People may take water for granted, but animals do not. They swim in it, drink from it, find their food in it, and live in it. American alligators, eastern newts, North American beavers, freshwater crayfish, and chinook salmon are just a few of the animals that depend on water. But these animals give back as much as they take. 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 keystone species.