In this post, I want to share a bit more about the technical details of my research and explain my motivations. I promise I'll try not to put you to sleep! There are photos at the end as a small reward.
A primary objective of my thesis research is to describe how the many Syzygium species at Tuanan differ in terms of pollinators and timing of flowering - together, these are two of the most important aspects defining the reproductive niches of flowering plants. But why Syzygium? Beyond having many attractive and useful tree species, Syzygium is the most diverse genus of trees on earth. The genus has speciated super rapidly (Low et al, 2022), and in many Asian tropical forests, there are far more Syzygium species than there are of any other tree genus (Cannon & Lerdau, 2015). The Syzygium story has recently come to the attention of a broader community of ecologists, thanks to the insights provided by molecular analysis (i.e., looking at their DNA), which led to a lot of shuffling of tree species previously not thought to be Syzygium into the genus.
There are many lineages of plants that reach truly incredible diversity in tropical forests. A few of these have been the focus of much ecological research and hypothesis building for eons. The diversity of figs (genus = Ficus), for example, can be really high in forests all around the tropics (Shanahan, 2016). Orchids are another great example... the huge diversity of sometimes impossibly rare orchids found in tropical forests has fascinated ecologists all the way back to Darwin (Darwin, 1862). There are also several diverse genera in the Arum family, such as Anthurium (>1000 species) and Philodendron, that form really diverse communities in tropical forestsin the Americas. It turns out that all of these plant lineages have one thing in common - they all have narrowly defined reproductive niches. Figs species are each pollinated by one, or sometimes a few, tiny wasp species that visit only visit one (or, again, sometimes a few) fig species. Orchids have evolved a whole array of ways to trick pollinators into visiting them specifically, and then getting them to simultaneously transport their pollen long distances while avoiding that pollen being lost or deposited on other flowers. And while some co-occuring Araceae may share the same pollinator species, they stagger the timing of their flowering during the day and night so that they aren't attracting the same pollinators at the same time (I highly recommend checking out the 'In Defense of Plants' podcast's interview of Florian Etl if you want to learn more). These strategies all lead to several outcomes that may be important in explaining the high diversity found in these plant communities: 1) competition between species for pollinators is decreased, 2) the likelihood of pollen being transported between two flowers of different species is lowered, and 3) the distance over which pollen is successfully carried between flowers of the same species may be increased.
If your still awake you may be wondering what this has to do with me studying Syzygium. Well, Syzygium trees generally don't have narrow reproductive niches. Studies of Syzygium species everywhere from Northern Australia to India's Western Ghats, and from Benin to Mauritius, find many different species of animals visiting flowers, and many of the same pollinators visiting multiple Syzygium species. Flowers also often persist for long periods, through multiple days and nights. So, what gives? Why are Syzygium species so diverse? Does reproductive niche overlap not matter for diversity, or are Syzygium just an exception? How much do Syzygium reproductive niches really overlap anyway?
Indonesia, Malaysia, and Papua New Guinea have the highest Syzygium diversity in the world... and I'm right in the center of it all at Tuanan, in Indonesian Borneo. One characteristic of the forests in this region is that they are wet and equatorial, so that seasons are hardly perceptible and plant growth and reproduction can occur year-round. The forests also have a really diverse set of potential pollinators... in Borneo researchers have recorded instances of plants being pollinated by birds, bees, bats, beetles, mammals, thrips, cockroaches, wasps, flies, etc. etc., you get the idea. This led me to wonder whether, maybe, instead of having narrow reproductive niches, Syzygium species experience reduced reproductive niche overlap because they collectively occupy a really broad reproductive niche space. That is, sure, two species may flower at the same time of day, but they only flower at different times of year; or two species may attract multiple pollinator types, but the pollinator types still differ from each-other. These details are lost in studies that use short term data, or group pollinators at large taxonomic resolutions.
To try to answer this question, I plan on using 20 years of existing data on the timing of flowering of the Syzygium species here, and I'm collecting new data on who visits Syzygium flowers and what they do once they get there. It's still too early to share any conclusions, but I have learnt that the flowers are all beautiful, and not all visited by the same pollinators, and that species differ from each-other in nuanced ways. Scent, for example, really varies across species: one species' flowers smell distinctly of watermelon rind, and another could pass for eau-de-cologne. Here are some photos of Syzygium flowers so you can share in my excitement and wonder:
(P.S. Yes, I know they are all white and look 'sort of' the same, so what?)
Further reading:
Low et al., 2022 - Genomic insights into rapid speciation within the world’s largest tree genus Syzygium | Nature Communications
Cannon & Lerdau, 2015 - Frontiers | Variable mating behaviors and the maintenance of tropical biodiversity (frontiersin.org)
Shanahan, 2016 - Gods, Wasps and Stranglers: The Secret History and Redemptive Future of Fig Trees by Mike Shanahan | Goodreads
Darwin, 1862 - Fertilisation of Orchids - Wikipedia
In Defense of Plants Episode 383 - Florian Etl — In Defense of Plants