Rewilding death: The plan to restore the necrobiome

ve walked extensively across the UK's national parks, marvelling at the rolling hills and panoramic views, occasionally surprised by a deer bounding out of the brush. What I have never seen is a large dead animal.

The widespread eradication of top predators across Europe and the systematic removal of wild and domestic herbivores for food or sport, coupled with our fear of disease and squeamishness have gone a long way to sanitise the land of the spectacle of death.

But in removing unsightly bodies from natural spaces, a key part of the ecosystem has also been scrubbed out. Animal carcasses and the species that depend on them are collectively known as the necrobiome. Scientists are still fully getting to grips with this complex system, including understanding its impact on animals, birds, invertebrates, fungi and microorganisms involved in decay and decomposition. And crucially, whether leaving more carcasses out in the open could help us to nourish ailing ecosystems.

Decomposing vertebrate carcasses are just naturally repulsive to most people. Even for those of us who study it, it's not pleasant – Eric Benbow

The many species of the necrobiome perform essential roles returning organic matter and nutrients to the food chain, and removing potential sources of infectious disease. Some play other important roles in the ecosystem, for example as pollinators.

For Eric Benbow, a forensic entomologist and microbial ecologist who coined the term necrobiome and edited a landmark book on carrion ecology, studying decomposition is a challenge in more ways than one. In closely observing death and decay, there is the inevitable sense of confronting our own mortality. And then, of course, there's the smell. "Decomposing vertebrate carcasses are just naturally repulsive to most people," says Benbow. "Even for those of us who study it, it's not pleasant."

There are also practical difficulties. The speed of the decomposition process is one problem, says Jennifer Pechal, an entomologist at Michigan State University and a colleague of Benbow's. Nature is very efficient at removing and breaking down dead bodies, which makes them tricky to study and puts them out of sight from the wider public. Scavengers also are naturally resourceful and can damage or remove bodies while they are being researched, and replicating experiments accurately can be tricky, says Pechal.

For all these reasons, carrion has "not historically been a hot topic for ecological studies" and it can be difficult to get funding to research it. A study she co-authored concluded that there is a lack of data on the volume of carrion biomass for most biomes and animal species with "no clear pathway towards improved knowledge in this area".

Despite such hurdles, in recent years the field has blossomed. Early studies in the 1950s and 60s found a general and predictable process of decomposition as a carcass moves from death to bones, and there are now researchers in every environment and biome – from deep sea abysses to mountain ranges – studying how carrion decomposes, and the effects of that decomposition on ecosystems.

Pechal says technological advancements in biology, chemistry and computational sciences have helped. "[These] have vastly improved our capabilities to answer specific questions with increased accuracy and precision."

From what we do know, there is reason to worry. A paper in Global Change Biology examining scavenger studies from around the world found that hunting and the loss and modification of habitats had a big impact on the number and diversity of vertebrates feeding on animal carcasses.

The researchers warn that scavengers are unlikely to be performing their roles in the ecosystem as well as they could be, and make the case for management and conservation actions to preserve scavengers worldwide.

The motto of German national parks is 'let nature be nature', but we skipped out this natural process of what's happening with a dead animal – Peter Südbeck

In national parks across Germany, from the mountainous Alps in the south to the mud flats in the north to the dense forests of Bavaria, a plan is being hatched to address this. By leaving animal carcasses decaying on the ground, researchers want to gather evidence about the impact of carrion on a variety of ecosystems to see how important they are as a food source for scavengers, for wider biodiversity and soil health.

Peter Südbeck, head of the Niedersächsisches Wattenmeer (Lower Saxony Wadden Sea) national park and a lead proponent of the project, says there have not been any systematic studies on the impact of carrion in Germany, but it is a clear gap in the ecosystem. "There are no animals lying out in the field anywhere at the moment," says Südbeck. "The motto of German national parks is 'let nature be nature', but we skipped out this natural process of what's happening with a dead animal."

From September 2022 – as long as full funding can be found – roe deer and other locally significant species will be left in chosen places in the parks and researchers will study what happens using camera traps for bigger scavengers and pitfall traps for invertebrates, as well as soil sampling and genetic tests.

The project will build on research undertaken in the Bavarian national forest, led by ecologist Christian von Hoermann, into the best ways of supporting carrion species. Von Hoermann has found that deliberately leaving carcasses out attracts a wide range of scavengers and endangered species – especially in the winter. The scavengers are not too picky about the species left out, or whether the carcass had been fresh or frozen – but size does matter. And leaving carcasses in regular fixed places instead of random ones attracts a greater diversity of carrion insects.

Von Hoermann describes carcasses as "biodiversity hotspots" and hopes Germany's project will answer important questions about the necrobiome and how human activities are changing it. We do not know, for example, how sensitive carrion species are to changes in the volume or variety of carcasses. Nor has research explored how climate change will affect the composition and abundance of scavenger species or the rate of decomposition.

One accidental experiment in death ecology might hold lessons for the German project.

In the Oostvaardersplassen nature reserve in the Netherlands – one of the oldest rewilding sites in Europe – the sudden death of thousands of large herbivores during a cold snap several years ago showed researchers that large carcasses can significantly increase the numbers of invertebrates. It benefits vegetation too; the research found a big increase in plant matter around the carcasses months after the animals had died. But the deaths also sparked public concerns about animal rights and the number of animals in the reserve has since been limited.

Separate research into a herd of reindeer killed by lightning in Norway in 2016 found that the animal carcasses provided a hugely important temporary source of nutrients for a wide range of species including golden eagles, ravens and wolverines. The strategy of artificially supplying carrion "is not without controversy", the study noted.

"One of the problems is it's not fully understood how the food webs work, how nutrients are cycled and how important the carcasses are in linking all these things together," says Debbie Fielding, a research assistant in community ecology at the James Hutton Institute. "There's quite a lot of perceived benefits, but there's not actually that much evidence for it. And that's not necessarily because there aren't the benefits [but] it's unlikely to be a win-win situation."

Fielding was part of a research team that left red deer carcasses on a variety of upland estates in Scotland to study their impacts on local ecology. Although the project was curtailed due to lack of funds, it did find that carcasses on estates focused on conservation were quickly taken away by scavengers such as foxes while on more heavily managed estates with greater predator control the process of decomposition was slower and dominated by insects.

She stresses that cultural factors make leaving carcasses out a complex decision. "In Scotland the deer are traditionally hunted for trophy hunting and venison. If they're going to shoot it, they want to make sure that it's being used properly so they want to recover it; just to leave a carcass out on the hill rotting is seen to be very disrespectful."

Projects also need to be careful to avoid unintended consequences. Some vulture "restaurants" in southern Africa, for example, led to complaints from tourists, attracted unwanted scavengers, caused problems for farmed animals and even resulted in illness as people ate food left out for birds that was unfit for human consumption.

Südbeck admits the German project, which is "a little bit dirty", will involve some public relations work and sensitive discussions with the hunting community. "We have to talk about what does it mean in terms of nature conservation, in terms of the national park's overarching aims, and of course in terms of society," he says.

Since earlier attempts to deliberately leave carcasses out in projects across the world were criticised for ignoring the wider social and economic context, Südbeck says the national parks will share details online and in information centres, and some will also install webcams. Live feeds from cameras on carrion spots in the Bavarian forest have already shown animals such as lynx feeding and proved very popular with the public.

Decisions about where to leave carcasses also need to be handled sensitively. In Wattenmeer, carcasses will be left on uninhabited islands rather than busy beaches where holidaymakers are sunbathing.

Südbeck hopes the German carcass project will be the first step in demonstrating the value of moving towards a healthier ecosystem. Having studied ornithology, he admits it is easier to justify making changes on behalf of charismatic species like sea eagles than it is for tiny beetles, "but the responsibility of national parks is to look at the whole biodiversity".

Von Hoermann stresses that leaving carcasses out is a rather artificial way of dealing with the problem, and says that, once the benefits have been demonstrated, the job should be really done by natural predators like lynx and wolves. "In the Yellowstone National Park, they produce carcasses all over the year in a natural way and support the diversity of other carrion-associated species from bacteria, fungi, insects up to vertebrate scavengers."

Ultimately, he believes that people will come to terms with dead animals in national parks, just as they are increasingly with deadwood. Although it focuses on exploring the ecosystem benefits of leaving carcasses to support the necrobiome, Germany's experiment is perhaps as much about changing perceptions of the place of death in nature. As von Hoermann puts it, it is about getting the public used to "the very natural situation of dying animals in the landscape".