Abstract
Habitat fragmentation has strong effects on genetic diversity and overall species decline by isolating populations and keeping population sizes small. One way that we can address this decline is through translocations, which are the human-assisted movement of plants or animals from one location to another to found a new population or supplement an existing one. The IUCN cautions that sourcing translocations from populations with insufficient genetic variation may lead to reduced reproductive output and survival, and increased inbreeding depression. An increased focus on genetics-based translocation planning could be the key to improving translocations, but these considerations are not always seen in practice in Aotearoa. One potential roadblock of genetics-based practices is the lack of base genetics knowledge of the target species.
Here, I use toutouwai | North Island robin (Petroica longipes) to exhibit how genotyping-by-sequencing (GBS) data can be used to reveal population structure of the species and create a model to visualize different translocation strategies. Toutouwai are one of the most frequently translocated birds in Aotearoa, and there is a need to increase the success of their translocations to reduce or reverse their declining population numbers. In Chapter 2, I conduct a phylogeographic study to explore the genetic diversity of eight populations, with a focus on gene flow, connectivity, isolation, and the effect of past translocations on modern population structure. I emphasize the role that translocations play in mimicking a connected metapopulation by facilitating gene flow and maintaining genetic variation. In Chapter 3, I model single- versus multi-source translocations to predict their effects on genetic diversity at the release site. The simulations showed that multi-source translocations have improved genetic diversity outcomes at the release sites, and they can be used to strengthen the sourcing potential of a lower diversity source. I conclude that future toutouwai translocations should strive to maximize genetic diversity by using multi-source translocations; sourcing from genetically healthy sources should always be prioritized, but it is especially important when only single-source translocations are feasible.
The phylogeographic study largely demonstrated that toutouwai genetics follow the expectations of population genetics theory. Individuals from the same populations are much more genetically similar with each other than to other populations. Toutouwai have limited dispersal, and habitat fragmentation has further isolated these populations, so gene flow is much higher within populations than between populations. However, populations which are closer together have increased genetic similarity compared to populations that are further apart, reflecting past connectivity between populations. The two populations which are geographically the closest, Mangatutu and Pureora, suggest that the contiguous habitat between populations results in increased gene flow, which may contribute to their relatively larger effective population sizes and overall genetic diversity. Conversely, the two natural island populations, Te Hauturu-o-Toi and Kapiti Island, are the most differentiated from all other populations, reflecting their 7,000-year isolation from the mainland and lack of gene flow. There were observed genetic differences in populations which had been founded by translocations relatively recently. Tāwharanui and Tiritiri Matangi have differentiated from the mainland and from each other in a relatively short period of time, suggesting that founder effects may have acted more strongly in these translocations to isolated habitats. Conversely, the founded population in Bushy Park may have maintained better heterozygosity and effective population size via increased connectivity and comparatively decreased isolation in central mainland. The translocation from central mainland populations to Tiritiri Matangi may have caused the observed reduction in genetic diversity (heterozygosity) in the resulting Tiritiri Matangi population; however, it was unexpected to find that the serial translocation from Tiritiri Matangi to establish the Tāwharanui population did not result in further reduced genetic diversity.