To say that vultures are underappreciated would be putting it mildly. With their diet of carrion and their featherless heads, the birds are often viewed with disgust. But they have long provided a critical cleaning service by devouring the dead.
Now, economists have put an excruciating figure on just how vital they can be: The sudden near-disappearance of vultures in India about two decades ago led to more than half a million excess human deaths over five years, according to a forthcoming study in the American Economic Review.
Rotting livestock carcasses, no longer picked to the bones by vultures, polluted waterways and fed an increase in feral dogs, which can carry rabies. It was “a really huge negative sanitation shock,” said Anant Sudarshan, one of the study’s authors and an economics professor at the University of Warwick in England.
The findings reveal the unintended consequences that can occur from the collapse of wildlife, especially animals known as keystone species for the outsize roles they play in their ecosystems. Increasingly, economists are seeking to measure such impacts.
A study looking at the United States, for example, has suggested that the loss of ash trees to the invasive emerald ash borer increased deaths related to cardiovascular and respiratory illness. And in Wisconsin, researchers found that the presence of wolves reduced vehicle collisions with deer by about a quarter, creating an economic benefit that was 63 times greater than the cost of wolves killing livestock.
“Biodiversity and ecosystem functioning do matter to human beings,” said Eyal Frank, an economist at the University of Chicago and one of the authors of the new vulture study. “And it’s not always the charismatic and fuzzy species.”
Dr. Frank first learned the story of the vultures in India from Dr. Sudarshan.
The country was once home to tens of millions of vultures. Growing up in New Delhi, Dr. Sudarshan remembers seeing large flocks on his way to and from school, when his bus would cross a river with tanneries on each side.
“The river would be lined with these birds, huge birds, which would come down and eat those carcasses,” Dr. Sudarshan said. “When they disappeared, which was very rapid, that change was quite visible.”
For years, the vulture deaths were a mystery. But in 2004, scientists discovered that an anti-inflammatory medicine used in cattle, diclofenac, was highly toxic to the birds. A decade earlier, its patent had expired, leading to cheaper generic versions that farmers started using widely.
Conservationists pushed to get the drug banned for veterinary use, succeeding in 2006. But by then, India’s vultures had declined by more than 95 percent. In ecosystem terms, they had gone functionally extinct.
To assess the consequences on humans, Dr. Frank and Dr. Sudarshan first used range maps to determine where the vultures had lived and where they had not. Comparing human death rates between those districts would be key, because places that had never been home to a significant number of vultures would act as a kind of control to the terrible natural experiment wrought by diclofenac.
When the economists looked at the raw data plotted on graphs, they almost couldn’t believe how precisely it lined up with what they had predicted based on anecdotal reports. In districts where vultures had lived, human death rates started ticking up in 1994, the year after the price dropped on diclofenac, and they continued rising over the next several years. The districts that had not been home to vultures, on the other hand, remained remarkably steady.
To test what they were seeing, the economists examined other evidence, such as changes in water quality and sales of rabies vaccines.
Kelsey Jack, a professor of environmental economics at the University of California Berkeley who was not affiliated with the research, praised its importance and creativity.
“While this is a case study of a single species in a single country, it highlights the potentially large value of ecosystem services from keystone species,” Dr. Jack said. “This helps inform policy decisions, where policymakers have to trade off the costs of conservation with the benefits.”
Vultures in India are now at less than 1 percent of their previous populations, and appear to have stabilized at that new low, said Chris Bowden, who helps coordinate SAVE, a consortium of groups working to save South Asia’s vultures from extinction.
Four species are critically endangered. Despite the ban on diclofenac in livestock, it continues to be used illegally, and other anti-inflammatory drugs have been approved that are toxic to vultures, Mr. Bowden said.
In Pakistan and Nepal, the birds were also devastated by diclofenac. But in Nepal, where the ban on veterinary diclofenac use has been more effective, vulture populations have increased significantly over the past seven years, Mr. Bowden said.
Scientists warn that governments in Europe and South Asia are still not regulating veterinary drugs sufficiently to protect vultures.
In Africa, vultures are often unintended victims of poisoned carcasses meant to kill predators like lions and hyenas. Populations there have also plummeted. In North America, vultures are generally stable or increasing, though California condors remain critically endangered.
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