We have developed a General Ecosystem Model (GEM) that is designed to
simulate a variety of ecosystem types using a fixed model structure. D
riven largely by hydrologic algorithms for upland, wetland and shallow
-water habitats, the model captures the response of macrophyte and alg
al communities to simulated levels of nutrients, water, and environmen
tal inputs. It explicitly incorporates ecological processes that deter
mine water levels, plant production, nutrient cycling associated with
organic matter decomposition, consumer dynamics, and fire. While the m
odel may be used to simulate ecosystem dynamics for a single homogenou
s habitat, our primary objective is to replicate it as a ''unit'' mode
l in heterogeneous, grid-based dynamic spatial models using different
parameter sets for each habitat. Thus, we constrained the process (i.e
., computational) complexity, yet targeted a level of disaggregation t
hat would effectively capture the feedbacks among important ecosystem
processes. A basic version was used to simulate the response of sedge
and hardwood communities to varying hydrologic regimes and associated
water quality. Sensitivity analyses provided examples of the model dyn
amics, showing the varying response of macrophyte production to differ
ent nutrient requirements, with subsequent changes in the sediment wat
er nutrient concentrations and total water head. Changes in the macrop
hyte canopy structure resulted in differences in transpiration, and th
us the total water levels and macrophyte production. The GEM's modular
design facilitates understanding the model structure and objectives,
inviting variants of the basic version for other research goals. Impor
tantly, we hope that the generic nature of the model will help allevia
te the ''reinventing-the-wheel'' syndrome of model development, and we
are implementing it in a variety of systems to help understand their
basic dynamics.