Back when electronic games were new, one very popular game was
called "Space Invaders." It was pretty simple: You just had to
fight off aliens who were trying to land on the Moon. And
battling those simple digital aliens was a piece of cake.
Scientists at the Agricultural Research Service (ARS) are
playing their own version of "Space Invaders." But when they
play, the invaders aren't little creatures that beep when you
zap them. Instead, the invaders are all kinds of plants—like
grass. They might be flowers. They might even be water plants.
Some of these plants are newcomers to North America. Those
newcomers are called "exotic" plants, even though they aren't
really very glamorous. Lots of times they're just clumps of
grass, like this cheatgrass, which has made itself at home on
western rangelands in the United States. They're so comfortable
here that they're pushing out plants that have been here all
along—the ones we call "native" plants. And sometimes it's
native plants who are doing the pushing. For examp