[New Paper] Birds in paradise: biogeography in the subtropics

Light-vented bulbul (Pycnonotus sinensis), one of the species of interest in the Ryūkyū archipelago.

Biogeography is often more complicated than the species-area relationship as discussed in a recent Journal of Animal Ecology paper testing multiple extensions of island biogeography theory. Sam Ross, lead author of the study, describes how this work fits into the long history of biogeography research.

The species-area relationship is considered one of the only ‘rules’ in ecology. We have observed more species on larger ‘islands’ (whether true islands or simply some habitat patch of interest) in studies of different plants and animals all around the world. When MacArthur and Wilson (1967) proposed this pattern and the pioneering biogeographical principles which underpin it, they acknowledged that a piece of the puzzle was missing: species identity.

Biogeographers have since recognised that species aren’t randomly distributed across the globe. We now believe there to be ecological factors which predict where species occur. For example, predators can only live in habitats where their prey are sufficiently abundant, otherwise they’ll starve. This led Dominique Gravel and colleagues to predict that larger islands should have more complex food webs, since smaller islands support fewer prey species and so can in turn support fewer, if any, predators (Gravel et al. 2011). They then proposed that predators should be more influenced by island size than their prey, producing steeper species-area relationships for higher trophic levels. They called this idea the ‘trophic theory of island biogeography.’

We tested this empirically using checklists of bird sightings across the Ryūkyū archipelago running from southern mainland Japan to Taiwan. We separated birds by their trophic groups and found that contrary to the trophic theory of island biogeography, our predatory birds didn’t really differ in the slope of their species-area relationship from our herbivorous birds. This wasn’t really what we expected to find but the trophic theory hasn’t yet been tested across a range of different study systems, so our test helps us to understand whether communities may be structured by trophic level or not.

Expectation versus reality of our test of the trophic theory of island biogeography with the birds of the Ryūkyūs.


Another way species’ identities might structure communities is based on the idea of environmental filtering. These filters are thought to be strongest on small islands, where there is little opportunity to just scrape by. Small islands are harsh; there are many ways populations can go extinct on small islands, but particularly life on these islands is strongly affected by environmental conditions. This means that only species particularly suited to the environment are likely to survive and thrive on small islands. By expanding on the work of Claire Jacquet and colleagues (Jacquet et al. 2017), we could then predict that small islands would have species which are similar to each other and are all adapted to the local environment, whereas larger islands are more likely to contain random species from the regional pool of all species which could possibly live there.

Another longstanding idea predicts the opposite pattern. Because smaller islands have fewer resources, species must compete for those finite resources to survive. This means that on small islands, we might expect species to be widely different from each other to minimise competition for food and space. If there’s only one small grasshopper population on the island for example, it seems more likely that we’ll find five species of birds that all eat different things than five that are competing for the chance to eat this one grasshopper. So, we might expect that competition results in distinctive species on smaller islands and that as competitive pressure relaxes on larger islands, these islands again are more likely to contain a random assortment of species.

Blue Rock Thrush (Monticola solitarius) pictured at Cape Zanpa, Okinawa—the edge of the island.


We tested whether either of these two processes structured the bird communities of the Ryūkyūs by calculating the functional and phylogenetic diversity of birds on each island using two global databases. We used the global phylogeny of birds and a database of functional traits to measure the observed functional and phylogenetic diversity of birds on each of our study islands. We also tested whether this observed diversity was higher or lower than expected by random chance by shuffling the names of species on the phylogeny and functional trait matrix. Together, this meant we could test whether diversity was lower than expected by random on small islands and increasing to a random sample of the regional pool (trait-based assembly), or whether competitive assembly occurred, where diversity was higher than expected on small islands and closer to random on larger islands.

We found no clear overall pattern of either trait-based or competitive assembly of bird communities in the Ryūkyūs, but we did find some differences among our trophic groups in whether communities were structured randomly or not. The insectivorous intermediate predators showed patterns of trait-based community assembly since their phylogenetic and functional diversity was lower than expected on small islands and increased to random on larger islands.

Community assembly processes across our trophic groups of birds. We found no clear patterns for apex predators or herbivores, but intermediate predators followed the predictions of trait scaling by Jacquet et al. (2017).


Overall, we tested multiple extensions to the theory of island biogeography which have been rarely tested, and certainly not extensively across a range of study locations and focal species. In the subtropical Ryūkyū archipelago, we found that bird communities did not clearly conform to the theories laid out by recent extensions to island biogeography theory, but that some held true. For now, we encourage others to continue testing these hypotheses in a variety of study systems to see whether our subtropical bird communities show the same biogeographic patterns as animal communities around the world.

This post is written by: Sam Ross, a PhD student at Trinity College Dublin studying ecological responses to global change and a visiting research student at the Arilab.

More Info:

Ross, S. R. P-J., Friedman, N. R., Janicki, J., & Economo, E. P. (2019). A test of trophic and functional island biogeography theory with the avifauna of a continental archipelago. Journal of Animal Ecology. DOI: 10.1111/1365-2656.13029

You can read the full paper here.

New paper on Melissotarsus, one really strange ant

We have a new paper out led by Adam Khalife and Christian Peeters in Frontiers of Zoology, about the very strange ants from the genus Melissotarsus. These African ants live exclusively in tunnels they excavate from living wood. They get their food from diaspadid scale insects they cultivate in their tunnels. Tunnelling through living wood is a pretty tough thing for ants to do, but Melissotarsus has some famously weird things about it that seems to help. The most obvious is their middle legs, which point upward instead of down like all other ants. It’s been known for a long time that due to these legs, they can’t walk on a 2D surface, they just fall over.

So, what’s going on here? In this study we used micro-ct along with other techniques to investigate adaptations to the skeletomuscular system, and the mandibles and legs in particular, that allow this ant to live this unusual lifestyle. Melissotarsus have unusually shaped heads, packed with muscle to close their powerful jaws. Their apodemes are expanded allowing for increased muscle fiber attachment. Turns out, their opener muscles also are enlarged, which is unusual for ants. This appears to help it chew through wood with its zinc-fortified cone-shaped mandibles. One needs both closing and opening power topush mandibles in and pull them out of wood.

And what about those legs? Ants hardly have trouble walking through tunnels on normal legs. Melissotarsus legs appear to be evolved to help brace the ant in the tunnel to apply force to the biting motion. If you are tunneling into a hard surface you need to push down on that surface, and if the ant is not anchored somewhere, the powerful mandibles would just push it backward. So we think that is why they have such odd legs, to anchor itself in the tunnel. Aside from their orientation, legs themselves are also highly modified, apparently for bracing.

There’s a lot more in the paper, and a lot more cool things about this ant that have evolved to allow it to exploit a unique (to ants) ecological niche that our team and others will be exploring in future papers.

New review paper on the future of tropical biodiversity

Out in Nature this week: “The future of hyperdiverse tropical ecosystems”. We were happy to contribute to a broad review of the importance of the hyperdiverse tropics to global biodiversity, ongoing threats to tropical biodiversity, as well as strategies to help mediate those threats. Particularly pleased to see ants being used as an exemplar taxon alongside birds and mammals and the rest, a main goal of our (and many others’) efforts over these last years. This stunning photo was taken by the late Rodrigo Baleia.

Proceratium in China

We have a new paper out in Zookeys (https://doi.org/10.3897/zookeys.770.24908) that revises the taxonomy of the very rare and cryptic ant genus Proceratium in China. We recognized 8 species from China and described 3 of them as new to science. The most spectacular species from Yunnan we named Proceratium shohei in honor of Dr. Shohei Suzuki (1979-2016), a marine scientist from OIST who lost his life in a tragic research diving accident. The study was led by our colleague Michael Staab, and included Paco and Cong from the lab, along with Zheng-Hui Xu from China.

This study continues our lab’s line of research integrating x-ray microtomography (micro-CT) scanning, computer-based 3D reconstructions, and several downstream 3D data products (such as 3D surfaces and videos) into ant systematics. We used virtual 3D surface models based on micro-CT scans for in-depth comparative analyses of specimen morphology in order to overcome the difficulties of examining the rare and valuable physical material. Since these ants are extremely hairy, thus often very dirty, we basically “shaved” them virtually.

 

New paper on global ant diversity patterns

We have a new paper out in Nature Communications on testing hypotheses for latitudinal gradients in ants. This is the first paper to really present and analyze the full scope of the GABI database of all ant species distributions. To complement the species range data, we did a very extensive phylogenetic and dating analysis, including implementing the fossilized birth-death process with 500 fossil taxa informing the dating. We analyzed the geographic and phylogenetic data together to test hypotheses for the latitudinal gradient, including variation in diversification rate and time. We generally found the latter to be most consistent with explaining the gradient.

We also did a complementary study focusing on Pheidole, which deals with emergence of the gradient on more recent timescales. A preprint of that one is available on bioarxiv.