436 S. F, MacLean, Jr. 



of the Collembola found in the coastal tundra at Barrow, the feeding ac- 

 tivity of this group is biased toward saprophagy. 



The litter and soil-dwelling prostigmatid and mesostigmatid mites 

 are mainly predatory (Wallwork 1967); many Prostigmata feed upon the 

 eggs and juvenile stages of Collembola. The Oribatei include species 

 which feed upon microbial tissue, dead plant litter, and combinations of 

 these (Luxton 1972, Behan and Hill 1978). 



The Diptera include a variety of trophic types. Larvae of Pedicia 

 hannai are predatory and have been observed preying upon Enchytrae- 

 idae in culture (MacLean 1973). Larvae of aquatic Tipulidae are com- 

 monly indiscriminate detritus feeders (e.g. Hall and Pritchard 1975). In 

 British moorland blanket bog larvae of Tipula subnodicornis feed upon 

 Hverworts, and thus are herbivores (Coulson and Whittaker 1978). Smir- 

 nov (1958, 1961) examined gut contents of invertebrates in a Sphagnum 

 bog and found large quantities of Sphagnum leaves only in Tipula lar- 

 vae. In the coastal tundra at Barrow, Prionocera gracilistyla is restricted 

 to mossy depressions and Tipula carinifrons is commonly found in dry 

 moss hummocks. Although many of the invertebrates found living in 

 moss do not actually consume living moss (Smirnov 1958, 1961), more 

 than the estimated 25% of the energy consumed by these craneflies may 

 come from living plants. 



The majority of the remaining Diptera larvae are probably sapro- 

 phagous (Raw 1967, Healey and Russell-Smith 1970), although microbial 

 tissue is undoubtedly digested as it is consumed along with plant Utter 

 and humus. 



ENERGY STRUCTURE OF THE DETRITUS-BASED 

 TROPHIC SYSTEM 



Estimates of trophic function can be applied to biomass and ener- 

 getic estimates to infer the trophic structure of the invertebrate fauna of 

 this coastal tundra ecosystem. The result (Table 11-8, Figure 11-8) is 

 strongly determined by the division of trophic function assigned to En- 

 chytraeidae, which makes the largest contribution to three of the five tro- 

 phic categories: saprovores, bacterial and algal feeders, and fungivores. 



Canopy-dwelling herbivores are virtually absent from the fauna, the 

 only exception being sawfly (Tenthredinidae) larvae, which feed upon 

 the prostrate willows that occur in drier habitats. Even with a fraction of 

 the biomass and activity of the large and abundant cranefly larvae as- 

 signed to herbivory, invertebrate herbivores are of minor importance in 

 this ecosystem. Consumption of 15.1 kJ m"^ yr"' (Figure 11-8) amounts 

 to less than 1 gdw m"^ yr"', a negUgible part of annual net primary 

 production. 



