Ref. No. [UMCES] CBL 99-0073

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 Ecosystem, FY 98: The Mangrove Ecosystem

Robert E. Ulanowicz, Cristina Bondavalli, Johanna J. Heymans, 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 31, 1999

Technical Report Series No. TS-191-99 of the University of Maryland System Center for Environmental Science, Chesapeake Biological Laboratory

EXECUTIVE SUMMARY

A 94- component budget of the carbon exchanges occurring during the wet and dry seasons in the mangrove ecosystem of South Florida has been assembled. These trophic networks will serve as independent benchmarks against which the performance of the ATLSS multi-model, now under construction, will be assessed. 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:

Study of the indirect dependencies within the system reveals that the mangrove ecosystem for the most part is functioning like a detrital- based ecosystem. (This was not the case with the cypress ecosystem.) Most of the predator compartments depended ultimately upon detrital carbon for their sustenance. Exceptions to this pattern were the mammals, which depended more upon herbivorous rather than detritivorous prey. The mangrove ecosystem was observed to depend mainly upon internal fixation of carbon and receives only a small subsidy from elsewhere. Most of this subsidy is imported by the birds, which obtain 20% of their sustenance outside the mangroves. There were over 200 instances of "beneficial predation", i.e., instances where the positive indirect trophic benefits of a predator more than outweighed their direct negative impacts. In a full 30% of these instances, the prey had a net overall negative (malefic) effect upon its predator. That is, with at least 60 of the trophic processes in the mangrove ecosystem, the net overall effects of predation are exactly opposite to the direct effects. Like with the Florida Bay ecosystem, the birds fed at the highest trophic levels and on average feed a full trophic level higher than they do in the cypress swamps. This accords with the underlying trophic efficiencies, which ran only slightly lower than their counterparts in Florida Bay, but some 15% higher than in the swamps. No significant differences between wet and dry seasons were observed in the trophic statuses of the various mangrove compartments.

There were an enormous number of pathways for recycle of carbon in the mangrove ecosystem (more than could be easily handled by the computing machinery available), but fully 97.5% of all recycle activity occurred along only 15 cycles. Like with other estuaries, these cycles occurred among the benthic compartments. The main route for recycle was mediated by the sediment bacteria and the meiofauna, with auxiliary routes passing through the benthic flagellates and ciliates. The proportion of activity devoted to recycle (Finn Index) was 17.2%, which is comparable to other estuaries, comparable to Florida Bay, but much higher than in the cypress swamps.

From a whole- system perspective, the mangrove ecosystem shows virtually the same rate of trophic activity as is found in Florida Bay. The trophic network is not quite as rigidly organized as that in the Bay, however, because the system ascendency of Bay community exceeds that of the mangrove ecosystem by about 25%. As with the cypress system, the rare feline predators contributed the most to the community ascendency per unit of their aggregate activity. Whole- system information indices painted a picture of the mangrove ecosystem as being subjected to heavy natural stressors (presumeably osmotic in nature.) By comparison, the cypress swamp exhibited depressed activity, probably due to nutrient limitation. A surprising result was that Florida Bay, despite its recent erratic changes in biota, seemed to be the least stressed of the three ecosystems. Of all the three systems studied to date, the mangrove community showed least change between seasons.

PAST WORK

This report covers work done during the third year of a four year task under ATLSS to quantify the trophic processes in South Florida ecosystems. During the first year, 1996-7, a 69-compartment network of trophic exchanges in the cypress wetlands was constructed and analyzed. The analysis revealed that the higher trophic populations in the ecosystem were not as dependent on cypress litterfall, as had been assumed. Rather, most of the litterfall was being buried in the sediments of this peat- building system. Many of the upper trophic components were being supported instead by the production of the understory vegetation. Relatively little recycling occurs in this ecosystem. Despite the lack of physical advection in the horizontal dimension, most system activity resembles a pass- thru system in the vertical direction, i.e., litter falling and being buried in the sediments. An attempt was made for the first time to assess the "intrinsic value" of each ecosystem component in terms of the amount it contributes per unit of activity to the overall performance of the system at processing mass and energy (the ascendency.) This evaluation revealed that rare and endangered species, such as the Florida panther, were contributing more per unit of activity than some taxa that feed at higher trophic levels. Finally, the key role of the American Alligator in maintaining the species diversity of this ecosystem was highlighted and quantified through a method called "impact analysis." Eleven items in the diet of the alligator derived overall (indirect) benefit from the alligator’s eating habits.

During the 1997-8 reporting period a 125- compartment network of the trophic flows through the ecosystem of Florida Bay was estimated for both wet and dry seasons. These networks stand as the most highly- resolved and complete foodwebs ever to be assembled. Analysis of the network revealed that seagrasses are the ultimate source of reduced carbon for most of the rest of the system during the wet season, but that epiphytic periphyton supports most ecological activity during the dry period. Although 37% more activity transpires in the Bay during the wet season, most species feed higher on the food chain during the dry months. Concatonations as long as 15 exchanges can be identified among the network of trophic exchanges in the Bay. The recycling of carbon in the Bay ecosystem is representative of most estuaries. Over 14% of total system activity is devoted to recycling, and most of these processes involve the pelagic and benthic flagellates.

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