Ecological risk assessments are studies to determine the effect of selected contaminants on the biotic system. Such assessments are required for certain hazardous waste sites. These sites are often accompanied by non-contaminant-related events, such as vegetation removal, ground disturbance, and changes in local hydrologic conditions, all of which can in themselves affect wildlife populations. In many cases, it may be difficult to attribute observable effects (that is, a measurement endpoint such as no fish in a stream) to the contaminant versus other environmental conditions.

Ecological risk assessments for fish and wildlife must focus not only on the toxicity of the contaminants, but also on the degree to which organisms are exposed to the contaminants. These factors must then be related to the concentration of the contaminant shown to have a negative effect on that species of fish or wildlife. Scientific literature regarding the effects of specific contaminants on fish and wildlife are limited in scope, both with regard to the species investigated and the contaminants studied. Another key aspect of ecological risk assessment is a knowledge of the basic biology of the organisms, and a focus on those species that are present or likely to be present in the contaminated area.

The process starts by identifying the contaminated media (for example, water, soil, or sediment), along with the species or food chains likely to occur in the contaminated area. The next step is to determine the extent of exposure at each link in the food chain. Time of exposure is important, that is, are the animals present year-round or only part of the year? Are they present only during migration, or only during the breeding season? Location of activity in a habitat is also important; does an animal nest in a tree cavity or on the ground? Is the animal likely to come into direct contact with contaminated media, such as a fish in contaminated water, or indirectly through its food source? Food habits are very important with regard to exposure. Does the animal feed on seeds and grains growing in a contaminated area, or does it feed on earthworms living in contaminated soils?

To answer such questions and provide the necessary biological input requires the services of trained ecologists. All of the above, and many other factors, will affect not only the likelihood that an animal is exposed to contaminants, but also the severity of any such exposure. As a result, ecological risk assessments can prove to be much more complex, and subject to a greater degree of uncertainty than human health risk assessments, where one is dealing with only one receptor species - humans.