Productivity as a Population Performance Index in Life-Cycle Toxicity Tests

Toxicity at the population level is often measured by the intrinsic rate of population increase (r_m). In this paper, the biomass turnover ratio (P/B) of a population is introduced as another measure of population performance under toxic stress. An expression is derived by which the intrinsic P/B ratio may be calculated from life-table data. Unlike r_m, P/B depends also on the individual growth curve. The theory is exemplified by experimental data on the effect of cadmium on Orohesella cincta (Collembola) from forest soil. The results are compared with seasonal fluctuations of productivity in an unstressed field population. Toxic effects of cadmium appear at a much lower exposure level than would be deduced from the P/B ratio. Some mechanism of compensation seems to be present in that the high natural mortality (by predation) obscures the sublethal effects on growth. This may explain why Collembola can inhabit forest soils which are contaminated to levels far above the no effect level for individual growth.