【英语时差8,16】食物链守则（在线收听）

 Food Chain Regulation Biologists who study nature’s food chain agree about the basic links in the chain: sunlight provides fuel for plants, plants are consumed by herbivores, and herbivores are eaten by carnivores. But researchers aren’t sure about all the factors that control the chain. Traditionally, they have thought that food-chain populations are regulated from the bottom up. According to this idea, the availability of plants as food determines the size of animal populations in an area. But a study of a food chain in Isle Royal National Park, an island in Lake Superior, has shown that food-chain control can work in both directions. In the island’s food chain, balsam firs are eaten by moose, which, in turn, are eaten by wolves. When researchers tracked the effects of the moose and wolves on the balsam firs, they actually found that the number of wolves in the area affected the growth of the firs. During the study, researchers recorded the width of the tree rings of balsam firs. Each ring marked a year of the tree’s growth. A narrow ring indicated that the tree’s growth had been suppressed that year. The narrow rings appeared only after wolf populations decreased, allowing a greater number of moose to survive and eat more balsam firs. The researchers concluded that wolves play an important role in maintaining the complex and delicate balance of the island’s food chain. This food chain appears to be regulated from the top as well as from the bottom.