More than one year after aerial baiting to rid Macquarie Island of its destructive rabbits, rats and mice, there are encouraging signs that the eradication effort has been successful.

The seven year program began in 2007 with the aim of restoring the island’s biodiversity to a natural balance, free of the impacts of introduced species. Pest eradication project teams around the world have been watching the project on the sub-Antarctic, World Heritage-listed Macquarie Island, with interest. It is the world’s largest eradication project for three species at one time and the logistical challenges of undertaking the project 1500km from Tasmania, accessible only by ship, has added immense complexity to the undertaking.

Keith Springer, the Macquarie Island Pest Eradication Project manager for the Tasmania Parks and Wildlife Service is cautiously optimistic about the removal of both rabbits and rodents and the recovery of the island’s habitats.

‘Following the completion of aerial baiting in July 2011, the hunting phase began with the objective of removing the small number of rabbits that were expected to survive the baiting. A total of 13 rabbits were found and killed, with the last rabbit killed in November 2011,’ Mr Springer said.

‘I’m confident that rabbit numbers are now extremely low, and we estimate that there may be fewer than five remaining on the island. The last rabbits killed were a doe and kittens, but the father of that litter has not been located, and rabbits responsible for grazing damage in a couple of areas have also not been located and dispatched.’

The focus for the current eradication team is clear — to locate and dispatch any remaining rabbits that found the bait unappetising and survived. The island is divided into six hunting blocks, with dog handlers and hunters tackling one block for a four-week period. Their search patterns are recorded on GPS units and since August 2011 they have clocked up an incredible 33 412km as they search every accessible nook and cranny of the 12 785 hectare island’s varied terrain.

The going will invariably get tougher for the hunters with each month that passes with no further sign or rabbit kills. Physically, travel is more gruelling as the vegetation recovers and becomes denser and increasingly wetter, as it retains more of the atmosphere’s moisture in its leaf mass. Spotting rabbit disturbance is also more difficult as the ground cover increases. The psychological challenge increases too with each month that passes with no sign of rabbits.

The news is also encouraging in regard to the rodents, but Mr Springer is reluctant to claim success at this early stage.

‘I am pleased by the lack of rodent sign and I’m pretty confident the rats are gone because we know they take the bait well and we spread enough bait for all the target animals to access some. But mice eradications have failed on much, much smaller islands. We don’t fully understand the reasons for this, but there is a pattern that when rats are present, the success rate of mouse eradication decreases,’ he said.

As a precautionary measure, baiting continues at huts and station buildings and the island’s coastal caves. Rodent detection dogs will be part of the hunting team that arrives in February 2013. These dogs will help the team to confirm that the eradication of rodents has been successful. A two year interval after baiting allows any surviving remnants time to breed up to detectable levels, before eradication success is declared.

With the removal of rabbits, rats and mice, the island’s vegetation, insect and bird life are showing signs of recovery. Recently, visitors to the island have observed areas of dense spider webs in recovering vegetation, shimmering with moisture droplets.  It’s a sight not previously recorded on Macquarie Island and is testimony to the rapid recovery of spider populations in the absence of mice predation.

Populations of some bird species are slowly increasing, with the island’s burrowing petrels being the main beneficiaries. In particular, blue petrels, previously restricted to breeding on offshore rock stacks due to rat predation, have begun to breed again on the main island. Likewise, grey petrels have shown increased breeding success and have fledged greater numbers of chicks; both positive indicators even in these early days. In the first breeding season since baiting finished, Antarctic terns are now breeding on the island’s cobblestone beaches in far greater numbers than previously, when they were restricted to less accessible rock stacks.

Botanists too are watching the recovery process with interest and hope. Senior ecologist with the Tasmania Department of Primary Industries, Parks, Water and Environment, Dr Jennie Whinam, is among those who have seen the island at its worst and are now keenly documenting the recovery.

Department scientists started monitoring Macquarie Island’s vegetation in the 1980s. Their exclosure plots and photo-monitoring sites have provided graphic visual evidence of just how badly the island was degraded under the pressure of a rabbit population estimated at more than 100 000. It was a landscape-scale catastrophe that saw increased incidence of landslips resulting from the island’s denudation and subsequent erosion.

‘The island had changed to a very simple island. It was a much less exciting and complex mosaic of landscapes and vegetation. The specialness of it had gone with the loss of iconic species such as the unique megaherbs, the Macquarie Island cabbage (Stilbocarpa polaris) and silver-leaf daisy (Pleurophyllum hookeri),’ Dr Whinam said.

Dr Whinam is heartened to see the island may once again be worthy of its nickname, ‘The Green Sponge’. But she also warns that as the island regains its ecological equilibrium, there will be winners and losers.

‘In five years’ time the island will look significantly different from today. It will have a lot more green, a lot more colour in general. What I can’t predict is how long it will take to become stable. There’s a big difference between initial recovery with the grazing pressure taken off, and I expect that will be fast and lush, but it’s likely to be 20 years before we can start talking about what the new equilibrium is like,’ Dr Whinam said.

‘In this new world, there will be winners and losers. The sleeper is what will happen with the weeds. There are three weed species on the island and we’re expecting some of them to increase initially and then hopefully decrease as the native species re-establish. There may also have been new arrivals of weeds that have gone unnoticed simply because it’s been so hard to identify plants that were so heavily grazed.’

Coupled with the joy of the island’s vegetation recovery is caution. Reports from other island eradication projects indicate that even in the same archipelago, different plant species have responded quite differently to the removal of pest species.

‘Their message is that we should expect the unexpected,’ Dr Whinam said.

Whatever changes occur, if in another few years the eradication is deemed to have been successful, it will set a new benchmark in island eradications for its size, multiple species, remoteness and challenging environment.

LIZ WREN

Manager Media and Communications, Tasmania Parks and Wildlife Service