The Herbivore-Based Trophic System 399 



Nearly all production of caribou derives from summer range, so nu- 

 trient retention can be considered only in relation to summer forage. Re- 

 tention of nutrients in the diet, the P/I ratio in Table 10-9, shows that ni- 

 trogen, phosphorus, calcium and sodium must be concentrated strongly, 

 but not as strongly as for lemmings (Table 10-4). Nevertheless, nutrient 

 availability could be as important for caribou as for lemmings. 



No model has been constructed to explore the tactics open to cari- 

 bou to maximize nutrient intake and to minimize nutrient loss by conser- 

 vation and recycling processes as has been done for lemmings. However, 

 if studies made on reindeer apply to caribou, then caribou may conserve 

 nitrogen through urea recycHng (Wales et al. 1975), and this mechanism 

 may conserve use of energy, water and glucose as well (White 1975). 



COMPARISON OF GRAZING SYSTEMS 



Now that the main features of the herbivore-based food chains have 

 been described, it should be clear that microtine rodents and ungulates 

 represent very different approaches to herbivory. This section will com- 

 pare the main features of these two grazing systems and point out their 

 consequences for the coastal tundra ecosystem as a whole. 



Population Characteristics 



Perhaps the most conspicuous difference between microtines and 

 ungulates is body size. Although that may seem to be a trivial observa- 

 tion, many life history characteristics of Lemmus and Rangifer, which 

 determine characteristics of populations, appear to be a function of body 

 size (Table 10-10). 



The ratio of body size between lemmings and caribou remains nearly 

 constant from birth through adulthood; caribou weigh about 1500 times 

 as much as lemmings. Both species need to grow by a factor of 20 from 

 birth to adulthood, but lemmings grow relatively faster. Thus, lemmings 

 double their birth weight in four days and reach adult weights in 120 days 

 whereas caribou take four times as long. The association of higher meta- 

 bolic and growth rates with smaller body size is well known (Kleiber 

 1961). Generally, the efficiency of growth does not change. Small and 

 large animals produce the same amount of new tissue for each unit of 

 energy digested, but small animals produce the new tissue more rapidly. 



Two other life history characteristics associated with small body size 

 are a high reproductive rate and a short life span (Smith 1954). These 

 relationships are dramatic in the lemming-caribou comparison. 



Because lemmings have a large mean litter size (seven) and a rapid 



