For the last 15 years, Tasmanians have been arguing in pubs, parliament and online about whether or not there are foxes on the island. Recent research republished in PLOS One suggests that if they were here, foxes are now extinct in Tasmania.

So was the Tasmanian government’s A$35 million program to eradicate foxes worthwhile? Critics have argued that it was an expensive folly with no basis in evidence. But given the damage foxes can do to wildlife and farming, early action to eradicate them was wise.

Another Tasmanian tiger hunt?

The saga began in 1999 with reports of a deliberate introduction of foxes to Tasmania. The discovery of several road-killed carcasses, many claimed sightings from the public and DNA evidence all pointed to a fox presence.

Independent experts were alarmed and urged an eradication program. The state government responded with systematic poison baiting in areas most likely to be occupied by foxes.

But critics argued that the evidence was flawed or hoaxed. They accused wildlife authorities of profligacy, self-interest and deception. The weightiest point on this side of the argument was the lack of “proof”, in the form of an animal trapped or photographed in the wild, of foxes in Tasmania.

After many fruitless attempts to catch or photograph a thylacine, we have concluded that they no longer exist. Why was the same conclusion not drawn about the Tasmanian fox?

DNA fingerprints

The DNA evidence is especially significant to this problem. It can be very hard to detect animals, and thus confirm the existence of a population, where they occur in low numbers. Testing scats (faeces) for DNA is probably the best way to confirm the presence of foxes when they are still extremely rare.

State government scientists implemented this test by collecting carnivore scats across the potential range of the fox in Tasmania. After DNA extraction and testing of scats, most turned out to be devils, quolls or cats, but 61 tested positive for fox. This indicated a small but widespread population.

This DNA evidence was criticised in two papers, which featured in an ABC Radio National Background Briefing report on the fox controversy.

The first paper argued that the scats identified as fox were false positives - that is, they were some other species but tested positive because the test was not specific enough. But this critique considered only the first step in the test, which amplified DNA fragments likely to be fox, and ignored the second step, which sequenced that DNA to confirm identification.

The second paper considered the possibly surprising fact that no individual fox’s DNA was detected more than once. If searchers found one scat of a certain fox, it seems likely they should have found more scats from the same animal in the same area. The critics concluded that because this didn’t happen the DNA test was not detecting foxes at all.

But they did not account for the fact that each area was not completely searched: a new analysis shows that this means an absence of repeat detections is quite likely.

Scientific uncertainty

So both papers exaggerated problems with the DNA evidence. But the debate does emphasise uncertainty over the presence of foxes in Tasmania.

This uncertainty may now be as high as ever, given that no carcasses or DNA-positive scats have been found since mid-2011. Perhaps foxes were here but have gone extinct, or maybe they were never here and the whole thing was a hoax or a stuff-up. As with many scientific problems, our evidence falls short of absolute proof.

We think the most interesting aspect of this issue is not the search for proof, but what decisions to make on the basis of incomplete and contested evidence. Let’s imagine that there was a low-density population of foxes in Tasmania during the mid-2000s, as the DNA evidence suggests. This population could have done one of three things:

• grown exponentially

• gone extinct due to chance factors, as small populations often do

• persisted at low numbers for a while, and then either grown exponentially or gone extinct.

If the first scenario had happened, we would be in no doubt that there had been a small population of foxes, because it would now be a big population that is easy to detect (but impossible to eradicate).

If the second had happened, it would be difficult to prove retrospectively that the population had ever existed.

And if the third scenario happened, we would face the dilemma of deciding how long to wait before concluding that the absence of exponential growth meant that the population had gone extinct (or, possibly, never existed).

Did the government make the right choice?

Now, the crucial question: given the evidence available, what ought the state government have done? It could have done nothing, while waiting for stronger evidence, or it could have taken pre-emptive action in the hope of preventing population growth and increasing the likelihood of extinction.

The first option would have involved the least cost in the short term, but high risk of long-term damage from an established fox population. The second would have reduced long-term risk, while exposing authorities to the criticism of acting on incomplete knowledge.

Our view is that the weight of evidence supports the existence of Tasmanian foxes back then. We hope they are now gone, as suggested by a new analysis of the carcass data. But, in any case, the early response was wise.

Tasmania’s wildlife authorities have copped plenty of criticism over this. In weighing this criticism, we should reflect on the costs of doing nothing in the event of an incursion of foxes: a damage bill of millions of dollars a year for Tasmania’s livestock industries, and the extinction of species that the fox wiped out on mainland Australia and which now survive only on this blessed isle.

Chris Johnson is a member of the Tasmanian government Fox Technical Advisory Panel. This is a voluntary position.

Menna Jones conducts research funded by the Australian Research Council on fauna in Tasmania that is vulnerable to fox predation.