The Detritus-Based Trophic System 457 



abundant saprovores, and microbivory is the most important trophic 

 function of soil invertebrates. Invertebrate respiration accounts for 

 about 130 kJ m"^ yr'', which is 3.5% of the annual input of detritus. 

 Consumption of microorganisms is greater than average biomass and ac- 

 counts for 33 to 37% of estimated microbial production. Near the tundra 

 surface the entire annual production of microorganisms may be con- 

 sumed by invertebrate microbivores. Evidence suggests that grazing 

 upon detritus and microorganisms by soil invertebrates stimulates the de- 

 composition of organic matter and accelerates the turnover of energy 

 and cycling of mineral nutrients in the ecosystem. 



Soil invertebrate populations, especially Diptera, support an abun- 

 dant and diverse community of breeding birds; four sandpiper species, 

 the red phalarope, and the Lapland longspur are the most important of 

 these. Bird breeding is timed so that the young can feed on the adult Dip- 

 tera that emerge in early and mid-July. In June and August dipteran lar- 

 vae, especially those of craneflies, are the most important prey. Energy 

 requirements are determined by body size and duration of residence on 

 the tundra. When breeding density is also considered, longspurs are the 

 most important consumers of tundra arthropods. Birds may consume 

 35% of the annual production of the cranefly Tipula carinifrons, and 

 50% of the peak emergence of adult craneflies. This level of predation 

 must influence the evolution of life cycles and contemporary population 

 dynamics of tundra Diptera. 



