ANALYSIS OF POPULATION DEVELOPMENT IN DAPHNIA AT DIFFERENT TEMPERATURES

¹ From the Biological Laboratories, Harvard University, Cambridge

1. The development of populations of Daphnia magna was followed at two different constant temperatures. Sixteen populations were maintained at 18° and 21 at 25° C. The 50 cc. of pond water which served as medium were renewed every other day and always contained an excess quantity of the food-alga Chlorella.

2. Population development at 25° proved oscillatory in nature, four peaks occurring in 234 days, with a maximum population size of 126 animals. In the 174 days of observation at 18° , one major peak was observed (maximum 241) followed by a decrease and virtual stabilization at a population density of about 135.

3. Analysis of the oscillation disclosed that it is due to a delay in the expression of the effects of population density upon birth and death rates.

4. The mechanism of oscillation at 25° is an alternation of fluctuations in numbers of births and numbers of deaths. The mechanism at 18° is the fluctuation in the number of births about a nearly constant number of deaths.

5. Experiments with a series of population densities artificially maintained constant showed that birth rate at 25° is an inverse function of population density. At 18° the effect of density is similar but less severe.

6. Under these conditions of constant density, mortality at 25° is in general a function of population density, although the minimal mortality occurs at a density of 5. At 18° mortality is but little affected by conditions of density, and is apparently least at about 75 animals/50 cc.

7. The mean of population size at 18° was two and one half times as great as that at 25°.

8. 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 oxygen supply) is presumably never operative in the ocean, although it may be operative in some fresh water situations.

9. The possibility of accounting for the greater mean size of the 18° populations by reference to the direct effect of temperature upon longevity is considered but rejected. A basic fallacy is pointed out in the theory which attempts to explain by such a direct effect of temperature the greater density of asymptotic populations in polar than in tropical regions.

10. It is concluded that the influence of temperature upon mean population size observed in these experiments is indirect: the temperature difference exerts its effect only by modifying the action of population density.

This article has been cited by other articles:

				P. J. Wangersky and W. J. Cunningham Time Lag in Population Models Cold Spring Harb Symp Quant Biol, January 1, 1957; 22(0): 329 - 338. [Abstract] [PDF]