How should humans decide which endangered animal species to focus on saving? That's the question being posed by an international coalition of scientists in a new paper documenting 100 of the world's most at-risk species. The scientists fear that these plants and animals are at the greatest risk of extinction because, quite simply, they don't offer any obvious or immediate benefit to humans. Here's what you should know:

Who is behind the report?

The report, Priceless or Worthless?, was published at the World Conservation Congress in South Korea. Some 8,000 scientists from all over the world put together the first-of-its-kind study for the meeting of the International Union for Conservation of Nature (IUCN), which convenes every four years.

What kinds of animals are on this list?

Take the pygmy three-toed sloth, which is "no bigger than a newborn baby" and is found exclusively on a single island off the coast of Panama, says Fiona Harvey at Britain's Guardian. Or take the willow blister, a parasitic fungus that grows on twigs in a tucked-away corner of Wales; it's the rarest type of fungus in the world. Then there's the Red River giant soft-shell turtle. Thanks to pollutants in the water and destruction of its habitat, there are only four left.

Why are these animals endangered?

First, the usual culprits: Habitat destruction, pollution, hunting, and climate change. But there's another problem: Unlike cuddly or "charismatic" endangered animals like pandas and tigers — or even the appetite-suppressing hoodia cactus, which has obvious medicinal uses — these endangered species are dangling precariously close to extinction because of the simple fact that they don't offer humans any clear benefits. In many cases, people don't care enough about these species to intervene.

So... what now?

The question remains: Are these species worth devoting limited resources to saving? "The donor community and conservation movement are increasingly leaning towards a 'what can nature do for us?' approach, where species and wild habitats are valued and prioritized according to these services they provided for people," says Jonathan Baillie, director of conservation for the Zoological Society of London. Well, saving every single species is an "enormous undertaking," says Sybille Klenzendorf of the World Wildlife Fund. It's often better to save "umbrella species like tigers, elephants, and rhinos" in order to protect the habitats they share with other endangered creatures. "We won't be able to save every species," says Cristian Samper, head of the New York-based Wildlife Conservation Society, "but if we are smart we can save many of them."

Sources: Daily Mail, Guardian, NBC News, The Register