Zooplankton 281 



The enhanced growth efficiency plays an important part in a 

 "regulatory mechanism" of birth rates (Stross and Chisholm 1975). A 

 population with individuals of differing rates of food intake can bring 

 about a disproportionate allocation of food resources. The inequality of 

 food distribution compounded by an increased growth efficiency allows 

 some members of the population to reproduce while others enjoy only a 

 marginal maintenance intake (Stross, et al. 1979). The important point 

 here is that reproductive effort reflects both food availability and the 

 efficiency with which the food is utilized. The concept of an efficient 

 predator with a non-linear efficiency (Slobodkin 1960, 1974) emphasizes 

 the significance of balance between consumer and the concentration of 

 food. The model SUNDAY dramatizes that relationship for Daphnia in 

 arctic ponds. 



Simulation of Daphnia is based on a carbon equivalent. Computed 

 growth efficiency which is the ratio of accumulated biomass to food 

 ingested shows the nearly 3-fold increase that results from the appropriate 

 ratio between consumer density and food concentration (Figure 6-15). For 

 example,at a food density of 3,000 cells ml ~ ^ the ratio of yield at the end of 

 the first pregnancy to assimilation since birth is about 3%. At 7,000 cells it 

 has increased to 17% in the model. Figure 6-15 depicts the relationship of 

 efficiency to initial length at various food concentrations. 



Another kind of delicate relationship in arctic Daphnia is indicated by 

 output from the simulation model. The growth efficiency of an adult 



1.5 2.0 25 3.0 3.5 



Maternal length in ttie 6th instar, mm 



4.0 



FIGURE 6-15. Simulated growth efficiency of Daphnia 

 populations as a function of length of the female in instar 

 6. Each point represents a population begun at one initial 

 length. Each line connects a series of six groups of ani- 

 mals which were all fed the same density of food particles 

 (given in thousands of cells ml'' so that 3K equals 3000 

 cells ml''). 



