Oisin Sweeney: Rewilding in general is a movement I guess that seeks to recognise that humans can't possibly manage every interaction in ecosystems, there's just too much going on and too much that we don't know.

James Bullen: Oisin Sweeney is a senior ecologist with the National Parks Association of New South Wales.

Oisin Sweeney: It also recognises that ecosystems are not static in time, so they have no start-point or endpoint, so over ecological timescales, ecosystems will change as climatic conditions change, as the species mix changes. Rewilding really seeks to recognise that and to say if we restore functioning ecosystems with the maximum interactions between species, then perhaps they will be resilient over time through change.

James Bullen: Rewilding has been growing in popularity over the past few years, especially in Europe. There are different types. Passive rewilding is one.

Oisin Sweeney: Passive rewilding is really a term that has been coined in Europe to describe the changes in biodiversity that have resulted from decreased human management of land, particularly in marginal farming upland areas of Europe.

James Bullen: Another more active approach is trophic rewilding.

Oisin Sweeney: Trophic rewilding is really a focus on reintroducing species that have a very important role in ecosystems. So a trophic level in a food web essentially describes a feeding level, so the term 'trophic rewilding' really describes a species whose influence extends beyond its own trophic level and down ultimately to plants.

James Bullen: And usually the species that exerts the strongest top-down influence on an ecosystem is a predator. In the Australian context, that could be the Tasmanian devil.

Oisin Sweeney: So there's great enthusiasm amongst some in the scientific and the NGO world to see would it be possible to replicate some of those trophic influences on mainland Australia with Tasmanian devils. So the research in Tassie shows that devils indirectly protect smaller animals, like quolls, from introduced species like cats, and that the loss of devils to devil facial tumour disease is actually having knock-on cascading consequences on these other species. So yes, it would be very exciting to see whether the introduction of Tassie devils on the mainland could result in some trophic rewilding, and could indeed benefit some of the small mammals on mainland Australia that are at the moment put at risk from predation by foxes and cats.

Michael Westaway: The modelling that has been done by ecologists in more recent years is heavily biased towards our understanding of the distribution of Tasmanian devils today.

James Bullen: Dr Michael Westaway is a Future Fellow at the University of Queensland in the School of Social Sciences.

Michael Westaway: So we know they're in Tasmania, there are closed wet forests, cooler conditions. When you look at the fossil record, a very different picture of its biogeographical range emerges, and we can see devil fossils south of Alice Springs, we can see the devil fossils from the northern tropics, we see devil fossils in the high country, we see them in semi-arid New South Wales. Their distribution is extraordinary, they are in every major habitat in the country. This indicates that Tasmanian devils were a much more diverse and adaptable predator than were previously considered.

James Bullen: Thousands of years ago they weren't Tasmanian devils, they were distributed across the Australian continent. Then about 3,000, 3,500 years ago the devil disappeared from mainland Australia. Some think the appearance of the dingo around that time is to blame, but no one can say for sure. Now all that's left are the bones. And some poo.

Michael Westaway: You don't always get the actual record of the carnivore itself, the fossil remains of the carnivore. So what we do get however in this instance is its fossil poo.

James Bullen: A couple of years back, Dr Westaway and his team were digging in Willandra Lakes in far west New South Wales.

Michael Westaway: The really interesting site that we came across was a large small-fauna site that has been known by archaeologists for quite some time. There were questions about how did all this fauna get here, what was the catalyst for the accumulation, the large number of fossils and coprolites (fossilised poo) found across that site? What are excavations revealed is that it was a series of burrowing mammals, probably bettongs, their burrows. And what we found throughout that whole burrow complex were loads of coprolites. There was very obvious evidence of a lot of activity from small marsupial carnivores.

James Bullen: That's the devils. And you might be wondering why these scientists were so excited by the voluminous number of coprolites they found in the burrows. It turns out you can learn a lot from fossilised devil poo.

Michael Westaway: We would see fragments of burrowing bettongs and bilbies and even small wallabies, fish, birds. Devils are carrion eating animals but they also are predators and so they were going through these burrows and consuming bettongs. We saw half a bettong face in one of these coprolites.

James Bullen: The poo gave them some idea of what food webs from thousands of years ago in the Willandra Lakes looked like. Devils preyed on smaller marsupials, but the predator/prey relationship went on for millennia, so it's not like devils wiped out the smaller creatures.

Michael Westaway: Tasmanian devils would interact and sustain healthy populations of burrowing bettongs, stick nest rats, bilbies, a whole range of fauna that are now on the critical endangered species list.

James Bullen: That's a key difference between the devil and introduced predators like cats and foxes who are wiping the smaller guys out.

Michael Westaway: It's clear that in the Willandra Lakes the exotic predators (cats and foxes) had had an absolutely devastating impact on these small borrowing mammals, stick nest rats and hopping mice, and all the small fauna has been absolutely devastated as a result of these exotic predators. Only several thousand years before, which in the geological timescale is a flicker of the eye, there are many more taxa, many more species existing in that environment. So the food web provided an alarming insight into how devastating these predators had been in that environment.

James Bullen: These pieces of evidence are why Dr Westaway believes we should look at reintroducing the Tasmanian devil to the mainland.

Michael Westaway: Tasmanian devils were distributed across the country. The fossil record shows that they coexisted because of their co-evolution with all these small critical weight ranged fauna. We think that there is a case that Tasmanian devils, if they were brought back to the mainland, could apply pressure on cats and foxes, reduce the biomass of those exotic predators and start to restore ecosystem function in Australia.

James Bullen: So at one point, devils lived across Australia. But what do we know about how they'd interact with animals on the mainland today? Professor Mike Letnic from the Centre for Ecosystem Science at the University of New South Wales is also in favour of reintroducing the has the Tassie devil to mainland Australia. His team used modelling to compare different ecological scenarios. In some, devils are reintroduced to the mainland, and foxes and dingoes are controlled, or eradicated in the case of the dingo particularly. Under those models, populations of smaller endangered marsupials increase over time.

Mike Letnic: So we think that the devil could perform many of the functions of an apex predator, much like they're doing in Tasmania, so they could help keep cat numbers down which will then help to alleviate cats' effects on wildlife.

James Bullen: But the team's analysis has been contested.

Dr Bronwyn Fancourt is a research fellow at the University of New England and has worked in Tasmania looking at populations of devils and feral cats. She says we simply don't know what the effect of devils on foxes will be because their interactions have never been observed.

Bronwyn Fancourt: We can only guess what would happen, and that's because no one alive has seen the interactions between foxes and devils. Devils are in Tasmania, foxes are on the mainland. They just haven't been in the same place at the same time for people to observe what interactions would be.

Mike Letnic: We know almost nothing about devils and foxes, except that foxes have been introduced at various stages to Tasmania and they've never made it. But there are all sorts of reasons why they could fail. One reason that is plausible is that devils put a stop to it.

Bronwyn Fancourt: When we just don't know, you've got to adopt the precautionary principle and be conservative in our estimates, but in this proposal the assumption is that devils will have a strong negative impact on foxes. But there's actually nothing to support this.

Mike Letnic: Dr Fancourt also disputes the idea that cat numbers would drop when devils are introduced.

Bronwyn Fancourt: The assumption that devils will suppress feral cats to a significant degree, that assumption comes from a subset of spotlight counts in Tasmania which showed a slight increase in cat detections after devils started to decline.

James Bullen: Spotlighting is a method of surveying. Often researchers drive along a road, shining a light into the bush and counting the animals they see.

Bronwyn Fancourt: But those spotlight counts are performed across the entire state every year by the Tasmanian government.

James Bullen: Dr Fancourt says different regions have different results, and the overall picture doesn't support the idea that more devils means fewer cats. She also says that detecting more cats doesn't necessarily mean their numbers are increasing. It might just be that their behaviour is changing.

Bronwyn Fancourt: So I looked at that exact question as part of my PhD research. So I was investigating why eastern quolls have declined in Tasmania, and one of the hypotheses was that with devils declining, cat numbers might have increased, and that might have put increased pressure on eastern quolls. So to understand whether cats had actually increased, I looked at the relationship between cat and devil abundance and I found that cat abundance was the same regardless of whether devil populations had been devastated by the disease or not. So areas with intact healthy devil populations have the same cat abundance as areas where devils have all but gone from the landscape.

Mike Letnic: I think there is some reasonable research that shows that devils suppress cats or at least change cats' behaviour and cats are submissive to devils, I'm confident with that. We can focus on the devil/cat case and the evidence for that, but if we look more broadly and if we look worldwide, what we do see is that larger predators invariably suppress smaller predators because they kill them and they compete with them for food, and the larger predator nearly always does better.

James Bullen: The evidence here is complex and contested. Different research papers suggest different results. But Professor Letnic says a small-scale reintroduction of the devil is an experiment that he'd love to do.

Mike Letnic: I think the risks of bringing devils back are very low, given what we know about them in Tasmania. So I can't see why we probably couldn't give it a go. Maybe one way to give it a go would be to at least trial it first under relatively controlled conditions, and then see how that goes and then bring them back. I think it would be fun.

James Bullen: One suggestion that has been floated by a number of researchers is to introduce a male-only population of devils to the mainland so they can't breed, and monitor their progress. But Dr Fancourt says we need a stronger scientific foundation before the reintroduction of the devil is considered.

Bronwyn Fancourt: I totally agree that we need to have some bold thinking and some bold action, but that doesn't justify putting a species into an ecosystem when all the evidence suggests it's not going to be beneficial. So you would do that in situations where you were satisfied that you could make sure that devil was safe and make sure it's not likely to any more harm. There's bold, and then there's ethical, and to me that's not necessarily an ethical move.

David Nogués: We are living obviously in times of biodiversity crisis and tonnes of scientific data pointing to that. I think the question here is how do we achieve the aim of protecting what we have in a better way, where we do the maximum impact with the amount of funding that is available.

James Bullen: Dr David Nogués is an Associate Professor in the Centre for Macroecology, Evolution and Climate at the University of Copenhagen. He's seen the growing popularity of rewilding in Europe but is concerned the movement is extending beyond its scientific foundation.

David Nogués: There is obviously a lot of good science going on to try to understand what happens when we introduce a carnivore or when we introduce a herbivore, what happens to the ecosystem when we remove it. We have really strong theories about life, the architecture of life is made based on ecological networks. So we of course are in a position in which we are gaining more and more now an understanding of the potential impacts of our interaction or an interaction, but I will say that we are far yet to really be able to predict with a high probability what might be the final result or the outcome of a conservation project.

James Bullen: And with that comes the risk of unexpected knock-on effects.

David Nogués: My interest in rewilding comes because a significant part of my research has been trying to explore how past natives reacted to different factors of pressures like climate change or human impact. I have been studying species like woolly mammoths or also other carnivores in the Pleistocene, and what I learn from there is that it's quite difficult to predict how ecological communities react to changes. I know that because what you see the past, I can validate predictions from my models. So when the idea for rewilding came to the debate and I took an interest, basically I started to understand that we might expect some reaction to an ecosystem when we introduce a new species, but it may be also unforeseen impacts. And ecosystems might maybe change in directions that we might not expect.

James Bullen: Dr Nogués says an example of that was seen when beavers were introduced to Patagonia in 1946.

David Nogués: The beaver was never a native species in Patagonian rivers. The populations grow really fast, and what are basically happened later on is there was a massive amount of dams built by beavers. Rivers don't heal those dams, they dry up, killing the forest and basically drive into extinction the habitats that birds and insects and other animals and plants were in need of. I think those are two good examples about all the considerations should be behind the translocation of the Tasmanian devil into the continent.

James Bullen: The key difference between that beaver example and the devil is we know the devil isn't an introduced species in a typical sense. Not too long ago it was a native on the mainland. But it still gives Dr Nogués pause.

David Nogués: I only ask for actions in the fields that implement any kind of conservation approach, not only rewilding of course. It has to be a very careful plan and needs to have the all of the scientific background work behind. I think maybe my overall message from this interview may be that before implementing actions in the field, there should be a lot of planning and scientific work behind.

James Bullen: And while some experts advocate caution, others argue we need to push ahead in the face of a biodiversity crisis in Australia.

Bronwyn Fancourt: The concern is we don't have complete information, no one has complete information, so we have to make assumptions. But we have to make educated assumptions I guess. And when there is data available and all that available data is not actually being used to feed these models, then obviously the output is not going to be reliable, and the concerns are that the species that are…the hope that they are relying on these species to conserve other species and protect other species may be unfounded.

Michael Westaway: I think conservation biology needs to be quite bold in this country. Certainly it shouldn't be reckless, and we are not advocating an idea that we just release these things without any control, but experiments need to be conducted on the mainland to see if they can actually suppress the activities of predators and to see if they can also coexist as they did in pre-history with native marsupials.

James Bullen: That's Dr Michael Westaway, a Future Fellow at the University of Queensland in the School of Social Sciences.