Yellowstone Grizzlies and the Controversy Over Food

Grizzly bears in Greater Yellowstone may be getting the food they need, for now. But evidence emerging from 1978 to 2023 support a concern that the bear’s food supply is at risk, plausibly jeopardizing the bears’ population.

Authors of the June 2023 report published in the journal Global Change Biology looked at foods available for Yellowstone grizzlies from 2000 to 2020. During that time, they found that while several foods key to the bears’ diet have declined, their body conditions indicated that “sufficient food sources were available on the landscape.”

“However,” they add, “synergistic effects of continued climate change and increased human impacts could lead to more extreme changes in food availability,” with consequences for the population.

Precedents for this latest scenario of climate, food and the grizzly were established in 1978, and again in 1985.

The August 31, 1978 issue of Nature published former MSU wildlife management professor Harold Picton’s pioneering analysis of climate and grizzly bear reproduction in Yellowstone National Park. The late Picton’s study indicated that smaller litter sizes were more attributable to foods exposed to unfavorable weather than to the closure of Yellowstone’s food-waste dumps, which park officials at the time believed was the leading cause of fewer cubs.

Renowned bear biologist David Mattson has argued that since Yellowstone’s cutthroat trout and white bark pine are such significant foods for the ecosystem’s grizzlies, threats to the trout and the tree might be enough to preclude delisting the bear in Greater Yellowstone.

The food and climate scenario was presented again in May 1986 a Grizzly Bear Habitat Symposium in Missoula discussed Climate, Carrying Capacity, and the Yellowstone Grizzly Bear, of which Picton was lead author. Joined by three leading grizzly researchers as coauthors, a key finding was that when foods are scarce, grizzlies wander over wider areas, encounter people, and die.

Renowned bear biologist David Mattson was one of Picton’s coauthors of the climate and carrying capacity analysis. He’s argued independently that, since Yellowstone’s cutthroat trout and whitebark pine are such significant foods for the ecosystem’s grizzlies, threats to the trout and the tree might be enough to preclude delisting the bear in Greater Yellowstone.

His concerns were met with equivalents of “Bah, humbug.” The opposing argument was at least twofold.

Some quickly—and correctly—pointed out that Glacier National Park had seen an extensive whitebark die-off, it’s grizzlies had never relied on trout, and the bears were doing just fine without trout and tree.

It was also argued—again correctly—that grizzlies have many, many foods in the wild. There are hundreds of them. So, why wouldn’t these be more than enough to make up for the loss of just two?

Both arguments rest on an assumption that their truths of the moment will hold true even as plant food-based grizzly habitat of the Northern Rockies gets more heat, less snow, and drier soils. All three variables have been shifting to increasing extremes as we persist with the routine combustion of fossil fuels.

To that predictable set of changes, add an increasing number of people bringing new stressors of several kinds including significant changes of land ownership, and the coinciding sprawls of development and recreation demand.

In their 1986 presentation, Picton, et al, urged a look ahead and at least some thought to a worst-case scenario for the bear. Their thinking was anathema, heresy for at least a few in the world of grizzly biology and conservation. Even my modest grapevine buzzed with rumors of who of us were saying what.

The 1980s food debate’s liveliness had profile enough at the time to earn some ink in the print media. And it’s still lively enough to have persisted right up to June of 2023, a déjà vu in that Global Change Biology report that grizzlies have had it pretty good with the food thing up to now, despite some setbacks on the supply side. And authors of that 2023 report agree that “synergistic effects of continued climate change and increased human impacts could lead to more extreme changes in food availability.”

Does anyone—other than me—wonder if it’s possible to be overconfident about the bears’ food supply as their habitat changes under more intense heat and drought? Is there still some risk of smaller litters of cubs where foods are scarce? Is there any persistence of risk that, forced by shrinking food supply, bears wander more widely, encounter people, and die?

If only the plot didn’t get thicker

Digestion generates body heat, known as metabolic heat, with known consequences for another large mammal, this of the bovidae family. On hot enough days, cows just quit eating. The food can be there, but the cows aren’t interested. They’re more attuned to staying as comfortable as possible in the heat, at the expense of food.

Digestion of any food in the heat is as likely to affect grizzlies in the same way.

Exertion also generates body heat. Running in a marathon during a recent heatwave led to hospitalization for a group of Australians. There are likely similar risks for a horse running on a torrid day.

A grizzly in pursuit of a running elk calf on a hot day might just slow and give up, unless the calf does first. A grizzly digging for voles on a really hot day might even be at risk of heat stroke and collapse if its digging is vigorous enough for long enough, and temperature all around it high enough.

We may not be seeing those temperatures in the Northern Rockies currently, but we might need to think about the 121 degrees F reached in British Columbia in 2021 as a plausible harbinger for the food thing.
If only the plot didn’t get thicker

BC’s 120 F might also serve as an extreme heat benchmark for the risk added by metabolic heat.

Metabolic rate and heat have long been familiar to physiologists and physicians. Wildlife conservationists maybe not so much, but the fundamentals are fairly straightforward: Metabolism and metabolic rate are basic to just staying alive and doing things or, as the UK’s long-established Royal Society journal says it, the “Basal or standard metabolic rate reflects the minimum amount of energy required to maintain body processes, while the maximum metabolic rate sets the ceiling for aerobic work.”

Their further comment leapt off the page at me: “It is clear that metabolic rates are targets of selection, but more research is needed on the fitness consequences of rates of metabolism and their plasticity at different life stages, especially in natural conditions.”

In “natural conditions,” shaking off dangerous heat can be done with a dip, where water enough is available—and readily accessible.

Authors of a recent article in another British journal, Functional Ecology, report that “Our results suggest that the costs of heat dissipation, which are modulated by climate, may impose constraints on the behaviour and energetics of large endotherms like grizzly bears, and that access to water for cooling will likely be an increasingly important driver of grizzly bear distribution in Yellowstone as the climate continues to warm.”

These authors tried a look ahead, using the model of a future climate. Based on what the modeling had to tell them, they also conclude that, “the availability of water for thermoregulation increased the number of hours during which lactating females could be active by up to 60% under current climatic conditions and by up to 43% in the future climate scenario. Moreover, even in the future climate scenario, lactating bears were able to achieve heat balance 24 hr/day by thermoregulating behaviourally when water was available to facilitate cooling.“

In other words, if I understand correctly, a lactating grizzly female in a hotter and drier world, but with opportunity to cool off, will be functioning well enough to go looking for food, if the food is there to be found.

It seems important to note this data from the same Functional Ecology article: “… the heat dissipation limit theory posits that allocation of energy to growth and reproduction by endotherms is governed more by their capacity to dissipate heat than by their ability to harvest energy from the environment.”

Distance to water and, then, ready access to it may rise in urgency along with the food thing as we continue our normal daily routines of firing up fossil fuels—and it will affect some of us on the planet more than others.