The Herbivore-Based Trophic System 407 



TABLE 10-13 



Nutrients (kg ha~^) Accumulated 

 During a Standard Lemming Cycle (3 

 Years) as Calculated by a Simulation 

 Model of Nutrient Transport by 

 Lemmings 



Note: Four cases were run: with nutrient levels in forage high or low and with 

 nutrient levels in forage the same or different in the three microhabitats at the 

 beginning of the run. 



such data, we have tried to determine the feasibiHty of the hypothesis by 

 using computer models of lemming population dynamics and nutrition 

 to calculate deposition of urinary and fecal nutrients in the various mi- 

 crotopographic units. The activity of lemmings in the various microtopo- 

 graphic units was distributed according to time spent in nests and per- 

 centage of shoots clipped in each unit. We assumed that excreta are 

 formed and deposited continually. Thus, if lemmings spend 50% of their 

 time in their nests, 50% of their excretions would be deposited in 

 troughs, and the remaining amount would be distributed according to 

 the amount of time spent foraging in each microenvironment. If the fig- 

 ures are adjusted for the relative area of each unit, the removal and depo- 

 sition of nutrients can be put on an areal basis. 



The results of these simulations indicate that net transportation of 

 phosphorus from rims and basins to troughs only occurred when plant 

 nutrient concentrations were high and the same in all habitats at the be- 

 ginning of the run (Table 10-13). This would be the situation as poly- 

 gonal development began in a recently drained lake basin. 



In all cases the concentration of nutrients in lemming carcasses was 

 sufficient to alter the results, depending on where those carcasses were 

 deposited. Most carcasses found on tundra are in the troughs, near or in 



