The Herbivore-Based Trophic System 357 



TABLE 10-4 Relative Concentration of Energy (kJ 



gdw'^) and Nutrients (mg gdw'J in Forage 

 and Lemmings 



•Data from Tieszen (unpubl.); Chapin et al. (1975). 

 tData from Bunnell (unpubl.). 



Note: Percentage of forage nutrients retained in lemming production given by ratio 

 of production to ingestion (P/f) calculated from equation given on p. 356. 



should be considered. Relatively large amounts of the calcium and phos- 

 phorus in forage are retained as lemming production, and these nutrients 

 may be limiting for lemmings. If assimilation is low with high fecal loss, 

 or if recycling within the body is inefficient with high urinary loss, the 

 animal is in danger of entering negative nutrient balance, particularly 

 during pregnancy and lactation when nutrient demands are high. 



A simulation model of lemming nutrition was constructed to explore 

 the nutrient balance in lemmings (Barkley et al. 1980). The model calcu- 

 lates the amount of nitrogen, phosphorus and calcium that a lemming 

 could absorb from a given diet; the amount of nutrients required, given 

 the lemming's body size, growth rate and reproductive condition; and 

 the changes that occur in the lemming's nutrient pool. Normal nutrient 

 pools were calculated from data of Bunnell (pers. comm.). Because little 

 information was available on lemmings' ability to absorb nutrients or on 

 their endogenous loss rates, which determine minimum nutrient re- 

 quirements, this part of the model relied heavily upon nutritional infor- 

 mation for the laboratory rat. In rats, and presumably in lemmings, ab- 

 sorption and losses are functions of ingestion, fecal output, metaboHc 

 rate and nutritional condition. Relatively high absorption rates and low 

 loss rates were used, so the model was conservative and biased against 

 the production of nutrient deficiencies. 



The total amount of forage consumed was based upon lemmings' 

 energetic requirements, food habits and forage digestibility. The stand- 

 ard model used a diet consisting of 80% graminoids and 20% mosses 

 during midsummer and 60% graminoids and 40% mosses during mid- 



