Despite the damage humans are causing to the planet, in some cases wildlife can benefit from the presence of humans. The Tasmanian devil, for example, often feeds on road debris left by humans.
But our new research published in Scientific Reports suggests that this apparent benefit may come at a cost.
We compared the diets of Tasmanian devil populations living in three types of habitats by examining their whiskers. We have found that in many cases, Tasmanian devils primarily eat food that has been inadvertently provided by humans. Access to this food changed the behavior of Tasmanian devils – and potentially put them at risk.
Our findings are particularly important given the risks to Tasmanian devils posed by aggressive facial cancer disease. If we are to protect this endangered species, we must preserve environments untouched by humans.
What do Tasmanian devils eat?
The Tasmanian devil is the largest carnivorous marsupial in the world. It used to be found on mainland Australia, but now wild populations are only found in Tasmania.
Tasmanian devils rarely hunt prey. But they are extremely effective scavengers thanks to their keen sense of smell, bone-crushing jaws, and energy-efficient movement.
Animals that forage for food are “opportunistic feeders” — in other words, they eat whatever they happen to find. This usually means scavengers have a varied diet.
But our previous research has found that Tasmanian devils have a remarkably restricted diet. To find out why, we examined Tasmanian devil whiskers. A single whisker can provide a window into the animal’s past.
We used a technique called “stable isotope analysis” that allowed us to measure nitrogen and carbon incorporated into the devil’s whiskers during growth. We matched the whiskers’ chemistry to potential foods to determine what the devil ate weeks or months ago. Then we looked at how this differs between individuals living in different habitats.
The technique has been used to describe the diets of early humans and extinct species. It has also been used to study the migration patterns of widespread birds and marine mammals.
And the results?
We found devil populations in highly disturbed landscapes, such as cleared farmland, feeding on only one type of food — medium-sized mammals like the Tasmanian pademelon.
This is perhaps not surprising. Pademelons are very common in agricultural areas and often end up as road killers. So Tasmanian devils have little reason to look for other types of food.
We also studied the diet of devils in eucalyptus forests that have been cut down and regenerated. These animals also had relatively restricted diets. The result suggests these forests may not have had time to develop mature features such as tree cavities to protect birdlife, a process that can take up to 140 years.
However, the results were different for devil populations in old-growth rainforest habitats that were never logged. There, devil diets were varied. Larger devils tended to eat mammals such as Tasmanian pademelons and bush possums, and smaller devils consumed birds such as green rosellas.
These populations may provide insight into devil foraging behavior prior to European settlement.
Save wild landscapes
One would think that reliable access to food accidentally provided by humans would benefit Tasmanian devils. In fact, however, it may come with hidden dangers.
The presence of roadkill poses risks for devils; They can be drawn to roads and become roadkill themselves. In 2021, more than 100 devils were reportedly killed by motorists on just one stretch of road in north-west Tasmania.
And if members of the same species interact around a smaller number of carcasses – or in the case of Roadkill, the largest and most coveted carcasses – it could encourage the spread of the devil’s facial cancer disease.
Over the past 25 years, the disease – an aggressive, transmissible parasitic cancer – has caused Tasmania’s devil population to decline by 68%. And this year, the disease was first detected in north-west Tasmania, in the same population as many devils in our study.
A vaccine spread by edible bait is being developed. But in the meantime, a more varied diet could reduce a devil’s risk of spreading or contracting the disease to others.
Only in ancient rainforests did devils have a varied diet that lived up to their reputation as opportunists. The results suggest that preserving these wild landscapes is critical to protecting the Tasmanian devil.
Anna C. Lewis et al., Life in human-altered landscapes narrows the dietary niche of a specialized mammalian scavenger, Scientific Reports (2023). DOI: 10.1038/s41598-023-30490-6