The Herbivore-Based Trophic System 353 



the energy used by a nonreproductive female with a similar nest of 12-cm 

 outer radius. The aditional energy cost of reproduction is rather low 

 compared to earlier calculations because nonreproductive lemmings have 

 a high rate of energy use at such low temperatures, even with a nest. 



Total population energy demand for a high year (September of a 

 pre-high year through August of a high year) has been calculated by sum- 

 ming energy requirements for the individuals of each category present 

 each month. Monthly changes in density, population structure and re- 

 productive intensity were estimated for a standard cycle in which trap- 

 pable densities reached 150 ha"' (Batzli 1975a). The low population was 

 assumed to have a similar structure, but average densities for the whole 

 year were 400 times less than for the high population (Table 10-3). 



Grodziriski and Wunder (1975) reported that production, the sum of 

 all energy deposited in new tissue, averaged 2.3% of respiration for ro- 

 dent populations in general, a value slightly higher than the 2% predicted 

 by Turner (1970) for vertebrate homeotherms. When rising to a peak, 

 production for a population of Lemmus at Barrow was 5% of respira- 

 tion. This value is the highest known for a homeotherm and occurs des- 

 pite the high energy requirements of thermoregulation. The high produc- 

 tion results from nearly year-round reproduction and the high popula- 

 tion turnover rate of Lemmus. Values calculated for Dicrostonyx are 

 more similar to those for other small rodents. 



Digestion and Ingestion 



In order to calculate ingestion rates for the population, we must 

 know the mean com.position of the natural diet and the digestibility of 

 natural forage as well as energetic demand. The information on the food 

 habits of lemmings presented above has been averaged across habitats, 

 and overall summer and winter digestibilities calculated (Figure 10-18). A 

 mean digestive efficiency of 33 Vo for energy is derived from these values 

 and applied to assimilation to give an estimate of ingestion. 



The digestive efficiency of Lemmus is strikingly low, much lower 

 than that of Dicrostonyx and most other herbivorous mammals (Batzli 

 and Cole 1979), and dramatically elevates the ingestion rate. The materi- 

 al within the plants may be separated into structural carbohydrate, com- 

 posed of cellulose, hemicellulose and lignin, and nonstructural carbohy- 

 drate. Nonstructural carbohydrate is material contained in the cell cyto- 

 plasm and should be much more easily digested than structural carbohy- 

 drate. Total nonstructural carbohydrate (TNC) in graminoid shoots var- 

 ied between 30 and 40% of the biomass for most of the growing season 

 on moist meadows (Chapter 5, Figure 5-5). Lipid concentration found in 

 shoots varied from 5 to 15%. Therefore, the observed 36% digestibility 

 of graminoids can be accounted for by digestion of TNC and lipids. 



Although digestibility of tundra graminoids by brown lemmings is 



