In many cases, endangered creatures hang on at low numbers until a small event, like an untimely drought, tips the balance toward death. But strong efforts, such as the campaign to protect the American alligator, save endangered species too, depending on the species. The alligator breeds early and often and produces dozens of eggs, so when hunting was banned, the gator rebounded. The Florida panther and the West Indian manatee share some of the alligator's habitat, but they need much more territory and breed far more slowly. Both species could disappear well before 2020.
Some organisms are beyond saving because they can no longer reproduce in the wild.
A cutting flown to the Royal Botanic Gardens at Kew in London lived, but it can't set seed because of a faulty stigma. Eventually, it will die off. We're burning down nature's Louvre, and no one pays attention. Even if we keep a species alive in the zoo, it doesn't mean we can keep the thing alive. You need several hundred animals to keep a healthy gene pool.