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Introduction
The Everglades, located at the southern tip of peninsular Florida, is the most famous wetland in the United States and one of the most distinct in the world. The Everglades is unique among the world’s large wetlands because it derives its water from rainfall. Other large and famous wetlands, such as the Pantanal of South America, the Okavango of Botswana, and the Llanos in Venezuela and Colombia, derive most of their water and nutrient inputs from river flooding. The unique sheet flow, the slow flow of water over shallow, broad tracts of marsh, inspired Douglas to name the Everglades, River of Grass. As important as sheet flow is, the groundwater connections of the Everglades to Lake Okeechobee, the second largest freshwater lake entirely within the U.S., are also essential for the maintenance of the wetland. The linkages between the Everglades, Lake Okeechobee, and the Kissimmee River, which provides 80 percent of the surface flow into Lake Okeechobee, illustrate the importance of connectivity among ecoregions to maintain integrity.
The boundaries of this ecoregion extend to include the Big Cypress Swamp to the northwest, the southern edge of Lake Okeechobee to the north, and the Atlantic coastal ridge to the east. The Everglades climate has been classified as subtropical, featuring hot humid summers, when 80 percent of rainfall occurs, and mild winters. Rainfall varies spatially across southern Florida so that the inland marshes and Lake Okeechobee only receive about 60 percent of the rainfall levels recorded in the coastal areas. The most important climatic feature is also the most important natural disturbance factor: the recurrent hurricanes that strike most frequently from August through October. Extensive habitat destruction can occur from high winds, storm surge, and rainfall. Frosts also limit the northern distribution of many tropical species to this ecoregion and help to further define its boundaries.
Many observers have identified the Everglades as one of the most endangered of North American ecoregions as a result of clearing for agriculture, diversion of water flow, and other developments. Recovery efforts are now underway, supported by a broad association of environmentalists active in the region.
Biological Distinctiveness
he extraordinary biological richness of the Everglades has been well-documented, particularly the spectacular wading birds, alligators, crocodiles, snail kites, and mangrove species. The habitats that support this rich assemblage of species include: ponds, sloughs, graminoid (grass-like wetlands), and forested wetlands. The forested uplands of the Everglades also harbor distinct assemblages of species, dominated by many trees with tropical Caribbean affinities. Ponds occur throughout the Everglades on the lowest elevational sites and are important areas for alligators. Typical aquatic vegetation may include water lilies (Nymphaea spp.) spatter dock (Nuphar advena). Sloughs are found on the wettest sites which experience nearly constant inundation and are underlaid by peat soils. Floating aquatics such as white water lily (Nymphaea odorata) are common and a submerged complex of plants is dominated by bladderwort (Utricularia spp.). Perhaps the most famous habitat of the Everglades is the sawgrass (in reality a member of the sedge family rather than a true grass, Cladium jamaicense) that dominates marshes in terms of abundance and biomass. Sawgrass is well adapted to fire and fluctuating water levels but is adversely affected by prolonged high water. Wet prairies are dominated by rushes.
Also of great biological interest are the diversity of forest types also called tree islands due to being surrounded by a sea of sawgrass. They include swamp forest dominated by red bay (Persea borbonia), pond apple (Annona glabra) forests, Cypress (Taxodium ascendans) forests, and hardwood hammocks with a variety of tropical species, including palms, that occur in the U.S. only in southern Florida. The tropical hammocks contain a diverse assemblage of local endemic tree snails of the genus Liguus and tropical butterfly species are also common.
Fire is an important feature in the Everglades that along with water flow, helps to maintain early successional habitats. Increases in soil surface elevation leads to greater dominance of hardwoods and scrub.
As with other peninsulas, animal species richness tends to decline in most taxa from the mainland connection to the tip. Southern Florida is no exception, with reduced numbers of vertebrates and invertebrates from north to south. About 70 breeding bird species occur, 17 species of mammals, 30 species of reptiles, and 14 species of amphibians. Ponds and creeks are important as dry season refugia for many of these species. Sufficient water levels are crucial for a number of taxa to carry out breeding.