Vultures circling in the sky may actually be a sign of good health.
Despite their negative image, large scavengers like vultures and hyenas help limit pathogens that can harm humans by eating carrion, the meat from decaying dead animals. Unfortunately, a Stanford study has found that their populations are declining worldwide.
The research also found that smaller scavengers like rats, raccoons, and feral dogs have been increasing in the absence of their larger peers - a process called "ecological release." However, the smaller scavengers do not consume as much carrion and often carry zoonotic diseases, those that can jump from animals to humans.
For the study, published in the journal Proceedings of the National Academies of Sciences, researchers analyzed data on more than 1,300 vertebrate scavenger populations, finding that 36% were threatened or declining. That drop was disproportionately higher among apex scavengers, the large-bodied animals that primarily eat carrion.
Rodolfo Dirzo | Harrison Truong
"The evidence we found is very clear," said senior author Rodolfo Dirzo, the Bing Professor in Environmental Science and professor of biology at Stanford's School of Humanities and Sciences. "Scavengers are in decline, but it's not homogeneous. It is particularly the large and specialized ones. At the same time, this allows space for the smaller scavengers, which are problematic because they are themselves sources of zoonotic diseases. They are also not capable of compensating as they cannot consume as much carrion."
A global loss
The decline of scavengers is part of the ongoing loss of many animal populations, which Dirzo and other researchers have warned represents a global "defaunation." While previous studies have focused on the decrease in individual species or broad categories of animals like mammals or birds, this study focused on a functional group of animals (scavengers).
To do this analysis, the researchers first identified a diverse array of mammals, amphibians, fish, reptiles, and birds that rely solely or partially on carrion. The list includes well-known scavengers like vultures, as well as animals less recognized as scavengers, ranging from great white sharks and lions to tortoises and pond turtles.
The researchers then investigated the animals' population status and ecosystem impacts using prior studies as well as databases, including the International Union for Conservation of Nature's Red List of threatened species.
They found numerous examples showing that the loss of apex scavengers has reverberating effects on ecosystems and humans. One of the most dramatic is the collapse of vulture populations in India in the 1990s. Their absence allowed feral dogs to proliferate, resulting in an estimated 39 million additional dog bites and 48,000 rabies-associated human deaths from 1992 to 2006. The use of the veterinary drug diclofenac in livestock was identified as a key contributor to vulture deaths, and after India banned the drug in 2006, the vultures started to recover.
"We're starting to understand exactly how we depend on each individual species," said Chinmay Sonawane, the study's first author and a doctoral student in Dirzo's lab. "I think as we become more aware of these connections, we're going to be more inclined to protect these species, because, in turn, we are ultimately protecting our health."
Apex scavengers play a critical role not just in low- and middle-income countries, but also in high-income countries, Sonawane said. For instance, one previous study found that eagles and other scavengers disposing of elk miscarriages curbed brucellosis bacterial infections among elk herds in Wyoming - and by extension helped reduce risk of transference to livestock and humans.
The current study identifies three factors in the decline of apex scavengers: habitat loss, intensive livestock production, and the wildlife trade. These work in several, often compounding, ways to impact scavengers, such as removing habitat to make way for cattle grazing, killing scavengers that are viewed as preying on livestock, and big game hunting of predators that also scavenge, like lions and tigers.
The authors argue for policies to help apex scavengers recover, such as protecting habitat and reducing poaching. It is also important to remove the stigma around these animals, said Dirzo, who is also a professor of Earth system science at the Stanford Doerr School of Sustainability.
"There is this prejudice that these scavengers - vultures and hyenas and the like - are nasty animals and that we should get rid of them, but they are important not only for the functioning of ecosystems, but also for human well-being," he said.
