Study of a new method of estimating extinction risk for wild populations.

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To evaluate the environmental risk of toxic chemicals, risk benefit analysis has been introduced. In these, human-health risk is counted but not the risk to ecosystem (natural biological environment). Here we develop a new method to evaluate the ecological risk based on the harmful effect to wild populations, by shortening the mean extinction time. We first develop a method to estimate the mean extinction time of a density-dependent population, and how this will shorten when it is exposed to toxic chemicals. Then we apply this method to a particular population of Herring gull at New York State in 1950's, when DDT concentration in water was very high. We estimated that the extinction risk caused by the DDT exposure at this level is equivalent to the reduction of carrying capacity of 20-30% for a gull population of 250 individuals. This method will be useful in quantifying different risk factors using the same currency of risk equivalent.