Prehistoric human-induced range contraction of southern elephant seals in Australasia

Abstract

Human migration across much of the globe has often coincided with both climate change events and the decline of large animal species. Due to the temporal overlap of these events, it has often proven difficult to confidently identify the causes of megafaunal extirpation. New Zealand (NZ) and Australia represent two islands that have very different histories with respect to human colonisation times and megafaunal decline. Australia experienced ancient human colonisation, coinciding with major climatic transition periods, with early megafaunal declines. NZ, by contrast, experienced relatively recent human colonisation, coinciding with recent megafaunal declines. Pinnipeds represent a megafaunal assemblage particularly sensitive to human overexploitation. The largest among the pinnipeds is the southern elephant seal (Mirounga leonina), which has a circumpolar distribution, breeding on islands around Antarctica. M. leonina’s demography is of particular interest as both climatic and anthropogenic effects may have contributed to its recent history. Since M. leonina does not breed or haul out on ice, its distribution may be highly constrained by sea ice coverage, which reached a maximum during the Last Glacial Maximum, 18,000 years ago. Past and present diversity within species and populations can be inferred and compared using ancient DNA techniques. The mitochondrial hypervariable region 1 (HVR1) is a fast-evolving DNA region that has previously been sequenced from populations across the entire breeding range of M. leonina. This study focuses on the past and present HVR1 variation of M. leonina populations in the Australasian region and how they were affected by climatic and anthropogenic impacts. By sampling ancient DNA of M. leonina specimens from Tasmania and NZ, it was found that ancient M. leonina samples from New Zealand and modern samples from the Antipodes Islands shared the highest homology with the Macquarie Island breeding population. Tasmanian samples were found to be genetically distinct, further cementing their status as representing a prehistoric breeding population. These findings broadly confirm a substantial phylogeographic distinction between ‘Australasian’ and ‘South American’ populations of this species, consistent with separate glacial refugia. Collection records of M. leonina in an archaeological context show a decline of M. leonina remains after human colonisation in both NZ and Tasmania. Despite the NZ extinct population being genetically similar to extant populations, its rapid extinction seems attributable to human impacts. The presence of M. leonina remains for extended periods of Tasmania’s arhcaeological record might indicate a large reservoir population on King Island that was later driven to extinction by European sealers. The use of both genetic and archaeological data show that a multidisciplinary approach can be used to answer complex questions regarding ancient population expansions and contractions of a highly mobile species such as M. leonina.