Global Warming is Changing How Bears Eat—and That Could Mean Trouble

A warming climate is transforming grizzlies' diets. Will it endanger their future?
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A warming climate is transforming grizzlies' diets. Will it endanger their future?

Usually, early summer at Kodiak National Wildlife Refuge in Alaska means scores of grizzly bears crowding streams, scooping up so many sockeye salmon that fish carcasses soon litter the grassy banks. The red-bodied, spawning salmon pack the waterways so densely that even cubs can easily claw them from the water. William Deacy and Jonathan Armstrong, ecologists with Oregon State University, spent years studying how grizzlies “surf the wave” of salmon by moving from one stream to the next, gorging on fish.

So it came as a surprise to Deacy and Armstrong in 2014 to find streams teaming with sockeye, but not a bear in sight. Whole salmon carcasses floated downstream or caught on the gravel, untouched. Instead, the bears’ radio collars beckoned from the hills. An unusually warm spring had spurred elderberries to ripen earlier, synchronizing with the salmon run, and the bears opted to become herbivores.

If one source of food is good for bears, it stands to reason that two would be better. But to Deacy and Armstrong, the change spelled danger.

“Previously, [the bears] had this series of really good food: the early run of salmon, then the berries would come up, then you’d have a late run of salmon,” Deacy says. “In warmer years, it’s berries and salmon at the same time, then a gap, then the late run of salmon. The question is, can they fill that gap with something that’s nearly as good, or is [the change] harmful?”

Shifts in food cycles don't just affect grizzlies. After bears go for the energy-rich salmon pieces—humps on the males and eggs from females—they leave much of the leftovers on the banks, where the carcasses feed smaller animals like foxes, weasels, and gulls, and then fertilize the soil. If the bears aren’t there to pull the fish from the water, smaller predators go without.

As spring temperatures increase, elderberries ripen earlier by an average of two and a half days each decade, according to Deacy and Armstrong’s research, which was published in the journal Proceedings of the National Academy of Sciences in August.

By 2070, Deacy says, they’ll regularly align with the salmon run. (Salmon's schedules don't seem to be changing with the temperature so far)

“What’s a strangely warm year now will be the average,” he says. “If that happens, we can expect to see the trickle-down effects being more permanent, and the other wildlife populations will probably be affected.”

The bears themselves, at least on Kodiak, are expected to work it out. What sounds like a poor trade, like swapping a turkey sandwich for just a jar of jelly, isn’t all that bad from a nutritional standpoint: Salmon are packed with protein, but take more energy to break down and convert to weight. Research on how bears balance nutrients suggests they take just 17 percent of their energy from protein, a close match with elderberries, which are about 13 percent protein; Deacy estimates the bears may actually put on more weight eating just the elderberries. Plentiful options on the island allow grizzlies to ride out a changing climate by swapping one high-quality food source for another, but bears elsewhere with fewer options may run a higher risk of going hungry.

Inland, in the Greater Yellowstone Ecosystem, grizzly bears have been responding to a decrease in whitebark pine seeds, caused largely by climate-change-triggered beetle infestations, by consuming more plants and berries. Whitebark pine seeds, high in fat and protein, are a top-tier food for grizzlies, and how much bears fatten up before winter directly correlates with how many cubs they produce. A female grizzly can carry half a dozen fertilized eggs for months after mating in the spring, but how many eggs implant is dictated by how much fat she stores up over the summer months. Where food is plentiful, that could mean as many as four cubs; when food runs short, it might only be one.

Stepping down to berries and plants may not immediately provoke a catastrophe for bears around Yellowstone, but the decrease in food options puts them on shakier ground.

A new research method could shed light on just how much grizzlies rely on each food source, says Jack Hopkins, an assistant professor at Unity College in Maine. Hopkins uses isotopes from grizzly hair follicles to assess their food consumption for an entire season and estimate “assimilated diet”—what food actually goes into tissue.

To collect data, Hopkins and his team picked a location where bears still consume a lot of whitebark pine seeds to assess that contribution to their diet; the technique can also be used to illuminate how grizzlies are adapting to the loss or decrease of other food sources, like elk populations that declined after wolves were reintroduced, or cutthroat trout that have been wiped out by invasive fish. Previous research has correlated whitebark pine to successful reproduction, Hopkins says, but using isotopes could indicate if bears are even more adaptable than we think, or if shorter menus are putting them in danger.

“When we think about climate change, we often think about these iconic species that are threatened, like a polar bear starving on a melting iceberg, but in reality, we’re likely to see that a little bit of warming causes a lot of stuff to go out of whack,” Deacy says. “Just two degrees of warming can create fairly profound effects...In most of North America, it’s not going to be these species overheating, it’s going to be small shifts in the natural schedules causing ripple effects through the ecosystems.”

The best insurance for grizzlies, Deacy says, comes back to early wilderness advocate Aldo Leopold’s statement about the first rule of intelligent tinkering: keep all the pieces. The more options in the grizzlies' pantry, the better their chances of surviving.

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