But while the association can remove accreditation, which it does from time to time, it has few other enforcement powers. So decisions on just how conservation-oriented to be — from how many imperiled animals to save to how much money will be spent on species still in the wild — are largely made zoo by zoo.
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As standards for animal care rise and zoos install larger, more natural-looking exhibits, there is room for fewer animals. In the 1970s, the primate house in St. Louis held 36 species of monkeys and apes. Now it has 13.
And that narrowing of the species list is likely to continue for another reason. Zoos have come to understand that for animals to reproduce successfully for the long term without inbreeding, they need to maintain much wider gene pools for each animal. There are 64 polar bears in captivity in American zoos, far short of the 200 considered optimal for maintaining the population over 100 years. So zoos have been adding to the numbers of some species while culling others at the same time. St. Louis says it houses 400 more animals but 65 fewer species or subspecies than it did in 2002.