Ref. No. [UMCES]CBL 97-075

Annual Report to the United States Geological Service Biological Resources Division University of Miami Coral Gables, FL 33124

Network Analysis of Trophic Dynamics in South Florida Ecosystems, FY 96: The Cypress Wetland Ecosystem

Robert E. Ulanowicz, Cristina Bondavalli, Michael S. Egnotovich

University of Maryland System, Chesapeake Biological Laboratory, Solomons, MD 20688-0038

E-mail: ulan@cbl.umces.edu
Tel: (410) 326-7266
FAX: (410) 326-7378

May 30, 1997

EXECUTIVE SUMMARY

A 68- component 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 multi-model, now under construction, will be assessed. During the construction of these networks, it soon became clear that the received wisdom, 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 system, and phytoplankton is especially important during the dry season. The structure of the lower trophic level ATLSS module for the cypress wetlands will need to be revised in the light of these new findings.

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 less efficient by comparison. The result is a peak in trophic 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%.

One may employ the contributions by the various taxa to the overall measure of system ascendency to create an index of that compartment's "intrinsic value" to overall system performance. These new criteria nicely spotlight the potential values of rare and endangered species, such as the Florida panther, to system functioning.

Relatively little cycling takes place in the cypress ecosystems, even by comparison with physically more open systems, such as the Chesapeake estuary. What recycle as does occur, however, is exceedingly complex. In particular, the predation on eggs and juveniles of higher trophic elements by lower 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.

Finally, ecologists are accustomed to thinking that the norm for this ecosystem occurs during the wet season, because water level appears to be the limiting factor for the dominant vegetation, the cypress trees. Network analysis reveals, however, that the system is actually somewhat more organized during the dry season. Viewed with this as background, the high water levels of the wet season appear more in the light of a defining stress to the ecosystem.


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