Vivian La

Published in Science on July 15, 2024

By Vivian La

Vultures have long been associated with death, and perhaps for good reason. With their hunched shoulders, hooked beaks, and signature bald heads, they fly around looking for dead and decaying animals to scavenge. But they also serve an important role in protecting human life, a new study finds.

The near-extinction of the birds across India in the 1990s led to the spread of disease-carrying pathogens from an excess of dead animals, killing more than a half-million people from 2000 to 2005. The study, currently online as a working paper that will be published in an upcoming issue of the American Economic Review, puts the monetary damage from the related public health crisis at nearly $70 billion a year.

“This [paper] will be a classic in the field,” says Atheendar Venkataramani, a health economist at the University of Pennsylvania who was not involved with the study. “It’s going to generate a lot of new science.”

Vultures are a keystone species in India, essential to the functioning of many of the country’s ecosystems. The birds of prey don’t just clean up disease-ridden carcasses; by removing food, they reduce the populations of other scavengers, such as feral dogs that can transmit rabies. What’s more, without vultures, farmers dispose their dead livestock in waterways, further spreading disease.

And that’s exactly what happened. In 1994, farmers began giving a drug called diclofenac to cattle and other livestock for pain, inflammation, and other conditions. But it was poisonous to the vultures that fed on these animals, destroying their kidneys. In just a decade, Indian vulture populations fell dramatically, from 50 million individuals to just a couple thousand.

Anant Sudarshan saw the impacts firsthand. As an adolescent in India, Sudarshan—now an environmental economist at the University of Warwick—says the bodies of cattle accumulated outside tanneries and city limits, where fields became carcass dumps for feral dogs and other less efficient scavengers such as rats to feed on. When the remains piled up, the Indian government required tanneries to use chemicals to dispose of the waste, causing toxic substances to leech into waterways used by people.

To uncover the impact of all of this on humans, Sudarshan and co-author Eyal Frank, an environmental economist at the University of Chicago, overlayed maps of vulture habitats onto those of Indian administrative districts. The team looked at health records for more than 600 districts, controlling for water quality, weather, and number of hospitals.

Before 1994, human death rates in districts surveyed averaged about 0.9% per 1000 people, a baseline that accounted for whether there were a lot of vultures in a particular district. But by the end of 2005, areas that were traditionally home to large number of vultures saw a 4.7% hike in human death rates on average, or roughly 104,386 additional deaths per year. Meanwhile, death rates in those districts that weren’t the typical homes of vultures remained stable at 0.9%.

To calculate monetary damages, the team relied on previous research that calculated the economic value of what Indian society is willing to spend to save one life at roughly $665,000 a person. That put the total economic damages from the loss of vulture populations at $69.4 billion a year from 2000 to 2005.

The numbers themselves aren’t surprising, says Andrea Santangeli, a conservation scientist at the Institute for Mediterranean Studies who wasn’t involved with the research. He and others have sounded the alarm on biodiversity loss for decades. But the new, dramatic stats could help convince lawmakers to act, he says. “If you give them flashy figures, it’s probably easier to push forward policy and conservation measures.”

The “natural experiment” detailed in the new study could be applied to other species, says Marshall Burke, an environmental economist at Stanford University also not involved with the work. Such methods might encourage environmental economists to look at other species with a known impact on human health, he says.

The Indian government banned the use of diclofenac in 2006, but it’s unlikely that vultures will completely recover, Sudarshan and the team say. That makes it all the more important to address species loss before it gets out of hand, he says—even if it means paying attention to a species like vultures that don’t usually get the spotlight.