14 - Population growth rate as a basis for ecological risk assessment of toxic chemicals

Summary

Introduction

Ecological risk assessment tries to predict the likely impacts of human activities on ecological systems (USEPA 1992). In the case of toxic chemicals, the raw materials for ecological risk assessment involve exposure assessment based on predictions or measurements of environmental concentrations of toxic chemicals and an assessment of hazards, i.e. the potential of those chemicals to cause ecological harm. Hazard assessment is generally based upon observations on survival, growth or reproduction in a few individuals in a few species. We shall refer to these responses as individuallevel variables. Variability in responses among species is expressed only in terms of differences in these traits as measured under standard laboratory conditions and hence only reflects physiological variability in sensitivity to chemicals. It is presumed that these kinds of observations are relevant for protecting populations and ecosystems. However, this raises at least three different questions, as follows.

1. (i) To what extent do individual-level variables underestimate or overestimate population-level responses?

2. (ii) How do toxicant-caused changes in individual-level variables translate into changes in population dynamics for species with different life cycles?

3. (iii) To what extent are these relationships complicated by population-density effects?

We have addressed these questions, which go to the heart of the ecological relevance of ecotoxicology, using the population growth rate as an integrating concept. We have limited our attention to modelling the links between development, fecundity and survival to population growth rate.