Why dying vultures mean dying humans, and why we shouldn't … – iNews


Remember in secondary school biology, when you learned about ecosystems? You might remember drawing diagrams of the “food web”, where each species either preys upon, or is preyed upon, another species; if you remove one (say, foxes), you might get too many of another (say, rabbits), which has further knock-on effects all around the system (say, more grass than usual, and an increase in insects).
You might even remember the idea of a “keystone species”, which is one whose loss could cause serious problems for the whole ecosystem.
New research has given us a perfect example of a keystone species – and the story is a pretty macabre one.
The species in question is the vulture (or, to be exact, three different species of vulture), and the ecosystem is India. There used to be millions of vultures in India, but wildlife surveys showed that their populations collapsed during the mid-90s – to the point that there are just tens of thousands left nowadays, and all three types are critically endangered.
Vultures serve a useful—though grisly—purpose: they eat the bodies of dead livestock. And they eat them fast: they can completely strip a cow carcass in less than an hour, leaving very little rotten carrion behind.
That’s particularly helpful in a country where there’s no easy access to incinerators to hygienically dispose of corpses, and where dead livestock either lie around in their fields or are dumped into rivers.
That first option is bad because it attracts wild dogs and rats—often rabid in India—and increases their numbers since they have more food. The second option is bad for obvious, water-poisoning reasons.
You can see how the vultures are a keystone species: remove them, and you create big, negative consequences for humans: more risk of rabies from dogs and rats, and more polluted water. In other words, remove the vultures, and more humans will die.
Or at least, that’s the story. What’s the evidence? To see that, we have to understand why the vulture population declined so much. It’s to do with a painkiller called diclofenac.
It turns out that vultures who eat even a tiny amount of diclofenac rapidly get kidney failure and die. And it also turns out that farmers in India started giving diclofenac to their livestock in the mid-90s.
Putting two and two together isn’t quite enough: we have to properly test this hypothesis. A new study from this month—still a “working paper”, so not yet peer-reviewed—used a clever method to do just that.
They noticed that the main reason for farmers starting to use more diclofenac was that the drug company’s patent ran out, and it became generic, and thus much cheaper.
In their analysis, they showed that the human death rate crept up just after the diclofenac patent ran out – but only in areas where there were a lot of vultures, and a lot of livestock. Indeed, they found that, in the most vulture-friendly areas, there could have been more than 100,000 additional human deaths per year after the birds began to die out.
They looked into other datasets from India to find that rabies vaccine sales and feral dog populations both increased at the same time as the vulture population dwindled, while water quality declined.
None of this is direct evidence: it’s not like it’s feasible to do an experimental trial where you deliberately kill vultures in randomly-selected parts of the country and see whether more people die in those areas. But it’s a nice example of how scientists can use circumstantial evidence—if there’s a lot of it, and if it all points in the same direction—to make a compelling case that A (in this case vulture population collapse) causes B (in this case more human mortality).
Of course, the fact it’s not an experimental trial does mean that it leaves us with more uncertainty. It’s possible that some other alternative theory will come along, showing that the increase in livestock painkillers was linked to the human death rate for some reason other than the collapse in vulture populations. But since the painkillers don’t seem to be toxic to humans, it’s difficult to imagine what that might be.
Farmers in 90s India couldn’t have known that using diclofenac in their herds (which has been banned since 2002, though many still use it regardless) would eventually lead to human deaths. But it’s a reminder of what we know—all the way from high school—about ecosystems and keystone species. It’s hard to predict which is the keystone in advance, but the risk of unintended consequences means we should interfere with the system at our peril.
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