When the construction of a hydroelectric dam on Venezuela’s Caroni River was finally completed in 1986, it flooded an area twice the size of Rhode Island, creating one of South America’s largest human-made lakes: Lake Guri. As floodwaters turned hilltops into islands, a key group of animals—predators such as jaguars, harpy eagles, and armadillos—disappeared from the islands. Some swam or flew away. Others drowned or starved to death. In the predator’s absence, their prey—howler monkeys, iguanas, leaf-cutting ants—began multiplying. Soon these plant-eaters had devoured most of the once pristine forest.