SLTaE Bits 1sTROL LE Se [Vou, 91 as samples he bone ash inimals ecoltained Sr-90 ent to > 10 -90 in their e older age ? years, the ig the first © bably were tout from the Percent Soluble 10.9 6.2 13.0 66.7 50.3 71.8 tls of areas ntration of ed by these dated with not linear. ils may be jack rabbit $ in other : explained et al. 1960, ’ of Sr-90 illout eon- ‘availabilbeen dem‘les < 44y 1964] MARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION 313 in diameter (Larson et al. 1960), that plants are selective collectors of < d4y particles (Romney et al. 1962a)}, and that the solubility of fallout particles in 0.1 N HCI tends to inerease with decreasing particle size (Larson et al. 196?, Lindberg et al. 1954, 1959). During the first few weeks after fallout, the jaek rabbits living in a contaminated environment feed on vegetation containing relatively large amounts of Sr-90 which can be assimilated and deposited in bones. The period of maximum assimilation of Sr-90 by jack rabbits probably coincides with the period that fallout particles are retained by plant foliage. After the loss of externally contaminated foliage, the Sr-90 content of desert plants is probably quite low. External contamination by redistributed fallout debris is negligible, and the uptake of Sr-90 by desert shrubs from contaminated soil has never been demonstrated (Larson et al. 1962). Probably, the availability of Sr-90 for ingestion by jack rabbits is greatly reduced during the first year after fallout. If the average life-span of jack rabbits is 3.2 years, the population 3.7 years after fallout should be composed largely of individuals born more than 6 months after fallout. Most of the Sr-90 assimilated by these animals (assuming no further depositation of fallout on vegetation) is probably derived from soil materials accidentally ingested while feeding or while preening after dust baths, or from plants contaminated by world-wide fallout or other means. The maintenanceof relatively high Sr-90 levels in a jack rabbit population for a period of 2 to 3 years after fallout seems to indicate either a steady-state condition in which input (assimilation) is approximately equal to output (excretion) or else a condition in which both are negligible. In the case described above, a moderately high input value could have been maintained by stratospheric fallout from previous detonations of high yield. According to this rather speculative hypothesis, the Sr-90 level in a jack rabbit population sampled 4 to 5 years after a single contaminating event should be relatively elose to equilibrium. Also, the level at which this equilibrium is established should be related to the amount of biologically available Sr-90 in the soil. But, a non-linear relationship between the Sr-90 content of jack rabbit bones and the Sr-90 content of soil (total and acid soluble) might indicate that the rate of stratospheric fallout is more important in determining the rate of Sr-90 assimilation by jack rabbits than either the total or the potentially available Sr-90 in or on the soil. EcoLoGicaL MODELS, 1. Discrimination Factors, Strontium-90 and caleium are chemically similar and may follow the same routes of transfer in an ecosystem. According to Langham and Anderson (1958, 1959), there is little or no discrimination between Sr-90 and Ca in the transfer from soil to plants. Se, the Sr90/Ca ratio in plants divided by the Sr90/Ca ratio in