Human-caused extinctions are normally thought to result from overexploitation or habitat alteration. A third possible cause of extinction is the loss of interactions associated with keystone species. Using recent and historical information on sea otters and kelp forests, we show that the extinction of Steller's sea cow from the Commander Islands in the mid-1700s would have been a nearly inevitable consequence of the overhunting of sea otters, which occurred a decade earlier.
