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Habitat critical to the survival of the Golden Bandicoot and Golden-backed Tree-rat

On the Kimberley mainland the Golden Bandicoot is now only recorded in rocky sandstone habitats and vine thickets within the medium to high rainfall area (700 to 1200mm). In the Pilbara, Golden Bandicoots survive on two islands with an arid climate and on one island the Northern Territory where they occupy heathland on sandstone.

There are remarkably few records for the Golden-backed Tree-rat from the NT and this provides an inadequate basis for assessing habitat critical to the species survival. In the Kimberley, presence/absence data suggest a range of habitats including rugged King Leopold and Warton sandstone with Eucalyptus sp. open woodland over hummock grassland and the ecotone between monsoon forest patches and some savanna woodland types.

Given the large declines shown by both species all areas that have extant populations should be regarded as habitat critical to the species. This is particularly the case with island populations because of their isolation from threatening processes such as feral cats.

Threats

Australias terrestrial mammal fauna is particularly susceptible to declines and extinction (McKenzie and Burbidge 2002). Twenty-two species of mammals are extinct in Australia with eight other species remaining only on continental islands (McKenzie and Burbidge 2002). The causal factors most frequently cited as the cause of decline are predation and changed fire regimes. In northern Australia, although large-scale extinction has not occurred, there is a pattern of general decline (Woinarski et al. 2001) and the causes of this decline are unknown. Woinarski et al. (2001) and McKenzie and Burbidge (2002) considered the most likely cause of mammal decline in the north to be changed fire regimes and pastoralism. Recent research on fire impacts in Kakadu National Park has been inconclusive (Watson and Woinarski unpubl. data) because of complex and inconsistent patterns being exhibited. Mammal declines in southern Australia have been linked to the fox, which is not present in northern Australia. However, the feral cat Felis catus is present. Cats have been shown to prevent the introduction of mammals to arid areas (Gibson et al. 1994; Short and Smith 1994), and cause major mortality for the endangered Barred Bandicoot Perameles gunni (Seebeck et al. 1991). Cats have caused the extinction of populations on islands (Delroy et al. 1986; Dickman 1993).

The extinction of the Golden Bandicoot from Hermite Island (near Barrow Island) before 1912 has been attributed to the introduction of the feral cats (McKenzie et al. 1995; Burbidge et al. 2000). Species can recover on islands once feral cats have been removed (Dickman 1996). In northern Australia, predation from cats, in conjunction with intense fires, could be causing the decline of mammals. Intensive fires, by opening up the undergrowth, may make animals more susceptible to predation.

Recent studies in northern Australia have confirmed that relatively intense late dry season fires are having a significant impact on rainforest patches and obligate seeder species, particularly in rugged sandstone areas (Russell-Smith et al. 1998; Russell-Smith et al. 2001). Many obligate seeder species in this rocky habitat require a five years fire free period to reproduce (Russell-Smith et al. 2001). Fire history for the Kimberley mapped from coarse resolution NOAA satellite images inferred that large hot dry season fires are occurring in some places about every two years in this rugged sandstone habitat. There is anecdotal evidence of increasing Sorghum loads in these rocky areas matched by a decrease in Triodia (spinifex) cover potentially due to annual or biannual fires occurring in this habitat. Recent assumptions that areas of the Top End and north Kimberley provide a refuge for a range of mammal species (Woinarski and Braithwaite 1990) would appear to be overly optimistic (McKenzie and Burbidge 2002; Palmer et al. in prep).

Golden Bandicoot

The greatest threat to the island populations in both the NT and WA is the deliberate or inadvertent introduction of cats.

Predators of the Golden Bandicoot include the feral dog, dingo and feral cat, and native species such as pythons and monitor lizards. Bandicoots on Marchinbar Island were in the past hunted occasionally by Aboriginal landowners. Native predators are not considered a threat to healthy populations. Feral dogs have been present on Marchinbar Island for around 30-50 years, and these are known to take some bandicoots. However, this predation is considered low level. Marchinbar Island has no feral cats.

In Western Australia there is no information on the level of mortality from feral predators. Feral cats, dogs and dingoes are present on the mainland and a dingos have been recorded on Uwins and Augustus Islands. Middle and Barrow Islands are free of feral predators. Barrow Island is subject to strict environmental protection procedures that controls damage to vegetation and prevent the invasion of exotic species (McKenzie et al. 1995).

Golden-backed Tree-rat

The decline of the Golden-backed Tree-rat from the Northern Territory and drier areas of WA is symptomatic of a more general decline occurring in many mammals in northern Australia. The causes of this decline are unknown. The patchy nature of food resources, and their susceptibility to disturbance, could explain the decline of tree-rat populations, particularly in the more inland areas of their distribution. Grazing by introduced cattle and buffalo and changes in fire regimes since European settlement may have reduced the understorey trees and shrubs that the animals rely on for food and opened up the understorey making animals vulnerable to predation by feral cats. These factors probably had a more severe impact in the drier areas of the species distribution leading to a contracting of populations to the higher rainfall coastal areas.