Biogeography and macroecology of Australian marine molluscs
Latitudinal diversity gradients are the highest-order biodiversity patterns on Earth. Despite a large body of research into them, there is no consensus on their drivers. Furthermore, studies of latitudinal gradients tend to either ignore or simplify biogeographic structure in their analyses, despite the usefulness of biogeographic schemes in regional conservation planning. As such, there is a disconnect in our understanding of patterns of diversity and faunal composition. In this thesis I reach for a new synthesis, simultaneously establishing both biogeographic and macroecological patterns for a highly diverse group, marine molluscs, with a focus on the coastline of Australia. In the first chapter of this thesis, I give a brief account of how previous research has analysed the latitudinal diversity gradient. I also discuss various global and regional bioregionalisations and their varied methodological approaches. In the second chapter, I use factor analysis to produce a global biogeographic scheme for molluscs that improves on previous research by quantifying gradients between provinces. I then use these province definitions to examine the drivers of their boundaries, showing that as coral reefs become more common in an area its composition changes systematically. The third chapter then applies the same methods to a study of the coastline of Australia, producing a regional scheme that complements previous work. I use it to investigate spatial patterns of diversity around the continent, relating the observed changes in biogeographic structure to diversity patterns. I show that commonly flagged environmental correlates are only indirectly tied to diversity via their control of biogeographic transitions. In the following chapters I utilise a large field collection drawn from across the eastern coastline to quantify diversity. In the fourth chapter I compare the biogeographic and diversity patterns present in the field collection to those generated from routine macroecological datasets stemming from online databases. I find that latitudinal gradients are stronger in the field data, but there is much variance depending on the choice of diversity estimator and the inclusion or exclusion of observational data. In chapter five I show that despite large sample sizes and a rigorous study design, there is no latitudinal signal in body size distributions or abundance data in the field data, contrary to previous work. The approaches taken in this thesis highlight how several generalisations about latitudinal diversity gradients do not apply to Australia. Instead, new methods applied to generating biogeographic schemes may help to explain the drivers of observed patterns. Additionally, these methods demonstrate how to retain more information in bioregionalisation schemes, potentially aiding future assessments of optimal marine reserve placement.