Population consequences of chronic toxicity: incorporating density dependence into the analysis of life table response experiments

Data from chronic toxicity tests are usually analysed by calculating how the toxicant reduces population growth rate. The contribution of effects on different parts of the life history to this reduction can be determined using sensitivity analyses and life table response experiments (LTREs). These provide a convenient descriptor of what happens within the laboratory toxicity test, but are only a good predictor of likely consequences in the field if density dependence of population dynamics can be ignored. Here I show how sensitivity analysis and LTREs can be applied to density dependent populations and illustrate the methods with data on the toxicity of dieldrin to Eurytemora affinis. I also outline the extension of the approach to populations which experience variations in vital rates. Stationary population sizes in density dependent populations at equilibrium mean that vital rates late in life are usually more important than in the density independent analysis. Substantial reductions in some vital rates can have little impact on the population if they are compensated by reductions in the intensity of density dependence. This represents one aspect of the assimilative capacity of ecological systems.