The Northern Quoll is endangered marsupial in Northern Australian. Its main problem, leading to its population decline, was eating the invasive and toxic cane toad. A population of them was introduced to a couple of islands (free of both cane toads and predators), where in 13 generations a large population was built up from which animals were reintroduced to the mainland. Problem is that the quolls lost their fear of predator scent and those that were introduced were quickly eaten. This is considered a genetically encoded fear of the scents which was lost due to lack of selection to maintain it during the 13 generations on the islands. Science news article here. This is not so different from problems of putting farm raised fish into wild waters, where they breed with the wild fish and introduce genetics not adapted to the local wild situations.