2. Henry David Thoreau (1817-1862), American writer, naturalist, and philosopher. Another transcendentalist leader, Thoreau wrote about his reverence for the natural world and his belief that humans and nature share a divine spirit.

3. John Muir (1838-1914), American naturalist, explorer, and writer. Muir was a founder of the Sierra Club, an organization dedicated to protecting the natural environment and wildlife. His environmental lobbying also contributed to the establishment of Yosemite National Park and Sequoia National Park, both located in California.

4. Theodore Roosevelt (1858-1919), 26th president of the United States. Roosevelt worked with U.S. Forest Service chief Gifford Pinchot to set aside more than 220 million acres of public land during his administration, more than any previous president.

5. Lucy Braun (1889-1971), American botanist and ecologist. Braun's extensive research on plants was a major impetus in establishing plant ecology as an academic discipline. Her commitment to conservation led to the eventual preservation of more than 10,000 acres in Ohio.

6. Rachel Carson (1907-1964), American marine biologist. Carson's 1962 book Silent Spring helped prompt a widespread ban of the chemical pesticide dichlorodiphenyltrichloroethane (DDT) and raised awareness of the negative effects of pesticides.

7. Jacques Cousteau (1910-1997), French marine explorer. Cousteau's films and writings drew attention to the plight of the world's oceans.