ABSTRACT
The Migratory Bird Treaty involving the US, Canada, and United Kingdom is one such case that prohibits the taking of individual birds of specific listed species. More often, effects to individuals are measured with the intent of predicting consequences to natural populations: the primary goal of most ecotoxicologists is assuring persistence and vitality of populations within ecological communities. Knowledge of activation mechanisms for carcinogens may be sought to support inferences about causal structure in studies of disease prevalence in field populations. A population is a group of individuals of the same species occupying a defined space at a particular time. External factors influence the size, nature, and distribution of nonhuman and human populations. The simplest models of population response treat all individuals identically and predict change in total number or density of individuals over time.
