Photo Ark: capturing endangered wildlife before it’s too late

US photographer Joel Sartore is spending a decade taking portraits of the 12,000 species in captivity worldwide - with an emphasis on those facing extinction

" See a gallery of Sartore's portraits here

As the world goes through what scientists have called its "sixth great extinction", with animal species vanishing at a rate unheralded since the dinosaurs were wiped out 66 million years ago, the sense of helplessness can be acute among those who revere nature.

Joel Sartore spent several years taking evocative photographs of endangered species for National Geographic before realising he needed to change tack if he was to drum up public concern about Earth's diminishing wildlife.