Jolie Lee

USA TODAY Network

A northern white rhino named Suni died last week, leaving only six of this rare subspecies left in the world, according to a statement from Ol Pejeta Conservancy in Kenya.

The 34-year-old Suni was one of four northern white rhinos at the conservancy. Worldwide, he was only one of two breeding males, according to Ol Pejeta.

There are no known northern white rhinos in the wild, the conservancy says.

The northern white rhino is one of two subspecies of the white rhino. The other subspecies, the southern white rhino, is estimated to number at about 20,000, according to the World Wildlife Fund.

The white rhinos' biggest threat is poaching. They are easy targets because they live in herds and are not aggressive, according to WWF.

It's unclear yet how Suni died, but the conservancy has ruled out poaching.

Rhinos are important in keeping certain plants in check, reports National Geographic.

"It's not just another charismatic animal — it's also a species that has a very clear ecological role, and we need to be very worried that we have lost that," Stuart Pimm, a conservation ecologist at Duke University, told National Geographic.

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