Humans destroy precious habitat--the natural environment of a living thing--when they fill swamps and marshes, dam rivers and cut down trees to build homes, roads and other developments.
Oil spills, acid rain and water pollution have been devastating for many species of fish and birds.
Many animals are over-hunted because their meat, fur and other parts are very valuable.
When animals or plants arrive into a new habitat from a foreign place they sometimes introduce diseases that the native species can't fight. These "exotic" species can also prey on the native species.