Long after the dinosaurs died out some 65 million years ago， many giants continued to roam the Earth. Some mammals were big as backhoes！ Mammoths grazed on grasslands， and elephant-sized ground sloths tore apart tree branches with sharp claws. Millions of whales cruised the oceans.

Collectively， these megafauna — the scientific term for very large animals — ate mega-size meals. Then they went on to shed mega-quantities of poop.

And that was a good thing. The huge quantities of feces shed by the big animals contained leftover nutrients from the food on which they had dined. That poop fed a global cycle that moved nutrients like phosphorus and iron from the ocean to the land. These nutrients fed plants， animals and other organisms around the world.

But this cycle has shrunk dramatically， a new study finds. By virtue of their size， big animals can quickly move lots of nutrients across large distances. Most of the world's giant mammals， though， are now extinct. There are a few left， like rhinos， elephants and whales. But there were far more in the past. " Almost everywhere looked like an African savanna，" notes an ecologist of the University of Oxford in England.

Since then， however， humans have killed off many of the big animals， mostly for food. Large animal species that still exist are far fewer in number than they once were. Plus， people have been altering ecosystems in ways that can make it harder for important players in the nutrient cycle to survive.

As the numbers of whales and fish have been falling， fewer ocean nutrients have been reaching the land. Of nutrients that do， fewer are traveling inland as fast as they once did. Without those nutrients， plants won't be able to grow as well. As a result， the total weight of all the Earth's vegetation will decline， observes Joe Roman. A coauthor of the new study， Roman is a biologist at the University of Vermont in Burlington.

The decline in vegetation will have a cascade of effects. Animals will have less to eat. That will lead to fewer animals， and a further drop in the spread of nutrients. That's the conclusion Doughty， Roman and their colleagues reported in the Proceedings of the National Academy of Sciences.

Less plant growth means less carbon dioxide will be removed from the air. Carbon dioxide is a greenhouse gas and a major cause of global warming.

"We need to keep as many animals on the planet as possible，" says Roman. And livestock， such as cows and sheep， won't work. Most of those animals are kept fenced in， so that they cannot participate in the long-range recycling and dispersal of nutrients. What the planet needs， he argues， is wild， free-roaming， big animals.

1. Which of the following animals is not megafauna？ ____.

A. mammoth B. rhino C. elephant D. goat

2. Which of the following causes the extinction of megafauna？ ____.

A. Human activities， such as killing and ecosystem alteration

B. Bulk body size hard to hunt for food

C. Natural selection — the survival of the fittest

D. Drop in the spread of nutrients

3. Which of the following is the correct sequence？ ____.

a. the reduction of vegetation on earth

b. more carbon dioxide

c. drop in the population of whales and fish

d. killing off large animals and altering ecosystems

e. global warming

A. bceda B. cbade C. dcabe D. bcdea