ABSTRACT
Our focus to this point has been on an individual organism. It is essential to be able to extrapolate from effects of a contaminant on individuals to effects on populations. This extrapolation is complex for at least two reasons. First, the mortality caused by a pollutant to some individuals of a population may actually beneˆt survivors, because more resources such as food and habitat will be available to them. This phenomenon is known as population-level compensation. Second, many pollutant effects on individuals will be sublethal; that is, they will not lead directly to the death of the individual. Instead, they may decrease its efˆciency in feeding, avoiding predators, and reproducing. Therefore, an assessment of the effects of a pollutant on a population will involve determining how various levels of individual impairment contribute to increased probabilities of mortality or reproductive failure through a variety of more direct factors.
