The Large Blue Butterfly, Phengaris arion, was declared extinct in the UK in 1979. Populations still existed in Europe and its reintroduction from those populations has been successful. The UK is now thought to have the largest population of Large Blue Butterflies in the world.
What is responsible for the remarkable success? Most important was understanding the life history of the butterfly. The Large Blue depends on ants to collect the early stage larvae, haul them to an ant nest and feed the butterfly larvae until they reach maturity. This understanding led conservationists to create and manage preserves to support robust populations of both ants and butterflies.