When U.S. lawmakers passed the Endangered Species Act in 1973, many of North America's most iconic plants and animals were teetering on the brink of extinction. Some of those have since rebounded thanks to three decades of federal protection, and several dozen have even been "delisted," including American alligators, bald eagles, brown pelicans, eastern Stellar sea lions, Tennessee purple coneflowers and the Oregon chub.

Despite such success stories, however, the global effort to stop Earth's sixth mass extinction is far from over. The U.S. still ranks No. 2 worldwide in endangered species by country, behind only Ecuador, with about 1,500 domestic species listed. (Those are also joined by nearly 600 foreign species, which are listed in the U.S. to help control international wildlife trafficking). Up to 200 species around the planet now become extinct every 24 hours, according to some estimates, a pace that's roughly 1,000 times the historical "background" extinction rate.

Americans have already watched — and helped — several ancient species die out during the country's brief history, perhaps none more notoriously than the passenger pigeon. The U.S. government is now trying to protect hundreds more from that fate, a task that involves exhaustive research into things like population trends, habitat size and reproduction rate, not to mention collaboration with local governments and landowners. The number and urgency of endangered species also vary widely by state, as the infographic above illustrates, requiring customized plans for each one's recovery.

Hawaii has less than 0.2 percent of U.S. land area, for example, yet more than 25 percent of all federally endangered species are found there. Most are plants and birds, threatened mainly by invasive species and habitat loss. Nationwide, flowering plants represent more than half of listed species, followed by fish, birds, mammals and clams. But that still isn't the whole picture, since 145 "candidate species" are still waiting to join the list, a limbo that can last years.

Russell McLendon is science editor at MNN. Follow him on Twitter and Google+.

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