Ref. No. [UMCES] CBL 00-0176

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 99: The Graminoid Ecosystem

Robert E. Ulanowicz, 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

September 1, 2000

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

A 66 component budget of the carbon exchanges occurring during the wet and dry seasons in the graminoid 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:

In the graminoid system, the breakdown of carbon into detritus is very important, but the recycling of detritus seems not to be as important as one might expect of wetland ecosystems. Most of the carbon sinks out of the system. In contrast, periphyton seems to be very important to the higher trophic levels.

An analysis of beneficial predators and malefic prey in the graminoids indicates that the living POC (microbiota attached to labile and refractory detritus) and living sediment (microbiota attached to sediment carbon) compartments actually receive indirect benefits from a large number of their predators. On the other hand, living POC appears to impact many of its predators negatively via indirect routes. Thus, it seems that the bacterial component of the graminoid system affects the rest of the system in a negative way, even though it is affected positively by most of its predators. One thereby infers that the sediment is a natural sink for carbon in this system.

The ratio of detritivory to herbivory is very high in the graminoid system. However, a low value for the Finn Cycling Index reveals a relative unimportance of cycling in this system. This seeming paradox is explained by the fact that much of what is produced by the primary producers seems to make its way into the detritus (sediment carbon, labile and refractory detritus), which is then consumed by the bacteria in the living POC and living sediment. The total dependency analysis shows, however, that the carbon in the detrital and bacterial compartments is not recycled to the higher trophic levels, but seems to be deposited as peat.

The cycle analysis also supports the theory that cycling in the graminoid ecosystem is confined primarily to the sediment and water column detritus. The link between the detrital cycles and the higher trophic levels is very weak, which is to say there is little interaction between the microbial loop and upper trophic levels in the graminoid system.

The fact that the graminoid ecosystem is a source of food to many of the migratory species that reside in the cypress and mangrove systems has influenced the systems properties of the graminoid model. The analysis of beneficial predation is a good example of this interaction. In the graminoid system there are 13 beneficial predators in the wet season and 17 in the dry season. These numbers are much fewer than the instances of beneficial predation that occur in the cypress, mangrove and bay systems, where beneficial predation was more prolific. It may be that excluding the wading birds and other birds that do not roost in the graminoids, but still feed in the system reduces the number of beneficial predators.

Even though the graminoid system has the fewest compartments, it is far more active than the cypress, mangrove and Florida Bay communities. Its total system throughput is an order of magnitude larger than that of any of the other systems, and consequently, the development capacity of the graminoid system is significantly higher than its counterparts in the other systems.

Table of Contents

This report covers work done during the last 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 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-through 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 "impact analysis". Eleven items in the diet of the alligator derived overall (indirect) benefit from the alligator’s eating habits.

During 1997-8 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 food webs 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. Concatenations 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 the total system activity is devoted to recycling, and most of these processes involve pelagic and benthic flagellates.

During 1998-9, an 87-compartment network of the mangrove ecosystem was estimated for both wet and dry seasons. The study showed that the mangrove ecosystem mostly functions like a detrital-based ecosystem. Most of the predator compartments depended ultimately upon detrital carbon for their sustenance, while mammals depended more upon herbivorous rather than detritivorous prey. The system depends mainly on internal fixation of carbon and receives only a small subsidy from elsewhere, mostly imported by the birds, which obtain 20% of their sustenance outside the mangroves. There were an enormous number of pathways for recycle of carbon in the mangrove ecosystem, but 97.5% of all recycle activity occurred along only 15 cycles - among the benthic compartments. The main route for recycling was mediated by the sediment bacteria and the meiofauna, with auxiliary routes passing through the benthic flagellates and ciliates. The rare feline predators contributed the most to the community ascendency per unit of their aggregate activity. Whole-system information indices paint a picture of the mangrove ecosystem as being subjected to heavy natural stressors. Of all the three systems studied previously, the mangrove community showed least seasonal change.



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