The Detritus-Based Trophic System 429 



enchytraeid species was well above those of the two cranefly species 

 studied, and this is reflected in much higher P/B ratios in the En- 

 chytraeidae (Table 11-6). Thus, the difference in energetic activity of En- 

 chytraeidae and Diptera is even greater than the difference in biomass. 



Evolution of Life Cycles 



Life cycles lasting more than one year occur in many arctic inverte- 

 brates (Chernov 1978, MacLean 1975a). Given the short growing season 

 and low temperatures of the Arctic, few species may be able to complete 

 growth and development in a single season. Those species unable to over- 

 winter and renew growth in the following season will be eliminated from 

 the arctic fauna. An invertebrate species might be able to complete devel- 

 opment in many, even most seasons; however, an obligate annual life 

 cycle demands successful development and reproduction every season for 

 maintenance of the population. Thus, a sequence of severe summers 

 could eliminate annual species from the fauna. Many herbivorous insect 

 species have obligate annual life cycles that are closely tied to the phen- 

 ology of the plants upon which they feed. This may contribute to the 

 shortage of foliage-dwelling insect herbivores in the coastal tundra at 

 Barrow. 



In the Arctic, life cycle length is determined by both temperature 

 and length of the active season, that is, by growth rate and duration of 

 the growth period. Thus, were the season lengthened with no change in 

 mean temperature as, for instance, occurs in the subantarctic islands 

 (Rosswall and Heal 1975), Tipula carinifrons and Pedicia hannai might 

 achieve annual life cycles. 



Relative growth rate of both Tipulidae and Enchytraeidae declines 

 in larger individuals. This observation is not unique to arctic inverte- 

 brates. Consider a boreal and an arctic species characterized by the 

 growth rate functions g, and gi, respectively, with ^2 < g\ due to lower 

 temperatures in the arctic regions (Figure 11 -5a). Such a growth rate 

 function results in the pattern of growth shown in Figure 1 l-5b. Let W„ 

 be the weight at maturity. The slower growth rate (^2) of the arctic popu- 

 lation requires a prolonging of the development period to reach W„. This 

 increases the period of exposure to mortality, and may lead to a smaller 

 population, M (Figure 11 -5c), at maturity. Thus, many species that 

 might be able to grow and complete the life cycle in the Arctic are unable 

 to maintain a population due to the total mortality during the long devel- 

 opment period. This may be one factor contributing to the reduced diver- 

 sity of northern ecosystems. Reduction of development time and de- 

 creased generation mortality would help to explain the steep increase in 

 species diversity found along climatic gradients away from the immedi- 



