NEARLY 20,000 species of animals and plants around the globe are considered high risks for extinction in the wild. That’s according to the most authoritative compilation of living things at risk — the so-called Red List maintained by the International Union for Conservation of Nature.

By generalizing from the few groups that we know fairly well — amphibians, birds and mammals — a study in the journal Nature last year concluded that if all species listed as threatened on the Red List were lost over the coming century, and that rate of extinction continued, we would be on track to lose three-quarters or more of all species within a few centuries.

We know from the fossil record that such rapid loss of so many species has previously occurred only five times in the past 540 million years. The last mass extinction, around 65 million years ago, wiped out the dinosaurs.

The Red List provides just a tiny insight into the true number of species in trouble. The vast majority of living things that share our planet remain undiscovered or have been so poorly studied that we have no idea whether their populations are healthy, or approaching their demise. Less than 4 percent of the roughly 1.7 million species known to exist have been evaluated. And for every known species, there are most likely at least two others — possibly many more — that have not yet been discovered, classified and given a formal name by scientists. Just recently, for instance, a new species of leopard frog was found in ponds and marshes in New York City. So we have no idea how many undiscovered species are poised on the precipice or were already lost.

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