The Everglades are a complex system of interdependent ecosystems. Marjory Stoneman Douglas described the area as a "River of Grass" in 1947, though that metaphor represents only a portion of the system. The area recognized as the Everglades, prior to drainage, was a web of marshes and prairies 4,000 square miles (10,000 km2) in size.[35] Borders between ecosystems are subtle or imperceptible. These systems shift, grow and shrink, die, or reappear within years or decades. Geologic factors, climate, and the frequency of fire help to create, maintain, or replace the ecosystems in the Everglades.