Population Dynamics of Daphnia Pulex as Modified by Chronic Radiation Stress

Twenty—five self—regulating populations of Daphnia pulex were exposed to different levels of external 60 Co gamma radiation for 18.5 hr per day for 55 weeks. Dose rates ranged from 0 to 516 R/day. Although the population at the 3 highest dose rates became extinct, the remaining 22 populations established quasi—equilibrium densities which decreased with increasing dose rate. The maximum tolerable dose rate was 436 R/day. Average turnover rates increased with dose rate. An approximate inverse relationship between mean population size and mean turnover rate suggested that net production might be unaffected by the radiation. Estimates of yield to decomposers indicated that net production was, indeed, unaffected. Radiation stress became a limiting factor for population size and turnover rate at much lower dose rates than those needed to limit net production. Within the entire range of indefinitely tolerable dose rates, the population consequences of radiation stress stem almost entirely from effects on individual fertility and survival, whereas individual metabolism is apparently unaffected. Increased individual growth is an indirect effect, due to reduced fecundity at a given food supply per individual or increased food consumption at a given food supply per population.