Many species associated with New Zealand native forests have experienced extreme range declines and population size reductions due to habitat alterations and introduced predators. Removing immediate threats should be the main conservation objective, but as population sizes begin to stabilise, genetic factors play an increasingly important role in maximising the evolutionary potential and adaptability of these species via maintenance of genetic diversity. To understand the genetic effects of isolation and corresponding implications for management of fragmented species, I studied the genetic variation of mohua or yellowhead (Mohoua ochrocephala), a New Zealand endemic passerine which has experienced dramatic declines in population size and fragmentation into isolated patches, currently existing in only 3% of its original range.
I assessed changes in genetic diversity and genetic structure over time by comparing extant mainland populations with historic museum samples and compared genetic diversity among extant populations. I then developed a stochastic model to estimate the number of translocated and released individuals needed in threatened species reintroductions in order to preserve genetic diversity and compared this model to genetic data from two island reintroductions of mohua.
Evidence from historic and contemporary DNA of mohua showed that a significant amount of genetic diversity has been lost over the last hundred years, with most loss due to extirpation of entire regions and some occurring within extant populations. Comparisons among extant populations allowed me to identify which mohua populations have maintained high levels of genetic diversity (e.g. Dart Valley) and would serve as the most suitable sources for reintroductions. Comparisons of genetic structure between the two time periods revealed that a clear pattern of genetic structure among contemporary populations was explained by anthropogenic factors, not by historic pre-fragmentation population structure that would have carried evolutionary significance. This allowed me to establish that mixing contemporary populations can be appropriate. However, because the differences among contemporary populations were mostly due to differences in allele frequencies, augmentation of gene flow is not necessary. I suggest that when assessing the evolutionary significance of current population structure in declining species, it is necessary to consider historic levels of population structure, highlighting the importance of museum specimens in conservation genetics research. Allele retention modelling for reintroduced populations allowed me to estimate that a minimum of 60 individuals should be released in threatened species translocations and reintroductions in order to not only establish populations successfully, but also to ensure that high levels of genetic diversity are conserved.
In conclusion, maintenance of genetic diversity should be an essential management consideration during the recovery of threatened species that have faced population size reduction and fragmentation. This can be accomplished through protection of and sourcing from large genetically diverse mainland populations as well as releasing large numbers of individuals in reintroductions. Preservation of genetic diversity in these species will allow for increased evolutionary and adaptive potential and maximisation of long-term viability.
