The primary purpose of this project is to support the modeling efforts of the Across Trophic Level Systems Simulation (ATLSS) Project by providing quality data on the workings of the south Florida wetland ecosystems that can be used to calibrate and verify that the ATLSS model is a reasonable simulation of how these ecosystems work.

Its secondary purpose could well surpass the primary goal in importance – namely, by discovering several major facets of ecosystem behavior, the network analysis could provide very important strategic information useful to those engaged in management and restoration efforts.

The full suite of eight networks (wet and dry season networks for each of the four major biomes) are completed and on the Web at http://cbl.umces.edu/~atlss/.

The first step in any modeling project is the “lexical” phase, or the identification of the primary biological components that are to be modeled. The next step involves connecting the selected compartments to one another via feeding and detrital pathways. This topology is determined from information about the diets of each taxon. The purpose here is not merely to formulate a qualitative "foodweb", but also to quantify the connections. Toward this end, it is useful to concentrate first on assessing the densities, or stocks of the participating taxa. Knowing the concentration of biomass is the key to scaling all the activities of a particular population.

The biomasses of most species are known to reasonable precision. It is usually just necessary to find those estimates from the available literature or from the opinions of experts. For example, the number of animals per cubic meter or liter is available for many of the compartments. As the standard units used in network analysis (NA) are grams of carbon per square meter, the available data had to be transformed to maintain dimensional consistency. Towards this end, information on the average weight (grams) of animals was gathered from technical manuals. The percentage of carbon per gram of dry weight was then combined with wet weight/dry weight ratio to obtain gC/m³ (in the case of number of animals per liter we just need a simple equation to transform gC/l in gC/m³). By assuming an average water depth, the carbon biomasses in the required units (g/m²) was calculated. In the case of primary producers, most sources reported data on biomass in grams per hectare or square meter. In this case only the wet weight/dry weight ratio and the percentage of carbon per gram of dry weight were necessary to convert the biomasses into the correct units.

Once the biomasses had been estimated, the total demand by each compartment was estimated by searching for data on consumption per unit biomass. This total consumption then had to be apportioned among the various items in that taxon’s diet. On the output side, the fraction of the consumption that is dissipated was estimated from data on respiration per unit biomass. The remainder was available as either production to meet the demands of predators or was recycled into the detrital pools. Unfortunately, the dietary components of some taxa were available only as a list of species. In such cases the total input was apportioned to the list of prey in proportion to the standing stocks or production of those populations.

The two seasonal networks were assumed to balance over each period. Although this assumption is not entirely realistic, balance is required for the critical input/output phase of NA.

At this point, the balance is almost complete. It remains to estimate the exchanges of carbon with the outside world. Exogenous imports occur in three different ways: (1) Carbon from the atmosphere may be fixed as biomass through the process of photosynthesis. The magnitude of this import is assessed by multiplying the standing stock of the autotroph by its primary productivity per unit carbon, as mentioned above. (2) Biomass may enter or leave the system advected by water flow into and out of the study area. These exchanges can be estimated from the overall water budget for the wetland area, which includes figures for gross advective water exchange with the surrounding areas. Multiplying these water exchanges by the concentrations of carbon suspended in the water column provided an approximation for these exogenous transfers. (3) Biomass may enter and leave the system as animal populations migrate across the boundaries of the study area.

Proceeding in this manner, it was usually possible to balance carbon around each taxon to within a few percent. Final balance was cast using a software routine DATBAL, which forces balance using the assumption of donor control. (It should be borne in mind that this unrealistic assumption was invoked only for the purpose of obviating very small discrepancies.)

Once the networks had been estimated, they were analyzed using a suite of algorithms, collectively known as NETWRK. Four types of analyses are performed by NETWRK. First, so-called input-output structure matrices are calculated. These allow the user to look in detail at the effects, both direct and indirect, that any particular flow or transformation might have on any other given species or flow. Next, the graph is mapped into a concatenated trophic chain (after Lindeman, 1942). Then, global variables describing the state of development of the network are presented. Finally, all the simple, directed biogeochemical cycles are identified and separated from their supporting dissipative flows. NETWRK 4.2a and its accompanying documentation may be downloaded from the World Wide Web at: http://www.cbl.umces.edu/~ulan/ntwk/network.html.

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(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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