Bird diversity on shelf islands does not benefit from recent land-bridge connections.
Above: Brown-throated Sunbird Anthreptes malacensis, a species very commonly found on islands in Sundaland.
When studying biogeography, I make a constant effort to tell myself that what I am witnessing today is just a mere snapshot in time. I’m still in my 20s, while the planet is quite a number of times older than me. Seemingly strange observations hence make much more sense once I consider various historical influences of Earth’s climatic variations and land movements. When I first started this project, I fully expected the data to tell such a story: that species distribution and composition of birds on islands in Sundaland are primarily determined by historical factors too.
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Sin, Y. C. K., Kristensen, N. P., Gwee, C. Y., Chisholm, R. A., & Rheindt, F. E. (2022). Bird diversity on shelf islands does not benefit from recent land-bridge connections. Journal of Biogeography, 49, 189-200. https://doi.org/10.1111/jbi.14293
Satellite images of Sundaic Southeast Asia show that the region comprises many islands today – roughly 17,000 of varying shapes and sizes. Had such technology (and us) existed half a million years ago, we would have witnessed a completely different landscape: no islands scattered across the region, and the whole of Sundaland completely exposed as land instead. It was only ~400,000 years ago that consistent subsidence of the entire shelf led to its partial drowning, and since then, sea level fluctuations have been constantly changing the land configuration. During interglacials, high sea level causes land to be broken up into islands, while during glacial periods, the sea level is ~120 metres lower than it is today and Sundaland gets “reunited” by land-bridges. While we live in a period of interglacial, the last glacial maximum occurred only as recently as ~20,000 years ago. Furthermore, during the Holocene sea level peak ~7,000 years ago, the sea level was 3-5 metres higher than it is today. This rise causing low-lying islands to be completely submerged before they re-emerged again when the sea level returned to its current form. The configuration of Sundaland we see is truly ephemeral: a majority of the islands are shelf islands that get connected to the mainland by land-bridges during glacial periods, while a small subset are deep-sea islands that have no historic connection to the mainland.
As a consequence of the Sundaland’s complex geologic chronicles, we would expect biodiversity patterns on its islands to be heavily influenced by the past: an island that enjoyed more land-bridge connection to the mainland and used to be bigger throughout evolutionary history should harbour more species. This phenomenon where species richness in an area exceeds the expected value given its size is called supersaturation and is observed in lizard diversity on land-bridge islands in Baja California and birds on some satellite islands of New Guinea. In addition to supersaturation, recently submerged islands that just had terrestrial biodiversity obliterated should be expected to have species composition differing from similar-sized islands that did not drown.
Map of Sundaland delineated by the Sunda shelf in light blue. Study islands (n=94) are highlighted, with deep-sea islands in yellow and shelf islands in red. Many small islands are not visible at this scale and circle sizes indicate the number of study islands within a 50 km radius. The number of endemic species-level bird taxa, if any, is indicated adjacent to islands following the same colour scheme (only one shelf island—Kangean, in the extreme southeast of the shelf— has an endemic species-level taxon). In the case of the Mentawai islands (dotted yellow ellipse), three taxa are endemic to the whole island group which forms one connected paleo-island.
We investigated these hypotheses by analysing the effects of various past and present geographical variables on diversity patterns of birds on islands distributed across Sundaland. We found evidence that bird endemicity was mainly restricted to deep-sea islands. The lack of any historical land-bridge precluded gene flow between the bird populations and gave rise to endemic species. This initial result provided a strong clue that historical processes shaped diversity. However, contrary to our expectation, neither the duration of mainland connection in the past 20,000 years nor the average change in island area in recent geologic history was shown to influence the number of species breeding on the shelf-islands. Instead, the classic Island Biogeography parameters of island area and distance from mainland predominantly explained species richness. Our unexpected results suggest that once a landmass is disconnected from the mainland due to rising sea levels – effectively becoming an island – extinction kicks in rapidly, leaving no room for “excess” species as the island shrinks. We also found that in addition to island area and distance from mainland, the proportion of landmass surrounding an island played an important role in maintaining species richness. For example, a solitary island that is 50 kilometres from the mainland would have lower species richness than a similar sized island, also 50 kilometres away, but clustered together with a group of other islands. In Sundaland, avian species richness of an island equilibrates quickly based on extinction-immigration processes, which are in turn influenced by the island’s present-day geographical parameters.
We additionally discovered that species composition on similar-sized islands did not differ across recently submerged and unsubmerged islands. On recently submerged islands, the bird population present today could only have arisen from entirely new colonisers since the island was wiped clean when it drowned. These birds, by their capability of occupying brand-new islands, can be said to have high dispersal and colonising ability. On the contrary, islands that were never submerged should technically be able to host both colonisers and surviving populations from before the islands shrank. Yet our results showed that the two island classes had species composition unaffected by their history of submergence. This observation implies that only a subset of species is capable of persisting on the tiniest islands in Sundaland: the strongly dispersive birds. Our result makes sense when we think about the biology of these animals – their population density is generally low, hence, once an island shrinks below a certain size, an isolated island population is unable to sustain itself. Resultantly, small islands become home only to the most dispersive species as these birds are able to fly around and find partners in nearby islands. Having said that, a majority of these species were intriguingly not island specialists – they were also birds that do well on the mainland too, but all on edge habitats; forest dependent species are the first to disappear once an island shrinks.
Examples of non-island specialists that are commonly found on small Sundaic islands: top left – Collared Kingfisher Todiramphus chloris; top right – Olive-winged Bulbul Pycnonotus plumosus; bottom left – Ashy Tailorbird Orthotomus ruficeps; bottom right – Pink-necked Green Pigeon Treron vernans. Example of an island specialist that is found across islands in Sundaland: middle – Pied Imperial Pigeon Ducula bicolor.
Birds are known for their flight ability, and it might be a counterintuitive idea that endemicity and species composition is shaped due to their reluctance to fly to other islands. However, flying across water is actually a behaviour that many tropical birds generally avoid due to the high risks involved, and this is why only the most adventurous of species do well on tiny islands.
Our work shows that species richness of birds on shelf islands in Sundaland is predominantly determined by present-day geographical parameters. It turns out that, at least for birds, Sundaic islands themselves are also only witnessing a snapshot in time.
Written by:
Yong Chee Keita Sin
Research Assistant, Avian Evolution Lab, National University of Singapore
Additional information:
Instagram @okamoto_keita_sin
Acknowledgements:
I am greatly indebted to the co-authors of our manuscript Nadiah (Twitter @NadiahKrist; nadiah.org) , Chyi Yin, Ryan (ryanchisholm.com) and Frank (Twitter @avianevo; avianevonus.com) for the support and mentorship offered throughout the work. I would also like to thank everyone who helped out in the fieldwork and analyses of our manuscript and Geraldine Lee for comments on this blog post.