The Wolves of Yellowstone

It is almost ten years ago that the US National Parks Service introduced a total of 31 Canadian gray wolves into Yellowstone Park. This had once been prime wolf country, but the animals had been hunted out of existence in the 1920s. This month, Scientific American has published a truly fascinating article about the extraordinary effect these wolves have had on Yellowstone's ecology in just a short time.

While the effect on the region's elk population -- which had ballooned to more than 20,000 animals without serious predators -- might have been obvious,

"The wolf introduction has had numerous unexpected effects as well. The animals' impact on the flora and fauna in the park has been profound. Indeed, the breadth of change has been so far-reaching that researchers from around the country have come to study the alterations. "Wolves are shaping what you see here," says Douglas W. Smith, leader of the Yellowstone Wolf Project. "In 30 years, when you drive through the park, it will look very different."

One of the most visible signs already has been the recovery of the aspen and willow trees, the buds of which are favourite elk food and which had, therefore, be denuded as the elk population soared.

"Without young trees on the range, beavers, for example, had little or no food, and indeed they had been absent since at least the 1950s. Without beaver dams and the ponds they create, fewer succulents could survive, and these plants are a critical food for grizzly bears when they emerge from hibernation."

But the trees are now coming back and the scientists see the direct link between this recovery and the wolves.

"Trees are coming back most dramatically in places where a browsing elk doesn't have a 360-degree view; these willows, for example, sit below a rise that blocks the animals' view. A look at the plants shows they have not been browsed at all in several years. Elk don't feel safe here, Ripple contends, because they can't see what is going on all around and are nervous about spending time in this vicinity. Just 50 meters away, however, where the terrain is level and wide open and the elk enjoy a panoramic view, the willows are less than a meter tall and have been browsed much more heavily over the past three years. "It's the ecology of fear," Ripple says.

The tree recovery has led directly to teh recovery of beavers and this in turn creates a cascade of effects that includes changes to river shapes and fishing habitats:

"Just upstream is a small beaver dam, one of three--the first dams documented on the river in 50 years. Slough Creek, a tributary of the Lamar, has six dams. Both Ripple and Smith believe that because of the regrowth, beavers have something to eat again. "Their food caches are full of willow," Smith says. And other changes are in the offing. As more woody vegetation grows along the Lamar, it will stabilize the banks and stop some erosion. More vegetation, Ripple predicts, will also shade and cool the stream. It means, too, more woody debris in the Lamar, which will slow the river, cause water to pool, and improve the trout habitat, leading to more and bigger fish."

These changes have also altered life for other predators.

"The number of coyotes in the park is down 50 percent and in core wolf areas has dropped 90 percent. Male coyotes are smaller than they were before the wolves arrived, perhaps, Crabtree says, because "the larger ones were more aggressive and challenged the wolves and lost." With fewer coyotes, their prey--voles, mice and other rodents--have exploded in number. That has benefited red fox and raptors. But red fox prey on songbirds as well, and more foxes could mean a greater toll on birds.

Wolves have also thrown the doors to the Yellowstone meat market wide open. Rarely do grizzly bears or cougars attack full-grown elk, although they eat calves or feed on the winter-killed carcasses. Wolves, on the other hand, pull down big ones all the time. After they eat their fill, they wander away, meat drunk, to sleep it off, or they get pushed off the kill by a grizzly. The presence of wolves has meant that much more meat is available on the ground. All manner of scavengers make a living on these carcasses, and an increase in numbers of everything from grizzly bears to magpies reflects these newfound riches. The largest number of ravens on a wolf kill ever recorded (135) was here. "We see bald eagles, golden eagles, coyotes, ravens and magpies on every kill that's made," Smith says. "I don't know what they did before wolves showed up."

These extraordinarily rapid changes, though probably beneficial in this case, warn us clearly what risks we take when we mess with natural environments. A single change can rapidly create ever-wider shock waves throughout an entire eco-system.

"Wolves have brought other lessons with them. They dramatically illustrate the balance that top-of-the-food-chain predators maintain, underscoring what is missing in much of the country where predators have been eliminated. They are a parable for the unintended and unknown effects of how one action surges through an ecosystem. More important, the Yellowstone wolves are bringing into focus hazy ideas of how ecosystems work in a way that has never been so meticulously documented. Just as the actions of the wolf echo through Yellowstone, they will reverberate into the future as they help to increase the understanding of natural systems.

This is a well-written piece that deserves to be studied with care.