The Effect of Removal of Fixed Percentages of the Newborn on Size and Variability in Populations of <i>Daphnia pulicaria</i> (Forbes)

Abstract

Populations of Daphnia pulicaria were maintained under laboratory conditions of constant food supply and temperature. Varying percentages of the newborn were removed from the populations at four‐day intervals. Mean size of the residual populations decreased with increasing removal rate, while yield and yield per individual increased. Mean differences between the number of animals present at successive censuses after removal of the newborn decreased with removal rate while mean differences between successive yields increased. The sum of mean differences in residual population and yield was a constant, independent of removal rate. That is, total variability in the population‐removal system was not changed by removal rate, although the variability was divided between yield and residual population. The ratio of yield variability to population variability was dependent on removal rate and on the fact that only newborn animals were removed, but was independent of food supply and temperature differences between populations. This ratio also may be independent of the physiological peculiarities of Daphnia. Median life expectancy of the survivors was increased 13–15 days when the removal rate was increased from 25% to 90% of the newborn. The median life expectancy with 90% removal was 32 to 36 days.