The Herbivore-Based Trophic System 395 



age (White et al. 1975) were used to generate estimates of food and 

 energy intake and rumen function parameters with rumenmet. The 

 number of grazing events, the time spent grazing, and the rate of ruminal 

 volatile fatty acid production predicted by rumenmet agreed with field 

 observations in the Prudhoe Bay region (White et al. 1981) when the 

 rumen capacity was expressed as: 



Rumen capacity = 211^x18.9x0.57 = 226 kJ kg"' 



where 21 W is the dry matter capacity of the rumen in relation to body 

 weight (g kg"'), 18.9is the energy content of the forage (kJ g"') and 0.57 is 

 the proportion of energy assimilated. This capacity is about half the daily 

 energy requirement of an adult non-lactating caribou during summer. 



At the end of summer, green biomass at Prudhoe Bay declines 

 markedly to 10 g m'^ or less on all vegetational types except those on rims 

 of low-centered polygons and pingos, where 20 g m'^ remains (Webber 

 and Walker 1975, White et al. 1975). In most vegetation types a large 

 amount of standing dead graminoid leaves is available. But this material 

 is generally high in crude fiber and lignin, and expected digestibiUty 

 would be only 30 to 40<^o (White et al. 1975). At this time caribou could 

 continue to eat mostly green forage and maintain a relatively high digest- 

 ibility, or they could consume larger amounts of dead material. 

 RUMENMET was used to evaluate the effectiveness of these alternative 

 tactics. If caribou continued to feed only on green material, then metab- 

 olizable energy intake would be about 30*^0 of energy requirements. 

 However, if both green and standing dead material were ingested, the 

 model predicted that the daily metabolizable energy intake would be 

 about 75% of energy requirements. 



Even if caribou could consume almost all available food on exposed 

 ridges and at the base of feeding craters, the Prudhoe Bay region appears 

 to be poor winter range. In interior Alaska good winter range for caribou 

 is characterized by shallow snow and high lichen biomass (>100 g m"^; 

 Hanson et al. 1975). Supplementation with frozen green graminoids is 

 possible (Klein 1970, Hemming 1971), and maintenance requirements 

 can probably be met. In the Prudhoe Bay region exposed ridges and pin- 

 gos contain the highest lichen biomass. However, these areas constitute 

 only 5% of the vegetated area (White et al. 1975), and the biomass of 

 lichens is generally less than 10 g m'^ (WiUiams et al. 1975). 



Population energetics can be calculated from estimates of energy ex- 

 penditure by age and sex classes within the population combined with 

 estimates of productivity by age class. A flow chart was used to calculate 

 energy expenditures of adult male and female caribou (Table 10-8). 

 Average daily metabolic rate was computed using the model active 

 (Bunnell et al., unpubl.), which was constructed to simulate the grazing 



