Reading passage Go to the questions
The return of the wolves
A By the 1920s, the grey wolf had disappeared from Yellowstone, the oldest national park in the United States. For decades, wolves had been shot, trapped and poisoned, partly because they killed cattle and sheep on nearby farms, and partly because they hunted the elk and deer that visitors came to see. For roughly seventy years, the park had no wolves at all.
B Without their main predator, elk numbers grew. In winter, large herds gathered along the rivers and ate the young willow and aspen trees that grew there. For several decades in the middle of the twentieth century, park staff tried to control the problem themselves by shooting or moving thousands of elk, but the practice was unpopular and was eventually stopped. Some ecologists began to argue that the park was missing an essential part of its natural system.
C In 1995 and 1996, after years of public argument, 31 grey wolves captured in Canada were released into the park. Before their release, the animals were kept for several weeks in large fenced areas so that they could get used to their new surroundings. More wolves were brought from the neighbouring state of Montana in 1997. Many local ranchers opposed the plan, fearing for their animals, so a wildlife organisation agreed to pay farmers for any livestock killed by wolves.
D The wolf population grew quickly, and the number of elk in the northern part of the park fell sharply over the next twenty years. Wolves were not the only reason. Bears and mountain lions also hunt elk calves, hunting by people outside the park increased, and several dry years reduced the food available. Nevertheless, the arrival of a new predator clearly changed the way elk behaved and where they spent their time.
E Soon, a simple and attractive story was being told. Wolves, it said, had brought the whole park back to life: with fewer elk eating them, willows grew tall again along the rivers; beavers returned to use the willows; their dams created ponds for fish and birds. A short online video telling this story was watched by tens of millions of people.
F Many scientists now believe the reality is more complicated. In some places, willows have recovered well, but in many others they remain short. One explanation is that during the decades without wolves, elk ate so much vegetation that beavers left. Without beaver dams, the water level in the streams dropped, and the rivers cut deeper channels. Young willows now often grow too far above the water to thrive, even where elk are fewer. Ecosystems, these researchers argue, do not simply return to their old state when a missing animal is put back.
