ANALYSIS OF POPULATION DEVELOPMENT IN DAPHNIA AT DIFFERENT TEMPERATURES

Abstract

The development of 16 populations of D. magna was followed at 18[degree] and of 21 populations at 25[degree] C. The 50 ml. of pond water which served as medium were renewed every other day and always contained an excess quantity of the food-alga, Chlorella. Population development at 25[degree] was oscillatory, 4 peaks occurring in 234 days, with a max. population size of 126 animals. In the 174 days of observation at 18[degree], one major peak was observed (max. 241) followed by a decrease and virtual stabilization at a population density of about 135. The oscillation is due to a delay in the expression of the effects of population density upon birth and death rates. The mechanism of oscillation at 25[degree] is an alternation of fluctuations in numbers of births and numbers of deaths. The mechanism at 18[degree] is the fluctuation in the number of births about a nearly constant number of deaths. Expts. with a series of population densities artificially maintained constant showed that birth rate at 25[degree] is an inverse function of population density. At 18[degree] the effect of density is similar but less severe. Under these conditions of constant density, mortality at 25[degree] is, in general, a function of population density, although the minimal mortality occurs at a density of 5. At 18[degree] mortality is but little affected by conditions of density, and is apparently least at about 75 animals/50 ml. The mean population size at 18[degree] was 21/2 times as great as that at 25[degree]. This fact is compared to the supposed greater density of planktonic populations in polar than in tropical waters. The results of this study cannot be applied to the problem of marine plankton abundance since the limiting factor in the present case (the conditioning of the medium by the accumulation of metabolites and/or depletion of the dissolved O2 supply) is presumably never operative in the ocean, although it may be operative in some fresh water situations. The greater mean size of the 18[degree] populations is not due to a direct effect of temp. upon longevity. A basic fallacy is pointed out in the theory which attempts to explain by such a direct effect of temp. the greater density of asymptotic populations in polar than in tropical regions. The influence of temp. upon mean population size observed in these expts. is indirect; the temp. difference exerts its effect only by modifying the action of population density.