When people killed the vultures, people also began to die.

Remember studying biology about ecosystems in high school? You may think of “food web” patterns in which each species hunts or stalks the other; If you remove one (eg foxes), you may end up with too much of the other (eg rabbits) with further consequences for the whole system (eg more grass than usual and more insects).

You may even think of the idea of ​​a “key species”, a species whose loss can cause serious problems for the entire ecosystem.

A new study has given us a perfect example of a key species – and the story is pretty creepy.

We are talking about vultures (more precisely, about three different types of vultures), and the ecosystem is about India. There used to be millions of vultures in India, but wildlife studies have shown that their populations declined in the mid-1990s – to the point where only tens of thousands remain today, and all three species are endangered.

Vultures have a useful but ferocious purpose: they eat the carcasses of dead cattle. And they eat them quick: You can completely gut a cow carcass in less than an hour, leaving very little rotten bait behind.

This is especially useful in a country where there is no easy access to incinerators for sanitary disposal of carcasses and where dead cattle lie in their fields or are dumped into rivers.

The first option is bad because it attracts wild dogs and rats, which are often rabid in India, and increases their numbers because they have more food. The second option is bad for obvious reasons of water intoxication.

You can see that vultures are a key species: remove them and you create big negative consequences for people: higher risk of rabies from dogs and rats and more polluted water. In other words, remove the vultures and more people will die.

At least that’s the story. What is the evidence? To see it, we must understand it. Why the vulture population has been greatly reduced. It is related to a pain reliever called Diclofenac.

It turns out that vultures, having eaten even a small amount of diclofenac, quickly suffer from kidney failure and die. Also, it appears that farmers in India started giving diclofenac to their livestock in the mid-1990s.

Putting two and two together is not enough: we need to properly test this hypothesis. A new study this month – still a “working paper”, that is, not yet peer-reviewed – used a clever way to do just that.

They found that the main reason for the increased use of diclofenac by farmers was that the drug company’s patent had expired and it had become generic and therefore much cheaper.

In their analysis, they showed that the human mortality rate increased immediately after the diclofenac patent expired, but only in areas where there were many vultures and livestock. In fact, they found that in the most vulture-friendly areas, there could have been over 100,000 additional human deaths per year after the birds began to die out.

They studied other datasets from India and found that both rabies vaccine sales and feral dog populations are on the rise, while vulture populations are declining and water quality is deteriorating.

None of this is direct evidence: it’s impossible to do an experimental test where you deliberately kill vultures in randomly selected areas of the country and see if more people die in those areas. But this is a good example of how scientists can use circumstantial evidence – when there is a lot of it and when everything points in the same direction – to convincingly prove that A (in this case, the collapse of the vulture population) causes B (in this case, there is more human mortality). .

Of course, the fact that this is not an experimental attempt leaves us with more uncertainty. It is possible that another alternative theory will emerge, showing that the increase in cattle painkillers is related to human mortality rates for a different reason than the decline in the vulture population. But since painkillers are not toxic to humans, it’s hard to imagine what that might be.

Little did farmers in 1990s India know that the use of diclofenac in their herds (which has been banned since 2002, although many still use it) would eventually lead to death. But it’s a reminder of what we know—since school—about ecosystems and key species. It is difficult to predict in advance what will be the cornerstone, but the risk of unforeseen consequences means that we must interfere in the system at our own peril.