A female grizzly with a cub. Research shows the Greater Yellowstone Ecosystem could be reaching carrying capacity for grizzly bears. (courtesy of John Way)

New research published Oct. 22 in the Journal of Wildlife Management shows the decline in whitebark pine isn’t hurting grizzly survival in the Greater Yellowstone Ecosystem, and the bear population might have reached the ecosystem’s carrying capacity for the animals, said Frank van Manen, an author of the paper and team leader of the Interagency Grizzly Bear Study Team.

The research addresses concerns a federal court raised in 2009 when it reinstated federal protections for the Yellowstone grizzly population after the U.S. Fish and Wildlife Service removed the animals from the endangered species list in 2007. The court said the service failed to consider the impact of global warming and the reduction of whitebark pine, an important food source for the bears.

Research shows bears are resilient, switching their diet when whitebark pine trees and Yellowstone cutthroat trout disappear and ungulate populations change.

“Yet there’s no indication that those changes have affected grizzly bears at the individual level or the population level,” van Manen said. “They can adapt in terms of their diets and find alternative food resources. If they can weather these types of changes, I feel pretty good about the future of grizzly bears under various climate change conditions.”

This latest research is part of a larger food synthesis study van Manen and other scientists with the Interagency Grizzly Bear Study Team conducted to better understand how grizzlies are adapting to changing food sources. This study looked at whitebark pine and bear mortality. It also looked at mortality and bear density — or how many bears are living in the ecosystem.

Researchers used population data going back to 1983 when scientists began collaring bears and conducting aerial surveys to document cubs and yearlings. Data collection on whitebark pine began in 2000 when mountain pine beetles began killing the trees.

New research suggests that grizzlies in the Greater Yellowstone Ecosystem are good at switching their diet when whitebark pine trees and Yellowstone cutthroat trout disappear and ungulate populations change. (courtesy of Interagency Grizzly Bear Study Team)

Researchers found that while whitebark pine declines don’t correlate with mortality, density of the bear population does impact survival of cubs and yearlings, van Manen said. There have been more observed instances of males killing yearlings and cubs as the population — now estimated at 714 bears — grew.

Scientists believe males are killing cubs and yearlings more often because they are more frequently crossing paths. “For bears, space is important,” van Manen said. There aren’t numbers on how many males are killing young bears, because deaths often happen in the backcountry.

The lower cub survival rates in areas with more grizzly bears, along with the slow to stagnant growth of the whole population, indicates the grizzly has reached the maximum number of animals the ecosystem can support, van Manen said. Carrying capacity is a natural limit the ecosystem sets. It’s what is expected in populations when there is little human interference, he said.

“The bears themselves are telling us that they are at a naturally recovered level,” van Manen said.

David Mattson is a retired wildlife researcher with the U.S. Geological Survey and a visiting senior scientist at Yale University and a research associate with the Northern Rockies Conservation Cooperative. He said while the ecosystem might be reaching carrying capacity, it could be because of the state of the ecosystem.

“Bears are bumping their figurative heads against a ceiling that is collapsing on them,” Mattson said. “They probably are at carrying capacity, but carrying capacity is dropping.”

Carrying capacity is determined mostly by food quality and quantity, he said. If there is more food in an ecosystem, it can sustain a higher density. When there is more food, there is less competition for that food. A productive habitat also offers food sources throughout the ecosystem, instead of concentrated in specific areas.

Normally, males eat more meat and females gravitate toward fat-rich foods like whitebark pine seeds. These different food preferences normally keep females and males in separate areas and reduce bear-on-bear conflicts, he said.

“It’s not just food abundance and quality, it’s the kind of food and whether it’s the kind of food that attracts males and females together,” Mattson said.

Two of the four major food supplies for bears in the ecosystem have dramatically decreased — whitebark pine seeds and Yellowstone cutthroat trout. That leaves the animals competing for meat and army cutworm moths, which brings bears together in specific areas where those food sources are found.

And while bears are diverse omnivores, not all foods are equal in terms of nutrients, Mattson said. “Roots don’t equal pine seeds. Grass doesn’t equal trout. Dandelions don’t equal moths.”

If food quality was better, the ecosystem would have a higher carrying capacity Mattson said. He’s skeptical of research that isn’t duplicated by independent teams that also reach the same results. It’s not enough to publish findings in a scientific journal. It should be independently verified, he said.

It is important the best available science — studies and research that has been peer reviewed and published — guides management for the Yellowstone area bears, said Scott Christensen, the conservation director with the Greater Yellowstone Coalition. The latest research meets those requirements and addresses the federal court ruling, he said. But that doesn’t mean grizzly bears are safe and should be taken off the Endangered Species list.

“Does it tell us everything we need to know about grizzly bears in a future where climate change is a reality?” Christensen said. “No, it doesn’t. There is always going to be some uncertainty and anxiety.”

Christensen’s support for delisting the bears will depend on the conservation strategy, which will guide management. The strategy should include population thresholds and plans for if the population begins to decline, he said. He wants to know how the states will monitor bears, and assurances that the goal is to keep a stable grizzly population thriving on the landscape.

Managers also need to address connectivity between the Yellowstone bears and the Northern Continental Divide grizzlies near Glacier National Park. Core habitat in the Greater Yellowstone needs to be protected, and human-bear conflicts need to be reduced, Christensen said.

“There were 50 mortalities this year, and the bears are still listed,” he said. “These things are the population drivers and, whether listed or not, are going to dictate what’s going to happen for grizzly bears.”

Kelsey Dayton

Kelsey Dayton is a freelancer and the editor of Outdoors Unlimited, the magazine of the Outdoor Writers Association of America. She has worked as a reporter for the Gillette News-Record, Jackson Hole News&Guide...

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  1. Quoted: “It is important the best available science — studies and research that has been peer reviewed and published — guides management for the Yellowstone area bears, said Scott Christensen, the conservation director with the Greater Yellowstone Coalition. The latest research meets those requirements and addresses the federal court ruling, he said.”

    The latest research emphatically does not meet “those requirements”.

    Internal peer review as practiced by the IGBST doesn’t meet any standard of scientific quality. That would require non-governmental scientists to independently review the raw data, the procedures and practices involved in gathering that data, the analyses of that data and more.

    This requires IGBST to make its raw data, procedures and practices, and analysis available to scrutiny, review and analysis by non-governmental scientists, which it is has repeatedly and consistently refused to do.

    As to “best available science”, the most basic of the IGBST’s responsibilities, that of producing population estimates, clearly are NOT obtained using the best available science.

    While the official IGBST count of 714 grizzlies seems like a definitive number, it is not.

    The IGBST establishes its official count using a methodology known as Chao2. Like most methodologies it has a margin of error. Chao2 says “we believe there are approximately 714 bears, but there might be as few as 638, and there might be as many as 793”. So already we have a possible spread of 155 bears.

    IGBST says that Chao2 is biased low and becomes less accurate over time. Team leader Frank Van Manen said that he believes that “we are underestimating by about 40 percent.”

    So are there 638 or 714 or 793 or 1,110 (793 + 40 percent) grizzlies? No one knows with any certainty and now we have a spread of 472 bears.

    At this point IGBST’s “best available science” starts looking more like a wild-ass guess.

    How can you defend the ESA’s and the USFWS’s mandate to use the “best available science” when even the folks who do the counting freely admit that their estimates become less accurate with each passing year and may be off by as much as 40 percent?

    This problem will persist after delisting.

    How can anybody, with any certainty, presume to monitor what is happening to the grizzly population after delisting, if no one can say, with any certainty, what the population was at delisting?

    DNA hair analysis as practiced by Alberta bear managers is the current “best available science” and produces a much more definitive population count with a far better margin of error and confidence level.

    Political pressure from all the usual suspects is the driver of the current push to delist grizzlies, period.

    If we are serious about maintaining grizzly bears in the GYE, we must have a definitive population count that inspires confidence, not derision. The current population count is a “flexible fiction”.

    Kent Nelson