420 S. F. MacLean, Jr. 



expresses conditions encountered by an animal living in a three- 

 dimensional environment. For example, an average cubic centimeter 

 within the top 2.5 cm of the polygon trough contained 4.8 nematodes, 

 2.2 enchytraeid worms, 0.85 mite, and 5.8 Collembola. Although the 

 densities of soil invertebrates in some other ecosystems may equal or ex- 

 ceed those of the coastal tundra at Barrow, it is unlikely that many have 

 consistently greater concentrations of animals. 



The abundance and vertical distribution of soil invertebrates can be 

 compared with the vertical profiles of soil temperature and moisture, 

 plant biomass and production, microbial biomass and production, and 

 dead organic matter. The seasonal course of soil temperature in several 

 microtopographic units was presented in Chapter 2. Temperatures signif- 

 icantly above air temperatures occur in the top few centimeters of the 

 soil, but soil temperature declines rapidly with depth, and below 10 cm 

 rises only slightly above 0°C even at mid-season. Low temperatures 

 below 5 cm may contribute to the surface concentration of many soil in- 

 vertebrate species. Conversely, winter season temperatures are lowest 

 near the tundra surface. In temperate regions many soil invertebrates 

 descend into deeper layers of the soil to avoid the frozen soil and cold. In 

 tundra, this movement is prevented by permafrost. Soil animals could 

 gain some protection by descending to lower depths, but to do so would 

 result in a later onset of activity in the following spring. It appears that 

 the advantage of the longer and warmer near-surface growing season 

 outweighs the increased risk of mortality from winter cold, and soil ani- 

 mals remain near the surface during the winter. In fact, Enchytraeidae 

 must move toward the surface late in the summer season (Figure 11-2). 



Abundance of Nematoda, Enchytraeidae, and Acari all relate signif- 

 icantly to the moisture ranking of the microtopographic units, suggesting 

 that moisture or some correlated factor is important in determining their 

 distribution. The Enchytraeidae and Nematoda are basically aquatic or- 

 ganisms living in the soil interstitial water. The Enchytraeidae are rela- 

 tively large, thus requiring larger water-filled pores and cavities in the 

 soil. Increased soil moisture apparently increases the amount of habitat 

 available to them, thus increasing population abundance. The Nematoda 

 are much smaller, and can use the thin film of water that surrounds soil 

 particles even in relatively dry soils. Since their abundance is negatively 

 correlated with soil moisture some other factor must be involved. 



At high soil moisture levels the soil pore volume is filled with water, 

 and anaerobic conditions may develop. The commonly anaerobic condi- 

 tions occur in the 5- to 15-cm depth interval (Chapter 7). Shortage of 

 oxygen could well contribute to the observed depth distribution of Bar- 

 row soil invertebrates. The drier soils of high-centered polygons and rims 

 of low-centered polygons probably have sufficient oxygen throughout 

 the period of biological activity, and a larger proportion of soil inverte- 



