456 S. F. MacLean, Jr. 



pending upon the time demands of other activities, notably incubation. 

 Female pectoral sandpipers incubate their eggs 85% of the time (Norton 

 1972), leaving no more than 15% of the day for foraging. They must ob- 

 tain food at a rate of about 100 mg dry wt min"' during foraging bouts. 

 This might be satisfied by taking about 100 chironomid larvae, 20 P. 

 hannai larvae, or 4 larvae of T. carinifrons per minute (Figure 11-10). 

 The advantage of feeding on the large cranefly larvae is clear. Female 

 dunlin have a similar energy demand, but by sharing incubation with the 

 males they have much more time available for foraging, and their requi- 

 site rate of prey capture is about one-fourth that of the female pectoral 

 sandpiper during incubation. 



Shortly after arrival female longspurs forage about 80% of the day 

 (Custer 1974), and must find food at a rate of about 10 mg dry wt min"'. 

 During this period longspurs feed on seeds, Collembola, and small chiro- 

 nomid larvae, items too small to be used by other species with higher 

 energy requirements, or by longspurs later in the season when time avail- 

 able for foraging is reduced. 



Thus, as is so often the case in studying animal populations, the im- 

 pact of avian predators stated in relation to the total energy budget of the 

 ecosystem appears small (Figure 11-8); however, because of the concen- 

 tration of predation upon Diptera and particularly upon the craneflies, 

 avian predation may have a large influence as a force in the evolution of 

 life cycles and in the reproduction and population dynamics of Diptera 

 populations. 



SUMMARY 



The coastal tundra ecosystem supports abundant populations of En- 

 chytraeidae, Collembola, and Diptera, modest populations of Acari, and 

 small populations of Nematoda. Soil invertebrates are concentrated near 

 the surface of the tundra, where the number of individuals per cubic cen- 

 timeter can be quite high. Large differences in abundance are found 

 among the various microtopographic units that compose the coastal tun- 

 dra. These differences are related to soil moisture, aeration, and annual 

 input of detritus. Both invertebrate abundance and biomass and plant 

 production are inversely correlated with accumulated soil organic 

 matter. 



Tundra soil invertebrates have long life cycles, often extending over 

 several seasons. Craneflies require four years to complete larval develop- 

 ment and their ratio of annual production to average biomass is conse- 

 quently small. 



The energetics of the detritus-based trophic system in the coastal 

 ecosystem is dominated by Enchytraeidae. The fauna is lacking in large, 



I 



