[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.

Lab retreat to Iheya

Our lab decided to take a trip to Iheya island the second week of December 2017, in order to explore different parts of Okinawa, but also to celebrate the end of Cong’s PhD defense and Yuka’s proposal defense.

The island is full of goats and sculptures made from marine debris. We also visited a cave, a beach, hiked to the top of a small mountain, and had some good food and drinks.

Nick (our post-doc who does a lot of research on acoustics and birds) also did some bird watching on the island, and below is a list of birds he saw on Iheya:

Eurasian Teal
Chinese Turtle Dove
Japanese Bush Warbler
Pale Thrush
Blue Rock Thrush
Japanese White-eye
Grey faced Buzzard
Japanese Sparrowhawk
Japanese Wood Pigeon
Little Grebe
Great grey egret
Little egret
Pacific Swallow
Intermediate Egret
Zitting Cisticola
Daurian Redstart
Tufted Duck

Apart from bird watching, Takuma (our technician leading the field team who is a great taxonomist) also collected insects during our hike.

Until next time, Iheya!

(Most images taken by Cong Liu)

Listening to ecosystems: New study published using acoustic monitoring to study Okinawa’s “Soundscape”

At every OKEON site there is a small green box attached to a tree. These boxes are acoustic monitors, and they are recording natural sounds almost constantly. As part of the OKEON project, we use these natural sound recordings, or “soundscapes”, as a way of monitoring biodiversity.

Sam Ross sets up an acoustic monitoring device at the OIST field site.

We collect more than 1 terabyte of audio data every week. If you wanted to listen to all of the recordings we’ve made so far, it would take you about 8 years… if you listened all day and never went to sleep. To sort through all this audio data, we use two approaches. First, we break the sounds up into sounds at different frequencies (i.e., pitch). This lets us get a big picture view of when and where animals are active on Okinawa. Second, we use machine learning to train our computers to detect species in which we are interested. This helps us understand more about which particular species are in each area of the island, and how their behavior varies across the year.

In many parts of Okinawa, humans and nature live close together. Managing this interaction is important for preserving wild populations of plants and animals.

Ultimately, our project aims to understand the ways that human activity affects Okinawa’s wildlife, and how we can better protect these species in the future. For more information (including videos), please see the OIST press release. A link to the study can be found here.

Chillier Winters, Smaller Beaks

This honeyeater (Melidectes belfordi) lives in the montane forests of New Guinea. Photo credit: Charles Davies; Flickr. This photo was cropped from the original version.

(Article provided by the OIST media section)

Although Charles Darwin lived and worked in the 19th century, modern evolutionary biologists are far from exhausting all avenues of inquiry regarding birds and evolution. For example, in the 1990s, researchers such as Russ Greenberg, ornithologist from the Smithsonian Institution in the United States, began to explore a new question concerning the relationship between climate and the evolution of beak size. This question was inspired by Allen’s Rule, which states that warm-blooded animals living in cold climates will have shorter limbs and appendages than those that live in warmer climates. The biological mechanism behind this rule is thermoregulation—more body surface area helps animals to shed heat better whereas less surface area helps them to conserve it. Since a bird’s beak plays a large role in thermoregulation—it has lots of blood vessels and is not covered in feathers—researchers wondered whether hotter climates beget larger beaks and colder climates beget smaller ones. Indeed, studies revealed that climate has influenced beak size, but not which type of climate had more of an overall impact.

Past research left a question open at the end: “Which of these functions is under selection?” Dr. Nicholas Ryan Friedman, a researcher from the Biodiversity and Biocomplexity Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), comments. “Are birds with small beaks dying in the summer because they get too hot? Or are birds with large beaks dying in the winter because they get too cold?” In collaboration with scientists in the Czech Republic, Dr. Friedman designed a study to explore this question and ultimately found that winter, not summer, had more of an impact. The study is published the journal Evolution.

Dr. Friedman and colleagues chose to tackle the question by recording variations in beak size in Australasian honeyeaters and allies. What makes this group of birds a great subject for this study is that the region they inhabit, Australia, New Guinea, and the South Pacific, exhibits huge variation in climate and temperature—from the tropical forests of New Guinea, to central Australia’s arid deserts, to the temperate forests of Tasmania. This means that it is possible to compare differences between individuals of the same species that are living in wildly different conditions.

After measuring the beaks from 158 different species using specimens from the Australian National Wildlife Collection and comparing beak sizes to climate, the researchers found no correlation with summer temperatures but a clear one for winter—the coldest winters were associated with the smallest beaks, whereas warmer winters were associated with larger beaks.

The top graph shows a correlation between beak size and winter minimum temperatures, with the smallest beaks relating to the coldest winters. The bottom graph shows no clear correlation for summer maximum temperatures.

Before Dr. Friedman and colleagues reported this new environmental pressure on beak size, winter temperatures, feeding habits were believed to be the greatest driving force in beak evolution. For example, since the 1970s, Peter and Rosemary Grant, the famous duo who measured the process of evolution in real-time in the Galapagos, have been studying how beak size can change due to food availability over a short period.

“Which is exciting!” Dr. Friedman comments. “But it’s not yet clear from that whether adaptation to improve feeding efficiency is the only, or even the most important, factor in driving beak evolution across millions of years.”

What is unique about Dr. Friedman and colleagues’ study is that it allowed for a peek into an unusually broad evolutionary timeframe. By comparing many different species of birds, the researchers were able to delve into a very distant past and discover the morphological importance of winter temperatures. The next step would be to better understand the relationship between these two factors—feeding efficiency and winter temperatures—in the overall narrative of beak evolution.

The Helmeted Friarbird (Philemon buceroides), a member of the honeyeaters, lives in Northern Australia and New Guinea. Photo credit: Jim Bendon; Flickr.

By Anne McGovern (media@oist.jp)