The FBI says it is investigating a string of death threats made to hunting enthusiasts after a permit to hunt endangered black rhino was sold at auction.

The Dallas Safari Club, which organises what it calls "the greatest hunters' convention on the planet", sold a permit to hunt a black rhino in the wild in Namibia for $320,000 (£212,500) at an auction on Saturday – in order to raise money for wildlife conservation.

Ben Carter, director of the Dallas Safari Club, said the rhino hunt would have no impact on the survival of the species. He insisted the rhino to be hunted was an older male that is no longer able to reproduce and was likely to be earmarked for culling.

He said of the auction: "This fundraiser is the first of its kind for an endangered species and it's going to generate a sum of money large enough to be enormously meaningful in Namibia's fight to ensure the future of its black rhino populations."

The club claims the auction was organised at the request of the Namibian government on behalf of the Namibian Game Products Trust Fund, which will receive 100% of the money raised from the sale of the hunting permit.

The money raised will pay for 70 game rangers to work for a year, according to The Club. The Namibian Government offers five such hunting permits each year, and the one auctioned on Saturday was the first to be available to bidders from outside Namibia.

The black rhino is an endangered species. There are thought to be nearly 1,800 black rhinos in Namibia and an estimated 4,000 black rhinos left in the wild – down from 70,000 in the 1960s.

Conservation campaigners and animal welfare activists were appalled. Thousands of people have already signed online petitions against the auction.

Jeffrey Flocken, the North American regional director of the International Fund for Animal Welfare, said: "This auction is telling the world that an American will pay anything to kill. This is, in fact, making a spectacle of killing an endangered species."

Watch the black rhino in action in the YouTube video below: