Cooperative Ecosystem Studies Unit, Sout Florida-Caribbean CESU

archived

The Everglades ecosystem is a complex subtropical marshland, which is habitat for an extremely rich biodiversity. In this context, the huge flocks of wading birds with their rookeries are most conspicuous. This biodiversity is maintained by an underlying basic food web of small fishes and invertebrates, which is the foundation for higher trophic levels. Based on the rainfall pattern, the whole system expresses strong seasonality with its dry and wet phases, which also governs the distribution of flooded and non-flooded areas. Because of the flat landscape, small differences in mean water levels can alter the whole landscape dramatically. Together with seasonal changes in water depth, the trophic webs undergo significant changes in structure through the year. Larger food web components, such as planktivorous and piscivorous fish functional groups, can disappear in certain areas, while others can increase in biomass. If this is the case, dramatic trophic cascades involving top-down effects can occur in the food webs, eventually over several hierarchical levels. Our goal here is to understand how changes in hydrology may affect the seasonal dynamics of the Everglades food webs, particularly the community of small fish species, and what the consequences may be for the wading birds and other predators that depend on it. The model will meet the following specifications. It will contain a food web structure with the following levels: primary producer (periphyton), detritus, invertebrate detritivores, fish consumers and nutrients, which are recycled within the system. The model is spatially explicit on a 100  100 grid of square cells (though this size can be increased if needed) representing a segment of the Everglades hydroscape, each cell being assumed to be 100  100 meters. For example, the overall modeled landscape could consist of an area resembling the east-southern Taylor Slough in the Everglades. Hydrology is represented by rising and falling water levels through the year, following typical empirical patterns. In this food web structure, the fish are assumed to move seasonally, with a fraction being allowed to move up the gradient during floods and down the gradient during falling water. There are two basic parts to the model. The first is the structure of the food web and the equations describing fluxes between food web components. This set of equations is assumed to be the same for each of the 10,000 cells. The second part of the model is the set of rules for the movement of fish and crayfish species of the food web between the spatial cells.

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