The history and relationships of northern platypus (Ornithorhynchus anatinus) populations: a molecular approach

The aim of this study was to understand the distribution and genetic structure of platypus populations in Australia, and in particular to investigate the interactions of distribution and genetic structure. The research considered the entire distributional range of the platypus, but with a special focus on the scientifically neglected platypus populations of northern Queensland. Platypuses in north Queensland are smaller than their southern counterparts and have a more reddish colouration. There appears also to be a break in the distribution of the platypus between about Mackay and Townsville, which corresponds to the catchment of the Burdekin River and which geographically separates northern platypuses from southern populations. The relationship of northern and southern platypus populations of mainland Australia, together with the biogeographic significance to the platypus of the Burdekin break, was a binding thread throughout the study. However, before that relationship could be inferred there were several smaller gaps in the knowledge regarding the distribution of platypus that had to be filled. These gaps were represented by several intriguing questions: Where do platypuses occur and why is their distribution limited to those areas? And, how are local populations of platypuses structured and how do they relate to each other? With these key pieces of information it was possible to expand the scope of the study to a distribution-wide level. Using distribution modelling software (MaxEnt), climate data and 4,315 occurrence records, I produced a climate-based distribution model to describe the current distribution of the platypus. The two most important climate factors determining environmental suitability for the platypus were precipitation during the driest quarter (which was positively associated with platypus occurrence) and maximum temperature (negatively associated), to the near exclusion of all other variables (53.8% and 41.2% contributions respectively). This distribution map supported the existence of a significant distribution break occurring in northern Queensland. Separate modelling of the northern and southern distributions revealed differences in the limiting factors in each part of the range. To the south, precipitation during the driest quarter and maximum temperature remained the two most important factors (76.2% and 18.9% contribution respectively). However, in the north additional environmental factors were important. These were temperature seasonality, precipitation during wettest quarter, minimum temperature, and precipitation seasonality, with respective contributions to habitat suitability of 34.7%, 22.6%, 19.2%, 16.7% and 3.5%. The initial species distribution model was projected onto palaeo-climate data representing the last glacial maximum (c. 22,000 years before present). This palaeo-model indicated that overall conditions were less favourable for the platypus at that time, and that the gap between the northern and southern portions of the distribution would have been even more pronounced, although there may have been connectivity between Tasmania and the mainland via the Bass land bridge. The platypus distribution was also projected forward to predict the effects of anthropogenic climate change. An aggregated mean across the complex models involved in this suggested a likely decline in range of approximately 15% by the year 2070 with best/worst case scenarios depicting an increase of 3.5% or a decrease of 65% respectively. The areas affected by these distributional changes were the marginal fringes surrounding the main areas of distribution. After developing a reliable set of 12 microsatellite DNA markers for the study it was possible to investigate population structure and dynamics from a molecular perspective. At the finer scale of investigation (comparisons within and between adjacent river systems), I showed that despite individual sample sites within a river systems having some genetic differentiation, they generally exhibited a strong isolation-by-distance pattern within the system (e.g. Hawkesbury-Nepean system: r = 0.7315, p = 0.02). Moreover, significant differentiation between systems as suggested by pairwise Fst, AMOVA and Bayesian population clustering techniques indicates that the physical separation of river basins does limit gene flow and is responsible for local population structuring. The detection of several first generation migrants (13 of 120 samples) also provided a genetic indication that platypuses must move between river basins, which would require overland movement to occur more often than previously thought. I also showed that a large dam inhibited within-river gene flow and could lead to increased differentiation between populations: the construction of the Nepean Dam has lead to higher differentiation occurring within a single river (above vs. below dam pairwise Fst = 0.07681) then occurring between two rivers at three times the distance and requiring an overland crossing (Wingecarribee River vs. Nepean River pairwise Fst = 0.05978). Genetic analysis across the entire platypus distribution revealed three evolutionarily significant units within the platypus distribution that are in strong consensus with the observations gathered from the distribution modelling. These represent the isolated Northern Region, the Southern Mainland Region, and Tasmania. Within these evolutionarily significant units six discrete population clusters were identified, which formed the basis of five proposed management units for the platypus (two clusters were combined due to the presence of active gene flow). Attempts to investigate population sub-clusters within these clusters were futile as genetic admixture between local river systems rendered their level of distinctiveness below that of discrete conservation units. Future conservation and management planners will have to keep in mind that not all platypuses are created equal; there are distinct groups that must be considered independently in order to maintain the genotypic and phenotypic features that currently exist across the species.