Pesticides and other contaminants that get into the natural environment can affect wild plants and animals. The science of studying these effects is called ecotoxicology.¹ Ecotoxicology is a mix of ecology, toxicology, physiology, analytical chemistry, molecular biology, and mathematics. Ecotoxicology looks at the impacts of contaminants including pesticides on individuals, populations, natural communities, and ecosystems. Communities of living things and the environments they live in form ecosystems. Ecosystems include ponds, rivers, deserts, grasslands, and forests, and they too can be affected by pesticides. Ecotoxicologists also study what happens to the pesticides themselves, where they go in the environment, how long they last, and how they finally break down. This fact sheet will focus on pesticides in ecotoxicology.

A pesticide may directly affect something far from the site of application. Pesticides that are bound to soil particles may be carried into streams with runoff. Pesticide drift may travel many miles in the wind. Sunlight, water, microbes, and even air can break down pesticides.

Some pesticides last a long time in the environment, and may pose risks to living things many years after they were last used. Insecticides such as DDT, chlordane, and dieldrin don't break down easily, and they are still found in soil, plants, and animals. Persistent pesticides may travel long distances in the air or water, or even in living organisms such as migrating birds or fish. Researchers have found pesticide residues in alpine lakes and snow, many miles from where the pesticides were applied.^2,3 Pesticides have even been found in the Arctic and Antarctic environments, probably carried there by currents in the atmosphere or oceans.^4,5

The timing of exposure can greatly affect how much damage a pesticide might cause. Migrating animals may use a stopover site or staging area only briefly. At that time, those special locations may harbor a large proportion of the population or even the entire species. Other animals form breeding colonies for a few weeks or months of the year. Examples include some species of bats and swifts. If a pesticide is used when and where wildlife are clustered together, much greater harm could result than if that application occurred at another time even in the same place.

Risks may also increase at certain times in the animals' lives. Pesticides may pose greater risks to young animals or animals under stress from migration or breeding. The life stage of a plant may affect its risk of harm. An herbicide may not hurt a seed, or cause only small damage to a large, vigorous plant. However, it might kill a seedling.

Exposure risks may also depend on the conditions in a certain place. For example, barn owls eat voles when they are available.⁸ When voles are scarce, barn owls are more likely to eat other rodents such as rats and house mice. Rats and house mice are more likely to carry traces of pesticides. Dead and dying prey may be easier to catch and eat.⁹ That means the risks to barn owls depend on what is happening within the rodent community, which affects what prey the barn owls are likely to catch.

Effects can also occur on larger ecological scales than that of the individual. For example, predator-prey relationships may be changed by pesticides and other contaminants.¹¹ If the predatory wasps are more affected by an insecticide than the pests they feed on, the pest population may grow. The population of pests will often recover faster than the populations of predators following pesticide applications in agriculture.¹¹

When pesticides remove one of the species at the bottom of a food web, many other species may be affected. In this example of community-level effects, spraying for mosquitoes with Bacillus thuringiensis israelensis (Bti) reduced the populations of midges and mosquitoes, the favorite food of house martins. House martins in treated areas made fewer trips back to their nest with food, and raised fewer young, than house martins living in untreated areas. Spiders and dragonflies declined in treated areas, probably because they also eat midges and mosquitos.¹²

Declines in the number of one species may also affect plants or other organisms. For example, if a butterfly's host plant is affected by pesticides, they may not have enough places to lay eggs. If a pollinator species is lost, plants may not be able to set enough seed to maintain their numbers. These indirect effects can be very difficult to predict without doing experiments.

Pesticides and contaminants may affect more than just the populations of animals and plants that make up a community. They may also affect basic processes like nutrient cycling or the formation of soil. For example, nitrogen cycling may be affected if pesticides impact the bacteria and fungal communities in soil.¹⁶ There may be a time lag between the pesticide exposure and the ultimate effects. The pesticide could be gone before the damage it caused also disappears.

In the United States, the Environmental Protection Agency (U.S. EPA) reviews all pesticides before the products are registered for sale. They require companies to submit the results from standard toxicity tests as part of the application for registration. Depending on how and where the products are going to be used, companies may have to conduct more or different types of studies. A product meant only for use in someone's home may not require toxicity tests on fish. That information would be required for a product that will be applied on aquatic weeds. The U.S. EPA registers pesticides when the benefits outweigh the risks, assuming products are used according to the label directions.