The invaders are plants from other countries brought here to make gardens and yards look pretty. Ever since people started to arrive on America's shores, they've carried along trees, flowers, and vegetables from other places.
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. He's an entomologist (an insect expert) at the University of Delaware. He explains that almost all the plant-eating insects in the United States—90% of them—are specialized. That means they eat only certain plants.
Monarch butterfly caterpillars, for example, dine on milkweed. If people cut down milkweed and replace it with another plant, the butterflies will not have the food source that they need to survive.
But the trouble doesn't stop there, it goes right across the food web. 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 birds depend on insects to feed their young.
"We cannot let the plants and animals around us disappear," says Tallamy. "The way to preserve them is to give them food to eat. But when we plant non-native plants, we are clobbering the food web, because then we don't have the insects the birds need to live."
Fewer of the right plants mean fewer bugs, and fewer bugs mean fewer birds. And that's bad for the Earth, because we need a variety of living things to keep the planet healthy and beautiful.
The good news is, gardeners everywhere are working hard to protect native plants and get rid of the invaders. Many local garden centers sell native plants. "Just research 'native plants' and your location, and you can find out which plants really belong where you live," says Tallamy.
Planting the right things makes a real difference, and fast. He describes planting milkweed in a tiny city courtyard about the size of a living room one spring. By summertime, that milkweed patch had produced 50 new monarch butterflies!
Tallamy encourages kids to go out and plant native plants. "Adopt a bird species in trouble and see if you can't plant some things that will attract the insects they need," he suggests. "It will happen—insects move around a lot, and they will find the plants you put out there for them!"