This work has involved the modeling of intermediate trophic levels in the Everglades ecosystem as part of the Across Trophic Level System Simulation (ATLSS) package of models. The fish and herpetological communities play pivotal roles in the Everglades, acting as principal forage sources for such top consumers as wading birds and alligators.

Fish modeling: The purpose of this work was to develop a model of the dynamics of a key functional group of fishes in the freshwater in the Everglades/Big Cypress ecosystem. The work performed here developed a predictive model for the numbers, size and age structures, and biomass per unit area of small fishes in any selected part of the freshwater marsh. A key factor included in the model is the seasonal cycle of water depth. This environmental seasonality is echoed in patterns of production of fish biomass, which, in turn, influences the phenology of other components of the food web, including wading birds. Human activities, such as drainage or other alterations of the hydrology, can influence these natural cycles and result in changes in the fish production and the higher trophic levels dependent on this production. In the model the seasonal pattern of fish production in the freshwater Everglades/Big Cypress region is simulated on 5-day time steps. The model illustrates the temporal pattern of production through the year, which can result in very high densities of fish at the end of a hydroperiod (period of flooding), as well as the importance of ponds and other deep depressions, both as refugia and sinks during dry periods. The model indicates: (1) there is an effective threshold in the length of the hydroperiod that must be exceeded for high fish-population densities to be produced, (2) large, piscivorous fishes do not appear to have a major impact on smaller fishes in the marsh habitat, and (3) the recovery of the small-fish populations in the marsh following a major drought may require up to a year. The last of these results is relevant to assessing anthropogenic impacts on marsh production, as these effects may increase the severity and frequency of droughts.

This small-fish model has been extended to cover the whole Everglades landscape, in 500 x 500 meter cells, by the University of Tennessee. This predicts the distribution of fish number and biomass density across the Everglades landscape on 5-day time steps and allows the calculation of a breeding potential index for wading birds, for which fish is a main dietary component. The model is currently providing output that is being included in the ATLSS "Model Outputs for the Central and Southern Florida Comprehensive Review Study" (or C&SF Restudy). In addition, this model will be integrated in 1998 into the wading bird breeding colony models that are nearing completion.

Reptile and amphibian modeling: A food web "submodel" of the amphibians (frogs and salamanders) and reptiles (snakes and turtles) has been developed. The model has been parameterized for three ecosystem types within the Everglades/Big Cypress region, using data supplied by George Dalrymple. Linear programming methods have been used to ensure the consistency of these data in predicting biomass standing stocks and fluxes. The reptiles and amphibians of this submodel are important food sources of the alligators and wading birds. The purpose of the model is to predict the dynamics and production of these taxa across the landscape of the Everglades/Big Cypress under a variety of hydrologic scenarios. A primary reason for modeling them here is to predict the availability of biomass to higher trophic levels (for example, particularly alligators). In addition, the model will be able to predict the expected year-to year variability in these populations under realistic climatic conditions, and their responses to changes in system hydrology. This submodel will also be used to provide input data for an alligator model now in preparation.

Associated field studies (alligator diets): To help link these intermediate trophic levels with a key top predator, a four-year study of the diet of the alligator, Alligator mississippiensis, was conducted in the Everglades. The first three years of this study led to the amassing of a data set on alligator diets in the central slough area of the southern Everglades. These data will be extremely valuable in attempting to predict the responses of the alligator population to changes in landscape hydrology. On the basis of observations of the investigator and other researchers, the condition of alligators in the slough (away from canals) is far worse than that of alligators living in canals. This has been hypothesized to be a result of more food available to alligators in the canal. Stomach contents of alligators from the canals will be sampled to determine diets through the year, and compared with the diets of alligators in the slough. During the three years of this study, the investigator has tagged approximately one thousand alligators. These alligators are currently being recaptured in another project and statistical analysis will be used to estimate both the current total population size and mortality rate of alligators in the central slough area.

A significant part of the funding for this research was provided from the U.S. Department of the Interior South Florida Ecosystem Restoration Program "Critical Ecosystems Studies Initiative" (administered through the National Park Service) and from the U.S. Geological Survey, Florida Caribbean Science Center. Additional funding for the "Atlas Tropic Level System Simulation" was also provided by the U.S. Environmental Protection Agency and the U.S. Army Corps of Engineers.

REFERENCES

Barr, B., 1997, Food habits of the American alligator, alligator mississippiensis, in the southern Everglades: Ph. D. Dissertation, University of Miami, Coral Gables, Florida.

DeAngelis, D.L., Loftus, W.F., Trexler, J.C., and Ulanowicz, R. E., 1997, Modeling fish dynamics and effects of stress in a hydrologically pulsed ecosystem: Journal of Aquatic Ecosystem Stress and Recovery, v. 1, p. 1-13.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)