Julian Schrader is a postdoc at the Macquarie University, Sydney. He is an ecologist with special interest in plant functional ecology, biogeography and conservation biology. Here, Julian shares his recent work on functional traits in relation to the Equilibrium Theory of Island Biogeography.
Fieldwork: Julian is fascinated by life on small islands even though field work can be quite space-limited sometimes (small island in Raja Ampat Archipelago, Papua, Indonesia; see also Schrader et al. 2021 Journal of Biogeography).
Institute. School of Natural Sciences, Macquarie University, Sydney, New South Wales, Australia.
Academic life stage. Postdoc.
Major research themes. Ecology – Biodiversity – Biogeography – Ecosystem Functioning – Nature Conservation
Current study system. The core idea of this paper was to extend MacArthur and Wilson’s famous Equilibrium Theory of Island Biogeography (ETIB) to functional traits and test whether community trait values remain constant over time, similar to species richness, as predicted by ETIB. ETIB is among the most influential theories in ecology. Testing its core assumptions for traits for the first time was very exciting.
Recent JBI paper. Schrader, J., Wright, I. J., Kreft, H., Weigelt, P., Andrew, S. C., Abbott, I., & Westoby, M. (2023). ETIB‐T: An Equilibrium Theory of Island Biogeography for plant traits. Journal of Biogeography, 50(1), 223-234. https://doi.org/10.1111/jbi.14526.
Motivation behind this paper. Functional island biogeography – the integration of trait ecology to island biogeography – is a relatively new but rapidly expanding research field. Core ideas of functional island biogeography include testing whether island community assembly can be explained by species traits rather than species richness. ETIB provides the conceptual background for countless island studies and extending ETIB to traits has long been a research focus of our team. However, major advances have so far been hindered by lack of temporal data needed to test ETIB for traits. Repeated vegetation sampling over longer time spans is scarce in island ecology, especially when also trait data are needed. With the island vegetation surveys done by Ian Abbott and colleagues from the 1960s to 1990s on small islands off Perth in Western Australia and the recent AusTraits database (a trait database for Australian plant species), we could finally test whether island community trait values remain constant over time irrespective of local species immigrations and extinctions.
The Perth Archipelago includes many small to medium seized islands. The islands’ small size and often high disturbances by waves and wind lead to high local immigration and extinction events. Small plant species with small seeds are more likely to establish but also have higher local extinction risk.
Key methodologies. We used simple statistics to test whether community trait values and functional diversity remain at equilibrium over time. The patterns we found are thus easy and intuitive to interpret, clearly showing that ETIB can be extended to functional traits. Our simple analytical approach also makes the study easily repeatable in other archipelagos, hopefully leading to further insights from other regions, species and traits. Fortunately, all data needed for this study were available in published papers. This allowed us to extract data remotely and write-up the manuscript during the height of the covid-19 pandemic.
Unexpected challenges. The biggest hurdle of this research was certainly the covid-19 pandemic. None of the initial co-authors has ever visited the small islands off Perth, which made ecological interpretation of the results difficult. Fortunately, Ian Abbott was keen to join our team and help interpreting the pattern. Ian is a highly experienced island ecologist and has visited countless of the small islands in Western Australia since the 1970s. Also, some of the species occurrence data were collected by Ian. After the pandemic, I had the chance to visit some of the studied islands myself at last.
Major results. This study is the first, to the best of our knowledge, to test for temporal trait changes and trait equilibria on islands. We found that community trait values and functional diversity remain constant over time despite very high species turnover, which was as high as 60% in some islands. Further, we found that species most susceptible to turnover were on average smaller and had lower seed mass than persisting species. These results provide evidence that ETIB is extendable to functional traits.
Next steps for this research. I would certainly like to test the Equilibrium Theory of Island Biogeography for Traits at larger biogeographical scales. That is, to test whether the same patterns we found on the small island off Perth also occur across different archipelagos worldwide and under varying geo-environmental factors. For that, I would like to motivate colleagues and other island enthusiasts to revisit and resample islands studied in the past. It may also be interesting to test whether global change, in the form of climate warming and rising sea levels, has an impact on community trait composition on islands. This is something I would like to test in the near future.
If you could study any organism on Earth, what would it be? I would again turn my attention towards islands and their unique biodiversity. Islands are wonderful study systems and fun to visit and work on. I am especially fascinated by small islands as the ecological mechanisms maintaining their biodiversity can be studied and understood in detail and still yield novel and surprising results.
The largest island of the Perth Archipelago – Rottnest Island or Wadjemup – also supports a large variety of wildlife like this endangered Quokka (Setonix brachyurus). Luckily, on Rottnest Island Quokkas are still common.