An Experimental Approach to the Dynamics of a Natural Population of Daphnia Galeata Mendotae

Abstract

In general, populations have been studied either in the laboratory under experimental conditions with environmental variables controlled, or in their natural habitat where variables are uncontrolled. In laboratory investigations the analyses are often powerful and yield knowledge of the fundamentals of population growth but are limited to specific conditions seldom found in nature. In field studies, analyses are limited to correlations of population phenomena with environmental variables and frequently involve large errors in estimates of the inferred rates. The apparent paradox of many field and laboratory population studies can be reconciled in part by manipulating laboratory populations in such a manner that information appropriate to an analysis of the natural population is obtained. To manipulate experimental populations properly, some a priori knowledge of influential variables in the natural population is necessary. This study is directed to combining an experimental approach with a field description. The purpose of such an analysis is to predict natural population growth of Daphnia, and, consequently, to focus attention on the factors which control it. Because of the difficulties encountered in determining population rate processes and the role of underlying environmental variables, predictive models of population growth have been limited to controlled laboratory populations or unusual natural situations with relatively constant environmental conditions. Predictive schemes applied to natural populations are more likely to succeed the more information they utilize; but such models become hopelessly complicated, requiring vast amounts of empirical information. It is not yet clear whether simple models, requiring relatively little information, can adequately predict population growth in natural situations. However, comparison of a model with the observed population growth focuses attention upon the kinds and amounts of information absolutely essential for prediction. Inappropriate models may prove valuable by emphasizing the effect of disregarding important variables. Few investigations of this sort have been attempted on plankton populations. Elster (1954) in studying the population dynamics of the copepod, Eudiaptomus gracilis, utilized the ratio of eggs-to-females to obtain a reproductive index for the population. By determining the developmental rate of eggs at different temperatures he was able to estimate the increase of the population. Edmondson (1960) applied the experimental-field approach to several rotifer populations, placing great emphasis on the eggs-to-female ratio as a useful tool for determining the birth rates of populations. A model based on birth alone was then used to predict population growth and size. Although not concerned with zooplankton populations, 2 other studies are pertinent because the approaches are similar. Reynoldson (1961) made a quantitative population study of the triclad, Dugesia lugubris. Laboratory experiments allowed Reynoldson to assess the effects of temperature and food on the reproduction of Dugesia, and to conclude that population growth was often foodlimited. Subsequent field experiments strengthened his conclusion. Morris (1959) in a study of 2 spruce-defoliating insects constructed a predictive model based on a single key factor. The incidence of parasitism in a larval stage of a given generation of defoliators could be used to predict