In northern Australia, quolls' (squirrel sized marsupials that eat any animal smaller than them) population is dying. Their diet of cane toads (a poisonous species) nearly led them to extinction. But in Queensland, scientists found that the quolls there did not feed on cane toads and had successful populations. This was because they had a gene that prevented them from eating toads. Scientists found that if you bred the Northern Australia's quoll with the Queensland's quoll, their offspring had the toad-aversion gene and used this to increase their population. Scientists believe that this method can help increase declining populations of any endangered species.
While some argue that it is "unnatural" to mess with nature this way, I think that it is a great way to keep everything balanced. Science was discovered to help living things thrive, so it only makes sense that we use this knowledge wisely.