442 S. F. MacLean, Jr. 



tive to plant tissue (Coulson and Whittaker 1978), invertebrate biomass 

 contains less than 1% of the nutrients of the system. 



The high nutrient concentration of invertebrate biomass may be sig- 

 nificant as a source of nutrients for microorganisms following death of 

 the animal. Such a nutrient source might be sufficient to stimulate mi- 

 crobial decomposition of surrounding energy-rich but nutrient-poor or- 

 ganic matter. That is, local concentrations in an otherwise nutrient-poor 

 environment may lead to a higher overall rate of decomposition. 



For a brief period each summer the coastal tundra at Barrow is 

 aswarm with adult insects, mainly Diptera, that have emerged from the 

 soil to complete the life cycle. The emergence of the craneflies P. hannai 

 and T. carinifrons, alone, represents 55 mg m"^ (Table 11-4); the total 

 emergence of Diptera is, perhaps, twice this amount. Thus, each summer 

 about 100 mg m ^ including 1 mg of phosphorus and 10 mg of nitrogen, 

 leaves the soil and becomes mobile over the tundra surface. This provides 

 a considerable potential for nutrient transport. 



It is likely that insect death and oviposition in most microtopo- 

 graphic units approximates emergence, resulting in no net movement. 

 However, where insects develop in areas that are relatively uncommon 

 and disperse in search of other similar areas, there is Hkely a net move- 

 ment away from the preferred unit. This movement is increased when 

 predators intercept dispersing individuals. Thus, it seems certain that 

 there is a net movement of nutrients from pond sediments to surrounding 

 tundra caused by the emergence of adult chironomids (midges), which 

 are heavily preyed upon by terrestrial birds. This process would tend to 

 reverse the movement of dissolved nutrients and detritus from the land 

 surface into ponds with spring snowmelt each year. The net movement in 

 any year is undoubtedly small, but accumulated over many years this me- 

 chanism may contribute to current patterns of nutrient distribution. 



The action of soil saprovores in reducing the average particle size of 

 litter, thereby increasing surface area available for attack by microorgan- 

 isms, is frequently cited as an important factor in the decomposition pro- 

 cess (van der Drift 1959, Crossley 1977, but see also Webb 1977). Given 

 the estimated rate of consumption of detritus by saprovores, and even 

 adding an additional amount for consumption of detritus by microbi- 

 vores, it appears that less than 10<^o of the annual input of detritus is con- 

 sumed by invertebrates each year. In contrast, where large earthworms 

 dominate the fauna, as on the mineral soils at Moor House, consumption 

 may approach the annual input of detritus (Satchell 1971). 



In the coastal tundra at Barrow, the most important interaction of 

 invertebrates in the decomposition process is their consumption of mi- 

 croorganisms and consequent effect upon the composition, biomass and 

 activity of the microbial community. Direct evidence is scanty, but recent 

 research suggests that the activity of soil invertebrates can significantly 



i 



