Green invaders (入侵者) are taking over America. Not invaders from space, but plants! You might not think of plants as dangerous, but in this case they are becoming a great threat (威胁) to the nature's food chain (食物链) . The invaders are plants brought here by people from other countries to make gardens and yards look pretty. Now there are so many of those plants, they are crowding out the native plants that have lived here since before human settlers arrived.
And that's a problem, says Dr Doug Tallamy, an insect expert at the University of Delaware. He explains that almost all the plant-eating insects in the United States eat only certain plants. Monarch butterfly caterpillars (幼虫) , for example, eat milkweed. If people cut down milkweed and replace it with another plant, the butterflies will have no food to be alive. But the trouble doesn't stop there. When insects can't get the right plants to eat and they die off, then the birds don't have enough bugs for their meals. Tallamy points out that almost all migrating (迁徙) birds depend on insects to feed their young.
"We can't let the plants and animals around us disappear," says Tallamy. "The way to protect them is to give them food to eat. But when we plant non-native plants, we are breaking the food chain, because then we don't have the insects the birds need to live." That's bad for the Earth. We need different kinds of living things to keep the planet healthy and beautiful.
The good news is: garden centers sell native plants. "Just google 'native plants' and your location, and you can find out which plants really belong where you live," says Tallamy.
