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Sumatran RhinocerosWhat is the critically endangered Sumatran rhinoceros (pictured, a mother and baby in Lampung, Indonesia, in June) worth to you?That's the question posed by a new book, Priceless or Worthless?, which showcases the hundred most threatened species as chosen by International Union for Conservation of Nature (IUCN) experts. The book is available for free online as well as in print.Released Tuesday at the IUCN World Conservation Congress in Jeju, South Korea, the book "challenges the way we think about nature," Jonathan Baillie said at a congress press briefing. One of the book's authors, Baillie is conservation director at the Zoological Society of London. (See National Geographic's reports from the conservation congress.)If people continue to value nature only in terms of how it can help us, "these amazing species on this list here—we can't really justify their existence," said Baillie, who worked on the book with the IUCN.Instead, conservationists need to protect species for ethical reasons—because it's the right thing to do for the future of the planet, said Baillie.About half of the hundred threatened species lack conservation attention, which means there is no mechanism in place—such as captive-breeding programs or no-hunting zones—to protect them from extinction, Baillie said."We're now in a place where we have to decide. Are they priceless or worthless? And if they are priceless—which I definitely say they are—then we need to step up to the plate."—Christine Dell'Amore in Jeju, South Korea

Photograph by Ch'ien C. Lee, AP