Scavengers aren’t the most charismatic of animals, but they perform a vital task: by eating carrion, they remove dead animals from our environments. Yet large-bodied scavengers are declining worldwide, increasing the risk of the spread of diseases, according to a recent study.
Researchers found that 1,376 animal species have been recorded in the scientific literature to either partially or fully eat carrion as part of their diets. They range from vultures and hyenas, to tiger sharks and cane toads, and even some salamanders, orcas and shrews.
Of these documented scavengers, only 17 species, or 1%, are obligate scavengers, those whose diet is fully comprised of carrion. A further 50% are facultative scavengers, meaning carrion is just part of their diet. For the remaining 49% of scavenging species, their dependence on carrion isn’t well-described, although they’re likely to be facultative, the authors write.
The study found that about 36% of the known scavenging species are either threatened with extinction, or are declining in number, according to the IUCN, the global wildlife conservation authority.
What’s most worrying, the authors say, is that apex scavengers — the most efficient, large-bodied or obligate scavengers that consume the most carrion in an ecosystem — are declining. These include large scavengers in the marine world like some sharks, albatrosses and petrels, and those on land, such as hyenas and vultures. In India, for example, several vulture species have suffered catastrophic population declines of 97-99.9% since the 1990s.
On the other hand, some smaller-sized scavenging animals, or mesoscavengers, are thriving. This isn’t necessarily a good thing, the authors write, because they don’t consume the same amounts of carrion that apex scavengers do, and some mesoscavengers like dogs and rodents are themselves carriers of diseases.
“Scavengers are in decline, but it’s not homogeneous,” study co-author Rodolfo Dirzo, from Stanford University, U.S., said in a statement. “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.”
After the decline of vultures in India, Ethiopia and Zimbabwe, for instance, dog numbers increasingly filled the void as scavengers. With dogs far more likely to come into contact with people, deadly zoonotic diseases like echinococcosis, leishmaniasis and rabies also increased. Similarly, an increase in rat and mouse populations has been associated with more leptospirosis outbreaks over the past decades in the Caribbean, East Africa, Oceania and Southeast Asia.
Pablo Plaza, a veterinarian at the National University of Comahue in Argentina, who wasn’t involved in the study, told Science News that scientists still need to collect experimental evidence to understand how big the health impact of scavenger decline might be. But he added he hopes that studies like this one will promote conservation of large scavengers.
Banner image of vultures feeding on carrion, by Rhett A. Butler/Mongabay.
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