424 S. F. MacLean, Jr. 



unimportant in determining the distribution and abundance of tundra 

 soil invertebrates. Rather, it appears that physical factors, among which 

 temperature, moisture, and aeration are prominent, determine access to 

 the habitat and the food that it contains. 



Invertebrates reach their greatest abundance in the polygon troughs 

 and meadows, where they are part of a syndrome that involves relatively 

 little organic matter accumulation, rapid nutrient recycHng, and high 

 rates of primary production. Low invertebrate abundance is associated 

 with more organic matter accumulation and low rates of primary pro- 

 duction, suggesting a system that is constrained by a low rate of nutrient 

 release and recycling. 



This analysis does not show causation. A very large number of 

 cause-and-effect relationships are doubtless included in these broad pat- 

 terns. It would be of little value to debate whether an impoverished soil 

 fauna is cause or effect of low decomposition rate; surely feedback rela- 

 tionships make both points, in part, true. The same can be said for soil 

 nutrient concentration and decomposition rate. These data do suggest 

 that the activity of soil invertebrates is interwoven with the pattern of 

 production and decomposition that characterizes the coastal tundra 

 ecosystem. 



LIFE CYCLES OF TUNDRA SOIL INVERTEBRATES 



The abundance, seasonal dynamics, and energy requirements of in- 

 vertebrates derive, in large part, from the life cycle characteristics of the 

 species involved. Life cycle characteristics have been studied in the crane- 

 fly species Pedicia hannai (MacLean 1973) and Tipula carinifrons (Cle- 

 ment 1975), and in the three dominant enchytraeid species, Mesenchy- 

 traeus sp., Henlea perpusilla, and Cernosvitoviella atrata. Life cycles of 

 these groups differ in a fundamental way. The Diptera pass through four 

 larval instars in the soil and then undergo a complete metamorphosis in a 

 pupal stage to the adult form. The adults leave the soil to swarm over the 

 tundra surface, where reproduction is quickly accomplished. In Enchy- 

 traeidae development is gradual, and reproduction may occur over a con- 

 siderable part of the total life cycle. There is no marked change in habitat 

 or Hfe form associated with the onset of reproduction. 



Tipulidae 



The cranefly species require at least four years to complete larval de- 

 velopment. The early instars are completed relatively quickly, while the 

 final, fourth instar lasts about two years. As a result, the population at 



