The Detritus-Based Trophic System 421 



brate populations are found below 5 cm in these than in other microtopo- 

 graphic units. Soils of the meadows, troughs, and basins of low-centered 

 polygons may become strongly anaerobic in early summer when satu- 

 rated by snow meltwater, but less so, or even oxidizing, as the surface 

 soil dries out in mid-season. The soil may again become anaerobic fol- 

 lowing late summer rains. Seasonal changes in depth distribution of En- 

 chytraeidae follow the same pattern (Figure 11-2), suggesting that Enchy- 

 traeidae may use the resources occurring at depth only when the aeration 

 of the deeper levels allows. Springett et al. (1970) showed that surface 

 drying of peat soils in a British moorland resulted in downward move- 

 ment of the enchytraeids Cernosvitoviella briganta and Cognettia sphag- 

 netorum. In the present case, the fact that seasonal changes occur in even 

 the wettest areas suggests that access to the deeper strata, determined by 

 temperature and aeration, is responsible for population movements 

 rather than exclusion from the surface layers by desiccation. 



The basins of low-centered polygons are characterized by very low 

 densities and high surface concentration of invertebrates. This could be a 

 result of anaerobiosis developing when the basins are flooded in spring 

 by meltwater confined within the surrounding rims. Over 9Q^o of the in- 

 vertebrate biomass in the polygon basin consisted of Enchytraeidae, 

 which are known to be more tolerant of anaerobiosis than other inverte- 

 brates, as indicated by the very high densities achieved in sewage beds. 



Little is known of the feeding habits of soil invertebrates, making it 

 very difficult to discern the relationship of the animals to their food sup- 

 ply. Progress will probably require detailed examination on a spatial 

 scale much smaller than is reported here, and careful experimental and 

 manipulative study. We can, however, attempt to relate the abundance 

 of soil fauna in major habitat units to estimates of microbial biomass 

 and productivity. 



From direct counts (Table 8-1), it appears that abundance of bacter- 

 ia changes in the order rims » meadows > basins > troughs. Free-living 

 nematodes are largely bacterial feeders, and their abundance (Table 11-1) 

 shows a similar pattern, rims » meadows = troughs » basins, with the 

 exception of the very low abundance in the basins. 



Fungal biomass is greatest in dry habitats. The abundance of soil 

 Acari corresponds; however, the Enchytraeidae, which are probably the 

 major soil fungivores, are most abundant in wet habitats. Fungal bio- 

 mass is higher in the basins than in the troughs, but low in both; how- 

 ever, fungal productivity, estimated as the sum of positive biomass incre- 

 ments between sampling occasions (Chapter 8), is highest in the troughs. 

 This could account for the abundance of soil invertebrates found in 

 troughs. Soil algae may also help support the abundance of invertebrates 

 near the surface of the wet meadows and troughs. In all microtopograph- 

 ic units, abundance of soil invertebrates drops off much more rapidly 



