Factories and cars release chemicals into the air. The chemicals are deposited on land by rain, causing pollution, including what is known as acid rain. Acid rain weakens and kills plant life, decreasing the food supply for animals that eat the plants. Pesticides are another source of pollution. Farmers use pesticides to keep insects from eating crops.
Insects also carry pesticides.
The chemicals can disrupt physical functions like reproduction in these animals. We know less about other factors that probably contribute to the decline of biological diversity. We know little about how changes in our atmosphere, such as global warming or ozone depletion, is affecting other life forms. Disease and insect infestations, which are natural and non threatening phenomena in many ecosystems, can deal a death blow to populations weakened or depleted by other pressures. We know very little about how all these factors interact to affect plant and animal populations.