Staving off extinction

Abstract

“Extinction is the rule. Survival is the exception.” Carl Sagan's premonitory words are vindicated by the oft-heard statistic that over 99% of all species that ever lived are now extinct. But while his message is clear, it might be said to contain a tiny technical flaw, since no species ever became extinct that did not first survive. The true exception is not survival, but continued survival, something that requires a species to show ongoing pluck while enjoying continual and timely good luck. Any Bramble Cay melomys (Melomys rubicola; WebFigure 1) or Lord Howe Island stick insect (Dryococelus australis) – two species that until recently were found only on one small oceanic island each – could tell you that. Bramble Cay is a flat, Australian island in the Torres Strait, some 227 km north of Queensland's Cape York Peninsula. It's tiny: just over 300 m long by 100 m wide, less than half of it (an area the size of a soccer field) with any vegetation, all low-lying. Yet this speck of land, where the only fresh water is what falls as rain, has been home to generations of its eponymous melomys. In fact, the population of these 100-g rodents is estimated to have run, at times, into the hundreds (Aust Mammal 1983; 6: 77–79). How the species appeared on the island is a mystery, but that it survived and prospered is all but a miracle. If ever there was a mammal with uncommon pluck (read adaptability and tenacity) and incredible luck (while it lasted), surely it was this. A miniscule island, however, is no place to be when your luck runs out. Though Bramble Cay is among the first places in the world to welcome the New Year, there will be no 2017 for its melomys. A June 2016 report to the Queensland Government's Department of Environment and Heritage Protection (http://bit.ly/2f0AHgJ) records not a single specimen captured during a 2014 survey. The report's authors suggest the species to be the first mammalian taxon lost primarily to human-caused climate change, a victim of rising sea levels and more aggressive weather salt-lashing the vegetation on which it depended. Maybe. But were it not for luck, anything from genetic drift during lean times to a few extra days of drought or high ocean swell (the island's maximum elevation is just 3 m) could have wiped it out well before now. The authors recommended the species’ status be changed from “endangered” to “extinct” in Australia, but only suggested that “possibly extinct” be added to its “critically endangered” rating on the IUCN Red List – and the Lord Howe Island stick insect knows why. Back in the 1920s, these hand-long arthropods were extirpated from their home on Lord Howe Island, off Australia's eastern coast, by, ironically, an invasion of the Bramble Cay melomys’ cousin, the black rat (Rattus rattus). Descendants of animals that jumped ship from the British steamer SS Makambo, which ran aground on the island in 1918, the intruders set to work, decimating the stick insects which they found an appealing, crunchy snack. This stroke of rotten luck, it was believed, had driven the species to extinction. But 20 km away, on another tiny island (1100 m × 300 m), luck of a different kind was holding. Ball's Pyramid (WebFigure 2), an almost bare volcanic stack reaching over 560 m into the air, could not be more different from Bramble Cay. Nor could the fate of the few stick insects that somehow got there differ more greatly from that of the Bramble Cay melomys. In 1964, climbers tantalizingly found dead specimens on the island. Then, in February 2001, on a rock terrace 65 m above the waves, researchers discovered two adults and a nymph clinging to life under a small shrub (Melaleuca howeana) (Biodivers Conserv 2003; 12: 1391–403). Plant debris at the base of the shrub retained water from a seep, and provided their refuge. A year later, others were found: 24 in all. Like the melomys, no one knows how they reached the island. Like the melomys, they lived in precarious isolation. Like the melomys, their survival depended on being plucky. But unlike the melomys, they had not stopped being lucky. And their luck has since burgeoned; a program run by the Melbourne Zoo has bred thousands of them from a single pair, aptly named Adam and Eve (www.youtube.com/watch?v=Eg3dcYJ2oI4). It is even hoped that one day Lord Howe Island may be successfully repopulated. No such luck for the Bramble Cay melomys. Lord Howe Island stick insect (Dryococelus australis). Granitehighs; license: CC BY-SA 3.0 Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.