Humans have introduced plants and animals from the outside world. Goats were introduced in the 1850’s and immediately began to take food from tortoises and iguanas. On the island of Isabela, there are more than 200,000 goats, which are extremely adaptable and hard to exterminate. Cats kill young iguanas and chicks of birds; dogs eat turtle eggs and hunt adult iguanas. Pigs destroy bird nests; donkeys devour vegetation; rats eat eggs of the giant tortoises. Invasive species also include insects and plants, which are equally threatening.

But eliminating one species at a time is not practical. The vegetation that goats eat provides cover for feral pigs. Cats eat rats. Dogs kill cats. So there needs to be an integrated, well-designed program. Eradicating introduced species and keeping new ones from arriving is a never-ending and enormously costly struggle.