Island defences help at-risk mammals

Lianthawirriyarra Sea Rangers, above from left: Richard Dixon, Damien Pracy and Allan Charlie, and landowner Samuel Evans on South West Island. The survey was undertaken collaboratively with Aboriginal rangers and landowners

Above: a northern brown bandicoot, from Centre Island

In their isolation, plants and animals on Australia’s islands are protected from many harmful factors that may affect their mainland relatives. A new collaborative project has re-surveyed at-risk mammals on some of our northern islands—and the results may help us understand threats facing those on the mainland. The project team* reports

Fragility of the island refuge | Sir Edward Pellew Islands | Ocean defence breached | Island Ark program | Figure 1 | Figure 2 | More information |

Australia’s islands are extremely important for conservation. Largely because feral cats and foxes and other exotic plants and animals have not reached them, many islands have served as refuges for animals that have disappeared from extensive mainland ranges.

While many of our native mammals have become totally extinct over the last 200 years, at least nine more were saved from extinction solely because they retained small populations on offshore islands free of exotic predators such as cane toads, cats and foxes.

Many additional species have maintained only a claw hold on their mainland range, but persist still in good populations on one or more islands. In the tropical savannas, one example is the golden bandicoot Isoodon auratus . Just 200 years ago, this small bandicoot had an almost continental range, including deserts and tropical woodlands. Now it is present only on one island in the Northern Territory (Marchinbar), two islands off the Kimberley (Augustus and Bigge), Barrow Island (off the Pilbara), and a couple of small areas of the rugged Kimberley mainland.

Islands can give a glimpse of what our land was like before the advent of cats and foxes, pigs, cattle, horses and the myriad other plants and animals we released onto the Australian mainland. But of course, it is always a somewhat distorted glimpse, because each island has its own peculiarities.

Fragility of the island refuge

While the conservation value of many islands is high, experience has shown that this value is easily destroyed. Dodos disappeared rapidly from Mauritius Island; moas from New Zealand. Islands are small, and support generally small numbers of individuals of any species; and they generally offer no escape from newly introduced threats.

Northern Australia has many islands, including (after Tasmania) Australia’s second (Melville), fourth (Groote Eylandt) and fifth (Bathurst) largest islands. The island groups from Torres Strait to the Kimberley support some of the premier conservation assets in northern Australia. These values have recently been recognised through the NT’s Island Ark program (see box ).

Figure 1, above : Location of main survey sites during this survey (crosses) and the 1988 survey (dots).

Figure 2, above: Relative trapping success for each island, and over all islands combined, for 1966–67, 1988 and 2003 surveys. Note Vanderlin Island was not sampled in 1966–67.

Sir Edward Pellew Islands

One of those important island chains is the Sir Edward Pellew group off Borroloola, in the Gulf of Carpentaria (Figure 1). These Aboriginal-owned islands harbour important populations of some mammals regarded as threatened in the Northern Territory. These include the Carpentarian antechinus, which until recent discoveries around Mount Isa was thought to occur only on the Pellew islands; the canefield rat, a predominantly Queensland species known in the Northern Territory only on the Pellews; and the brush-tailed tree-rat, brush-tailed phascogale and northern quoll. We know these last three species have declined on the Northern Territory mainland, and to attempt to understand that decline, we have started a study of the populations of these five mammals on the Pellew Islands. Are these island populations also in decline? If not, what factors that operate only on the mainland have affected these mammals?

This is a collaborative project, involving the Tropical Savannas CRC, Parks and Wildlife Service of the NT, the Threatened Species Network, and the Aboriginal landowners and residents of these islands, through the Lianthawirriyarra Sea Ranger Unit of Mabunji Aboriginal Resource Association. In October 2003, we visited the five main islands in the Pellew group, sampling mammals through trap and release, and talking with the Islands’ residents. We could compare our results with those of two similar previous surveys, undertaken in 1966–67 and 1988.

Ocean defence breached

We found both good and bad results. The good is that we found more Carpentarian antechinuses, and that northern quolls are still present on the one island from which they were previously known.

But the not-so-good news is that populations of most mammals were down substantially from the two previous surveys (Figure 2). We failed to record the canefield rat, brush-tailed phascogale and brush-tailed tree-rat, but these were not recorded frequently in the earlier surveys, so our lack of records may not necessarily mean decline or loss.

We also found that cane toads had colonised all of the large islands, having floated tens of kilometres out to sea on floodwaters in the 2000–01 wet season (you really have to admire their design!). Feral cats are now present on most of the large islands, whereas they weren’t two decades ago. The isolation of these islands has been breached and it is likely that their previously protected native mammal fauna will decline. In particular, we expect that the northern quoll will be lost from these islands within the next two years, because this marsupial predator is so susceptible to cane toad toxin.

Our work here will continue; and we hope to pin down the status of all mammal species with further fieldwork over the next year. A feature of the work is the collaboration of scientists with Aboriginal traditional landowners and rangers, and the increasing awareness amongst residents and visitors to these islands of their conservation values, and of the shared responsibility and need to manage and protect these values.

* Project team

John Woinarski, Biodiversity group, NT Department of Infrastructure Planning and Environment
Rob Taylor, NT Parks and Wildlife Service
Allan Charlie, Richard Dixon, Damien Pracy and Felicity Chapman; Lianthawirriyarra Sea Ranger Unit, Mabunji Aboriginal Resource Association.