In the late 1990s scientists began to notice changes in Yellowstone National Park. Trees that had stopped growing for years began to grow again. Animals started behaving differently. Scientists believe it is the wolves that have caused these changes.
Wolves used to live in Yellowstone. But many people thought wolves would kill animals kept on farms, such as cows and sheep, and they would harm crops, too. In 1926 the last wolf in Yellowstone was killed.
For the next 70 years Yellowstone did not have any wolves. Scientists decided to bring them back to the area in 1995. The U.S. Fish and Wildlife Service brought in 15 gray wolves from Canada. Today between 250 and 300 wolves live in Yellowstone—and they have already left their mark.
Wolves have been hunting and eating elk. The elk leftovers (驯鹿被吃剩的部分) provide food for animals such as bears. With fewer elk, plants can grow better and taller. This new plant growth provides more trees for birds and more plant food for beavers. The number of beavers in northern Yellowstone has increase greatly since wolves were reintroduced.
Wolves have such a big effect on Yellowstone because they are a keystone species (物种). Keystone species are species on which a large number of other plants and animals depend. The removal of a keystone species can lead to the death of other species.
However, not everyone is happy with the wolves’ return. Farmers want to limit the number of wolves in the area, because if there are too many wolves, they may have not only their cows and sheep killed by wolves, but also their cats, dogs, and horses.
Still, many scientists believe that Yellowstone is starting to look more like it did before people began to kill wolves there.
【題組】46. The sentence “… they have already left their mark” most probably means in the passage that _____.
(A) they have brought about changes to Yellowstone
(B) they have made marks where they can get food
(C) they have left a lot of footprints in the park
(D) they have marked out the areas of their own
寫作批改
VIP與詳解卡管理
私人筆記( 0 )