They evolved in a given habitat in conjunction with other species that kept them in check. For example, rabbits have natural predators in Europe. Predator and prey evolved together which ultimately kept their population balanced. More predators caused rabbits to evolve to be more fertile so that they can reproduce quickly. But their numbers are kept limited by the predators, whose population increased as their prey species of rabbits increased. Then you go and release rabbits in Australia, which doesn’t have a significant predator species to eat the rabbits, so they breed outrageously fast and become a plague in Australia.
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