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Sugar gliders in Tasmania: an introduced predator or an elusive native?



The sugar glider (Petaurus breviceps) is widely distributed through Australia and Papua New Guinea and is found on a number of Indonesian islands. Historical records suggest that the species was introduced to Tasmania in 1835, as pets, from Port Phillip in Victoria. The population is now widespread but their provenance has not been investigated. Recently Tasmanian sugar gliders have been implicated in the predation on the highly endangered swift parrot (Lathamus discolour) causing nest failures of close to 100%. Without intervention to prevent this predation L. discolor is predicted to go extinct within two decades. To determine appropriate management of this presumed novel predator, there is an urgent need to establish the provenance of Tasmanian P. breviceps. Here we use sequencing of mitochondrial ND4 and ND2 genes to show that Tasmanian P. breviceps comprise only two sequences separated by just a single base pair. These sequences are identical to sequences obtained from P. breviceps sampled in Victoria. Genetic divergence is much lower than would be expected for a species isolated for around 10kya from the Australian mainland since the Australia-Tasmania land bridge disappeared in the last glacial period. These data suggest that P. breviceps in Tasmania have been introduced recently and have since filled a new ecological niche as an apex predator. Management of the species around the breeding habitats of the swift parrot can now move forward with suggestions of humane lethal traps and inventive forestry management.
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Populations on continental islands are often distinguishable from mainland conspecifics with respect to body size, appearance, behaviour or life history, and this is often congruent with genetic patterns. It is commonly assumed that such differences developed following the complete isolation of populations by sea-level rise following the Last Glacial Maximum (LGM). However, population divergence may predate the LGM, or marine dispersal and colonization of islands may have occurred more recently; in both cases, populations may have also diverged despite ongoing gene flow. Here, we test these alternative hypotheses for the divergence between wedge-tailed eagles from mainland Australia (Aquila audax audax) and the threatened Tasmanian subspecies (Aquila audax fleayi), based on variation at 20 microsatellite loci and mtDNA. Coalescent analyses indicate that population divergence appreciably postdates the severance of terrestrial habitat continuity and occurred without any subsequent gene flow. We infer a recent colonization of Tasmania by marine dispersal and cannot discount founder effects as the cause of differences in body size and life history. We call into question the general assumption of post-LGM marine transgression as the initiator of divergence of terrestrial lineages on continental islands and adjacent mainland, and highlight the range of alternative scenarios that should be considered.