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A long-term project was initiated in 1968 at oak ridge national laboratory to assess effects of mixed beta and gamma radiation from simulated fallout on a grassland ecosystem. Beta and gamma radiation dose rates in microhabitats of the experimentally contaminated enclosure were measured with lif thermoluminescent microdosimeters. Extensive statistical analyses of data on numbers of individuals collected for each 76 arthropod and 2 molluscan taxa have identified no lasting significant changes in similarity of species diversity of experimental versus control communities as the result of the long- term irradiation at low dose rates. Natural fluctuations in community dynamics obscured any possible radiation effects. Thus, the apparent threshold for mixed beta and gamma radiation inducing changes in community structure must be above the exposure rate range of 2.3 to 13 rad/day delivered during the 5 years observation. Establishing such a threshold is of importance in assessing the impact of communities subjected to chronic, low level environmental exposure to ionizing radiation.
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