This project aims to quantify the networks of trophic interactions that occur among the major taxonomic groups of the various Everglades habitats. The strategy is to use available information on levels of biomass and exchanges of carbon to piece together complete budgets for each ecosystem. Typically, this exercise results in a "tangle" or "bird's nest" of interactions that requires systematic analysis. The structural characteristics imbedded within these complicated pictures are revealed through application of a suite of algorithms collectively referred to as "Network Analysis."

Network Analysis was developed largely by the principal investigator, and can be used to reveal how each member of the ecosystem contributes to, and is dependent upon, each of the others. It also portrays the trophic "spectrum" of each participant (that is, the degrees to which each functions at the various trophic levels--autotroph, herbivore, carnivore, etc.). It depicts as well the trophic spectrum of the system as a whole. Furthermore, the analysis identifies and enumerates the manifold pathways for recycle within the system and constructs from them the composite structure for recycle in the system as a whole. Finally, the algorithms provide a set of indices grounded in information theory and thermodynamics that describe the activity level and organization of the system as a whole.

These products in concert provide investigators with a didactic image of how the ecosystem is functioning that is part of the overall package of models of the "Across Trophic Level System Simulation" (ATLSS) program, but that is independent of ATLSS simulation activities, so that it can be used to keep check on the reality of predictions from these latter activities. Working the other way around, the outputs from ATLSS can be used to construct trophic networks at particular times and places. The analysis of networks thus synthesized should provide a significant, if not essential, tool for tracing the causal roots that underlie ATLSS predictions of community responses to changing water and/or nutrient levels. Network Analysis for two components of the Everglades/Big Cypress region has been performed so far, the cypress wetland ecosystem and the Florida Bay ecosystem.

The cypress wetland ecosystem: A 68-compartment budget of the carbon exchanges occurring during the wet and dry seasons in the cypress wetlands of South Florida has been assembled. These networks of exchange will serve as independent benchmarks against which the performance of the ATLSS multimodel, now under construction, will be assessed. During the construction of these networks, it soon became clear that the perceived fact--that these systems are driven by cypress litterfall--is simply not true. Less than half the carbon reaching the higher trophic levels has spent any time in the form of detritus. Production by the understory of vines, epiphytes, and aquatic vegetation play the key roles in sustaining the ecosystem, and phytoplankton is especially important during the dry season. The middle trophic level fish and amphibians appear to exhibit the highest trophic efficiencies found in the cypress ecosystem. Taxa at either end of the trophic chain appear to be less efficient by comparison. The result is a peak in efficiency at trophic level four. The trophic levels of most taxa do not change appreciably between seasons. Most dietary replacements occur at the same trophic levels. In fact, the entire trophic structure does not change much between seasons, although the overall system activity during the dry season falls by some 25 percent. Relatively little carbon recycling takes place in the cypress ecosystems, even by comparison with physically more open systems, such as the Chesapeake estuary. What recycling does occur, however, is exceedingly complex. In particular, the predation on eggs and juveniles of higher trophic elements by lower trophic level species complicates the cycling structure enormously and provides tens of millions of new pathways for recycle. Such "ovi-predation" has been neglected in most trophic budgets, but it could be of enormous significance in forecasting the results of system impacts.

The Florida Bay ecosystem: A 125-component budget of the carbon exchanges occurring during the wet and dry seasons in Florida Bay has been assembled. These trophic networks are the most highly resolved and complete foodweb ever to have been assembled for any ecosystem. As is the case with such detailed, quantitative descriptions of ecosystems, the overall configuration of trophic transfers yields numerous clues as to how the ecosystem is functioning. An analysis of indirect contributions reveals, for example, that seagrasses are the ultimate source of carbon for the system during the wet season. However, epiphytic periphyton becomes the foundation that sustains system activity during the dry season. These primary producers fuel ecosystem activity mostly via indirect routes involving passage through detrital links. There is some 37 percent more trophic activity during the wet season, as compared with the dry interval that follows. Nevertheless, more species appear to feed higher on the trophic ladder during the dry season than they do during the wet period. The taxon feeding highest on the trophic ladder is the raptors, which, on the average, feed at level 4.6. Such averages hide the existence of some very long trophic feeding chains, which in a few instances reach 15 transfers in length. Such long concatenations, however, move only an insignificant amount of carbon along their whole length. A remarkably high percentage of carbon is recycled by the Florida Bay ecosystem. Over 26 percent of total system activity involves recycling (a proportion exceeded only be coral reel ecosystems), and, quantitatively, most of these processes are carried out by the pelagic and benthic flagellates. The topology of the Florida Bay ecosystem is remarkably stable throughout the year.

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 "Across Trophic Level System Simulation" was provided by the U.S. Environmental Protection Agency and the U.S. Army Corps of Engineers.

PAPERS AND REPORTS

Ulanowicz, R.E., Bondavalli, C., and Egnotovich, M. S., 1996, Network analysis of trophic dynamics in South Florida ecosystems--The Cypress Wetland Ecosystem: Annual Report to the U.S. Geological Survey, Biological Resources Division.

Ulanowicz, R.E., Bondavalli, C., and Egnotovich, M. S., 1997, Network analysis of trophic dynamics in South Florida ecosystems--The Florida Bay Ecosystem: Annual Report to the U.S. Geological Survey, Biological Resources Division.

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