Study shows Yellowstone grizzlies fairing well

BILLINGS – Although some key foods for grizzly bears in the Greater Yellowstone Ecosystem have declined in the past two decades, a recent study shows the big bruins have been able to maintain healthy amounts of body fat.

That’s essential for the animals to survive during winter hibernation, as well as for female bears to birth and nurse healthy cubs.

The study was conducted by lead researcher Andrea Corradini, a post-doctoral researcher at Fondazione Edmund Mach, an Italian research foundation.

The U.S. Geological Survey also took part in the study, along with information collected by state wildlife agencies in Montana, Wyoming and Idaho.

Grizzly bears, which can live up to 25 years in the park, are omnivores.

In Yellowstone they’ve been documented eating more than 260 foods, from bugs to plants to fish and young elk.

One of the shifts in bear diets researchers have seen is an increase in fall consumption of carcasses, whether from animals killed outside the park by hunters or male elk and bison dying following injuries incurred during competition in the mating season.

Research

“The new study found that lean body mass (total body weight minus body fat) was lower in areas with higher grizzly bear population density,” the USGS said in a news release.

“However, body fat levels stayed the same over the study period, regardless of bear population density. These findings suggest that grizzly bears were still able to gain sufficient energy reserves and able to cope with changes in food availability and increased competition by prioritizing body fat storage.”

Young female grizzlies had the hardest time building lean body mass, possibly because they are less likely than young male bears to disperse to more isolated areas, and therefore compete with large, dominant males for food, the study published in the journal “Global Change Biology” suggested.

Female grizzlies reach physical maturity at age 7; males don’t mature until age 14.

Foods

Some of the foods consumed by Yellowstone grizzly bears have declined, including: whitebark pine trees, which produce protein-rich seeds in high-elevation areas; cutthroat trout, which were accessible in spawning streams in the spring; and elk, whose population declined after wolves were reintroduced in 1995.

“I think there’s little doubt these animals have been able to adapt to food availability changes over the last couple of decades,” said Frank van Manen, USGS scientist and team leader of the Interagency Grizzly Bear Study Team in Bozeman.

“The fact that we’re still seeing similar body fat levels in these animals despite all of these huge changes in food resources tells me that is not an issue that is of concern anymore.”

The issue of declining food resources for grizzlies was raised after a federal judge in 2009 rejected the U.S. Fish and Wildlife Service’s first attempt to delist Yellowstone bears.

At the time, the court found that the USFWS hadn’t considered the effects of the loss of whitebark pine on grizzly bears. The trees are listed as threatened under the Endangered Species Act.

Pressure to delist the animals by lawmakers in Montana and Wyoming, as well as their congressional delegations, has mounted in recent years, following the USFWS’s foiled second attempt.

“It is past time to celebrate this recovery, delist the bears and return the management back to the states,” Montana Sen. Steve Daines said in a May committee hearing.

Interior Secretary Deb Haaland, who oversees the USFWS, told Daines that although grizzlies had recovered in some areas like Yellowstone, the courts require the agency to look at the entire population.

The big picture

In the portion of the 22-million-acre Greater Yellowstone Ecosystem where bears are monitored, the USGS has estimated their population at 965 animals under newly revised counting methods.

In May, van Manen said grizzly bear populations had retracted in some regions and bears were no longer expanding as they had since being listed as threatened under the Endangered Species Act in 1975, WyoFile.com reported.

“It’s fair to say we’re seeing population densities indicative of reaching carrying capacity in many regions of the ecosystem,” van Manen said, although some areas are still seeing a bit of growth.

Much of that growth was in more marginal habitat in forests surrounding the park, sometimes resulting in lethal encounters with hunters as they attempted to retrieve their elk, or government trappers responding to livestock depredations at ranches and on federal grazing allotments.

In 2021, the most recent report available, 78 grizzly bear mortalities were recorded in the GYE, 59 of which were attributable to human causes with many of those clustered outside the eastern edge of the GYE.

“The current status of the population certainly suggests we’ve reached biological recovery and that the population is healthy and in really good shape,” van Manen said.

“Of course, lots of things can change in the future, but the fact that they’ve responded so well and adaptively to what I would call pretty drastic changes over the last two or three decades, I think is also an indication that in the coming decades we should feel pretty secure that this population is going to be in good shape.”

Opposition

Grizzly advocate and former USGS scientist David Mattson said using lean body mass – a measure of muscle, bones and bodily fluids – along with body fat are “crude proxies for what’s driving population growth, what’s governing the demographics of a population.”

“They don’t tell you about what bears are eating that are critical to them and where,” he said. “It’s absolutely clear that bears are eating different things in different parts of the ecosystem with different consequences, not just in terms of their body composition or body size.

“By virtue of eating the foods that they do, how does that affect exposure to people … who are inclined to kill them or engender conflicts that lead to the bears dying?

“Bears were not starving when pine seeds were not available. What happened is that bears sought out alternate foods. Many of those alternate foods were in places that increased their exposure to humans, increased the number of conflicts, increased the number of dead bears.

“So the key aspect of the foods the bears eat is the extent to which by eating those foods bears are exposed to people, exposed to conflict, exposed to risk of mortality.

“There’s some major issues when you’re trying to make inferences or gain insight into any aspect of population dynamics to just look at bear body mass, or the composition of bear bodies.”

More important, Mattson said, is where high-calorie foods are in relation to where people are living and recreating.

And what foods bears are eating “can vary, dramatically from one part of the ecosystem to another. So all of that is kind of obscured by these broad-brush generalizations of bears in Yellowstone. It’s like saying people in the United States, as if all of us are the same. The places we live are all the same. Our diets are all the same.”

Bears

The study used measurements from 418 bears born between 1978 and 2019 and captured from 2000 to 2020.

This included 146 females and 272 males. Another 52 male bears were excluded from the study because they would have skewed the results.

The males lived near what Yellowstone calls “carcass redistribution sites.”

That’s where the Park Service places animals killed by cars or maybe brought down by a predator too close to a developed area. The agency’s staff relocates the dead animal to reduce the likelihood of a predator encountering a human, or vice versa.

According to the study, some of the outcomes of increasing grizzly bear density are a higher mortality rate for young animals; females reproducing at a later age; a reduction in the reproductive rate of adult females, and an increase in the death rate of adults.

There aren’t many places where a study with so much long-term data regarding an unhunted grizzly population could be collected, van Manen noted.

“This type of study shows to me the importance of investment in long-term research and monitoring,” he added. “I think how we’ve operated as a study team collaboratively … is a really successful model to investigate these important ecological relationships.”

The researchers noted what the future may hold is uncertain.

“Though the grizzly bears of the Greater Yellowstone Ecosystem have been able to maintain fat levels and overall body size as environmental conditions changed over the past two decades, it is not known how they will adapt to more extreme disturbances in the future, such as continued warming, changing wildfire patterns and increasing human development and recreation.”

One of the studies the USGS is conducting will look at whether grizzlies are spending more time feeding to maintain their lean body mass and build up fat reserves.

“Unfortunately, it’s too early to tell what the results will be,” van Manen said.