282 R. G. Stress et al. 



female may be related to her initial size. At 6,000 cells ml ' growth 

 efficiency increases from 10 to 16% when the initial (feeding power) length 

 is increased from 1.0 to 1.4 mm. The minimum length for maximum 

 efficiency decreases as food concentration increases (Figure 6-15). 



The buffering effect of a differential growth efficiency becomes 

 evident. When density approaches saturation, the population will have 

 only some of its individuals within the critical range of initial length. These 

 individuals will reproduce, while the major portion of the population can 

 gather only sufficient food for the maintenance requirement. In effect, at 

 high density the bulk of the population gives a part of its ration to the few 

 large individuals. These in turn will, with increased growth efficiency, 

 produce gametes. Thus in Daphnia, as in species with more elaborate 

 behaviors, a mechanism exists for stabilizing birth rates as the population 

 saturates its food resource. Viewed from outside the population, the large 

 Daphnia have a disproportionate effect on food resources for, say, a 

 potential competitor such as the fairyshrimps. Internally, however, the 

 population may be said to distribute its food resources unequally into a 

 size hierarchy that functions to "regulate" birth rate. The large variability 

 noted in the size of Daphnia (see previous section) and aphids (Taylor 

 1975) could represent a selective adjustment, one function of which is to 

 stabilize birth rates at high densities. 



Predation that is selective for the larger sizes may prevent such a 

 system from functioning, however. The absence of vertebrates in tundra 

 pools may be a factor in the observed size patterns. If vertebrate predation 

 results in an increased food density for herbivores, then the loss of the 

 more efficient larger sizes could be balanced by an increase in growth 

 efficiency of the population remaining, as is illustrated in Figure 6-15. 



Observed reproductive efforts of Daphnia and fairyshrimps in the 

 ponds at Barrow were less than expected and infer food-limited 

 populations. To determine how density of the population influences food 

 availability, populations were isolated in small ponds. These pondlets were 

 carved in the tundra turf, lined with polyethylene sheeting, and filled with 

 10 liters of water from Pond C. The experiment was begun when the 

 Daphnia {D. middendorffiana) were 1 or 2 days old, and continued until 

 they were gravid with the first and only brood of young. 



Isolated populations of Daphnia are much more productive than their 

 pond counterparts. Mean brood sizes are near the maximum for the first 

 brood. At 1 , 5 and 10 individuals liter " ' initially, mean brood sizes ranged 

 from 10.4 at 5 to 5.6 embryos at 10 individuals liter ' (Table 6-10). Brood 

 size was largest at the intermediate density, although the maternal length 

 was similar at both 1 and 5 individuals. Attention is drawn to a large 

 mortality in both pondlets at the intermediate density. 



The average number of embryos per brood was 2.5 times larger than 

 those o(\Daphnia in Pond C although the mean maternal length was the 

 same (2.9 mm). Brood size in the isolated population was reduced to that 



