A species is extinct when all of its members have died. Human actions have caused many species extinctions, including the great auk, the passenger pigeon, the dodo, the Monteverde golden toad, the Chinese paddlefish, and the Bali tiger.
Biologists try to avoid future extinctions by learning how the species above went extinct and by monitoring groups that are currently threatened. IUCN Red List provides summary statistics for organisms that may soon become extinct. I have further summarized some of these data in the graphic below.