Abstract
Genetic diversity is important as it enables the adaptation to changing conditions and ensures the long-term viability of populations. Populations that have become small and isolated (e.g., due to habitat fragmentation or predation pressure) are at risk of losing genetic diversity due to genetic drift and inbreeding. In the absence of gene flow, decreased genetic diversity and increased inbreeding can lead to reduced fitness in small and isolated populations (i.e., inbreeding depression). Genetic diversity can also be lost in translocations when a new population is established by a small number of individuals (i.e., founder effect). In Aotearoa New Zealand, translocations of birds to predator-free islands are a common management tool to protect declining species from the threats they are facing on the mainland. Furthermore, island populations often act as insurance to prevent species’ extinction. Consequently, these insurance populations should capture the genetic variation of their natural counterparts and be genetically viable.
The mohua (Mohoua ochrocephala) is a small, forest-dwelling bird endemic to Te Waipounamu o Aotearoa/the South Island of New Zealand. The species is currently listed as “At risk – declining” by the New Zealand Threat Classification System and populations are isolated. Despite ongoing predator control, most populations on the mainland are in decline. Conservation efforts have included translocations of mohua to predator-free offshore islands. While the island populations have increased substantially since their establishment, these populations were created by sourcing a small number of mohua from a single source. Additionally, some island populations were established by sourcing mohua from another island, thereby creating sequential bottlenecks and founder events.
Here I use genome-wide single nucleotide polymorphism (SNP) data generated via genotyping-by-sequencing (GBS) to investigate genetic diversity, inbreeding, population structure and gene flow patterns in mainland and island populations of mohua to inform species conservation. In addition to SNP data, I use DNA sequence of the mitochondrial control region and the cytochrome b gene to resolve the phylogeography of mohua and examine past distribution and gene flow patterns amongst mainland populations. Overall, I aim for my research to aid the conservation management of mohua by i) evaluating current and historical population structure and connectivity amongst mainland mohua populations; ii) investigating the loss of genetic diversity in mainland mohua populations over the last decade; and iii) identifying the suitability of island populations to act as insurance populations and sources for future translocations.
My analysis of SNP data indicated three genetic clusters of mohua on the mainland and suggested limited gene flow amongst these clusters as well as between some populations within each cluster. In contrast, the analysis of mtDNA did not reveal population structure and indicated gene flow between the sampled populations in the past, supporting previous findings that recent habitat fragmentation is driving the genetic patterns seen in today’s populations. My findings suggest that translocations to simulate gene flow amongst mohua populations are a suitable management tool as populations seemed to have been connected throughout the species range historically. I did not find evidence for a loss of genetic diversity associated with ongoing population decline over the past decade. However, my analysis identified mainland populations with low genetic diversity that would benefit from receiving individuals via translocations to simulate natural gene flow to prevent inbreeding and boost genetic diversity. Updated genetic sampling across the range of the mohua will be beneficial when planning further translocations to supplement mainland mohua populations.
My analysis of mohua from island populations found that island populations form a fourth distinct genetic cluster indicating differentiation from the mainland mohua. I also found reduced genetic diversity and increased inbreeding on the islands. My findings indicate that the island mohua populations analyzed in this study are currently not suitable as insurance populations as they do not capture the genetic variation found on the mainland. Consequently, additional mohua should be introduced to these islands to increase their suitability as insurance populations.
My research aimed to support the conservation management of mohua by providing guidance for the decision-making in future translocations. Here I have identified mainland mohua populations that would benefit from translocations to boost genetic diversity. Furthermore, I discuss which mainland source populations should be used to improve the suitability of island populations as insurance populations.