We see grass everywhere. It is in our yards, parks, fields, and many other places. Often it is just something to push the lawnmower across. But prairie dogs, bison, honey bees, white rhinoceros, and lemmings don't see it that way. They depend on grasslands for their homes and food. Although they need the grasslands, the grasslands need these animals even more. They are keystone species. Each species is so important to its habitat that its absence might cause the whole habitat to change or even disappear. The balance of nature that keeps our environments healthy depends on all keystone species.