Density dependence in ecological risk assessment

Summary

Most natural populations are affected by density dependence, either within the species (e.g. competition for resources) or in interaction with other species. Nevertheless, this mechanism is often ignored in ecotoxicology. Density dependence normally implies that an increase in density auses a decrease in demographic rates (survival, growth or reproduction per individual). Conversely, a loss of individuals may be compensated by increased survival or reproduction in the remaining population. Effects of toxicants on demographic rates may also be modified by density-dependent processes. As a consequence, population-level effects of toxicants do not necessarily correspond to individual-level effects. Moreover, density-toxicant nteractions may have two different outcomes: density dependence can compensate for the toxicant-induced loss (antagonistic interaction), but it can also amplify the negative toxicant effects (synergistic interaction). The outcome of the interaction depends on several aspects of both the toxicant stress and the density stress. For example, compensatory responses may be most likely if the toxicant reduces the population density moderately, and the remaining individuals have a net benefit from reduced density stress. On the other hand, if either density stress or toxicant stress has sublethal effects, then individuals may suffer more from the combination of the two stress actors than from either factor alone. In order to assess population-level risks of toxicants for natural populations, it is important to predict the possible density-toxicant interactions. This in turn requires a good understanding of the density-dependent mechanisms operating in the population. This paper presents methods for detecting and estimating density dependence in field and experimental populations, with focus on density-toxicant interactions.