The decline of the Tasmanian demons has an unusual knock-on effect: carcasses of animals will never devour in a short time by demons now taking a few days longer to disappear.
We made the discovery, which was published today in the Proceedings of the Royal Society B journal, by placing carcasses in various locations and observing what happened. We found that the reduced scavenging by Satan produced extra food for less efficient scavengers, such as wild cats.
The Tasmanian devil has struggled for two decades against infectious cancers that are usually fatal, which is called Satanic Face Disease. This disease has caused the devil population to decline by around 80% on average, and up to 95% in some regions.
Scavengers are carnivores that eat dead animals (carcasses). Almost all carnivores scavenge to a greater or lower level, but the devil is a Tasmanian dominant eater. Since the Thylacine's extinction, this is also the island's top predator.
In our study, we excreted carcasses from Tasmania (small wallabies weighing about 5kg) in various places, ranging from disease-free areas to large demon populations, to long diseased areas where demon numbers were very low. We then used a motion sensor camera to record all the scavenger species that ate the carcass.
Not surprising, especially the carcass is consumed by demons in areas where the demon population has declined. This has increased the availability of carcasses for other species, such as invasive wild cats, tailed spotted quolls and jungle crows. All of these species significantly increase their scavenging in places with fewer demons.
The response of original scavengers (quolls and ravens) is subtly different from wild cats. The amount of eating with quolls and crows depends only on how much each carcass has been consumed by the devil. Crows and quolls are smaller and less efficient than devils in eating carcasses, so they get the opportunity to feed only when the devil hasn't monopolized the carcass.
Tasmanian demons raised in captivity show they can develop in the wild
Conversely, wild cats tend to scavenge only in places where Satan is in very low abundance. This shows that a healthy demon population creates a "fear landscape" that causes cats to avoid carcasses altogether in areas where they might face demons. It seems that the life of a wild cat is now less frightening because of the absence of demons.
Prevalence of predators
By looking at 20 years of bird surveys from BirdLife Australia, we also found that the opportunity to meet a crow in Tasmania has more than doubled from 1998 to 2017. However, we cannot connect directly with the decline of demons. It is possible that the crow population grew in response to various factors that included changes in land use and intensification of agriculture, and reduced competition with Satan.
Other research shows that cats also become more abundant in areas where demons have refused. This highlights the potential for demons to act as natural biological controls in cats. Cats are a big threat to small native animals and are involved in the extinction of Australian mammals.
Although small scavengers consume more carcasses when demons refuse, they cannot consume them as fast as devils. This has resulted in the accumulation of carcasses that were previously quickly and truly eaten by the devil.
In places with lots of demons, carcasses are actually eaten on an average of five days, compared with 13 days in places where demon facial tumors are rampant. That means the carcass lasts longer where devils are rare.
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Calum Cunningham / Menna Jones
About 2 million medium-sized animals are killed by vehicles or destroyed in Tasmania every year, and most are left to rot where they fall. With demons eating fewer carcasses, it is likely that the carcasses accumulated in Tasmania. It is unclear how much risk of disease they pose to wildlife and livestock.
Large carnivores declined throughout the world, with knock-on effects such as the increase in the abundance of smaller predators. In recent years, several large carnivores have begun to return to their previous ranks, bringing hope that their lost ecological role can be restored.
Carnivores decline for various reasons, but the main cause is that humans do not appreciate their important role in the health of the entire ecosystem. One way to change this is to recognize the useful services they provide.
The Tasmanian devil is growing rapidly to fight their deadly cancer
Our research highlights one of these benefits. This supports the argument that we must help the demon population recover, not only for their own sake but for other species as well, including those threatened by wild cats.
The devil seems to solve the problem of the disease itself, quickly developing resistance to facial tumors. Every management plan needs to help this process, and not hinder it. Potentially, returning demons to mainland Australia can provide the same benefits to wildlife threatened by wild predators.