San Diego County’s unique environment is a hidden treasure that most locals don’t know enough about. San Diego is one of the most biodiverse counties of the United States, and one of the best biologically documented regions in the world. San Diego has 54 endemic, or nearly endemic plant species. With 524 observed bird species, San Diego County’s bird species list ranks at the top few among the U.S. counties. San Diego is also home to 122 species of mammals, both on land and in water.