Abstract
The Pleistocene glacial-interglacial cycles significantly reshaped global patterns of biodiversity. The New Zealand biota is no exception, with the environment of Quaternary New Zealand broadly characterised by glaciation of the Southern Alps and the expansion of shrub and grasslands during glacial periods, followed by deglaciation and the recolonisation of forest during interglacials. Molecular studies of New Zealand biota have revealed that phylogeographic structure is a common response to Pleistocene glaciation, but ancient DNA has not yet been fully leveraged to directly examine biological responses to past climatic and environmental change. Ancient DNA is a useful tool for such research, providing a valuable temporal framework. Analysis of ancient DNA from regions of the Northern Hemisphere have revealed that the Quaternary was a more dynamic and complex period than previously thought, but less is known about the impacts of Quaternary climatic/environmental change in the Southern Hemisphere. New Zealand provides an important system to study the impacts of past climatic/environmental change as humans arrived less than 1000 years ago during a period of stable climate, enabling the impacts of both climatic/environmental change and human arrival to be examined in isolation (in contrast to many continental ecosystems).
Here I sequence ancient DNA from New Zealand endemic avian species (bush wren, eastern moa, and takahē/moho) to investigate their responses to past climatic and environmental change. Each of these species were severely impacted by humans and introduced predators with the bush wren, eastern moa, and moho rendered extinct, while the takahē was reduced to a relict population within the southern South Island. Furthermore, these species cover a broad taxonomic range and exhibit different habitat preferences, making them a useful series of case studies for the study of the impacts of past climatic/environmental change on New Zealand’s avifauna. I find that each species exhibits different responses to Quaternary climatic/environmental change, possibly influenced by different habitat preferences and environmental tolerances. Bush wren are highly phylogeographically structured- this is potentially associated with the increased severity of the Pleistocene glacial periods following the Mid Pleistocene Transition. Additionally, morphological and molecular evidence suggests that the North and South Island bush wren subspecies should be recognised as separate species, and that the subspecies known as Stead’s bush wren be synonymised with the South Island form. In contrast, the panmictic eastern moa exhibit low levels of genetic diversity throughout the past 14 Ka, possibly due to a late Pleistocene population/genetic bottleneck, followed by rapid population expansion as the climate warmed and the Holocene epoch began. Analysis of ancient DNA from takahē subfossils provides direct evidence of a genetic bottleneck associated with human arrival. Additionally, no phylogeographic structure was observed between takahē populations throughout the South Island, although some temporal structure was observed. This temporal structure may be related to a loss of genetic diversity or genetic turnover through the Pleistocene-Holocene transition. Finally, phylogenetic analyses suggest that the takahē and moho are sister-taxa, resulting from a single invasion of New Zealand by a common Porphyrio ancestor, contrary to previous molecular studies.
This thesis highlights the individualistic responses of New Zealand’s avifauna to past climatic and environmental change, and that these responses can often be masked by more recent patterns and processes (e.g. anthropogenic impacts). It also demonstrates the use of ancient DNA to study the evolution of extinct species and inform the conservation management of endangered taxa.