As humanity grapples with COVID-19, world leaders are scrambling to put in place measures to lessen its devastating impacts. In doing so, the importance of accurate and reliable modelling has been pushed into the spotlight. This is because people want to understand the disease, how to best manage it, how severely it will affect their lives and how long they must live with the disruption.

With any disease there are generally three possible outcomes in the long-term: eradication of the disease, extinction of the host, or a state of coexistence between the host and the disease. Models are developed to work out which of these outcomes is most likely, and disease management decisions rest on these modelling results. Of course, working out which of these outcomes is going to eventuate is challenging.

Similar challenges have followed the story of Australia’s iconic marsupial: the Tasmanian devil. These ferocious mammals, sharp in both dentition and personality, were once widespread across the Australian continent and filled the ecological niche (the role and position of an organism in its ecosystem) of an apex predator. But since the arrival of Europeans, the beloved devil has disappeared from the mainland and is now only found on the island state of Tasmania.

Since the mid-1990s, the remaining devil population has faced a new threat, this time in the form of a viral cancer called devil facial tumour disease, or DFTD for short. The disease, as the name suggests, causes the growth of large and debilitating cancerous tumours on a devil’s face.

Anyone who knows anything about Tasmanian devils knows that they tend to bite one another during social interactions. Unfortunately for them, they might end up biting off a bit more than they can chew – biting down into a facial tumour is exactly how DFTD spreads.

The disease is almost always fatal, and it is usually the fittest individuals (i.e. the best breeders) who are exposed. As can be imagined, this has had a devastating impact on devil populations, with approximately 95% of the entire population becoming infected in just over 20 years, leading to an 80% drop in devil numbers.

The devastating impacts of this disease led to predictions of imminent extinction of the species. However, to date no local extinctions of any populations have been documented. So, this begs the question: why hasn’t this iconic species joined the ever-growing list of extinct species?

The answer lies in a crucial piece of information left out by earlier predictive models. When a Tasmanian devil is infected with DFTD, the number of transmissible tumours (i.e. tumours that can spread cancer between different individuals) increases as the infection goes on. So, an infected devil is more infectious a few months after contracting the disease than it was when it first started to develop symptoms. This additional factor of time can have a major influence on the dynamics of disease spread and has, until now, been left out of predictive modelling.

Previous models have also assumed that the disease is being spread in devil populations that are equally and wholly susceptible. But in reality, not every individual is equally susceptible, nor is every individual equally infectious.

Density and social behaviours also need to be considered when formulating disease models. The importance of density is aptly demonstrated by the rampant spread of COVID-19 in the world’s largest cities, as well as behavioural changes, such as social distancing being critical to contain the spread.

A recent study on DFTD has incorporated these factors of individual and spatial variability into a new predictive model, tells a different story of DFTD’s likely path. This study led by Dr Konstans Wells from Swansea University in the UK found that, contrary to previous models, eradication of DFTD was the most likely scenario, an outcome that was supported by 57% of their models, with the possibility of coexistence between devils and DFTD coming in second, supported by 22% of their models. The extinction of devils altogether ended up in last place with a 21% likelihood of occurring.

Not only do these new modelling results paint a much more hopeful picture for the future of Tasmanian devils, but there is also evidence that devils are beginning to rapidly evolve resistance to DFTD.

Why is this new information important? It demands a reconsideration of our conservation strategies for Tasmanian devils. Current conservation strategies include reintroducing captive-bred and disease-free animals that may not have evolved a genetic resistance to DFTD into wild populations. Doing this may do more harm than good, as it could dilute genetic resistance to DFTD in wild devil populations.

Why is their conservation important? As an apex predator, Tasmanian devils hold an especially important position in their ecosystems. They are essentially the ‘sharks’ of Australia’s terrestrial environment. There is evidence that they help to suppress feral cat numbers, so if they are lost this would spell devastation for Tasmania’s smaller native mammals. There is also potential to reintroduce Tasmanian devils to the mainland in an attempt to create a natural suppression control for the ecological disaster of invasive predators, such as foxes and feral cats.

So, where to from here for devil conservation? The modelling results of Dr Wells and his colleagues suggest that the devastation of devil numbers we have already witnessed has merely been the first of many peaks in a slow-burning disease. As Dr Wells explains, with DFTD “the initial dynamics can be rather different from long-term dynamics”, and this needs to be factored into current conservation strategies.

Dr Wells believes that captive management programs and conservation efforts in the wild “should move forward together rather than as independent conservation efforts”. Work is needed to assess whether releasing “healthy [captive-bred] devil individuals into wild and disease-affected populations would have positive or negative consequences in terms of population growth. This is because if healthy and introduced devil individuals would exhibit behaviour that facilitates disease spread, their introduction into wild populations would not necessarily be a sensible conservation measure”. Their predictive model also suggests that if large and undisturbed natural environments are effectively protected, we may be able to create a world where Tasmanian devils, and other species, are able to cope with threats such as infectious disease in the future, and all by themselves.