transfer of energy from terrestrial primary producers (i.e. grass) to brown lemmings is influenced 

 by the amount of grass and lemmings present, the reproductive status of the lemming population 

 (biotic factor), and the ambient temperature (abiotic facta). The effect of each of these on the 

 transfer of energy can be determined, quantified, and described by the equation. 



The change in the quantity of energy in any compartment over a period of time is the algebraic 

 sum of aU of the processes which add energy to or remove energy from that compartment. Thus, 

 the equations which describe the transfer processes may be combined to describe changes in the 

 distribution of energy within the system. If the quantity of energy in each compartment of the 

 system at the start of a time interval, and the values of the various abiotic and biotic factors which 

 influence transfer processes, are known, the equations can be used to calculate or predict changes 

 in the distribution of energy over a time interval. A model such as this, which predicts changes 

 in the system, is a dynamic model and contains the most information concerning the functioning of 

 the system. This model, in its most general terms, is presently being constructed for the tundra 

 ecosystem. The mechanics of model construction and testing is being done by personnel of the 

 Desert Biome Program at Utah State University, with frequent consultation with Tundra Biome 

 scientists. The model is being built around a set of differential equations which describe the 

 principal transfer processes indicated in Figure 6. 



Our knowledge of exchanges between some compartments, particularly the below-ground com- 

 partments, is far from complete. For the construction of this "first approximation" model, changes 

 in distribution of energy will have to be described as well as possible without full knowledge of 

 the various processes involved, i.e. a "black box" approach is required. Because of this, the 

 resolution of the first approximation model will not be great. The main function of this model is 

 not highly accurate prediction but the identification of compartments and processes that are 

 particularly important in the functioning of the system so that research effort may be directed to 

 the most critical areas. As our knowledge in these areas advances, the "black boxes" will be 

 replaced by more exact, mechanistic descriptions of processes in later versions of the system 

 model. The first approximation model will be constructed usit^ data collected in 1970 and prior 

 seasons. It will be completed by early 1971, so that it may be used to modify the 1971 research. 



Some processes of the system have been studied in such detail that accurate mechanistic 

 submodels can now be attempted. Detailed submodels have been constructed for primary production 

 (Miller), depth of thaw and soil temperature (Nakano and Brown), and gross near-surface meteoro- 

 logy (Lord and Pandolfo). The status of each of these is discussed elsewhere in this report. 

 Eventually, these and other submodels will be brought together in a final system model. It is in 

 the final model that accuracy of prediction will be stressed, so that the model may be used for 

 system simulation and problem solving. 



14 



