We're number two! The United States is home to 1,278 species at risk of extinction — the second-highest count worldwide — according to the latest update of the International Union for Conservation of Nature's Red List of Threatened Species. The update, released last week, brings the total number of species that have been assessed for their extinction risk up to 71,576. Of course, that's just a fraction of the total world biodiversity, but digging into the IUCN's numbers reveals some interesting data points, as well as some of the areas where our scientific knowledge still has room for improvement.

So let's look at those numbers. According to IUCN counts, the countries with the highest numbers of species at risk of extinction are Ecuador (2,301), the U.S., Malaysia (1,226), Indonesia (1,206) and Mexico (1,074). India, China, Brazil, Tanzania and Australia round out the top ten; each of those nations has more than 900 species at risk of extinction on the IUCN Red List.

What do those numbers actually tell us? Both a lot and not much. Ecuador has the highest number of endangered species — not because the species there are more threatened but because the country made an intense effort over the last 15 years to evaluate its biodiversity. You can see the result of this work most notably in the assessment of Ecuador's plants, which revealed that 1,843 of its native species are at risk.

The results of similar assessments are visible in other countries' data. The U.S., for example, has the highest numbers of fishes (236) and mollusks (301) known to be at risk, as well as the highest number of recorded extinctions (257) on the IUCN Red List. Indonesia has largest number of mammals at risk (185), Brazil has the most birds (151), and Madagascar has the highest number of at-risk reptiles (136).

Of course numbers alone don't tell the full story. Craig Hilton-Taylor, head of the IUCN Red List Unit, cautions that "you cannot use any of the country totals to show reliably which country has more threatened species than another" as they don't reflect a country's size, biodiversity levels, or other factors.

Meanwhile, an awful lot of species haven't been assessed at all, or have only been assessed at national levels and not throughout their range, which may include several countries. Projects underway around the world will eventually help to fill that knowledge gap. "Brazil and South Africa are busy assessing all of their species," Hilton-Taylor reports, "so those countries will suddenly have much higher numbers of threatened plants than all others because of the size of their respective floras." Brazil is home to about 30,000 species and South African almost 20,000, so the Red List numbers will probably swell over the next few years.

Even that's just a drop in the bucket. Earth is home to something like 1.9 million known species, with more being discovered every day. Estimates of the planet's total biodiversity go as high as 12 million. How many of those species are currently endangered or even going extinct? That's a number we don't have.

Photo by Forrest O. via Flickr. Used under Creative Commons license