Landscape genetics for conservation management : brushtail possums (Trichosurus vulpecula) in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology at Massey University, New Zealand
The negative impact of brushtail possums (Trichosurus vulpecula) on New Zealand ecosystems became apparent soon after their introduction from Australia in 1858. Possums not only denude native vegetation but prey on native birds and invertebrates. They also carry bovine tuberculosis (TB) impacting the dairy industry and consequently the New Zealand economy. New Zealand possum populations have spread from several introduction sites and densities have increased. The resulting complex patterns of gene flow influences regional diversity, and potentially the effectiveness of control measures. Currently, ~100 million dollars are spent on 1080 management per year, mostly in response to Tb risk, but there is little information about the migration rates associated with resulting population density fluctuations. To determine whether the potential for intermixing between populations since their introductions could have caused a homogenizing effect on the genetic diversity across New Zealand, I began a detailed population genetic analysis by genotyping possums from 19 locations using nuclear microsatellites and mitochondrial DNA haplotyping from across the country to estimate population structure. Initial introductions of possums from multiple locations resulted in genetic and fur colour diversity but, in comparison to natural Australian populations, it appears that only a subset of genetic variants was brought to New Zealand from Australia. Mitochondrial sequence variation analyses showed overall high haplotype diversity with substantial differences among samples in haplotype frequencies, but with relatively low nucleotide diversity.
Similarly, analysis of nuclear markers (microsatellite genotypes with Naïve Bayesian clustering) reveals that while there has been admixture between populations in various locations, indicated by shared genotypes, there are genetically distinct regional populations. Concordance of genetic and geographically distant sampling shows a well-developed population structure of possums across New Zealand. These results are also supported by pairwise Fst comparisons between all pairs of populations; although nearly all populations showed significant differences, there was no signature of isolation by distance as expected from their history of introductions.
This study provides a foundation for further research into spatial structure of brushtail possums which will enable the effective targeting of management and is essential for modelling population recovery, disease spread, and potentially the emergence of toxin resistance. Predator-free 2050 is an ambitious objective considering current circumstances. In order to achieve its goals, even for the targeted species, we need to efficiently manage our resources and improve the accuracy of control measures to maintain long-term effects.