Wildlife within protected areas is under increasing threat from bushmeat and illegal trophy trades, and many argue that enforcement within protected areas is not sufficient to protect wildlife. We examined 50 years of records from Serengeti National Park in Tanzania and calculated the history of illegal harvest and enforcement by park authorities. We show that a precipitous decline in enforcement in 1977 resulted in a large increase in poaching and decline of many species. Conversely, expanded budgets and antipoaching patrols since the mid-1980s have greatly reduced poaching and allowed populations of buffalo, elephants, and rhinoceros to rebuild.