Vultures are often seen as harbingers of death, their circling skyborne silhouettes a sign that a fresh carcass is nearby.

But it turns out these scavengers are also life savers, protecting millions of people from an early demise. The birds’ recent disappearance in India due to accidental poisoning has revealed their critical role in controlling disease, and the human toll that can come when a species is pushed to the brink of extinction.

“The vulture collapse in India provides a particularly stark example of the type of hard-to-reverse and unpredictable costs to humans that can come from the loss of a species,” said environmental economist Anant Sudarshan of the University of Warwick in the United Kingdom.

While there is widespread concern about the growing number of species threatened by extinction, it’s often difficult to calculate the loss in terms of human lives or dollars, measurements that can carry extra weight with policymakers. But quantifying costs is the specialty of economists like Sudarshan and his collaborator, the University of Chicago’s Eyal Frank.

The fate of India’s vultures provided these economists with a grisly natural experiment in how the loss of a species can ripple through an entire society. Once so numerous that biologists hadn’t tried to make an accurate count, South Asia’s vultures began dying in a mysterious plague starting in the mid-1990s. Within years, species that had once numbered in the millions were down to a few thousand.

It wasn’t until 2004 that scientists found a culprit. A pain killer, diclofenac, became widely used to treat cattle starting in the mid-1990s when an expired patent enabled cheaper generic versions to flood the pharmaceutical market. It turns out many species of Asian vultures can’t metabolize the drug. So when the birds dined on the remains of contaminated cows, it destroyed their kidneys. By the time this was discovered, vulture populations across South Asia had been devastated.

The loss highlighted the critical role these birds had played in disposing of dead animals in a country teeming with livestock. India has more livestock than any other country, more than half a billion, including 300 million cows. That translates into a lot of animal carcasses, particularly in a country with a Hindu prohibition on consuming cow meat, poor rural farmers and limited infrastructure to deal with massive numbers of dead animals. Farmers whose cows stop producing milk often set the animals loose to fend for themselves. Sudarshan and Frank estimate that in India there are around 30 million dead livestock per year.

In the past, vultures had been the linchpin to dealing with this Everest-sized mountain of dead bodies. With the vultures suddenly gone, what would that mean for public health?

To look for potential effects, the economists compared human death statistics in parts of India that had once been vulture habitat to areas where vultures weren’t previously found. The results were striking. Starting in the mid-1990s, just as diclofenac started spreading, death rates began rising in places where vultures had dwelled, but not in places without vultures. That higher rate plateaued in the early 2000s, around the time vulture populations had reached their low point.

Overall, the loss of vultures coincided with an increase of 48 deaths for every 100,000 people in places that were once vulture havens, the economists reported in a draft of a paper set to appear in American Economic Review. While that might sound like a relatively small number, in populous India it translates to more than 104,000 additional deaths every year between 2000 and 2005, the economists found. In true economist fashion, they calculated that represents a loss of nearly $70 billion per year, based on estimates of the dollar value of a human life in India.

“We were both surprised by the magnitude of the effect,” Frank wrote in an email. “But after looking more at other studies that found similar effect sizes from improvementsto sanitation, we think the effect size is very reasonable.”

The economists point to several potential reasons why a loss of vultures could translate into more people dying. The surplus of dead animals has been blamed for a surge in populations of feral dogs, which feed on the carcasses. That, in turn was thought to increase the risk of people contracting rabies from dog bites. The researchers found a sharp uptick in sales of rabies vaccines in India beginning in 1996 – right when diclofenac was becoming widely used.

Water quality data also showed an increase in water pollution in urban areas where vultures once lived, a potential connection to contamination from decaying carcasses.

Researchers estimate the death toll from 2000 to 2005 could be more than 500,000 people. But there is little reason to think things have changed for the better. Vulture numbers haven’t bounced back, in part because toxic pain killers continue to be used, despite a government ban, the economists note. Little has been done to find a way to cope with all the dead livestock, such as building incinerators. If nothing much has changed, that could mean nearly 2.5 million people in India have died since 2000 due to the loss of vultures.

Tackling the carcass problem won’t be easy. Sudarshan and Frank estimate it could cost nearly $800 million a year to operate enough incinerators to cope with all the dead animals, and that doesn’t include the damage from air pollution generated by all the burning.

But given the human and economic toll of not doing anything, even a solution that costly sounds like a bargain.

“We think that changing how dead animals are handled (through burial or incinerators) makes sense given the large health impact we estimate,” Frank wrote.

Meanwhile, there are some small glimmers of hope for South Asia’s vultures. The Indian government has expanded its bans to several other pain killers shown to be toxic to vultures. But undercover checks at Indian pharmacies continue to show the banned drugs are widely available, according to Saving Asia’s Vultures from Extinction (SAVE), a coalition of conservation groups. The group is seeing some success in efforts to create “vulture safe” zones through a combination of enforcement and public education, particularly in neighboring Nepal.

But vulture numbers in India remain dismally low. For the time being, people will likely continue dying because of a shortage of this once-reviled bird.

Frank, et. al. “The Social Costs of Keystone Species Collapse: Evidence From The Decline of Vultures in India.” American Economic Review. July 23, 2024.

Image: White-rumped Vulture (Gyps bengalensis) feeding at cattle carcass, India./ FLPA/Alamy Stock