Author Archives: ryanchisholm

Aloysius’s paper on estimating tropical plant litter decomposition published in Pedobiologia

Aloysius Teo completed his PhD in the lab in 2017, and a chapter from his thesis has just been published in Pedobiologia. The “tea bag method” was developed several years ago by our coauthor Joost Keuskamp for estimating decomposition rates of plant litter. The method relies on commercially available tea bags, thus facilitating standardisation of methods across studies. Aloysius explored the applicability of the method to tropical forests, where a particular problem is that abundant termites readily damage tea bags.

Aloysius found that tea bag attack rates by termites were large enough in tropical forests in Singapore to invalidate experimental results relying on the tea bag method. He trialed methods for excluding termites and, based on his results, recommended an extended tea bag method for future use in the tropics that relies on a combination of unmodified tea bags and termite exclusion treatments.

Teo, A., N. P. Kristensen, J. A. Keuskamp, T. A. Evans, M. Foo, R. A. Chisholm. 2020. Validation and extension of the tea bag index to collect decomposition data from termite-rich ecosystems. Pedobiologia (in press)

tea bags

Top: Physical termite exclusion barriers used for tea bags in the study, along with unmodified tea bags. Bottom: A tea bag undetected by termites (left) alongside two bags that were detected (centre and right).


Tak coauthors new genetic modelling paper with Frank Rheindt’s lab

A new paper led by Tang Qian and  Frank Rheindt from the Avian Evolution Lab at NUS, with Tak Fung as a co-author, has just been published in Molecular Ecology Resources.

The paper describes how they developed a new R package called ResDisMapper, which helps in the management of biological invasions and habitat degradation by allowing users to generate a map showing resistance to dispersal over a landscape, as defined using genetic data. The R package is novel because there are few programs available that map resistance to dispersal over the relatively short spatiotemporal scales required for the management of biological invasions and habitat degradation.

They tested ResDisMapper against two other programs (DResD and EEMS) using a suite of simulated datasets and found that overall, it performed substantially better. They further demonstrated the utility of ResDisMapper by applying it to genetic data collected for rock pigeons (Columbia livia) in Singapore and Golden-crowned sifakas (Propithecus tattersalli) in northern Madagascar, to identify regions with high and low resistance to dispersal.

Tang, Q., T. Fung, and F. E. Rheindt. 2020. ResDisMapper: An R package for fine-scale mapping of resistance to dispersal. Molecular Ecology Resources


Resistance map produced by ResDismapper for rock pigeons in Singapore, with annotations describing the meaning of the different colours and contours. A significant barrier/corridor refers to areas with resistance values that are higher/lower than those from a null distribution with high probability, and lie within the red/green contours. Areas that lie inside the blue contours have resistance values with high probability of being positive or negative (high “certainty”). The yellow circles indicate sampling points.

Meryl’s new paper on Singapore butterfly extinctions published in Biological Conservation

In a paper just out in Biological Conservation, we estimate that 46% of Singapore’s butterfly species have been extirpated since 1854. Our study, one of the most comprehensive of its kind for tropical insects, gives a window on to how insect biodiversity may suffer as habitat destruction and degradation continues across the tropics. The paper was led by Meryl Theng, who was a research assistant in our lab and has recently started a PhD at the University of Adelaide.

To make our estimate of Singapore butterfly extirpations, we first put together an extensive database of butterfly records in Singapore going back to the first major collections in 1854. This included records from the Natural History Museum London, to which we sent two staff members to search for Singapore butterfly records. We then applied statistical models, recently developed in our lab, that estimate the total extirpation rate accounting for both observed and unobserved species.

We also looked at traits associated with early detection and early extirpation among Singapore’s butterflies. We found that species with rare larval host plants tended to be discovered later and extirpated earlier. Additionally, species with small wingspans tended to be discovered later, and species that were forest-dependent tended to be extirpated earlier.

Our paper provides an informative and timely case study of tropical insect extirpations. The estimated 46% extirpation rate of butterflies in Singapore (95% confidence interval [41%, 51%]) is greater than that previously estimated for birds in Singapore (33% [31%, 36%]), and suggests that tropical insects may be suffering more than other groups from human impacts.

Theng M., W. F. A. Jusoh, A. Jain, B. Huertas, D. J. X. Tan, H. Z. Tan, N. P. Kristensen, R. Meier, R. A. Chisholm. A comprehensive assessment of diversity loss in a well-documented tropical insect fauna: Almost half of Singapore’s butterfly species extirpated in 160 years. Biological Conservation 242:108401

Update: Our work has been reported on in the Straits Times (paywall) and in the Star online.

Ancistroides gemmifer.png

Ancistroides gemmifer, a species of skipper butterfly last recorded in Singapore 1926 and presumed extirpated there. The species persists in other parts of the region, including Penang, Malaysia, where this photo was taken. Image Credit: Gan Cheong Weei

New paper critically examining Modern Coexistence Theory published in Ecology Letters

In collaboration with colleagues from Bar-Ilan University in Israel we have just published a new paper in Ecology Letters critically examining key aspects of Modern Coexistence Theory—a theory that seeks to understand which mechanisms allow large numbers of species to coexist in nature. Specifically, we examine the theory’s reliance on using a species’ mean invasion growth rate as a measure of its ability to persist in a community.

Modern Coexistence Theory assumes that higher invasion growth rates imply greater persistence. We found that although the sign of the mean invasion growth rate correctly characterises two qualitatively different domains of species persistence, the magnitude of the mean invasion growth rate is not a reliable indicator of species persistence. The underlying reason is that the mean invasion growth rate ignores the effects of temporal variations in species abundances on species persistence. We suggest further investigation of metrics of species persistence that incorporate temporal variations in species abundances.

The project was led by Jayant Pande, a post-doctoral researcher in Nadav Shnerb’s lab at Bar-Ilan University. It is part of our collaborative grant with Shnerb’s lab under the Singapore–Israel research grants programme.

Pande, J., T. Fung, R. A. Chisholm, N. M. Shnerb (2019). Mean growth rate when rate is not a reliable metric for persistence of species. Ecology Letters. [link:]

New paper on resource facilitation models published in Oikos

A new paper in Oikos by Lam Weng Ngai (who recently defended his PhD thesis in Hugh Tan‘s lab at NUS) and Ryan looks at theoretical conditions under which one species—a facilitator—can promote the persistence of another—a recipient—by providing it with resources. A classic example of a facilitator species is a “nurse” plant, which creates an environment suitable for the germination of other plant species under its canopy. In the paper, we were particularly interested in examining the stress gradient hypothesis, which predicts that such positive species interactions should be stronger in environments suffering greater resource stress.

We analysed two simple dynamical mathematical models of resource facilitation and found that positive interactions between a facilitator and a resource recipient species occur only when the facilitator-mediated resource conversion rate is higher than the background rate. We found limited support for the stress gradient hypothesis in our two models—the hypothesis holds only when certain mathematical conditions on the model parameters are satisfied. Our work on these simple models establishes a mathematical framework on which future studies can build and explore the robustness of our conclusions about when and how resource facilitation operates.

W. N. Lam and R. A. Chisholm. Resource conversion: a generalizable mechanism for resource-mediated positive species interactions. Oikos (in press)

Facts about Gorse in New Zealand

The introduced plant Gorse (Ulex europaeus) in New Zealand can act as a nurse plant for regenerating native forest by stabilising the soil and providing an understorey environment that shelters seedlings from excessive wind and sun.