The Ecological Role of the Karst Wetlands of Southern Florida in Relation to System Restoration
By William F. Loftus1, Maria Cristina Bruno2, Kevin J. Cunningham3, Sue Perry2, and Joel C. Trexler4
1 U.S. Geological Survey, Biological Resources Division, Everglades National Park. Homestead, Florida 33034. Bill_Loftus@usgs.gov
2 South Florida Natural Resources Center, Everglades National Park. Homestead, Florida 33034.
3 U.S. Geological Survey, Water Resources Division, Miami Subdistrict, Miami, Florida 33178.
4 Florida International University, University Park, Miami, Florida 33199
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With the recent funding of the Comprehensive Everglades Restoration Plan (CERP), the largest ecosystem restoration program ever attempted, there is a pressing need to be able to detect changes in natural habitats as a result of restoration actions. Human activities, particularly the construction of canals and levees that can either drain or flood wetlands, have affected the natural variability of environmental conditions (Gunderson and Loftus 1993). CERP intends to restore natural hydropatterns to areas that have been damaged by water management. Baseline data on constituent aquatic communities and their ecology are needed before, during, and after the restoration activities commence.
Freshwater fishes and invertebrates are important ecosystem components in the Everglades/Big Cypress system. They operate at several trophic levels in the wetlands, from primary consumers of plant material and detritus to carnivores and scavengers. Factors that influence fish and invertebrate numbers, biomass, and composition therefore affect energy flow through the wetlands. The ecology and life histories of these animals are intimately tied to the hydrology of the wetlands, which is determined mainly by rainfall, but increasingly by water-management practices. Because of the hydrological changes wrought by drainage and impoundment, and the loss of spatial extent and functioning of former wetlands to development (Gunderson and Loftus 1993), there is little doubt that standing crops and overall numbers have declined. Changes to the original ecosystem have also altered the timing and the areas of prey availability to predators. Non-native fishes have colonized natural and disturbed habitats during the past three decades. Non-native fishes have affected native animals through predation, nest-site competition, and habitat disturbance (Loftus 1988) and may divert food-web energy into biomass unavailable to top-level predators.
Aquatic animals in southern Florida wetlands have a variety of ways to cope with environmental variability. These include movements to find refuge from drying habitats in winter and spring, and dispersal away from those refuges with the onset of the wet season (Kushlan 1974, Loftus and Kushlan 1987). This pattern of movements among habitats with fluctuating water depths is common to seasonal wetlands in the tropics (Lowe-McConnell 1987, Machado-Allison 1993). The major natural refuge habitat most-studied by scientists in southern Florida is the alligator hole (Craighead 1968, Kushlan 1974, Nelson and Loftus 1996). Canals and ditches offer a relatively recent but spatially extensive form of artificial refuge for aquatic animals on the landscape (Loftus and Kushlan 1987). In this study, we are studying the function of other types of aquatic refuges in the Everglades.
The Rocky Glades, or Rockland, habitat is a karstic wetland unique to Everglades National Park (ENP) in southern Florida (Figure 1), although similar habitats exist elsewhere in Yucatan, Cuba, and the Bahamas. Approximately half of the original area of this habitat occurs outside of ENP where agricultural and urban development has forever altered its geological structure and ecological function. This region is a high priority for restoration in CERP because it is the largest remnant, short-hydroperiod wetland in the eastern Everglades. That habitat has been disproportionately lost from the ecosystem. Unfortunately, the habitat remaining in ENP has been degraded by water management (Loftus et al. 1992).
Figure 1. Locations of the study sites within the Rocky Glades and Atlantic Coastal Ridge in southern Florida. The numbers indicate the drift-fence arrays on the main park road, and the stars on the coastal ridge are the well sites with Miami cave crayfish.
The highly eroded karst structure of the Rocky Glades appears to be responsible for the persistence of aquatic-animal communities by offering dry-season refuge in thousands of solution holes of varying depths, (Loftus et al. 1992). Their work was the first to indicate a tight relationship among the biological, geological, and hydrologic components of this region. Loftus et al. (1992) also found evidence that aquatic animals disperse, feed, and reproduce on the wetland surface during the short flooding period, then retreat below ground for periods of months to years. They also reported that several introduced species, particularly the pike killifish (Belonesox belizanus), walking catfish (Clarias batrachus), Mayan cichlid (Cichlasoma urophthalmus), and black acara (Cichlasoma bimaculatum) were common in the Rocky Glades (Loftus et al. 1992). Unfortunately, their study was interrupted by Hurricane Andrew and not continued.
In this paper, we report the rationale and results of the first year of a new study in which the primary goal is to define the interactions of the aquatic-animal community with the geologic structure and hydrologic conditions of the Rocky Glades. We are addressing questions that have arisen from past work there. How do composition, size-structure, and recruitment of aquatic animals change during the flooding period? Are the dispersal patterns of animals related to water flow? Are the animals dispersing from the main sloughs to recolonize the Rocky Glades, or is the Rocky Glades a source of animal colonists for the sloughs? Do roadways act as barriers to movement? The objectives of this study segment are:
• Collect baseline ecological data on the epigean aquatic communities in the karst landscape of the Rocky Glades.
• Quantify the direction and degree of dispersal by fishes and invertebrates during the wet season.
• Document the seasonal changes in species composition, size structure, and reproductive patterns of animals on the wetland surface.
• Survey the topography of representative areas of the Rocky Glades, particularly around the sampling sites, to provide depth-distribution data for the simulation model of the region.
• Develop a visual survey method for sampling fish communities in open, rugged terrain to follow community dynamics in the Rocky Glades in the wet season.
• Identify the extent of near-surface voids.
The Atlantic Coastal Ridge is another area affected by urbanization and changing hydrologic management (Figure 1). Aquatic habitats, such as the transverse glades that cut through the Ridge, have been replaced by canals and will not be restored. Ground-water habitats and animal communities may have been less affected. As in karst areas elsewhere, deeper geological formations (>5 m) beneath the Rocky Glades and the Atlantic Coastal Ridge have voids of various dimensions known to house truly subterranean aquatic species (Radice and Loftus 1995, Bruno et al., this volume). These include the Miami Cave Crayfish (Procambarus milleri), known only from a few wells in southern Florida (Hobbs 1971). The composition, distribution, and abundance of other hypogean animals are poorly known. Ground-water withdrawal and saltwater intrusion (Leach et al. 1972), limestone mining, and pollution may threaten these communities before they have been fully catalogued. Elsewhere in the world, such communities are known to be very sensitive to changes in their delicately balanced physical environment. The second goal of this project is to identify the composition, distribution by depth and space, and ecological relations of this subterranean fauna. The objectives of the second study element include:
• Develop effective traps to capture invertebrates and possibly fishes from subterranean habitats.
• Inventory hypogean communities and relate the composition and distribution to environmental factors.
• Collect life-history data for the Miami cave crayfish from a large captive population.
This first project year has been a pilot study to test designs and methods. The study is divided into two elements with several components each.
Element 1: In the Rocky Glades, we selected four sites along the ENP main road (Figure 1) to test the use of drift-fence arrays to describe directional animal dispersal and community successional patterns in the wet season. The four X-shaped arrays had 12-m wings made of black plastic ground cloth (Figure 2) to direct animals into one of 3 traps that faced east, north, and west, based on the direction that they were moving (Figure 2). The road shoulder formed a barrier to the south of each array. The 3-mm mesh minnow traps were fished overnight for 24 h to provide data on fish relative abundances, movements, and catch per unit effort (CPUE).