Sean Pang, who completed his PhD in our lab in 2022, has successfully published a chapter from this thesis about conserving Southeast Asian tree species in the face of both climate and land-use change. He modelled nearly 1,500 tree species distributions and explored how they would fare under four future climate-change scenarios involving different assumptions about human mitigation efforts. As expected, the tree species overall fared best under the most sustainable climate pathway (SSP1-2.6), but surprisingly the intermediate climate pathways (SSP2-4.5 and SSP3-7.0) were worse for biodiversity than the least-sustainable climate pathway (SSP5-8.5). The main reason was that the intermediate climate pathways involved greater land-use change for biofuel production to support a partial transition away from fossil fuels. The results demonstrate how halfway efforts to prevent climate change can potentially have detrimental effects on biodiversity. The paper is just out in the journal Nature Sustainability.

Sean was originally a student in Edward Webb’s lab, but completed the last year of his PhD in our lab after Webb moved to the University of Helsinki. Sean is now a post-doctoral research fellow at Aarhus University in Denmark.

Pang, S. E. H., J. W. F. Slik, R. A. Chisholm, and E. L. Webb. Conserving Southeast Asian trees requires mitigating both climate and land-use change. Nature Sustainability

From 5th to 9th August 2024, Sankar attended the 10th World Congress of Herpetology, held in Kuching, Sarawak. This is a meeting of global herpetologists held every four years. Sankar presented a poster on his work tracking squamate (lizard and snake) extirpations in Singapore using the SEUX and MODGEE models developed in our lab.

The SEUX model takes in the first and last years of detection for the species in a group and estimates the rates of known extinctions and dark extinctions (i.e., extinction prior to discovery) that occur within that group. The MODGEE model takes advantage of the entire detection time series and can additionally estimate the extinction probability of individual species and the time of extirpation. A recent comprehensive compilation of herpetofaunal records in Singapore has made it possible to use MODGEE to obtain these estimates for squamates (snakes and lizards). The resulting assessments of which species are extinct can be valuable in planning species reintroductions in the future.

The conference was well attended (~1500 delegates) and there were many opportunities to engage and network with other herpetologists from around the world working on conservation translocations, capture-mark-recapture monitoring and population viability analysis.

At the end of the conference, Sankar was awarded the best poster in the multi-taxa category. Congratulations, Sankar!

In the first week of July, Tak attended the Joint Annual Meeting of the Korean Society for Mathematical Biology and the Society for Mathematical Biology, held in Seoul. During the conference, he presented some new results on deriving probability distributions of extinction time for a population model with a growth rate that varies stochastically due to environmental fluctuations. This is an important topic for conservation because many species are in danger of extinction and it is unclear how environmental changes over time affect their extinction risk.

Specifically, the environmental fluctuations in the model are assumed to follow a random process that is called 1/f noise, following empirical evidence that many environmental variables in reality follow such a random process. Analyses of the model show that the mean extinction time should increase with the autocorrelation time of environmental fluctuations, because of non-linear averaging of the proportional growth rate over different sets of environmental conditions (technically, this is an application of Jensen’s inequality to the proportional growth rate). Because environmental variables in marine ecosystems tend to have longer autocorrelation times than in terrestrial systems, our model predicts that the mean extinction time would generally be greater for marine versus terrestrial populations, provided that the populations are comparable in other aspects of their biology.

In addition, Tak engaged with numerous other researchers and exchanged thoughts on different areas in mathematical biology, including but not limited to ecology and evolution. In particular, he met up with Nao Takashina from the University of Tokyo, who collaborated with Tak and Ryan on a recent paper, published in the Journal of Ecology, on mechanistic partitioning of species richness in tropical forest tree communities.

The conference was held at the Seoul campus of Konkuk University (photo used under a CC BY-SA 4.0 license, with credit to Konkukeditor).

As part of a talk series on modelling physical, biological and social phenomena run by Alphabet (Google’s parent company), Ryan presented an online seminar about our lab’s work on ecological modelling. He talked about our Singapore extinctions project, our research programme on the maintenance of diversity, and potential novel applications of optimised individual-based ecological models to video games and virtual reality applications.

Over the last six months, three undergraduate Special Programme in Science students, Chin Zhen Jie, Andrew Teoh, and Cheng Jun Yuan (pictured left to right, below), have been conducting a project in our lab for their Integrated Science Project. They built a computational model to explore the joint effect of stage structure and conspecific negative density dependence on forest tree biodiversity. Their main result was that stage structure enhanced the diversity generated by density dependence. They recently completed their report, underwent a viva voce, gave a poster presentation, and gave a slide presentation at the Special Programme in Science student congress. For the latter, they won the best presentation award. Congratulations, guys!

The Special Programme in Science was established in 1996 at NUS to nurture enthusiastic students with a passion for science. The students take six tailor-made courses over the first two years of their degree, including the Integrated Science Project in the second year.

Infectious diseases exert a large burden on the well-being of human societies, as vividly illustrated by the recent COVID-19 pandemic. In response to an infectious disease, governments enact a variety of non-pharmaceutical interventions (NPIs) such as social-distancing guidelines, mask-wearing mandates and lockdowns. These NPIs reduce spread of the virus causing the target disease, and also reduce spread of other viruses that cause non-target diseases. For example, NPIs enacted during the COVID-19 pandemic reduced the spread of non-target respiratory diseases caused by influenza and respiratory syncytial virus (RSV). However, due to lack of pharmaceutical interventions aimed at these non-target diseases (such as vaccinations), they often rebounded to levels higher than pre-pandemic levels after the NPIs were relaxed. Thus, the net effect of NPIs on the long-term total disease burden of non-target diseases is unclear.

This knowledge gap was addressed in a new study by Tak, Ryan and Jonah Goh (former Honours student at Chisholm Lab), recently published in the Journal of Theoretical Biology. We considered a scenario where NPIs were enacted for a year and then completely removed. Under this scenario, we analysed a suite of four epidemiological models of varying generality and complexity, to quantify the net effect of the NPIs on the long-term total disease burden of a seasonal, non-target respiratory disease.

The simplest model of the four was a SIR model with seasonal disease outbreaks and temporary immunity of recovered individuals. For this model, we performed a rigorous mathematical analysis to show that the net effect of NPIs was always to reduce the long-term total disease burden. The number of susceptible individuals increased during the year when NPIs were enacted and this led to the number of infections rebounding to high levels after the NPIs were removed (Fig. 1). But this was more than offset by the number of infections prevented by the NPIs when they were being applied (Fig. 2). We found that the net reduction in the number of infections depended critically on the rate at which immunity was lost. In the extreme case of near-permanent immunity, the pool of susceptible individuals remained small and resulted in small outbreaks, such that the NPIs produced a small net reduction in the number of infections. However, as immunity was lost more quickly, the size of outbreaks increased commensurately and the NPIs produced increasingly larger net reductions in the number of infections, with the net reduction reaching large values of 70-100% of population size when immunity loss occurred quickly (on the order of months).

These key results from the simplest model were supported by simulations of the three more-complex models, which were parameterised for specific locations including Singapore and China. Overall, our study highlights a hitherto under-appreciated role of NPIs in reducing the long-term total disease burden of non-target diseases, which should be factored into cost-benefit assessments of NPIs in public health management.

Fung T., J. Goh, R. A. Chisholm. 2024. Long-term effects of non-pharmaceutical interventions on total disease burden in parsimonious epidemiological models.
Journal of Theoretical Biology 587:111817

Aloysius finished his PhD in our lab several years ago, and another chapter from his thesis has just been published. The paper reports on work he did assessing leaf litter turnover and nutrient dynamics in forests in Singapore, looking at differences between old-growth forests and novel forests. The novel forests have arisen in the last few decades from cleared farmland and villages. His main findings were that leaf litter decays much faster in novel forests than in old-growth forests, resulting in a litter pool only one third the size, and that phosphorus is highly elevated in the litter of novel forests. The differences may be attributable to dominance of exotic tree species in novel forests. The reduced litter pools in novel forests have negative consequences for ecosystem carbon balance and climate change mitigation. This may have broad relevance because of the increasing prevalence of novel forests across the tropics.

Teo, A, T. A. Evans, and R. A. Chisholm. Elevated litterfall phosphorus reduces litter and soil organic matter pools in exotic-dominated novel forests in Singapore. Journal of Tropical Ecology. 2024;40:e4