The Everglades are part of a huge drainage basin, an area of land that collects precipitation and water from rain and nearby streams that run downslope. The Everglades area is wide and shallow, and is comprised of over 10,000km2. Since the 19th century, the Everglades have continually been subject to agricultural development from sugarcane cultivation to canal construction. From the 20th century, they have seen real estate development and tourism. Both urban development and agricultural use have reduced the size of the Everglades substantially, causing chaos to ecosystems and disruption to business. Since then, the Everglades have undergone comprehensive environmental repair to remedy damage inflicted on the environment during the 20th century. Canals were constructed to prevent flooding of the Everglades from extreme weather. The canal instead caused disastrous damage to animal habitats that in turn impacted the water quality in the region. High levels of phosphorus and mercury were found in the canal waterways again arousing concern about water quality. Allocating responsibility was noxious and became politicised, the outcome of which was a commitment to decrease pollutant levels in the water. The high phosphorus level in the water killed native species and changed the ecosystems. The phosphorus was discovered to be from the sugar plantations, and thus state legislation was heavily resisted by sugar industry lobbyists. Likewise, high mercury levels infected fish, and thus bioaccumulated through the food chain, killing animals that included alligators and panthers. The source was from waste incinerators and fossil fuel power plants. Legislation was eventually passed that required these actors to not dump mercury into the water thereby enabling carbon emissions to decrease by 60-70%. The impact on the wildlife of the Everglades has been substantial. Some species are considered endangered, and many species have seen a dramatic loss in population. By the 1970s, bird numbers had decreased by 90%. Also, invasive species have changed ecosystems and displaced native species and vegetation.