The Detritus-Based Trophic System 431 



So W 



"m ' '■ 



Log Wg 

 of 

 Weight 



Time, years 



FIGURE 11-6. Factors influencing the evolution of multi- 

 annual life cycles of Diptera. S = summer, W = winter. 



traeidae is a much less radical change. The form of the animal remains 

 much as before, and growth continues. In fact, most of the biomass is 

 added after the onset of sexual maturity. 



In Diptera, successful completion of the life cycle is influenced by 

 constraints upon adult biology. The highly synchronous emergence of 

 adult Diptera suggests that these constraints are rigorously imposed. On- 

 ly flies emerging within a narrow time span successfully complete the life 

 cycle. The short period each summer during which successful emergence 

 may occur is indicated by the areas 5,, Si, Sy (Figure 11-6). In a particular 

 climate larvae growing according to the growth function g, reach W^ and 

 complete the life cycle in two years. In a somewhat more severe climate 

 growth is slowed to that described by growth function ^2. In this case W„ 

 is reached at 6 which falls between S2 and Si, however emergence at time 

 ti is disadvantageous. Some individuals in the population might retain 

 the two-year life cycle and pupate in 52, but at a smaller size {W2) than in 

 the less severe climate. Since fecundity in insects is related to body size, 

 this carries with it a cost of reduced fecundity. Other individuals in the 

 population may extend the life cycle and pupate in S3 at a size equal to or 

 even larger than IV,„. In this case full fecundity is maintained, but at the 

 cost of increased period of exposure to mortality. The strategy that maxi- 

 mizes expected reproduction (probability of survival x fecundity) should 

 prevail in the population. 



Body size and fecundity can change continuously with climatic se- 

 verity and growth rate. Life cycle length, however, changes discontinu- 

 ously, a year at a time. Thus it is likely that small changes in climatic se- 

 verity will result in changes in body size and fecundity (Figure 11-7). This 

 would explain the well-known decline in insect body size along elevational 



