A Population Study of Irradiated Desert Rodents

Abstract

An experiment was begun in 1963 to evaluate effects of long—term low—level radiation exposure on desert populations of plants and animals. Monthly censuses of four species of heteromyid rodents in three 9—ha enclosures and one non—enclosed area indicate great year—to—year changes in densities of the nonirradiated populations and even greater fluctuations in the irradiated populations. Although magnitude of density changes varied, timing of fluctuations was similar. In a good growing season a population may increase by a factor of 5 or more. In a poor growing season there may be no reproduction. Chronic radiation exposure to Perognathus formosus was measured to be 211—360 r/year. This reduced survival, particularly before the age of 6 mo. The instantaneous rate of death for the irratiated population during this period was 0.219, and for the control populations was .075 and 0.104. Future life expectancy at age 1 mo in the irradiated population was 9.2 mo, and for the control populations was 11.4 and 14.4 mo. The computed intrinsic rate of increase for the irradiated population was 0.314, compared to 0.493 and 0.498 for the two control populations. Demographic and histologic evidence indicate an increased birth rate in the irradiated population. A population consisting of animals 3 yr old and older, having been subjected to a lifetime of such radiation exposure, would likely be incapable of reproduction. Therefore, only in the most unfavorable periods, when reproduction is curtailed by natural factors, will the threshold levels of artificial radiation insult on the natural population be discernible. A rapid turnover of generations prevents a population from accumulating damaging effects of low—level exposure. The diets of rodents were analyzed, and the production of important foods was measured over 3 yr. Seeds and vegetative parts of plants made up approximately equal portions of the diet. All parts of the small annual plant Thelypodium were highly selected at all seasons. Total production of food shows a lag effect in relation to precipitation. Estimates of food requirements of the rodent population, based on laboratory feeding trials, suggest that food supply may be an important limiting factor. Populations respond strikingly to periods of favorable vegetative growth. Desert rodent populations are adapted to withstanding periods of unfavorable conditions. They have comparatively long life—spans and maintain fertility. These adaptations make them vulnerable to changed conditions, such as environmental contamination.