科学美国人60秒:植物为何长刺-托福听力下载

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　　科学美国人60秒中英文翻译：植物为何长刺

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　　Plants thrive all over the planet, despite the fact that many animals love to snack on them. Various hypotheses account for all those plants. One is that predators kill enough plant-eating animals to give vegetation a chance. Another is that plants develop physical and chemical means to defend themselves.

　　Now researchers have teased out some of these factors in an East African savanna.

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　　The impala—an African antelope—eats grasses and trees and is itself eaten by wild dogs and leopards. Impala often munch on a tree called the acacia. Some acacia have thorns, and some don't.The researchers found that the impala—perhaps not surprisingly—prefer thorn-free acacia. Also, the animals avoid woody areas where predators are more likely to hide.

　　And as a result, the thorn-free, vulnerable acacia are more plentiful in woody areas with plenty of predators. But the thorny acacia are more numerous on the open savanna, where they need to defend themselves. The study is in the journal Science.

　　The researchers say their findings show that both plant defenses and carnivorous predators help plants thrive. They also say that when humans influence—in part by eliminating large predators—we disrupt longstanding, complex systems. And that we should really try to better understand such systems, and our effects on them.

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