Mortality in a Population of Daphnia Rosea
1972
Dodson, Stanley I.
A Daphnia rosea population was observed over a 2—year period in order to compare mortality with natural predation pressure. A technique is presented for estimating the instantaneous death rate of Daphnia from population dynamics data as derived from quantitative, serial, plankton samples. The effect of predation by a specific predator (Chaoborus larvae) on daphniid mortality is shown by the comparison between (1) the number of daphniid deaths and number of predator—prey encounters (per unit time and volume), (2) the probability of daphniid deaths and the relative probability of a Daphnia encountering a predator, and (3) the Chaoborus feeding rate implied by the daphniid mortality and independently observed Chaoborus feeding rates. The Daphnia population had six generations per year. For the last four generations, Chaoborus predation accounted for about 93% of the daphniid mortality. Daphnia mortality due to salamander larvae (Ambystoma tigrinum) averaged 3.5%, never more than 10%, that of Chaoborus larvae. These results support the hypothesis that Chaoborus larvae are significant predators of the D. rosea population, and that the rate of the Daphnia population's growth is significantly influenced by predation pressure.
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