Feral animal are descendants from once domesticated animals that have escaped or were set free and may have interbred with native species. They are interlopers in their environment. They often have no natural predators to control populations and can destroy whole ecosystems.
Feral pigs have contributed to the destruction of one third of the bird species on the Hawaiian Islands and to the disappearance of the land iguanas and the poor survival rate of Galapagos tortoise on the Galapagos Islands. In Australia, more than 20 kinds of feral animals, including camels, dogs (dingoes), and water buffaloes are destroying habitat and threatening domestic animals.