Moreover, in some regions the threats to species survival are so severe that no hope exists for their long-term in situ maintenance. There are roughly 500,000 mammals, birds, reptiles, and amphibians in captivity in zoos throughout the world.
Zoos contribute in many ways to the conservation of biodiversity.
They propagate and reintroduce endangered species, they serve as centers for research to improve management of captive and wild populations, and they raise public awareness of biotic impoverishment. The contributions that zoos have already made to the conservation of biodiversity are dramatic. The potential role of zoos as a site for preserving species over the long term is limited by both space and expense, particularly in the case of vertebrates. Even if roughly one-half of the spaces in all the world's zoos were suitable for captive propagation of threatened species and if 500 members of each species was maintained, only 500 species could be kept alive and healthy in captivity. This sobering calculation aside, for two reasons the contribution of zoos to biodiversity conservation is likely to exceed this figure.