Bramble Cay melomys
Common name: Bramble Cay melomys
Scientific name: Melomys rubicola (Melo = Melanesian, mys = mouse; rubicola = Bramble (Cay)
Family: Muridae (native rats and mice)
Conservation status: This species is currently listed as endangered in Queensland (Nature Conservation Act 1992) and nationally (Environment Protection and Biodiversity Conservation Act 1999). It is considered a high priority for conservation under the Department of Environment and Heritage Protection Back on Track species prioritisation framework.
The Bramble Cay melomys was one of the mosaic-tailed rats (distinguished by the mosaic pattern of scales on its tail rather than the concentric rows of scales running along the length of the tail found in most other types of rats and mice). It was larger than the three other Australian species of melomys and about the size of a small rat (head and body length: 148-165 mm; tail length: 145-185 mm). Also distinguishing it from other Australian melomys was that its tail was obviously lumpy, with each scale on the tail being bulbous. It had reddish-brown fur with a paler underbelly, relatively small ears and a long tail with a prehensile tip.
Habitat and distribution
This species was only found on Bramble Cay, a small vegetated coral cay (a reef island composed of coral rubble and sand) roughly 340 m long by 150 m wide, but subject to seasonal changes in both shape and size, located at the northern tip of the Great Barrier Reef. This made it Australia's most isolated species of mammal. There has been speculation that the species may also occur on other islands in the Torres Strait or in Papua New Guinea (PNG), given the close proximity of the cay to the mouth of the Fly River, which regularly deposits large amounts of debris (e.g., logs and assorted driftwood, whole palm trees and other vegetation) on Bramble Cay. Further survey work on these islands and PNG, along with clarification of the taxonomic status of the Bramble Cay melomys in relation to PNG species, is required (Latch 2008).
Life history and behaviour
Little was known about the biology and ecology of the Bramble Cay melomys. Individuals had been observed foraging at night among the island’s herbaceous vegetation and were thought to be dependent upon plant material, particularly the succulent herb Portulaca oleracea, for food. It is possible the species also ate turtle eggs.
Trap data demonstrated that the Bramble Cay melomys had a strong female-biased sex ratio. Pregnant and lactating females, juveniles and subadults had all been trapped in July, suggesting an extended winter breeding season.
The Bramble Cay melomys coexisted with large numbers of shorebirds (noddies, terns and boobies) and nesting green turtles. The Bramble Cay melomys were noted to avoid areas in which high densities of shorebirds occurred.
In 1978, the population on Bramble Cay was estimated to comprise at most several hundred individuals. A formal population census of the Bramble Cay melomys conducted in 1998 estimated the cay’s population size was 93, based on the capture of 42 individuals. Surveys conducted for the species in 2002 and 2004 detected only 10 and 12 individuals, respectively. These results suggested there had been an ongoing decline in the abundance of the Bramble Cay melomys. A survey in 2011 and two surveys in 2014 failed to locate the species at all. The most recent of these assessments, conducted in August–September 2014, confirmed that the Bramble Cay melomys has been extirpated from Bramble Cay.
Searches of other Torres Strait islands have failed to discover another population of the Bramble Cay melomys.
Being confined to a single, very small, isolated location, the species was susceptible to a range of threats. It appeared to be inbred, an intrinsic problem that raised doubts about the long term viability of the population. The Bramble Cay melomys was also vulnerable to threats posed by the introduction of weeds, predators and competitors, and novel diseases. Certainly, anecdotal reports indicate at least some individuals were killed by domestic dogs that were released onto the island from visiting boats, but also that the species was hunted by indigenous people who visited from PNG on a sporadic basis. Although the cay demonstrates seasonal fluctuations in size (involving periods of erosion or deposition as a result of prevailing winds, waves and tides), phases of significant erosion may have impacted directly on the Bramble Cay melomys by reducing the area available for the species to occupy or limiting the availability of potential daytime refuges in caves and crevices in phosphatic rock outcrops at the south-eastern end of the island. Most critically, however, the extent of herbaceous vegetation on Bramble Cay decreased dramatically during the 10-year period following 2004, when the species was last captured. The primary cause of this significant decline in habitat was repeated seawater penetration of the island’s interior, which killed or damaged the vegetation. With an elevation of only 3 m above high tide level, Bramble Cay is particularly vulnerable to ocean inundation. This appears to be the threat that eventually sealed the fate of its resident melomys population. Available evidence indicates that the anthropogenic climate change-induced impacts of sea-level rise, coupled with an increased frequency and intensity of weather events that produced damaging storm surges and extreme high water levels, particularly during the last decade, were most likely responsible for the extirpation of the Bramble Cay melomys from Bramble Cay.
Nominations are currently being prepared to amend the conservation status of the Bramble Cay melomys from endangered to extinct in the wild under both Queensland and Commonwealth legislation.
Because the Bramble Cay melomys is now confirmed to have been lost from Bramble Cay, no recovery actions for this population can be implemented.
Nevertheless, surveys for the species in other locations are warranted. It appears unlikely that the species occurs on any other Torres Strait island, but there is a possibility that the species is extant in the Fly River delta of PNG, an area that has received comparatively little mammal survey effort to date.
Dennis, A.J. 2008. Bramble Cay Melomys Melomys rubicola. In The Mammals of Australia, Third Edition (Eds S. Van Dyck & R. Strahan), pp 673–674. New Holland Publishers, Sydney, NSW.
Dennis, A.J. 2012. Bramble Cay melomys Melomys rubicola. In Queensland’s Threatened Animals. (Eds L.K. Curtis, A.J. Dennis, K.R. McDonald, P.M. Kyne & S.J.S. Debus), pp 398–399. CSIRO Publishing, Collingwood, Vic.
Gynther, I., Waller, N. & Leung, L.K.-P. (2016) Confirmation of the extinction of the Bramble Cay melomys Melomys rubicola on Bramble Cay, Torres Strait: results and conclusions from a comprehensive survey in August–September 2014.
Latch P. 2008. Recovery Plan for the Bramble Cay Melomys Melomys rubicola. Report to Department of the Environment, Water, Heritage and the Arts, Canberra. Environmental Protection Agency, Brisbane.
Limpus, C.J., Parmenter, C.J. & Watts, C.H.S. 1983. Melomys rubicola, an endangered murid rodent endemic to the Great Barrier Reef of Queensland. Australian Mammalogy 6: 77–79.
Woinarski, J.C.Z., Burbidge, A.A. & Harrison, P.L. 2014. The Action Plan for Australian Mammals 2012. CSIRO Publishing, Collingwood, Vic.