The first studies of the effects of ionizing radiation and radioactive particles dealt with individual organisms and were conducted under controlled laboratory conditions. Effects were generally measured as the LD50 or ED50, doses of radiation that respectively either kills or produces a specific effect in 50% of the subjects tested. When applied outside of the laboratory under natural environmental conditions however, these measures are replaced by the ecological LD50 or ED50, which are generally lower than their corresponding laboratory-based measures of radiation effects. Radiation-induced effects which might not kill an organism outright under protected laboratory conditions (e.g., the retarding of wing feather growth in a bird) could prove lethal under free-living environmental conditions (e.g., young birds with stunted wing feather growth might not be able to fly and escape predators).
As ecological studies and theory began to deal more extensively with higher and more complex levels of biological organization such as populations, communities, and ecosystems, radiation effects studies have also begun to deal with these levels above that of the single organism. Rather than simply exposing individual organisms to a single (acute) or protracted (chronic) dose of radiation, entire intact natural ecological systems began to be studied under conditions of controlled exposure to radiation stress. Gamma radiation sources were established in a variety of natural communities, including a tropical rain forest, desert, temperate mixed pine-oak forest, and old field successional communities. After periodically shielding the radiation source, investigators would sample and quantify the vegetative structure and animal inhabitants of these radiationexposed ecosystems. In these studies radiation is usually delivered at a low dose rate over protracted periods of time (often years or more) to more realistically simulate the conditions that would follow a nuclear accident or the detonation of a terrorist's so-called 'dirty bomb' spreading radioactive contaminants in the environment. Not surprisingly, these studies showed that the overall response to radiation stress of these intact ecological systems was usually more than simply the sum of all of the radiation responses of its individual component organisms or populations. Since different species of vegetation may differ greatly in their sensitivity to radiation exposure, some may disappear quickly after exposure to only low doses and be replaced by an increase in the abundance of more radiation-tolerant species. Thus while overall vegetative structure may change significantly with distance from the radiation source (and thus the degree of radiation stress), measures of overall system function such as net primary productivity may be less changed as the carbon fixation of one species is replaced by that of another which replaces it across the radiation stress-gradient.
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