The Detritus-Based Trophic System 443 



influence microbial decomposition, and that the magnitude of this effect 

 is poorly represented by measures of direct energetic involvement (Chew 

 1974, Crossley 1977, Kitchell et al. 1979). Coleman et al. (1977) estab- 

 lished soil microcosms containing bacteria as decomposers with and 

 without amoebae and nematodes as grazers of the bacteria. Release of 

 CO2 and mineralization of N and P occurred more rapidly in the micro- 

 cosms that included the grazers. 



Parkinson et al. (1977) showed that selective grazing by Collembola 

 influenced the growth and colonizing ability of competing fungal species, 

 and that this effect was as marked at low experimental densities of Col- 

 lembola as it was at higher densities. Addison and Parkinson (1978) 

 found that addition of Collembola stimulated the release of carbon diox- 

 ide from tundra cores that had been sterilized, then inoculated with fresh 

 litter and microorganisms. Addition of the saprovore species Folsomia 

 regularis had a greater stimulatory effect than addition of the fungivore 

 Hypogastrura tullbergi or of a mixture of the two Collembola species. In 

 this regard, it may be significant that Folsomia species dominate the Col- 

 lembola fauna of the coastal tundra ecosystem at Barrow. 



Standen (1978) used litter bags to study the effect of soil fauna on 

 decomposition of litter from a British peat moorland. Bags containing 

 litter with either enchytraeid worms or tipulid larvae lost weight faster 

 and showed a higher rate of oxygen uptake than did bags containing only 

 the litter without animals. At Barrow, Douce (1976) compared the rate of 

 weight loss of litter on control and chemically defaunated plots in 

 polygon rim, basin, and trough habitats. Weight loss from litter was 

 reduced on the defaunated plots. Similar results from a variety of other 

 ecosystems were reviewed by Chew (1974) and Crossley (1977). 



These data indicate that invertebrate activity stimulates the decom- 

 position of organic matter. This is consistent with the correlation be- 

 tween invertebrate biomass and organic matter turnover rate. Because of 

 the limited vertical distribution of invertebrates, any effect of their activ- 

 ity occurs only near the surface. The rate of microbial activity is also 

 highest at the surface and drops off with depth because of the combined 

 effects of lower temperature, poorer aeration, and reduced substrate 

 quality. This produces an important interaction. A high rate of decom- 

 position near the surface (to which invertebrates contribute) limits the 

 proportion of the annual organic increment that reaches the lower 

 depths, where its decomposition rate would be reduced. Anything that 

 inhibits surface activity, such as reduced invertebrate populations, may 

 allow material to reach the deeper layers, and thus will contribute to ac- 

 cumulation of organic matter and reduction of rates of nutrient cycling. 

 The interaction of invertebrates and microorganisms in the near-surface 

 layers may have an importance for overall ecosystem function that is be- 

 yond the proportion suggested by the amount of energy that is actually 

 respired by animals. 



