As a young man in the 1990s, walking to school in New Delhi, Anant Sudarshan would watch the vultures perched along telephone wires, waiting for the discards of nearby leather tanning factories. So when the birds started to disappear, he couldn’t help but notice.

What Sudarshan, who now researches environmental policy and economics at the University of Warwick in the UK, did not realize at the time but would help discover decades later, was that the extinction of India’s vultures had far-reaching consequences for the humans who lived alongside the birds. In just a few years, the species’s disappearance contributed to the deaths of hundreds of thousands of his fellow citizens.

Together with Eyal Frank, an environmental economist at the University of Chicago, Sudarshan used his adolescent experience as inspiration for a new study being published in the American Economic Review. As in other developing nations, they found, the scavengers functioned as a natural sanitation system for communities with a less developed infrastructure than the US or Europe, helping control diseases that could otherwise be spread through the carcasses they consume.

Outside experts unaffiliated with the study say it will be a classic that unlocks further research on how the loss of critical species can have disastrous effects on human populations that depend on them, in often underappreciated ways. The findings should reshape how the public and policymakers alike relate to the world around us, and how we consider the unforeseen consequences of ecological destruction.

“We’re interconnected with the rest of the natural world,” Frank said. “I think for a lot of people, it’s this hippie, quasi-tree-hugger concept. Turning it into numbers and an outcome that people care about like mortality does change how people think about this statement: that we’re one with nature. What does that actually mean? It’s not a spiritual statement. It’s a statement about causal mechanisms.”

Sudarshan and Frank estimate that from 2000 to 2005, an additional 500,000 people died in India above the preexisting trend, after the rapid dying off of vultures in the 1990s. The near-extinction was an unexpected (and for a long time unknown) byproduct of the country’s farmers introducing a medication to livestock that had previously only been prescribed to humans.

Within a few years, 95 percent of the country’s vulture population was wiped out, dropping from tens of millions to a few thousand. A decade later, researchers discovered the drug led to kidney failure and death in the vultures when they fed on dead livestock that still had it in their system.

Sudarshan and Frank compared death rates in the years following the die-offs between regions that had previously been home to vulture populations and those that hadn’t, finding that people started dying at higher rates in areas where the birds had lived.

In the communities that lost vultures, there were an estimated 104,000 excess deaths annually — deaths that may be attributed to the species’ near-extinction — from 2000 to 2005, the years immediately following their dramatic decline that were the focus of Sudarshan and Frank’s study. It adds up to more than half a million deaths over five years, costing India an estimated $69 billion annually.

“I would not have guessed the effect would be so large,” Sudarshan said. But as he and Frank came to realize the various vectors by which diseases might spread without vultures around, Sudarshan realized the extinction was “the largest sanitation shock you could imagine, where you have 50 million carcasses every year not being disposed of.”

Ecologists and conservationists have long known that some species — called “keystone” species — play a pivotal role in their ecosystems. Scientists have also suspected that those species’ role is so important that their loss could have life-and-death consequences for human beings. That relationship, though, has been hard to prove.

There has been plenty of circumstantial evidence. In India, vultures are known to be extremely efficient scavengers, eating nearly all of a carcass less than an hour after finding it. Before the extinction, Indian regions that were home to vultures already recorded lower baseline mortality rates than those without them. After the birds died off, people in affected areas reported seeing more feral dogs and more rotting carcasses building up in fields.

Without vultures to consume them, there were more dead animals lying around, which sometimes ended up in rivers or other bodies of water, tainting local water supplies. The absence of vultures became an opportunity for other scavengers, such as rats and dogs. India did not attempt a census of feral dogs until 2012, well after the study period. But when they did, there were more of the animals in the areas previously hospitable to vultures, which Sudarshan and Frank argue implies the dogs may have flourished after the birds were eliminated.

Dogs and rats are less efficient than vultures at fully eliminating flesh from potentially disease-carrying carcasses, creating more opportunities for a person to come in contact with infected remains. They’re also more likely to transmit diseases like anthrax and rabies to people. Orders of the rabies vaccines started to rise in the years after the vulture population plummeted.

“I was mind-blown that it happened so drastically, so quickly,” Frank said. “We often say that anecdotes are not evidence, but the amount of anecdotes about how people were negatively affected by the disappearance of the vultures, we read more and more and more of it and said, ‘Okay, this has got to show up in data.’”

Sudarshan and Frank have now provided a template for studying the impacts of species loss on human health, and researchers unaffiliated with the study told me that they expect more such research to follow. Frank hopes future work might be able to identify whether specific causes of death increase after the elimination of a keystone species.

The findings should inform conservation efforts in other regions, such as sub-Saharan Africa, where vultures play a similar sanitation role, the researchers argue. Small investments to support local populations could have big payoffs. More broadly, supporting species believed to be ecologically critical, of which vultures are only one, is a wise investment.

It is also clear that farmers and agricultural officials should consider the potential ripple effects when giving new medications to livestock. This is a textbook example of One Health, the public health paradigm that says we should protect animal and environmental health to protect the well-being of humans.

The drug in question, diclofenac, had been introduced because it was a cheap way to treat fevers and inflammation in farm animals. The medicine was banned once Indian officials learned of its role in the vulture die-off, but by then, the damage was already done.

Vultures remain critically endangered in India, with only a few thousand individuals. Sudarshan and Frank argue their findings should encourage conservation efforts in India, though vultures’ life cycles will make them difficult to restore: They lay, at most, one egg in a year and take years to sexually mature.

The enormous consequences of their near-extinction in India remind us that promoting biodiversity means embracing every species, not only those that look good on a T-shirt; they and we are all part of a whole.

“We need to really remember these connections. They are crucially important,” Andrea Santangeli, a conservation scientist at the Research Centre for Ecological Change at the University of Helsinki, told me. “We cannot live a healthy life without a healthy nature.”

A version of this story originally appeared in the Future Perfect newsletter. Sign up here!