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publications > paper > PP 1011 > ecosystems > freshwater and terrestrial > effects of man
Ecosystems of south Florida
Freshwater and terrestrial ecosystems
Effects of man
The most dramatic long-term effects on ecosystems have resulted from drainage. Wetlands originally occupied about 75 percent of south Florida; much of this land has been drained through the years. Drainage not only reduces productive wetlands but also allows other severe environmental effects including organic soil oxidation, damaging fires, and seawater intrusion. Drained lands usually become farms and urban areas that further alter the environment.
Early uncontrolled canal drainage in southeast Florida lowered water levels 1.5 to 1.8 m (5 to 6 ft) below the 1900 level and stressed natural systems. Uncontrolled fires modified, damaged, and eliminated much vegetation and soils (Alexander and Crook, 1973).
In the 1950's water control in the Everglades and urban coast began to take precedence over uncontrolled drainage. A water-management plan was adopted that involved the use of three conservation areas; these areas, which were completed in 1962, provide storage for water regulation. Water is stored during the wet season and released as needed during the dry season. As a result of impoundments, periods of inundation and water depths were increased in parts of the conservation areas, and changes in vegetation were thus caused. Some changes have been toward new assemblages of plants rather than toward previous pre-drainage plant groups (Alexander and Crook, 1973; Hagenbuck and others, 1974).
Natural systems in the west part of south Florida were less drastically altered during the first half of the century than those in the Everglades (Crowder, 1974e). Few large canals were dug in the west before 1940, and these (such as the Barron River Canal) had a less severe effect than those through the Everglades. In general, systems in the Everglades were adapted to longer and deeper inundation than those to the west. The Everglades systems thus suffered more from the reduced period of inundation than did the more terrestrial systems in the west, Also, organic soils were localized and less extensive in the west than in the Everglades, and therefore soil loss was less.
In the 1960's, however, drainage became extensive in southwest Florida. The Golden Gate Canal system inland from Naples effectively lowered water levels 0.6 to 1.2 m (2 to 4 ft) in 195 km2 (75 mi2) of wetlands in the western Big Cypress Swamp (Klein and others, 1970). The early ecological effects of this drainage are that pine and maple are replacing cypress forest (Alexander and Crook, 1973).
Once drained, land in south Florida is commonly farmed, and further disruption of the pre-drainage ecosystems results. Natural vegetation is usually cut and removed, and fertilizers and pesticides are utilized. Cattle grazing has a more moderate impact on the systems; not all natural vegetation is removed, and pesticide and fertilizer use is slight. Urban development often follows farming, and this development of course perpetuates and intensifies environmental alteration. Not only pesticides and fertilizers but toxic metals and polychlorinated biphenyls (PCB's) from agricultural or urban sources are now distributed in south Florida and are a threat to man and to the natural systems (Crowder, 1974h; Klein and others, 1973; Carter and others, 1973).
The forests of south Florida have been lumbered for many years. Selected harvesting of mahogany and buttonwood occurred along the southern coast as early as 1800 and continued until about 1940. Pines were first harvested extensively in the Western Flatlands in the mid-1920's and later in the Big Cypress Swamp. Cypress trees were cut mainly between 1940 and 1957. Lumbering had its greatest direct effect in the Big Cypress Swamp where almost all large slow-growing cypress were removed, as in the Fakahatchee Strand. Lumbering also subjects a system to indirect changes. It can increase the chance of fire by bringing in men and machines, by creating fuel from dead brush, and by drying the forest litter through canopy removal. Lumbering, fire, and drainage thus commonly act in combination to change a system.
Off-road recreational vehicles such as swamp buggies (vehicles with high tires), halftracks, and airboats damage wetland vegetation. The extent of the damage depends on type and number of vehicles and types of vegetation and soil. In the Everglades, drier areas are slower to recover than wetter sites, and wet prairie recovers more rapidly than sawgrass. On the average, 50-percent recovery of vegetation occurred in the Everglades marsh after 4 to 5 months of protection. Tree islands, however, are extremely vulnerable to long-lasting damage from halftracks (Schemnitz and Schortemeyer, 1972).
The use of off-road vehicles is increasing. In 1952, 484 airboat operator permits were issued as compared with 2,932 in 1972. The repeated use of off-road vehicles, particularly the large-wheeled or tracked ones, may create slow- healing scars (Schemnitz and Schortemeyer, 1972).
Man's introduction of exotic species is a major threat to the natural systems. Exotic species can often compete with and ultimately replace native ones. South Florida is particularly vulnerable because of its warm climate. The tropics harbor a numerous and diverse range of plant and animal species. These tropical plants and animals often die in cold weather and cannot survive and reproduce in temperate areas. In south Florida, however, climatic conditions favor the survival of many tropical species. Another factor that favors the spread of exotics in south Florida is the alteration of existing natural systems. Drainage and farming usually increase the chance for exotic plants to become established by stressing native plants and reducing their ability to compete.
Exotic aquatic plants such as water hyacinth and hydrilla have spread rapidly to inland waters, including parts of the conservation areas. Hydrilla is particularly difficult to control because it is submerged, grows rapidly, and sprouts readily. Each year about $10 million are spent in Florida to control aquatic plants, but infestations continue to grow (Crowder, 1974a).
Exotic vertebrates also pose a threat to the freshwater and terrestrial ecosystems. Species that are fecund, aggressive, and adaptive are particularly threatening in that they may replace native species and alter food webs. Of the 16 or so exotic fishes established in south Florida, the walking catfish, the pike killifish, and several species of cichlids are potentially the most dangerous to native fishes. About 20 species of exotic reptiles and amphibians are established, but they are localized. The marine toad, the knight anole, and the Cuban tree frog prey on smaller native counterparts. Birds are represented by a variety of exotic species, most of which occur along the southeast coast. They compete with native species, and some feed on fruit and grain.
Wildlife has suffered from man's impact on the freshwater and terrestrial ecosystems. Some species of amphibians, reptiles, birds, and mammals have declined in population (Rodgers and Crowder, 1974; Crowder, 1974b and c; Rodgers, 1974; Natl. Audubon Soc., 1973; Schemnitz, 1972). The alligator is listed as endangered and, until recently, was threatened with extinction by illegal hunting. They are now increasing in numbers (Rodgers and Crowder, 1974; Schemnitz, 1972). Seven other reptile and amphibian species, however, are declining in number (Crowder, 1974b). Eleven species of birds and three species of mammals native to the freshwater and terrestrial ecosystems are endangered (US. Dept. Interior, 1974). The most dramatic decline is in populations of panther, bear, and wading birds. A 1972 report estimated that only 92 panthers and 145 bears are left (Schemnitz, 1972). Wading birds decreased from about 2.5 million in 1870, to about 1.5 million in 1935, to about 300,000 in 1960, and to about 150,000 in 1973 (Crowder, 1974c; Natl. Audubon Soc., 1973).
U.S. Department of the Interior, U.S. Geological Survey
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