Study: Grizzlies near capacity in Greater Yellowstone Ecosystem

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.

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.”