Modeling 409 



9-month winter season. The interchange was also important for deciding 

 whether or not the output from the models was reasonable. We believe 

 strongly that in our present state of understanding the model output must 

 not be believed unless it is biologically reasonable. The biological insight 

 of the scientist must prevail and models must be judged incorrect if the 

 output is biologically unrealistic. This may seem trivial but it is evidently 

 all too easy to begin believing the model output and to use this as the basis 

 for rejecting contradictory data. 



It is obvious that the ecologists need to be intimately involved at all 

 stages in the modeling. The judgments on the processes to be included, on 

 which papers in the literature are to be believed, and on the reality of the 

 output all call for knowledgeable, experienced scientists who are critical 

 thinkers and completely up-to-date with the latest developments in the 

 field. 



WHOLE SYSTEMS MODELS 



Introduction 



It was originally hoped that the relatively simple aquatic ecosystems 

 of a tundra pond would allow whole system models to be constructed and 

 that these models would be good enough for testing hypotheses or 

 predicting the effects of perturbations. It is now clear that even an 

 ecosystem so simple and diligently studied as a tundra pond is not 

 understood well enough for predictive modeling. We believe that the fault 

 lies with our understanding of the biological processes and not with the 

 modeling techniques. 



As will be demonstrated, we have been able to simulate the complete 

 pond ecosystem using the initial conditions of biomass and concentration 

 actually observed as well as the measured parameter values such as half 

 saturation constants for photosynthesis, etc. The deterministic model, 

 however, proved very sensitive to small changes in certain 

 parameters — too sensitive, in fact, to be satisfying to biologists. Obviously 

 some ecological feedbacks and changes in parameters over the season are 

 not known; yet, these may be controlling the rates of predation or 

 adjusting the respiration rates. 



Some subsections of the model did contribute to our understanding of 

 the limnology of the pond by showing how various components could be 

 interacting. An example is the effect of light and temperature on algal 

 photosynthesis (Stanley and Daley 1976). This same study also used the 

 model to determine the importance and necessary values of downward 

 mixing of epipelic algae in the surface sediments. 



