Ninety-five percent of Biscayne National Park's 173,000 acres are covered by water, making it the largest marine park in the National Park System. Four major ecosystems are protected within the park. They are: - a narrow fringe of mangrove forest along the mainland shoreline of Biscayne Bay; - the clear shallow waters of Biscayne Bay itself; - the northernmost islands of the Florida Keys; and - the beginning of the world's third-longest coral reef tract. This mix of upland and marine environments provides habitat for hundreds of species of animals and plants. The park's proximity to Greater Miami, with a population approaching 2.5 million people, provides a unique set of challenges which include overfishing, boat groundings and water pollution, among others.