Some of Africa’s most endangered breeds of vulture have been driven closer to the brink of extinction after hundreds were killed in a mass poisoning in Botswana, conservationists said on Friday.

Poachers hunting for ivory in the north of the country laced three elephant carcasses with poison, killing 537 vultures and two tawny eagles in one of the deadliest such incidents on record. Among the dead were 468 white-backed vultures, one of the most endangered type of vulture, and of which only a few thousand are thought to remain in the wild.

Conservationists believe that the poachers deliberately targeted the birds for fear that the carrion-hunters, which circle at a height above a dying animal, might give away their position to rangers.

The scavenger, critical to curbing the spread of disease, is facing an existential crisis. “Vultures are the fastest-declining species at present,” said Kerri Wolter, founder of Vulpro, a conservation charity in South Africa. “There is a very real possibility that in my lifetime one or two vulture species [will] go extinct.”

The death toll also included 28 hooded vultures, 17 white-headed vultures, 14 lappet-faced vultures and 10 cape vultures. The death toll could end up being much higher as it is currently breeding season, and orphaned young are unlikely to survive.