Fallow deer have the potential to double their range in Tasmania with 56 per cent of the land mass providing suitable habitat, a new research paper has argued, with numbers increasing about 11.5 per cent per year for 35 years.
The paper from the University of Tasmania agrees with a common assertion from hunters that deer numbers had been relatively stable for 150 years, and that their range expansion in recent decades could have been driven by escaped or released deer from farms, increased irrigation and a decline in wool sheep.
Researchers analysed 5761 annual spotlight counts from 1985 to 2019 and 3225 camera traps across Tasmania covering 2014 to 2020 to develop maps of changes in deer numbers and range, finding they now span 27 per cent of the state.
Lead author, ecologist Professor Chris Johnson, said they then used combined Australian and European deer occurrence data - among other methods - to model habitat and climate suitability, which was then modelled to areas of potential expansion in Tasmania.
These areas of expansion were primarily in the North and East, with the potential to more than double their current range. Other suitable habitat areas were in elevated grass and shrublands in the north and north-east Wilderness World Heritage Area.
The paper, submitted for review with the journal Biological Invasions, argues that the priority must be on eradicating newly-established "satellite" communities.
"Decades of invasion ecology show that early eradication is the most cost-effective, long-term management strategy," the paper states.
"Next, we suggest that deer be prevented from expanding into protected areas, such as the Tasmanian Wilderness World Heritage Area. Habitats and pathways of likely spread could be identified and prioritised for monitoring and management.
"The trajectory of the Tasmanian deer population, together with climate and habitat suitability modelling, indicate that the population is likely to expand further, including into areas of high conservation value."
Deer population was stable for generations, then increased rapidly
Studies estimated that deer numbers in Tasmania were about 7000-8000 in the 1970s and had not ventured far beyond 60 kilometres from initial release regions in the Midlands from the 1830s.
By the start of the 21st century however, their numbers were closer to 10,000 before they increased rapidly.
An aerial survey carried out by the government in 2019 estimated 53,000 deer, which Professor Johnson believes was an under-estimate as it occurred at the end of a hunting season when about 30,000 deer were killed.
The paper argued that it was not uncommon for introduced species to remain in low number for long periods before expanding rapidly.
"It remains unclear why the Tasmanian deer population was relatively stable for over a century preceding the current increasing phase, but this is common for exotic species, which often exhibit time lags and exist at low numbers for decades before increasing rapidly," it reads.
A "gradual shift" in a climatic driver, increased irrigation, the end of poisoning and declining livestock numbers could have played a part.
Professor Johnson said they found an 11.5 per cent increase per year, which would result in a doubling of their population within six years.
Tasmania Deer Advisory Committee chairman Andrew Winwood said that human land use factors were among the reasons for the population increase, but disagreed that their numbers would continue to rise at such a rate.
"If they've only jumped to 53,000 in 200 years, why is it that they're going to get so out of control in the next 30 years? It just can't happen. The deer don't just jump on a bus from Ross and go to Sassafras," he said.
Managing deer numbers a complex problem
The Tasmanian Government is developing a wild fallow deer management plan and has released draft regulatory changes - including a new permit to cull deer over property damage - but environmental groups and hunters differ widely on the path forward.
The Invasive Species Council is urging the government to remove the partly-protected wildlife status for deer and declare them a feral species, claiming it would remove "red tape" for land owners and land managers, and "lay the groundwork" for a management strategy.
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Deer project officer Peter Jacobs said the protected status was intended to allow them to be managed as a hunting resource - like native ducks - but their numbers had grown beyond that.
"We're saying the numbers should be reduced to 10,000 around the Midlands area. The hunters would play a role in that, but it also requires pest managers. The only way is to have professional pest controllers ground shooting and aerial shooting, that can be integrated," he said.
"It's not about eliminating deer, but about keeping a traditional deer range so they're just in the Midlands."
Mr Winwood, however, said the partly-protected status was intended to manage the humans who hunt the prey, rather than the other way around, and he believed the current framework could work to control deer numbers.
"The best outcome is to have a management plan in place. You set goals, you tick them off as you go," he said.
"Where there's a small population you want to remove, then send in 10 hunters. They'll harvest these few deer and move away. That's a humane way to do it.
"That can happen right now."
He had animal welfare concerns regarding aerial shooting, particularly if tens of thousands of deer were proposed to be killed. At the moment, female deer cannot be culled during the fawning period, but Mr Winwood feared this would change if they were declared feral.
"It's inhumane to harvest deer when they have fawns at foot, because the female could leave the fawn somewhere in the forest and then get culled. That fawn will die a slow and cruel death," he said.
The government's draft regulation changes are open for public comment until September 30.
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