There's an old saying: "Never let the truth stand in the way of a good story." This weekend's read is the truth behind a really good story about habitat recovery in Yellowstone National Park. In case you're worried about where this is headed: The habitat recovery is real; the conventional wisdom about why it's happening, not so much.
In 1995, the process of reintroducing wolves to Yellowstone began. They had been hunted to extinction by the end of the 1920s, and in their subsequent absence, the elk population expanded (wolves prey on elk and other mammals). The elk, in turn, overconsumed willow and aspen trees in the park. In biology and conservation circles, this is known as a "trophic cascade," when the removal of an apex predator from an ecosystem has a ripple effect down through the food web, with some species expanding unchecked and others becoming depleted.
The popular version of the story, which you can find in many places on the internet and even in some well-watched videos, is that merely by reintroducing wolves, the elk behavior changed: They stopped browsing for food in risky open areas and stayed to more protected, wooded areas where wolves don't like to hunt. This allowed willows and aspens to reestablish themselves in the open areas. The trees stabilized river banks, keeping the rivers following the same route for much longer periods of time. Beavers have also started to return since there are now trees for them to cut down to make dams. And birds have reappeared since there are trees for them to nest in. So simply by reintroducing wolves, the entire ecosystem has been reshaped, pushing it back toward what it resembled before humans removed the wolves in the beginning of the 20th century.
Researchers at the University of Wisconsin, Utah State University, and the Yellowstone Center for Resources attempted to verify this story, and they found two big problems with it. First, there is very little evidence that elk changed their foraging behavior because of the risk of wolf predation.
If we simply do the math, we quickly realize that it's kind of a silly notion in the first place. In winter, there are about 4,000 to 6,000 elk in Yellowstone (in summer, there are 10,000 to 14,000; most migrate to higher elevations inside the park in warmer weather, and to lower elevations outside the park for the colder months), and there are 40 to 60 wolves. But Yellowstone is 3,472 square miles, which is roughly 75 square miles per wolf (and if you consider that they live and hunt in packs, that number goes up even higher). The vast majority of elk in the park will never see a wolf, let alone be hunted by one — and wolves prefer to hunt calves and older, weaker elk, rather than adults in their prime. Moreover, predator-avoidance behavior is very costly: It's a big investment of energy and time to be constantly on the lookout for wolves, and to pass up easy-to-get food because it might be higher risk.
The researchers confirmed this logic with empirical data by collecting statistics on elk density, wolf density, and the state of the aspens in several sample areas throughout the park. They found that wolf density was only weakly related to the amount of aspen browsing by elk in a given area; the same was true for predation risk in a particular area (for which wolf density is a factor, but also how open the land is). Other factors, such as the amount of snowmelt runoff, were much better correlated with aspen browsing.
Instead, they argue that the decline in aspen overbrowsing was because of the massive decline in elk population since the early 1990s. And to fully understand that, we need to rewind and revisit the park's history throughout the 20th century. Wolves were hunted to extinction in Yellowstone in the 1920s because the prevailing wisdom at the time was that predators were harmful to wild ecosystems. But after they were removed, it didn't take long for park managers to realize that the elk population was growing quickly — and from the late 1920s until 1968, they removed about 70,000 elk from Yellowstone. That's right, after wolves were hunted to extinction, the elk population declined — despite the absence of natural predators — because of direct human intervention.
Then, in 1968, the park managers reversed their policy and decided elk should no longer be culled and the population should adjust itself through "natural variation." But without natural apex predators, such as wolves, grizzly bears, and cougars (which had also been hunted to reduce their numbers), "natural variation" became explosive growth.
From 1995 to 1997, about 40 wolves were reintroduced to Yellowstone — and around the same time, the elk population started dropping. If that is the only evidence we look at, it's easy to draw the conclusion that the wolves' return was the cause. But park managers' practices were simultaneously causing the return of other elk predators, including black bears, cougars, grizzlies — and human hunters. It turns out that wolves and cougars each kill a fairly small percentage of the elk that are hunted in Yellowstone each year; black and grizzly bears kill far more, as do human hunters (mainly in the areas outside the park that elk migrate to during winter).
No one doubts that there has been a new trophic cascade in Yellowstone since the 1990s, and the ecosystem recovery is marvelous. But it seems we misunderstood what happened. Elk didn't change their behavior to avoid being hunted; in fact, they have been hunted quite effectively, and their numbers have dramatically declined since the 1990s. But wolves play just a small part in the management of the elk population.
We still don't really understand what's going on in Yellowstone or what happens from here. We know elk tend not to browse on aspens once they reach a height of about 100 centimeters; younger, shorter trees are more nutritious. So it follows that once an aspen reaches that height, it's pretty safe to continue growing. And with elk doing less aspen browsing, there are more and more aspens living and growing to full maturity. In 1988, massive forest fires devastated much of Yellowstone — and since that was the peak of the elk population, the aspen saplings that grew in their place were a feast for the elk. But today, with the elk population much smaller, the aspens are finally staging a comeback. They will probably stick around until the next big forest fire, making up an essential part of the new habitat. What effect will that have? There will probably be somewhat less open land, and more restored habitat for other species. But that's a guess; we know nature tends to surprise us.
Beyond the two big misunderstandings that this report tries to correct (that elk changed their behavior when wolves were reintroduced, and that the reintroduction of wolves was the trigger for the habitat restoration), there is a third misunderstanding that is left unsaid. We still describe the Yellowstone ecosystem as if wolves, bears, and cougars are the apex predators. In truth, for the past 140 years, humans have been the apex predator there: We killed the wolves, the bears, the cougars, and the elk. Since the beginning of the 20th century, every dramatic shift in the park's ecosystem has been triggered by human action, leading to a trophic cascade. One could argue that the "Pottery Barn rule" applies: We broke it, we bought it. But so far, our track record of trying to put it back the way we found it does not look very promising.
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