The Detritus-Based Trophic System 455 



11-12), the average value for longspurs is almost twice that of the next 

 species, the dunlin, and accounts for 35% of the energy removed from 

 the tundra by the complex of avian insectivores. 



The total food requirement for these birds (4.35 kJ m'^ yr~') amounts 

 to only 3% of the productivity of invertebrates in the detritus-based food 

 chain, which suggests a modest intensity of predation; however, a large 

 proportion of invertebrate productivity is accounted for by taxa, particu- 

 larly Enchytraeidae, that are not eaten by the birds. It appears that about 

 40% of the total consumption by avian insectivores is supported by the 

 cranefly species Tipula carinifrons, and about 60% by the three cranefly 

 species. Thus, about 35% of the annual production of T. carinifrons, 

 and 23% of total cranefly production, is taken by avian predators. 



The annual emergence of adult craneflies amounts to about 35 mg 

 m'^ in T. carinifrons and 20 mg m"^ in P. hannai, and is largely confined 

 to a three-week period. MacLean and Pitelka (1971) recorded the median 

 67% of the total captures within a period of 5.3 to 11. 6 days for T. car- 

 inifrons, and within 3.8 to 6.3 days for P. hannai. Thus, during this peak 

 period emergence of each species is on the order of 3 mg m"^ day"'. 



During this period 66% of the diet of adult dunlin and 79% of the 

 diet of juvenile dunlin consists of adult tipulids (Holmes 1966). In order 

 to satisfy the gross energy requirement of two adults plus three juvenile 

 dunlin (3.95 eggs clutch "' x 75% hatching success) at these proportions, 

 52 g of adult craneflies would be required each day, the equivalent of the 

 total emergence of adult Tipula carinifrons and Pedicia hannai from 

 9600 m^ At 0.11 dunlin nest ha'', even at the peak of emergence dunlin 

 alone consume about 11% of the daily emergence of craneflies. Each 

 longspur family (two adults plus an average of 3.25 chicks hatched per 

 nest) requires about 55 grams of prey each day. In early July, adult 

 craneflies compose about 70% of the diet (Custer and Pitelka 1978). This 

 removes another 11.6 g ha'' day' or 19% of the peak emergence. The 

 addition of other avian insectivore species may raise the daily intake 

 above 40% of the peak emergence of adult craneflies. It is easy to appre- 

 ciate the impact that these predators must have, particularly upon adult 

 insects that emerge into less than peak populations, when loss to preda- 

 tion must approach 100%. 



Earlier, I estimated the mortality of cranefly larvae prior to pupa- 

 tion at 134 mg m"^ for T. carinifrons and 120 mg m'^ for P. hannai. Over 

 the season, 76% of the diet of adult dunlin consists of larvae of T. carini- 

 frons. This results in a consumption of 202 g of T. carinifrons larvae 

 ha'', or 15% of estimated larval mortality. Addition of other avian spe- 

 cies might double this value. Predation by birds is heavily concentrated 

 on fourth instar larvae; mortality at earlier stages of development must 

 be due to other causes. 



Varying amounts of time are available to the birds for foraging, de- 



