What do the white-rumped vulture, the tiger shark and the spotted hyena all have in common? They are large animals that feed at least partly on carrion. And their populations are all shrinking.
This decline of large scavengers around the world is rippling through ecosystems and human communities in ways that can lead to more sickness and death in people, according to new research in the Proceedings of the National Academy of Sciences.
“The evidence we found is very clear,” said Rodolfo Dirzo, a Stanford University ecologist and co-author of the new paper. “Scavengers are in decline, but it’s not homogeneous. It is particularly the large and specialized ones.”
That discovery came from an analysis of the health of 1,376 scavenging species of vertebrates, ranging from tiny shrews to great white sharks. By comparing that list to information from the International Union for Conservation of Nature (IUCN), the scientists found that 36% of the species have shrinking populations. A disproportionate number are large-bodied. Animals that feed only on carrion, such as vultures, are also more likely to be in trouble.
The decline of what the scientists dubbed “apex scavengers” has implications for human health in a number of ways, none of them good.
For starters, researchers have found that a boom in smaller scavengers such as rats or feral dogs often can’t replace the work done by the top consumers. In Kenya, breakdown of livestock carcasses was almost three times slower when vultures weren’t present. A similar dynamic was found in Australia when Tasmanian devils weren’t around. In South Carolina, consumption of rabbit bodies fell by 90% without vultures.
While dining on decaying flesh might seem unappetizing, it’s a vital service. It disposes of bodies that can pollute waterways, harbor pathogens, and attract other kinds of disease-bearing animals. The loss of these apex scavengers can mean not just more rotting animals lying around, but also a boom in smaller scavengers that was found by Dirzo and colleagues.
The vultures of India are poster child for the benefits of abundant big scavengers – and the price of their loss. Vultures that once filled the skies in parts of the country began to mysteriously die starting in the mid-1990s. The cause was eventually traced to a new class of pain killers used on Indian cattle, which was lethal to vultures. Two economists looked at the vulture die-off and found that it coincided with a surge in the numbers of feral dogs, the sale of rabies vaccines, water pollution, and human deaths. They calculated the result was an additional 104,000 deaths per year, with bites from rabid dogs as a leading suspect.
But India isn’t the only place where this could be a problem. In Ethiopia, scientists calculated that spotted hyenas’ consumption of livestock carcasses near one city prevented five cases of either anthrax or bovine tuberculosis in people.
The authors of the new study point to several factors playing the biggest roles in the decline of these big scavengers, according to IUCN data: hunting and poaching, intensive livestock production, and the loss of habitat.
There is hope that growing awareness could help reverse the plight of at least some of these animals. There are signs the White-rumped vultures in parts of south Asia are slowly rebounding, likely thanks in part to bans on some of the painkillers and public awareness campaigns.
“We’re starting to understand exactly how we depend on each individual species,” said Chinmay Sonawane, a Stanford Ph.D. student and the new study’s lead author. “I think as we become more aware of these connections, we’re going to be more inclined to protect these species, because, in turn, we are ultimately protecting our health.”
Sonawane, et. al. “Global decline of apex scavengers threatens human health.” Proceedings of the National Academy of Sciences. June 16, 2025.
Photo: Getty Images for Unsplash+
