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The remaining natural component of the south Florida ecosystem includes freshwater and terrestrial systems such as ponds and sloughs, sawgrass marshes, wet prairies, hammock forests, bay heads, cypress forests, pine forests, mixed swamp forests, and dry prairies; and coastal systems such as bays, coral reefs, mangroves and saline marshes, and beaches and dunes.
The freshwater and terrestrial systems are controlled, in part, by the moisture in the soil, or by the duration and the depth of inundation; these in turn are determined by the amounts and frequency of rainfall, the infiltration capacity of the soil and underlying bedrock, and by land altitude. High areas that are seldom flooded usually support pine forests, hardwood hammock forests, or grassland systems. Low areas that are flooded part of the year are wetlands, which include prairies, marshes, or swamp systems. The hydrologic environment, however, is not the sole control on a system. Fire, tropical storms, frost and cold weather, saltwater intrusion, and man also affect systems.
Freshwater is a key environmental factor in that it not only affects a system directly but that it also affects other controlling environmental factors such as fire, soil, temperature, and saltwater intrusion. Freshwater is also a key factor manipulated by man. Many of the plants and animals of south Florida are adapted to and dependent on the seasonal fluctuations of freshwater levels. During wet seasons, aquatic plant production abounds; small crustaceans and fish feed on the growing plants or plant remains. With abundant food and space, aquatic animal populations increase. As water levels decline during the dry season, the small aquatic animals are forced to concentrate in scattered ponds, tributary creeks, and sloughs. The concentrated biomass then becomes a rich source of food for larger fish, alligators, snakes, birds, and mammals.
The coastal systems of south Florida are dependent on currents, tides, waves, and in most cases freshwater runoff to circulate and transport salts, nutrients, and other essential products. Freshwater runoff also dilutes seawater. Mangrove forests usually grow where freshwater runoff is greatest and salinity is seasonally reduced, whereas coral reefs occur in areas of little or no runoff and normal seawater salinity. In estuaries and bays, salinity varies during the year depending on the amount of runoff and evaporation. Within these systems, salinity is a major controlling factor on the distribution of organisms.
The coastal systems are also affected by other environmental factors such as cold weather, water turbidity, and tropical storms. Coral reefs, for example, flourish only in shallow clear warm seas and cannot endure temperatures below about 18°C (64°F). Bays and estuaries also benefit from clear water and are adversely affected by turbidity, which reduces sunlight penetration to sea grass and algal beds and thus reduces productivity. Tropical storms often alter coastal systems and are the major natural force that changes the distribution of mangrove forests.
Mangrove forests serve several important functions in the regional system of south Florida. They provide the nutrients and the shelter essential for maintenance of estuarine productivity and coastal fisheries. Tidal flushing carries mangrove leaf litter and seeds into coastal water where they are broken down by microorganisms and are made available as food to estuarine and marine species. The tangled mangrove roots and submerged trunks provide small animals with shelter from predators. Mangroves help to keep the water clear by trapping debris and sediment, and they also provide some protection to upland environments during tropical storms.
Shallow bays and estuaries are feeding and nursery grounds for many marine and estuarine species. Food is not only flushed into the bays and estuaries from mangrove forests and marshes but it also is produced in the bay by sea grass and algal communities. Juveniles of many marine species derive food and protection in estuaries and bays and thus require these habitats to complete their life cycles. More than 75 percent of marine commercial and sport fish of south Florida utilize and depend on bays and estuaries. The importance of these fish is stressed by the fact that marine commercial and sport fishing in the State generates more than $600 million annually.
Man has been altering the ecosystem of south Florida extensively for 70 years. About 35 percent (7,700 km2 or 3,000 mi2) of the natural habitat of the ecosystem has been destroyed by agriculture or urbanization. The remaining natural habitat is now threatened by exotic plants and animals, altered water levels and flows, severe fires, pollution, loss of animal and plant populations, and by further growth and development.
Man's most dramatic and long-term effects on the ecosystem have resulted from drainage. Wetlands originally occupied about 75 percent of south Florida, but through the years much of this land has been drained. In southeast Florida, drainage has lowered water levels as much as 1.5 to 1.8 m (5 to 6 ft) below the 1900 level and has disrupted the natural systems. In southwest Florida, drainage has lowered water levels as much as 0.6 to 1.2 m (2 to 4 ft) over a 195-km2 (75-mi2) area near Naples and has lowered them an undetermined amount over the remaining western Big Cypress Swamp. Drainage reduces productive wetlands, promotes organic soil oxidation and damaging fires, and has permitted seawater intrusion in some areas of excessive water-table lowering. Drained lands usually become farms and urban areas that themselves further alter the environment.
The introduction by man of exotic plants and animals has also altered parts of the natural systems of south Florida and threatens to alter much more. Exotic species often compete with and ultimately replace native ones and thus drastically change systems. Drainage and land clearing have increased the opportunity for exotic species to become established by stressing native species and reducing their ability to compete. Perhaps the most dramatic invasion is the rapid spread of cajeput, Australian pine, and Brazilian pepper.
The populations of many of south Florida's wildlife species have declined. Eleven species of birds and three species of mammals are endangered. Perhaps the most dramatic is the decline of wading birds from about 2.5 million in 1870 to about 150,000 in 1973.
The introduction of toxic chemicals, nutrients, and pathogenic organisms in increasing amounts threatens to further alter the ecosystem of south Florida. Most are transported to and accumulate in the aquatic environment where they degrade the quality of the water and alter or destroy life. Water quality has deteriorated over much of the region, and the deterioration is greatest in the east coast urban area. Most canals that drain urban or agricultural land contain toxic chemicals and high concentrations of nutrients. High nutrient concentrations favor the growth of dense stands of nuisance aquatic plants that clog canals and further degrade water quality. Canals also can serve as pathways to the estuaries for pollutants.
Dredge and fill and bulkheading are major threats to estuaries and marine fishery resources of south Florida. These activities not only reduce the vital littoral zone that serves as a nursery and feeding area but they also alter freshwater and nutrient inflow. Moreover, dredging increases the amount of sediment suspended in bay water; this sediment blocks out light necessary for photosynthesis. Tides transport the suspended sediment to other parts of the estuary where it may settle and suffocate plants and animals far removed from the dredging operation.
Agricultural and urban systems require freshwater to sustain them, and thus they compete with the remaining natural systems. Urban water shortages were indicated by the need to impose water-use restrictions in southeast Florida during the 1970-71 drought, and by progressive lowering of water levels each dry season in coastal parts of southwest Florida. The present difficulty in water supply is linked to urban demands that require increasing diversions of water from the natural and agricultural systems. Altered flow patterns of freshwater in the natural system adversely affect wetland and estuarine systems. The agricultural systems also depend on an adequate water supply, without which productivity drops.
Water-management methods have been proposed to increase the availability of freshwater to satisfy the future urban and agricultural demands and to alleviate the stress on the natural systems during prolonged drought. In southwest Florida a water-management method to increase availability of water would require, at this time, only reducing the discharge of water from canals. In southeast Florida, however, saving more water would require management techniques such as backpumping excess storm water and increasing the storage of Lake Okeechobee. Reducing canal flow by backpumping storm water into the conservation areas could save possibly 50 percent of the total canal runoff, which ranges from 28 to 193 m3/s (1,000 to 6,800 ft3/s) in southeast Florida. Backpumping, however, may have some adverse environmental effects on the natural system. For example, in parts of the Everglades it would raise water levels, prolong inundation, probably alter water quality, and thereby cause vegetative changes. Backpumping would also decrease the flow to the estuaries and thus alter salinity and nutrient input there. In addition, backpumping would provide some degree of flood protection for land now unsuitable for residential development and thereby enhance land values in formerly undevelopable tracts.
Agricultural and urban systems rely on the remaining natural systems not only for such commodities as land, but also for the benefits or "free services" provided by the natural systems. These services include maintaining a stable quality environment through oxygen production, recycling nutrients, dispersing and detoxifying pollutants, retaining biological diversity, and producing fish and wildlife. If the natural systems are destroyed or greatly reduced, the services they once provided free must be provided by the agricultural or urban systems, usually at a high cost.
Agricultural and urban systems require an input of energy in addition to that supplied by the sun, wind, and rain. Most of this energy is imported as fossil fuel or as a fossil-fuel product. Urban societies, in particular, require a large input of outside energy to build and maintain their complicated structure of transportation, schools, laws, communication networks, buildings, and public services. Continued growth or even maintenance depends on the availability of energy. As cities grow, so do energy requirements; if these are not met, stress occurs and the quality of life deteriorates.
In recent years, urban growth in south Florida has been rapid. The population increased from slightly less than 0.5 million in 1940 to 2.5 million in 1970. If present trends continue, by 2000 south Florida would have more than 4.4 million residents, or about an 80-percent increase from 1970.
U.S. Department of the Interior, U.S. Geological Survey
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