A Sumatran rhino, one of the most endangered mammals on Earth. The sixth mass extinction of wildlife on Earth is accelerating, according to an analysis by scientists, who warn it may be a tipping point for the collapse of civilisation. More than 500 species of land animals were found to be on the brink of extinction and likely to be lost within 20 years. (Photo by Rhett Buttler/Mongabay/PA Wire)
