Everglades: Location, Extent and Environmental Factors

The Everglades is a large mosaic of fresh water wetlands originally extending over more than 1 million ha (Davis and Ogden 1994) situated in southern Florida. Historically, the Everglades extended from the southern shore of Lake Okechobee southward to the mangrove fringe of Florida Bay and the Gulf of Mexico, west to the Big Cypress Swamp and east to the coastal ridge (Gunderson 1994). In addition, the marshes in the headwaters of the St. Johns River are floristically similar to the Everglades but are within a different drainage basin (see Marshes). Many of the processes that function in the Everglades also drive marsh systems in the headwaters of the St. Johns River. Other wetland systems such as the Kissimmee Prairie, Lake Okechobee, the Big Cypress Swamp and mangrove system fringing Florida Bay are ecologically linked, but are often studied as separate systems (Davis and Ogden 1994). The Everglades has been called the “river of grass” by Marjorie Stoneman Douglass, which aptly describes its character, an herbaceous dominated wetland of slowly moving water.

The Everglades has undergone extensive changes in the last century. Concurrent with population growth of non-aboriginal people, demands for water and land for agriculture and urban development have resulted in drainage of over half of the historical extent of the Everglades since the turn of the century (Davis and Ogden 1994). What remains of the Everglades is protected in Everglades National Park, about 1/5 of the original area of the Everglades and water conservation areas north of the park. Protected areas upstream of the park, the Water Conservation Areas, have been diked and impounded and water levels and flows are managed in an attempt to balance flood control needs, water supply needs for agriculture, urban and recreational uses and environmental needs. Drainage and nutrient enrichment have been two system-level alterations to the Everglades. For discussions of historical changes to the Everglades see Gunderson and Loftus (1993) and Davis and Ogden eds. (1994).

The climate of South Florida exerts a pronounced influence on the ecology of the Everglades. In tropical south Florida, temperatures are warm all year and freezes are uncommon, but those that do occur are instrumental in controlling the distribution of some plants and animals. Rainfall patterns are distinctly seasonal. The summer wet season extends from mid May to October, when 80% of rainfall occurs (Gunderson and Loftus 1993). The winter, dry season begins in October or November and extends to May. Summertime convective storms are the largest source of rainfall in the wet season. Occasional winter frontal systems and at times large inputs from tropical cyclones augment rainfall from convective storms. Lightning that accompanies convective storms is important in the fire ecology of the Everglades. The variable nature of summer convective storms results in localized rainfall events and at times, droughts that, too, are localized in time and space. Periodic droughts are important in many aspects, but perhaps most importantly because they influence fire patterns (Duever et al. 1994, Chen and Gerber 1990).

Under a natural hydrologic regime water sheet flows across the vast flat expanses of the Everglades often only at 0 to 1cm/sec (Gunderson and Loftus 1993). Rainfall and transpiration are the largest influence on water level fluctuations in Everglades wetlands. The natural pattern is high water levels through the wet summer months, a declining water table through winter and a rapid decline in spring as transpiration and temperatures increase (Duever et al. 1994, Gunderson 1994). Evapotranspiration is important in the water budget of the Everglades and in undisturbed wetlands it accounts for export of 70-90% of the rainfall entering the system (Duever et al. 1994). Hydroperiod and water levels in the Everglades vary by plant community and are the primary determinants of plant community development. The wettest communities are wet all year, most years, with water depths of approximately 30 cm (up to 1.25m) (Gunderson 1994, Wade et al. 1980). The driest grass-dominated wetland communities are wet from 3 to 7 months per year to a depth of approximately 10 cm (Gunderson 1994).

Relative water depths are controlled by elevation and thus, soil build-up. The Everglades are notably flat. The elevation change is approximately 3 cm/km going from north to south (Kushlan 1990, Gunderson 1994). Soils of the Everglades are generally shallow peats overlying limestone bedrock. Limestone outcrops at the surface are common features of some portions of the Everglades. Long hydroperiods with little seasonal drying promote the formation and build-up of peat. Deep soils at the northern end of the Everglades are on average 1.5 m thick and taper to about .75m at the southern end (Kushlan 1990), although pockets of deeper peat exist (up to 5.5 m thick) (Gunderson and Loftus 1993). Marl soils are largely restricted to the southern end of the Everglades and to peripheral marshes. Marl is formed by precipitation of carbonates by algae mats called periphyton that form on sites with a moderate hydroperiod and seasonal drying (Kushlan 1990). The waters and soils of the historic Everglades were naturally nutrient poor (oligotrophic) and phosphorus is generally the most limiting nutrient (Gunderson and Loftus 1993).