As for survival in the wild now, the example of the peregrine falcon is instructive. A few decades ago--within the lifetime of most of the folks in this forum--they were extinct as a breeding species in the eastern U.S. The habitat was fine and dandy, except for the DDT we'd laden it with, which caused peregrine eggshells to thin to the breaking point. We banned DDT, and environmentalists started a captive breeding program to reintroduce peregrines to the wild. (In other words, human intervention "resurrected" the species in this part of the world.)
Well, not entirely. Peregrines still migrate to countries where DDT is not banned, and DDT is still in "the system" even in the Canada and the U.S., due to them being apex predators. The side-effects are still there (thin egg shells I know, but were there birth defects too?), although now the thinness I think is just 10-12% thinner than they were before DDT use 40 years ago. Still, 40 years is a hell of a long time, and it's a bit frightening to know that chemical is still effecting them now (although it doesn't help that other countries in their migration patterns are still using it).
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