Maculinea arion, a large blue butterfly, became extinct in Britain 30 years ago; it has now been successfully reintroduced. That achievement involved more, however, than just hauling in a bunch of replacement butterflies or caterpillars:
Crucial to the success of the re-introduction was a discovery that the life cycle of the large blue is completely dependent on a species of ant called Myrmica sabuleti.
Large blue caterpillars hatch on thyme buds and then trick M. sabuleti into thinking they are ant grubs. The ants then carry the caterpillars to their underground nests, where they feed on ant grubs for 10 months before pupating and emerging as butterflies.
Professor Thomas realised that farmers had been gradually shifting their livestock away from grazing, causing the grass to become overgrown... This in turn meant the soil was now too cool to support adequate numbers of M. sabuleti ants...
As part of the reintroduction programme by the conservationists, grazing was re-established on the sites chosen for the insects.
There are now over 30 colonies across south-west England, with the largest supporting some 5,000 butterflies.
Update: followup here.