in lawsuits by conservation organizations. About 15 months later, U.S. District Court Judge Dana Christensen ruled federal wildlife managers “acted illegally and illogically,” overturning the delisting rule.
“The Service does not have unbridled discretion to draw boundaries around every potentially healthy population of a listed species without considering how that boundary will affect the members of the species on either side of it,” Christensen wrote.
Connectivity of the species with bears in northern Montana was a large part of Christensen’s decision.
Meanwhile, van Manen reports that 42% of all grizzly bear mortalities happen outside the DMA; bears found well outside the invisible boundaries of suitable habitat and in conflict with humans are often euthanized.
There have been 46 known grizzly bear mortalities in the Greater Yellowstone Ecosystem in 2020. Most recently, an adult male was euthanized on Nov. 4 “for property damage and obtaining multiple food rewards” north of Cody near Pat O’Hara Creek. Of the 19 known mortalities outside the DMA, 15 of those bears were euthanized in conflict management decisions made by the U.S. Fish and Wildlife Service.
Van Manen will make another presentation about the population estimator research at the Yellowstone subcommittee’s spring meeting.