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Ecosystems of south Florida

Freshwater and terrestrial ecosystems

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Preface
Synopsis
History of the Study
Regional System
>Ecosystems
> Freshwater
and Terrestrial

  >  System relations
  -  Effects of man
  -  Canals & lakes
  -  Ponds & sloughs
  -  Sawgrass marsh
  -  Wet prairies
  -  Pine forests
  -  Cypress forests
  -  Mixed swamp forests
  -  Bay heads
  -  Hardwood hammocks
  -  Palmetto & dry prairies
- Coastal
- Man-dominated
Hydrologic Systems
Final Word
References
Appendices
PDF version
The natural freshwater swamp, marsh, and terrestrial systems of south Florida whose extents are shown in figure 5 lie within five physiographic regions (fig. 6): (1) the Everglades; (2) the Atlantic Coastal Ridge; (3) the Eastern Flatlands; (4) the Western Flatlands; and (5) the Big Cypress Swamp (Davis, 1943). The Everglades and the Big Cypress Swamp are predominantly freshwater marsh and swamp systems. The Atlantic Coastal Ridge is mostly terrestrial. The Flatlands are composed of swamp, marsh, and terrestrial systems. All these natural systems are affected to some degree by man.

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.

map showing vegetation in south Florida as it existed before manmade alterations map showing physiographic regions of south Florida
FIGURE 5. (left) Vegetation in south Florida as it existed before manmade alterations. Modified from J. H. Davis, Jr. (1943). [larger image] FIGURE 6. (right) Physiographic regions of south Florida. [larger image]

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.

photograph of a swamp forest photograph of a marsh
Marshes also require prolonged inundation.
Swamp forests grow where land is inundated by water for months. [click on images for larger versions]

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.

line drawing showing generalized vegetative profile showing the relation of relative water depth to vegetative community
FIGURE 7. (above) Generalized vegetative profile showing the relation of relative water depth to vegetative community. [larger image]
line drawing showing a sequence of vegetation change in south Florida
FIGURE 8. (above) A sequence of vegetation change in south Florida. Succession proceeds towards hardwood hammock. Fire or other factors may check the succession or reverse it. [larger image]
Under ideal conditions, systems undergo orderly, successional changes until a relatively stable situation is reached in which a system is in equilibrium with its climatic environment and is capable of self-perpetuation as long as the climate does not change radically. The stable system is called a climax; antecedent unstable ones are subclimax. Each system in this successional change has its own species, organization, and conditions, and these are different from all others. In addition, each system creates the habitat and conditions for its successor. The concept of orderly change ending in a climax has been applied mainly to vegetation but is generally applicable to ecosystems, which include biological components as well as the physical environment.

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).

flowchart illustrating generalized plant succession in south Florida
FIGURE 9. Generalized plant succession in south Florida. From Alexander and Crook (1973). [larger image]
South Florida's systems have both tropical and temperate components. Land animals are almost completely temperate-zone species, but the plants are derived from both zones. Temperature affects these plants in two ways: high temperatures restrict the southward migration of temperate plants, and low temperatures restrict the northward migration of tropical species. Tropical plant species are frost sensitive and some defoliate after exposure to 7°C (45°F). Damage increases with duration of cold (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.

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