Historical Biogeography and Avian Speciation in the Indo-Australian Region

Abstract

Biotic interchanges denote the turnover of floral or faunal species occurring between previously separated regions. One such biotic interchange is the Sunda-Sahul biotic interchange occurring in the Indo-Australian Archipelago (IAA). This biotic interchange involves the exchange of biota from two continental shelves: the Sunda and Sahul shelf. Around the two main continental shelves lie more than 20,000 islands, including the islands in the transition zone of 'Wallacea' as well as the East Melanesian arc islands off the coast of New Guinea.

Despite only taking up 3% of the world's total landmass area, the IAA holds a disproportionate amount of biodiversity. Its unique patterns of biodiversity have been a focus since the 1800s, when Alfred Russell Wallace observed marked turnover across the now-named Wallace's line: biota to the west of the line are more Asiatic, while to the east of the line are more Australasian. Many more biogeographical lines have been drawn since, and following these more historical biogeographical perspectives, current research has further shown evidence for it.

My thesis aims to go beyond drawing lines and describing patterns, to investigating the processes that drive these unique biodiversity patterns. I mainly focus on the avian diversity of the IAA, specifically the passerine birds (Chapter 2 and Chapter 3). I start my thesis by summarising the state of the field, including background on biotic interchange and the biogeographical history of the region, before focusing on recent avian perspectives. I then outline my following chapters.

In Chapter 1, I provided a comprehensive quantitative assessment of the marked differentiation of avian communities in the areas of the IAA. I show that there is indeed marked differentiation of bird communities between island groups in the IAA despite interconnectedness and that this is mainly due to the presence of sea barriers and not environmental distance. I also collated a geographical dataset of all the birds present in the IAA that was used for the following chapters.

To further investigate the detailed diversification history of birds in the IAA, a reliable phylogenetic framework is required. Thus, in Chapter 2, I provide a new supertree of all the passerines in the Indo-Australian Archipelago. Given advancements in genetic sampling across the region over the decade since the last supertree, the new phylogeny has a much greater representation of islands and Melanesia. I found high endemism in the region and highlighted the super-endemism in New Guinea and neo-endemism of the East Melanesia.

Lastly, in Chapter 3, I integrated geographic data (Chapter 1) and the updated phylogeny from Chapter 2, to investigate dispersal and diversification history of passerine birds in the IAA. I quantified dispersals and within-area diversifications in the last 60 million years. I look at source-sink dynamics, dispersal pathways, and the relative roles of islands vs continental systems in the diversification of passerines. The oldest dispersal events were Sahul-dominated before there is an increase in dispersals and lineages in South-east Asia, mainly from New Guinea. More recent dispersals occur mainly among geographically closer areas. Island groups also have comparable levels of within area diversification to continental groups. Furthermore, I identified nodes in the tree where upstream colonization was detected and highlighted the extent of lineages that have resulted from these island colonisations.

Finally, I conclude my thesis by summarising the results of the preceding chapters and how they intertwine in the broader context. I list the challenges involved in disentangling these factors in such a dynamic region, as well as future directions. Collectively, these chapters contribute to the ever-growing literature focusing on investigating processes behind the patterns of biodiversity in the IAA that have been observed for over a century. Show more