The dry interiors of most continents are dominated by vast grasslands, and wetlands worldwide are populated by stands of rushes, sedges, and cat-tails. This widespread distribution is not the only importance of the group, for many animals depend upon graminoids for shelter and food.
Many birds depend upon rushes and grasses for their nesting materials and for cover.
Prairie and savanna grasses provide both food and cover for a wide variety of rodents , birds, and other small creatures. The spreading grasslands replaced forest over large areas on several continents, changing the appearance and ecology of the land. The result was a selection for faster-running grazers and small burrowing animals. The environmental trigger which probably pushed grasslands onto the scene was the drying of the land. In North America, this is attributed to the rise of the Rocky Mountains, which blocked moisture from the Pacific.