Because the human population is growing so rapidly, we are crowding out other species. As people take up more space and use more resources, we destroy the habitats of other species. More species are going extinct because of habitat destruction today than ever before. Global warming is a threat that is causing a change of climate for animals, such as the polar bear. The polar bear is a threatened species. This means that it will become endangered if something is not done to protect it. An endangered species is one whose numbers are so low that it is likely to become extinct. Another threat to biodiversity is the use of pesticides or other chemicals. Pesticides are chemicals that people use to destroy organisms that are pests. These chemicals can also kill beneficial organisms. Chemicals can run off into streams and harm organisms that live there.