Apex scavengers are decreasing globally as mesoscavengers increase
A decline in large scavengers and an increase in medium-sized ones can potentially lead to more disease transmission to people.
“When we think about these sorts of animals, which include a lot of predators, we typically think of their predation as the ecological function that they’re providing to ecosystems, which is certainly huge and important,” said Chinmay Sonawane, a PhD student at Stanford University. “The scavenging role that they play is often underplayed, but we know there are huge amounts of meat waste around the world, both in terrestrial and in aquatic ecosystems.”
In a study Sonawane led in Proceedings of the National Academy of Sciences, he and his colleagues looked at how vertebrate scavengers are faring around the world and what implications that might have for disease transmission.
Looking over five data sets that provided information on the foraging habits of vertebrate species including fish, birds, reptiles, amphibians and mammals, the team generated a list of 1,300 species that reliably scavenged and didn’t just do it occasionally. The International Union for Conservation of Nature considers 36% of these scavengers threatened or declining. But apex scavengers—defined by the researchers as those large in size with a diet made up entirely of carcass waste—were disproportionately affected, Sonawane said.
Mesoscavengers, on the other hand, are small and don’t depend on scavenging for their main food source. “Apex scavengers are the ones that are struggling, whereas mesoscavengers are thriving,” he said.
Sonawane also noted that one caveat to the distinction is that a species might be an apex scavenger in one area—like coyotes (Canis latrans) in the San Francisco Bay Area—but a mesoscavenger in others, like Yellowstone.
Sonawane initially thought that mesoscavengers might compensate for the loss of apex carnivores by consuming more rotting meat. But they discovered otherwise.
Instead, past studies showed them that apex scavengers play an important role in leading mesoscavengers to a carcass or initially penetrating the skin of the carcass to start it off. “Mesoscavengers have to wait for that to happen before they can get access to the meat,” he said.
As a result, Sonawane said, without apex scavengers, carcass waste might proliferate and spread pathogens.
In addition, the team noted that mesoscavengers are more likely to introduce diseases to people because species like feral dogs or coyotes often live in closer proximity to us. For example, when the vulture population declined in India in the 1990s, millions of feral dogs popped up, looking for scavenging opportunities. “As a result, more people were bitten, and more people were dying from rabies,” Sonawane said.
Sonawane said while the study was conducted on a global scale, managers likely need to implement solutions at the local scale.
“But what we can concretely say is, if we want to address this crisis from its ultimate cause, then we have to address the threats to apex scavengers,” he said.
Those key threats are wildlife trade, deforestation and urbanization, and intensive livestock production practices. “If we can address these threats, then hopefully we can better support apex scavengers, which can control the numbers of meso-scale scavengers as well as reduce carcass waste in the environment,” he said
