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Ecosystems of south Florida
Freshwater and terrestrial ecosystems
Relations between systems
A seasonal abundance of freshwater in south Florida has favored the development of a number of swamp, marsh, and terrestrial systems. Each is 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 elevation. 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 (fig. 7). The hydrologic environment, however, is not the sole control on a system. Fire, tropical storms, frost and cold weather, saltwater invasion, and man also affect the systems.
Freshwater is a key environmental factor in that it not only
affects a system directly but it also affects other controlling environmental
factors such as fires, soil, temperature, and saltwater invasion. Freshwater is
also a key factor manipulated by man.
Water, sunlight, and nutrients are essential ingredients for organic plant production, which sustains each system. Marsh and swamp systems require seasonal flooding to maintain adequate levels of production. In the marshes, herbaceous plants and periphyton are the primary producers; in the swamps, trees are the primary, producers. Terrestrial plants, which do not tolerate much flooding, rely mainly on rainfall and soil moisture. Plant production sustains each System by providing the food for the two other major components of a system: the animals and the saprophytes (bacteria, yeast, fungi).
Many plants and animals are adapted to and dependent on the seasonal fluctuations of water level. 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.
Rainfall is the ultimate source of water in south Florida. It is maximal over the Atlantic Coastal Ridge (1,523 mm/yr or 60 in/yr) and decreases incrementally away from the ridge. The annual rainfall pattern, however, does not correlate with the physiographic regions and their ecological systems. These regions and systems are more closely correlated with the distribution of water, soil type, and land elevation.
In south Florida, rainfall is generally inadequate to allow native plants their characteristic successional patterns, but droughts have regularly stressed vegetation and altered this process. The plant systems have evolved in this pattern and are adapted to seasonal changes in water depth. Figure 8 and figure 9 summarize the main successional sequences. Although succession is reversible, as indicated by arrows in both directions, it ends ultimately, if not checked, in a hardwood forest climax. Because of droughts, fires, and, more recently, man's intrusions, systems seldom reach climax; most are limited in their development and are subclimax. In addition, the alterations imposed by man, such as the introduction of exotic plants, appear to have set in motion changes in the details of this succession (Alexander and Crook, 1973).
Fire has an important effect on freshwater and terrestrial
systems. It maintains some systems, such as pine forests or sawgrass marshes,
and limits others, such as hardwood forests. Lightning has always caused some
wildfires during the summer thunderstorms, but during the wet season wildfires
tend to be less severe than fires during the dry season because the moisture
protects roots and soil. During the dry season, fires often burn into the roots
and soil, killing even fire-resistant species. muck fires can burn for weeks
and months. Such fires occurred in the mid-1940's and resulted in reduced soil
levels and destruction of many tree islands and hammocks (Alexander and Crook, 1973).
Temperature is variable with soil type, water conditions, and land altitudes. Areas of sandy soils tend lo be warmer than areas of muck. Water moderates air temperature. Swamps and marshes tend to be warmer in the winter and cooler in the summer than forest and prairies (Alexander and Crook, 1973).
Hurricanes and tropical storms affect ecosystems through
their local and immediate destruction and through their more extensive and
long-term alterations such as salting of land by tidal flooding and changing of
coastal elevations and outlines. Coastal ecosystems are more susceptible to
these changes than are interior systems.
U.S. Department of the Interior, U.S. Geological Survey
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