Zooplankton 285 

 Predation 



Food may be limiting fecundity, and predation seems further to 

 restrict maximum population density. Predation can also restrict annual 

 productivity by removing the young before they reach full size. The 

 predator Heterocope seems quite effective in removing the young-of-the- 

 year generation. Yield and its efficiency are known to be related to age of 

 the individual at harvest (Slobodkin 1959, 1960). Mortality at an early 

 age, especially in the young-of-the-year Daphnia, may reduce the yield. 

 Most of the young are removed, presumably by Heterocope, within the 

 first 48 hours, long before the individuals have reached adult size (see 

 previous section). The argument that predation reduces annual 

 productivity can be supported since yields from populations at densities 

 larger than existed in the ponds were obtained in the pondlets. 



Other evidence indicates that Heterocope predation may have 

 reduced Daphnia production. In a correlation of the densities of the two 

 populations from five ponds for 3 years, the production of the two were 

 inversely proportional (Figure 6-16). The inverse relationship was easily 

 detected when the annual yields for the ponds on the IBP site were 

 compared. A separate regression for each of the 3 years was necessary 

 because the annual yields are on a unit-volume basis and were uncorrected 

 for evaporation. A drought in 1972 was an especially effective 

 concentrator of the pond volume, hence the yield was apparently larger. 

 Slopes ranged from —6.4 to —1 1.2. To achieve such a consistent range, 

 three data points in 1973 were omitted as indicated (Figure 6-16). The data 

 were less comprehensible for the year (1973) when fewest samples per data 

 point were available. The regression slopes could express the 6 or 1 1 units 

 of Daphnia necessary to produce one unit oi Heterocope. The relationship 

 is likely to be coincidental, however. 



Dodson (1975, see Figure 6-7) has shown that other predator-prey 

 relationships exist in the polygon pools at Barrow. All populations could 

 suffer a restricted production from predation, while conceivably having a 

 fecundity limited by food concentration. The point is both significant and 

 vital to the coexistence of controls by predator selection and by size 

 efficiency. 



Food Production and Community 



Trophic level interactions are likely to modify quantity and quality of 

 food available to the zooplankton in tundra ponds. The Daphnia 

 population in pondlets was more fecund than was its counterpart in the 

 original pond, but only at an intermediate density. At least two 

 explanations are possible. One is that nutritive quality is diluted by the 

 detritus in suspension. When the detritus is heavily cropped, it is re- 



