Changes in animal populations, whether increases or decreases, can indicate pollution. For example, if pollution causes depletion of a plant, animal species that depend on that plant will experience population decline. Conversely, overpopulation may be opportunistic growth of a species in response to loss of other species in an ecosystem. On the other hand, stress-induced sub-lethal effects can be manifested in animal physiology, morphology, and behaviour of individuals long before responses are expressed and observed at the population level. Such sub-lethal responses can be very useful as "early warning signals" to predict how populations will further respond.
Pollution and other stress agents can be monitored by measuring any of several variables in animals: the concentration of toxins in animal tissues; the rate at which deformities arise in animal populations; behaviour in the field or in the laboratory; and by assessing changes in individual physiology.