The Detritus-Based Trophic System 439 



produced poor or no growth only at low temperature (10 °C). Plant de- 

 tritus, the food source of saprovores, contains little protein and consists 

 largely of carbohydrate in the form of cellulose and other long-chain 

 polysaccharides. 



The quality of the diet may also provide an explanation for the rela- 

 tively great abundance of prostigmatid compared with oribatid mites. 

 Luxton's (1972) review of data for Oribatei indicates that approximately 

 25% are wholly (macrophytophages) and 50<7o are partially (panphyto- 

 phages) dependent upon low-quality dead organic matter for food. In 

 contrast, Prostigmata feed on microorganisms and on other animals and 

 their eggs, a higher quality diet. 



Larvae of the cranefly Pedicia hannai are the dominant soil carni- 

 vores in the coastal tundra at Barrow, followed by the predatory mites. 

 Predatory beetles, Carabidae and Staphylinidae, were abundant only on 

 the drier areas. Spiders were not accurately sampled and are not included 

 in this analysis. The spider fauna is poorly developed, consisting almost 

 entirely of small web-spinners of the family Linyphiidae that probably 

 contribute minimally to energy flow. The total consumption by inverte- 

 brate carnivores of about 16.7 kJ m"^ yr'' amounts to about 12% of the 

 productivity and 27''7o of the average biomass of their prey, figures that 

 indicate a modest level of predation. 



The total consumption by invertebrates is about 700 kJ m"^ yr'. 

 Consumption by carnivores represents energy consumed at least twice by 

 animals. Subtracting this, the equivalent of about 35 g m"^ of input to the 

 ecosystem passes through invertebrate animals each year, 34 g of this in 

 the detritus-based trophic system. This is approximately 19% of the an- 

 nual input, assuming an input of 190 g to the detritus-based system. 



The fractions of consumption that appear as feces and as produc- 

 tion remain within the detritus-based system. Feces and unharvested pro- 

 duction are recycled through the dead organic matter pool (Heal and 

 MacLean 1975). Through respiration, animals in the detritus-based tro- 

 phic system are directly responsible for the loss of 128.5 kJ m~^ yr"', 

 which is the equivalent of less than 7 g of input. Assuming that annual 

 accumulation of organic matter is insignificant, 3.5% of the total annual 

 input of 190 g m'^ is dissipated by respiration of invertebrates, and the 

 remaining 96.5% by microbial respiration. Allowing for accumulation of 

 up to 15 g organic matter m"' yr"' changes these figures only slightly, to 

 3.8% for animal respiration and 96.2% for microbial respiration. Inver- 

 tebrate respiration varied between 65.3 kJ m"^ yr"' in the low polygon rim 

 and 242.3 kJ m"^ yr"' in the adjacent trough, with the difference due pri- 

 marily to Enchytraeidae; however, because of the relationship of inverte- 

 brate biomass to primary production (Figure 1 l-3a), it is unlikely that the 

 proportionate contribution of invertebrates to total community respira- 

 tion changed considerably between habitats. 



