Population‐level ecological effect assessment: estimating the effect of toxic chemicals on density‐dependent populations

We examined the relationship between individual‐level and population‐level effects of toxic chemicals, employing the equilibrium population size as an index of population‐level effects. We first analyzed two‐stage matrix models considering four life‐history types and four density‐dependent models, and then we analyzed ecotoxicological and life‐history data of the fathead minnow ( Pimephales promelas ) and brook trout ( Salvelinus fontinalis ) as real examples. Our elasticity analysis showed that toxic impacts on density‐dependent populations depended largely on the differences in density‐dependence and in life histories of the organisms. In particular, the importance of adult survivability was considerably increased in iteroparous organisms with density‐dependent juvenile survivability or fertility. Our results also suggested that population‐level effects, as indicated by the percentage reduction in equilibrium population size, were often greater than the percentage reductions in vital rates of individuals. Our analysis indicates that assessing population‐level risk and developing a risk‐reduction strategy without considering density‐dependence can be risky.