Mumbai is one of the densest urban centers on the planet, but a peculiar ecosystem has emerged around the city's immense Sanjay Gandhi National Park.
Leopards—about 40 of them—have been correlated with lower incidences of rabies, a disease that kills about 20,000 people in India every year.
The reason for the correlation? Feral dogs carrying the disease are being eaten by the large cats.
A new study published in the journal Frontiers in Ecology and the Environment looked at how leopards in the park inadvertently impact animals and people in the region.
One of the study's authors, Alex Braczkowski, was in the park filming for a National Geographic channel show in 2015 where he was struck by the stark border of the park—on one side lies preserved green space, and on the other lies a dense city.
When a leopard eventually walked by, "every dog in Mumbai started barking," he says.
Previous studies showed that dogs were the local leopards' primary source of prey, but they also feed on wild animals in the park like pigs and deer.
Based on the amount of food needed to sustain a leopard and the size of the dogs, the researchers now estimate that approximately 1,500 dogs are killed every year by the leopards.
India's Feral Dogs
An estimated 30 million stray dogs live in India, and a 2014 report published by the Indian government estimated just under 100,000 of those live in Mumbai.
It's illegal to kill a dog in India, and the government is undergoing a massive sterilization effort to mitigate the enormous population. If, as the study concludes, leopards kill about 1,500 dogs every year, it saves the government about US $18,000 annually.
Dog density, the scientists found, was about 10 times lower near the park than it was city wide.
Using statistical averages of dog bites and rabies cases in the region, the researchers also estimated that the park's leopards prevent 1,000 bites and 90 possible rabies cases.
"This is an ecosystem service we don't think about," says Braczkowski.
The potential financial and medical benefit to having leopards in the region could strengthen the push to conserve Sanjay Gandhi National Park, adds Braczkowski.
At just over 31 square miles, the park is one of Mumbai's most important green spaces, but proposed infrastructure projects threaten to fragment this leopard habitat.
The Importance of Predators
It's important to note that carnivores can be dangerous to people, too. While it's statistically low, they're known to occasionally attack or kill people, and they eat livestock that are essential to peoples' livelihoods.
Still, conservationists say it's important to protect the animals' space so they can be kept away from conflict with people, instead of killing off individual leopards. The provide an important service by stabilizing prey species and keeping ecosystems in balance.
The new study theorized that a decline in vulture populations led to an increased amount of carrion, or decaying flesh, and thus attracted more stray dogs. Vultures in India experienced a catastrophic decline in the past 20 years after a drug used to treat cattle was introduced into the environment and proved fatal for the birds.
Similar effects of human impact have been seen around the world. In the U.S., the decline of cougars had dangerous ripple effects.
A 2016 study found that the declining cougar population in the east caused a boom in white-tailed deer populations. That in turn led to a spike in car accidents and is thought to at least partially explain the rise in Lyme disease, which is carried on ticks that ride white-tailed deer.
Yellowstone National Park experienced positive impacts when one of their most famous carnivores, the gray wolf, was reintroduced in the mid-1990s. The predators helped increase beaver populations and forest growth. Previously, the decline in wolves led to fewer elk roaming to avoid predators. Thus they spent more time in winter eating willow trees, which beavers need to survive.
The effect is known as a trophic cascade, in which the removal of a key species has ripple effects throughout the ecosystem.
As for leopards, they've lost more than 80 percent of their range, but they remain protected in Sanjay Gandhi National Park—for now.
"We don't know the realities of living with these animals," Brazckowski says of the fear of living near these deadly cats. But he hopes his paper shows the indirect way they can potentially save lives.