Abstract
Human-induced climate change is a singularly pressing preoccupation for researchers in
all fields, especially those who study the natural world. It is an increasingly recognised factor in determining species distributions, abundances, and extinction risk, especially in birds. In Aotearoa New Zealand, birds are particularly vulnerable to climate change effects, as the isolation of the landmass from other regions and its highly heterogeneous landscape make it difficult to track habitat changes via range shifts. However, little is known about the ways in which the endemic birds of Aotearoa New Zealand respond to climate change. Most research has been focused on the palaeoecological impacts of the Pleistocene, but no attempt has been made at understanding the impacts of modern-day climate change. In addition, much of this research is couched in an implicit binary of forest birds and everything else, with little acknowledgment of birds in other habitats. This body of work represents the first broad examination of the relationships between the birds of Aotearoa New Zealand and climatic conditions, and simultaneously interrogates the usefulness of ecotypic classifications in this context.
Since predictions about the impacts of climate change are notoriously difficult to both
generate and verify, prehistoric climate change events provide valuable insights into the
responses of biological communities to environmental perturbation. I begin my research with an investigation into the effects of Plio-Pleistocene environmental change on the settlement of Aotearoa New Zealand by bird lineages. Using mitochondrial genomes sequenced from modern and ancient samples, I generate dated phylogenies of lineages spanning Aves to determine the relationships between endemic species and their closest overseas relatives. I assessed the distribution of divergence dates through time in different habitats. I find that generalist and open-habitat species diverge from their closest relatives more often during the Pliocene and Pleistocene, right as open habitats were expanding due to cooling climate conditions. Furthermore, I highlight the importance of Australia as a biodiversity source, and highlight the similarities between Pleistocene and modern-day settlement patterns. Environmental change during the Pleistocene was not limited simply to the establishment of open habitats; glacial cycling resulted in continual retraction and expansion of forest cover. To explore the impacts of the dramatic fluctuations in landscape on bird populations, I generated whole genomes for 16 endemic species across a variety of habitats and estimated the trajectories of their demographic histories throughout the Pleistocene. I find that species responded to glacial cycles in idiosyncratic and sometimes surprising ways. Most species, but not all, experienced severe declines in population in the early Pleistocene regardless of their
habitat preferences, especially lotic species like ducks. Some species responded dynamically to glacial cycling, while others experienced increasing effective population size regardless of the timing of glacial cycles. Overall, habitat preferences (in terms of vegetative cover) proved a poor predictor of demographic trajectories.
To more clearly define the relationship between climate conditions, vegetative
composition, and species demographics, I developed ecological niche models to generate
predictions about the amount of climatically suitable space for 17 endemic species at the height of the Last Glacial Maximum and into the future (2061-2080). To begin disentangling the effects of climate directly on vital rates, and indirectly through its impact on vegetative cover, these models were generated with two different sets of climate predictors: a set of variables individually selected for each species, and a shared set based on variables known to influence vegetative composition. I find the two predictor sets often make opposing predictions about total suitable space, highlighting the complexity of the impacts of climate change. I note the vulnerability of the alpine zone, highlight decreasing suitability of the North Island in the future for many species, and comment on the poor correlation between habitat-based classification
schemes and predicted distributions.
This body of work highlights the influence of climate change in determining the
abundance and distribution of endemic Aotearoa New Zealand birds, as well as the rate of settlement of new bird species from nearby landmasses. Furthermore, it provides clear evidence for the poor suitability of generic vegetation-based classifications in making predictions about the impacts of climate change, past and present, on Aotearoa New Zealand bird populations. The effects of climate on the evolution and future of our birds are complex and should not be disregarded, especially in light of additional pressures from humans and their commensals.