Pesticides present many hazards to wildlife - birds, fish and small mammals have been poisoned by pesticides used to control insect outbreaks. Herbicides and pesticide residues on plants are passed up the food chains from herbivores to carnivores, and in the process their concentration is often increased. The effect of pesticides on wildlife involves a number of factors, such as toxicity and persistence of a chemical compound; its stability and bioaccumulation as it is transferred along the food chain; the type of vegetation and habitat in which it is applied; and the characteristics of species of affected wildlife and wildlife at risk.
Some adverse effects on wildlife have been brought about by certain persistent chemical compounds, used as pesticides, which have spread from their point of application into the general environment; they are transported by air and water, washed out by rain or snow, accumulated by soils and disseminated by living organisms. For example, in 1992 US scientists discovered the use of certain pesticide chemicals, such as DDT, dioxin and PCBs plays a significant role in disrupting the hormonal systems of animals. Observations of seabirds that consumed fish contaminated with such chemicals led to discoveries of related problems, such as malformed sexual organs, changes in sex-linked behaviour, decreased fertility and immune system suppression.