Earth has already experienced five major extinctions in its history, and scientists now believe we are in the middle of, and causing, a sixth. There are more than 20,000 species that are threatened with extinction today, and the past century has seen perilous declines in the populations of plants, insects, and vertebrates. Robin Smith has spent his career in science education, as a high school biology teacher and as a Park Ranger with the National Park Service. He has done research on caribou and wolf populations in Denali National Park, and on bighorn sheep in Rocky Mountain National Park, among many other National Parks. He has also served as an environmental scientist on the Board of Directors of a number of non-profit conservation groups, including the Greater Yellowstone Coalition, Citizens for a Healthy Community, and WildEarth Guardians.