Bramble Cay melomys have been classified as extinct by a new study indicating that climate change may have been one of the major factors behind the extinction of these rodents.
One of the mosaic-tailed rats, the Bramble Cay melomys, was larger than the three other Australian species of melomys and about the size of a small rat. Endemic to Bramble Cay, a small vegetated coral cay, located at the northern tip of the Great Barrier Reef.
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.
The species was confined to a single, very small, isolated location, and because of this it was susceptible to a range of threats. One of the reasons behind its demise could be inbreeding. Also, this species was also vulnerable to threats posed by the introduction of weeds, predators and competitors, and novel diseases.
Although the cay demonstrates seasonal fluctuations in size 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.