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Falling vulture numbers in the Americas could have serious implications for public health and ecosystems, new research has found.

A first-of-its-kind study conducted in Costa Rica has revealed that pig carcasses take twice as long to decompose if vultures are not present to provide their rapid recycling services. Notably, other scavengers do not fill the void left by the absence of the birds, except for flies, whose numbers were seen to double with the reduced competition.

The study, in Ecology and Evolution, and led by scientists in the Center for Ecology and Conservation at the University of Exeter, Cornwall, and OSA Conservation in Costa Rica, shines a light on the underappreciated role played by vultures. It also raises questions and concerns about the potential impact of increased fly populations on the environment and human health.

"This research underlines the unique and irreplaceable role that vultures play as nature's 'cleanup crew,'" says Julia Grootaers, co-author and Behavioral Ecologist at the University.

"Unloved and unappreciated—even to the point of persecution—the importance of vultures is stark when you remove them from an ecosystem. These apex scavengers have specialized and highly acidic digestive systems that can destroy many of the pathogens found in carrion, thus mitigating the spread of disease from carcasses to other wildlife, livestock and humans."

The study was conducted as part of Grootaers's master's thesis for her MSc in Evolutionary and Behavioral Ecology at the university. The fieldwork took place between September 2023 and June 2024 in the OSA Conservation Campus, situated within the Golfo Dulce Forest Reserve in the southern Pacific region of Costa Rica.

The research team placed domestic pig carcasses within 500m of each other, in both forest and grassland areas, over a series of tests spanning the wet and dry seasons. Half the carcasses were secured but left open to all scavengers, while the remainder were placed inside a cage with netting, making them inaccessible to larger animals. At each site, the researchers installed cameras and insect traps to record animal presence and monitor the number of flies and dung beetles.

Studying the footage, the team identified black vultures, turkey vultures and king vultures at 15 of the 16 carcass sites, with few signs of other potential scavenging species, such as pumas, ocelots, caiman, or coyotes. When they measured the rates of decomposition, they found that the carcasses open to vultures lost an average of 9.5kg of mass each day, compared with just 4.8kg for those that were placed in the cages.

The researchers also recorded a near doubling of the number of flies at those sites where vultures were excluded, but no discernible variation in the numbers of dung beetles.

"Our results demonstrate that intact scavenger communities, with vulture, invertebrate and microbial decomposers, efficiently decompose carcasses at double the rate compared to when it is insects and microbes alone," said Dr. Chris Beirne, Director of Wildlife Programs at OSA Conservation. "And it's apparent that in Costa Rica, other large vertebrate scavengers do not drive decomposition in the food chain—and that is consistent with what we see in other regions, including Kenya and northern Spain."

The researchers say that the increased amount of time it takes for carcasses to decompose when vultures are not present enables flies to reproduce and for their larvae to reach maturity. This, they say, increases the likelihood that they might become vectors for diseases such as botulism, anthrax, e-coli and salmonella.

"While dramatic vulture declines have already been documented across Africa, Asia, and Europe, this study raises concerns about the overlooked vulnerability of their American counterparts," adds Greta Hernandez, of Universidad de Panamá. "And the findings highlight the urgent need for deeper research into vulture population trends in the Americas, the critical ecosystem services they provide, and the conservation action needed to safeguard these silent stewards of the environment."