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Keystone Species: How Predators Create Abundance and Stability

The day came clouded and wind-tossed, with 5 inches of fresh snow in the valley and a lot more piling up overhead on the peaks. It was early December in Montana in Glacier National Park. Although winter wouldn’t officially start for another two weeks, blizzards and bitterly cold temperatures had long since sent the bears into their dens.

But not every bear.

Very large, very fresh paw prints on the trail in front of me said at least one grizzly wasn’t ready to call it quits for the year.

Sleeping in underground dens keeps bears safe and insulated through the snow-smothered months while they live off reserves of fat. The biggest and most powerful ones — adult male grizzlies — sometimes leave their hidden chambers to roam about during midwinter thaws. Before, few naturalists realized these heavy-bodied bears could stay out through much colder conditions as long as they were able to take in more energy from food than they burned trying to find it. Then wolves returned to the American West.

The Food Web Surrounding Wolves

After an absence of half a century, wolves came back to Glacier during the 1980s, trotting across the border from neighboring Canadian wildlands. Suddenly, this Rocky Mountain landscape held more carcasses of deer, elk and moose, and those of us who frequented the slopes began to discover a few scavenging grizzlies later and later into the frozen season. One valley, with prime wintering grounds for hoofed herds, hosts a big male silvertip grizzly that I’m not sure ever holes up to snooze anymore.

Wolverines, with their unsurpassed nose for leftovers, can find more meals now as well. So can wintering bald eagles and golden eagles, along with northern ravens, which often follow wolf packs on the prowl. Wildlife biologists tracking the wolves discovered them taking over fresh kills made by mountain lions. In many cases, the packs seemed to be honing in on the sight of circling ravens or the birds’ excited calls in order to find the stealthy cats and drive them off their prize. Before wolves were reintroduced to Yellowstone National Park in the mid-1990s, cougars had expanded their range to include broad valley bottoms. After the wolves’ return, the cougars retreated to the steeper, more broken upland terrain they had normally hunted.

Animals ranging from jays and magpies to martens and black bears glean scraps from wolf-killed carcasses. Coyotes are on that list of opportunists as well, but the wolves are lethally intolerant of their closest kin. In fact, Yellowstone’s new wolves quickly reduced the number of resident coyotes by nearly half, and forced survivors to take up life in smaller, more scattered territories. Coyotes behave much the same way toward their smaller relatives, suppressing the numbers and territories of foxes.

So, where wolves move in, coyotes end up marginalized while foxes thrive. That in turn shifts the odds of survival for coyote prey such as hares and young deer, as well as for the small rodents and ground-nesting birds the foxes stalk. These changes affect how often certain roots, buds, seeds and insects get eaten, which can alter the balance of local plant communities, and so on down the food chain all the way to fungi and microbes.