How can captive breeding contribute to biodiversity and the health of ecosystems?

Captive breeding and subsequent re-introduction of a threatened species is an important and in some cases very successful tool for species conservation. Critics point to the need to conserve/restore habitat, list examples of failures, decry the cost, and argue we should rescue species before they are on the brink of oblivion. Fair enough. But, captive breeding saved the bison. Wolves roam Yellowstone and the Upper Peninsula of Michigan, the Peregrine Falcon is off the endangered species list, golden-lion tamarins thrive in the Brazilian forests, whooping cranes perform their mating dances along river banks in the west, and many more species might similarly be rescued. Zoos, botanical gardens and aquaria have found new purpose and direction, providing a safety net when other protective measures have failed.