Evaluation of the Intrinsic Rate of Increase as an Endpoint for Ceriodaphnia Chronic Tests

Abstract

Fecundity and survival data can be used to calculate a number of demographic measures which characterize populations including net replacement rate, generation length, life xpectancy, and reproductive value. Another measure is r, lternately called the intrinsic or instantaneous rate of population increase. This statistic is a measure of the capacity of a population to increase in an unlimited environment and is in estimate of ecological growth potential. The equation of Lotka (1925) for calculating the intrinsic rate of increase is 1=∑x=0nlxmxe−rx Where n is the number of age classes, lx is the probability of survival from birth to Age Class x, and mx is the average number of births per individual in Age Class x. The intrinsic rate of increase, r, has been calculated for >50 chronic toxicity tests on effluents from publicly owned treatment works (POTWs), metal refinishers, and acid-mine drainage. The hypothesis was that the intrinsic rate of increase is a sensitive endpoint for estimating toxicity to populations exposed to toxicants in laboratory experiments. Results of analyses performed on seven-day Ceriodaphnia tests indicate that r is actually less sensitive than the other chronic endpoints examined for toxicity tests with limited observation days which serve as age classes. These results suggest that for the type of toxicity test examined (short-term chronic), the calculation of r does not add to the interpretation of toxic response that can be made from evaluating fecundity and survival data as separate measures. Although r is a useful chronic endpoint, it is not cost effective to calculate when simpler estimates of toxicity are available.