Their removal affects the survival of other members of the chain. Fish, frogs, and ducks are some of the animals that depend upon aquatic invertebrates for food. Similarly, pesticides sprayed on crops and forests to reduce pest species may also kill invertebrates that are important in field and forest food chains. Populations of vertebrate wildlife, such as song birds, hawks, mice and shrews, may be reduced. When a pesticide is used repeatedly the species may develop a resistance to it.
People then develop more powerful pesticides to kill the resistant pests.
These in turn cause a development of populations resistant to the new pesticides. It seems that man cannot stay ahead of the pests. Although the use of pesticide has greatly increased over the last decades, the damage caused by pests is about the same as it was before the use of chemical pesticides. Many pesticides remain in soil or water long after they are used or needed. Invertebrates, resistant to the pesticide, take it in, and when they are eaten, pass the pesticide along.