Polar bears and hippos for the first time join more than 16,000 species threatened with extinction, according to the World Conservation Union.

The Switzerland-based nonprofit, known as IUCN, released its 2006 Red List of Threatened Species on May 2. The list shows a significant increase in the number of species on the brink since the last list was released in 2004.

The Red List now marks polar bears as vulnerable, largely because of habitat loss linked to global warming. Due to decreasing sea ice in the Arctic, "polar bears are predicted to suffer more than a 30% population decline in the next 45 years," the group wrote in a press release.

The following images highlight other Red List species, from pink pigeons to blue poison frogs.

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