Documenting every animal in the world’s zoos and wildlife sanctuaries might seem like a big project, but National Geographic photographer Joel Sartore is well on his way.

Sartore just photographed the 7,000th animal for National Geographic’s Photo Ark, a multiyear effort to document every species living in captivity to inspire people to care about and protect them.

WATCH: 'Forest Fairy' Joins as 7,000th Animal in National Geographic's Photo Ark

The animal is the Leadbeater’s possum, an adorable little marsupial native to the acacia forests of central Victoria in Australia. It’s known as a “forest fairy” for the way it flits through the undergrowth at night and nests in hollowed-out trees. The biggest threat to the animals is the loss of these trees to decay, fire, and land-clearing. The species was missing in action for more than 50 years before being rediscovered by botanist Eric Wilkinson and two of his colleagues as they ventured into the woods near Marysville, Australia, on the evening of April 3, 1961.

This particular Leadbeater’s possum was photographed at the Healesville Sanctuary, a bushland zoo specializing in Australian wildlife that’s part of Zoos Victoria in Australia.

With some estimates for the lowland population as low as 50 possums, the species has recently been upgraded from endangered to critically endangered. Zoos Victoria is hoping to start breeding them soon to increase their population.

View Images The tiny possums are speedy and feisty. Photograph by Joel Sartore, National Geographic Photo Ark

The National Geographic Photo Ark is attempting to document Earth’s biodiversity, find innovative ways to save endangered species, and protect those species’ habitats. Sartore, who founded the project, has visited 40 countries to create moving portraits of the animals involved in the Photo Ark.

“I want people to care, to fall in love, and to take action,” he says.