The Herbivore-Based Trophic System 401 



the maximum and the mean expected hfetime, are about 14 times greater 

 in caribou than in lemmings. Remarkably, the usual ages at first repro- 

 duction bear a similar relationship, which again suggests compensation 

 between length of life and speed of development. Of course, the expected 

 life span varies with any changes in the life table, so seasonal and annual 

 differences will make its ratio between lemmings and caribou much more 

 variable than that for maximum length of life. 



The consequences of life history differences can be immense when 

 considered at the population level. For instance, if we assume that sur- 

 vival is 100%, the potential population increase for lemmings during one 

 year (A^^J is a factor of 1300 while that for caribou is only 1.5 (Table 

 10-10). High reproductive potential allows lemmings to respond quickly 

 to temporarily favorable conditions, but such high local densities are 

 reached that they cannot be maintained. Thus, population densities fluc- 

 tuate wildly. Caribou cannot respond quickly to short-term changes in 

 the environmental conditions, and their population densities change 

 slowly in response to long-term changes in environment. 



Because of the great discrepancy in size of the major grazers, density 

 figures (Table 10-3) do not give a good comparison of the two grazing re- 

 gimes. The extreme changes in density of lemmings are modified some- 

 what if the time of residence is included (line 2), but the best comparative 

 figure of the amount of grazers present is probably biomass residence 

 (line 4), which compensates for both body size and time of residence. 

 Even this measurement shows that the annual grazing population on the 

 coastal tundra at Barrow is much more variable than that in the Prudhoe 

 Bay region. The annual biomass residence at Barrow may vary between 

 twenty times less and five times more than at Prudhoe Bay. 



Energy flow through biomass is disproportionately large in lem- 

 mings compared with caribou because of the small body of the lemming. 

 Maximum annual respiratory rates of lemming populations at Barrow 

 are 50 times those of caribou in the Prudhoe Bay region, and relative 

 production rates are even higher. Lemmings do not produce young any 

 more efficiently than caribou, but in relation to their size they produce 

 more. Among lemmings, Lemmus produces more than Dicrostonyx 

 because litters are slightly larger and breeding seasons are longer (Batzli 

 1975a). The result of these relationships is that even in the Prudhoe Bay 

 region, where caribou account for most of the biomass of grazers, the 

 respiration and production of lemmings is often greater than that of cari- 

 bou. Population efficiency and turnover time reflect the same relation- 

 ships; lemming production efficiencies are greater than those of caribou, 

 and turnover times are about 40 times shorter (Table 10-3). 



