418 



S. F. MacLean, Jr. 



ties up to 15,000 ind m"^ in the wet meadows; very few were found in the 

 drier areas. In both cases these must be regarded, cautiously, as mini- 

 mum estimates since no special effort was made to census these animals. 



Microtopographic units differ markedly in the abundance and com- 

 position of their invertebrate faunas, even using the gross taxonomic 

 units considered here. Total invertebrate biomass differed by a factor of 

 about three between the most productive polygon troughs and the least 

 productive basins and rims of low-centered polygons and tops of high- 

 centered polygons, although these units are separated by only a few 

 meters distance and about 20 cm of relief. We must be very cautious in 

 referring to a "mean" or "average" unit and its soil fauna in the very 

 heterogeneous coastal tundra at Barrow. Rather, the tundra is a repeat- 

 ing mosaic of polygon troughs, rims, tops and meadows that are quite 

 distinct habitats for soil invertebrates and microflora (Chapter 8). 



Even within microtopographic units, populations of all major soil 

 arthropod groups were significantly clumped or aggregated in their pat- 

 terns of dispersion. There was a significant tendency for coincidence in 

 the aggregations of total mites and Collembola (MacLean et al. 1977), 

 plant-parasitic and free-Hving nematodes, and prostigmatid and oribatid 

 mites, indicating that these groups respond similarly to microhabitat 

 suitabiUty or richness. 



A very consistent feature of the tundra soil fauna is the concentra- 

 tion of animals in the near-surface horizons of the litter and soil (Figure 

 11-1). In both Acari and Collembola over 90% of the individuals 



Q. 



a> 

 Q 



Percent of Population 



"■ "I 



s- K.J 



-c 5 



J I L 



100 



lO-J 



Acarina 







100 



I 



Collembola 







100 



'3 



^ 10 



Q 



15-U 



100 



-u — I 1 



Free-living 

 Nematodes 



10 



15-'-' 



100 

 I J I I 



Plant- parasitic 

 Nematodes 



100 

 J I . I 1 



'0| Enchytraeidoe 



FIGURE 11-1. The annual mean depth distribution of representative 

 groups of soil invertebrates. 



% 



