#isabelle gerretsen

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A 2021 study by the University of Michigan in the US found that scavenging hyenas provide significant health and economic benefits to the African cities they roam. Spotted hyenas collectively remove 207 tonnes (456,000lb) of animal carcass waste annually in Mekelle, the capital of northern Ethiopia's Tigray region, the researchers calculated. The hyenas feed on livestock carcasses dumped in landfill sites on the outskirts of Mekelle. The researchers wanted to understand whether by devouring these carcasses, hyenas helped prevent pathogens from spreading to people and livestock. They compared the public health and economic outcomes of times when hyenas were present to when they were absent, and found that their scavenging annually prevented five infections of anthrax and bovine tuberculosis (TB) in Mekelle's residents and 140 infections in cattle, sheep and goats. "We found that hyenas are contributing to the management of disease and controlling the spread as well," says Chinmay Sonawane, lead author of the study who is currently completing a PhD in biology at Stanford University in California. Anthrax and bovine TB are responsible for about 6,000 human deaths and 500,000 cattle deaths annually in Ethiopia, the study authors note. "These are just two diseases out of dozens, if not hundreds, that might be out there," says Sonawane. "The sanitation services provided by hyenas might also be preventing the transfer of novel diseases to people." The hyenas' scavenging also saved Mekelle $52,000 (£44,000) annually in health treatment costs and livestock losses avoided, the study found. "There are really big economic benefits for the local community," says Sonawane. People living in villages around Mekelle therefore "reap the benefits" of having hyenas around, he says. Communities on the fringes of Ethiopian cities "have coexisted with hyenas for as long as they've both been present, and certainly understand the sanitation services they provide", says Sonawane. "But that additional step of disease control might not be as tangible for them."

Isabelle Gerretsen, ‘Why we should value scavengers’, BBC

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In India, the feral dogs can't match the efficiency and speed of vultures when it comes to disposing of carcasses, says Bowden. This means that many carcasses are left to decompose naturally or, in some cases, are incinerated. Carcasses release greenhouse gasses when they decompose, including CO2 and methane. But most of these emissions are prevented if the vultures dispose of them, according to a study released earlier this year by scientists at the National University of Comahue in Argentina. The study estimated that a single vulture consumes between 0.2 and 1kg (7 and 35oz) of decaying carcass meat per day, depending on the species. When it's left to decompose naturally, each kilogram of carcass meat emits about 0.86kg of CO2 equivalent, or CO2e. Using these estimates, the scientists calculated that the 134-140 million vultures worldwide could be preventing tens of millions of tonnes of emissions per year, an amount comparable to the emissions of some countries. "This ecosystem service contributed by vultures to humans and nature cannot easily be replaced by other species, including humans," the study notes. However, due to the absence of vultures, rotting carcasses in India are now often buried, dumped at landfill sites, or thrown in the river, where they risk contaminating the water and result in odour pollution around cities, says Prakash.

Isabelle Gerretsen, ‘Why we should value scavengers’, BBC

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India's feral dogs soon benefitted from the vultures' decline, he says. Due to the fall in vulture populations, cow carcasses piled up and the number of feral dogs surged as they became the main scavengers. This had a knock-on effect on human health as it led to a rise in rabies deaths, says Prakash. "There has been an increase in rabies in India over the past 20 years," he says. According to a study by scientists at the University of Bath, the vulture decline made way for at least 5.5 million extra feral dogs in India, responsible for an additional 38.5 million bites between 1992 and 2006. Using a national survey which showed that 123 out 100,000 people who are bitten by dogs die of rabies, the scientists calculated that at least 47,395 people died as a result of feral dogs becoming the dominant scavengers in India. Taking account of the average cost of treating rabies patients and dealing with the additional deaths, Taylor and his colleagues concluded that the vulture decline caused by diclofenac indirectly cost India $34bn (£28bn) between 1993 and 2006, equivalent to 3.6% of the country's GDP in 2006. "It is important that vultures be restored, as they are a valuable resource for India and hold an important place in the ecosystem," says Taylor.

Isabelle Gerretsen, ‘Why we should value scavengers’, BBC

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The decline of India's vulture populations began in the 1990s and happened very quickly. In the early 1990s, there were between 100 and 160 million vultures in the country. "They were absolutely everywhere in India," says Bowden. "They were doing an amazing job of cleaning up carcasses on a huge scale." But by the turn of the century, tens of millions of the birds were mysteriously dying. The populations of India's three most common vulture species together fell by more than 97% between 1992 and 2007. The vultures were tested for heavy metals, pesticides and other pollutants but no single cause of death could be identified. Eventually, autopsies revealed that many of the dead vultures had suffered from a type of gout caused by kidney failure. Researchers realised that vultures were being poisoned by the painkiller diclofenac – widely used as a veterinary drug to treat cattle – when they were feeding on livestock carcasses. "There's something unusual about vultures' metabolism...they can't [process] diclofenac and that causes kidney failure," says Bowden.

Isabelle Gerretsen, ‘Why we should value scavengers’, BBC