Tag Archives: conservation

Sankar’s paper on squamate extirpations and recoveries in Singapore published in Biological Conservation

Singapore has experienced substantial habitat and biodiversity loss since 1819. Nevertheless, the last century has also seen some forest recovery, as well as the natural re-establishment of several previously extirpated species like oriental pied hornbills and smooth-coated otters. Animals that can fly and swim can re-establish from source populations in Malaysia, but snakes and lizards—also known as squamates—are not able to recolonise as easily. How can we use mathematical models to think about which species to reintroduce to recovering forests? In 2023, we published a paper applying the novel MODGEE (matrix-of-detections-gives-extinction-estimates) model to detailed historical bird, butterfly and plant datasets from Singapore to study extirpation rates for these groups. In a new study, led by Sankar and just published in Biological Conservation, we applied this model to squamates in Singapore.

We compiled a detailed historical dataset for squamates in Singapore (largely based on records from Figueroa et al., 2023) and used the MODGEE model first to generate an extirpation timeline. Our estimated extirpation rate for squamates was 17%, which was lower than previously estimated rates for birds (40%) and butterflies (46%). This implies that squamates may be more resilient to habitat degradation than birds, perhaps because birds generally have lower population densities, and butterflies, perhaps because of butterflies’ reliance on host plants.

Our model estimated that about 17% of squamates have been extirpated from Singapore over the last two centuries. Primary forest species (red) experienced higher extirpation rates than secondary forest-tolerant (green) and parkland species (yellow).

We also used the results to inform a conservation translocation triage for extirpated squamates in Singapore. Given that squamates were never systematically hunted at scale (no pun intended) in Singapore, their extirpation timeline can serve as an indicator of each species’ sensitivity to habitat loss. Since highly sensitive species would have been lost early on in Singapore’s developmental history, recently extirpated species would likely be better candidates for reintroduction to Singapore’s recovering forests. Through this method, we identified the hulk forest gecko (Gekko hulk) as a fitting potential candidate species for reintroduction to Singapore from stock populations in Peninsular Malaysia.

We identified the hulk forest gecko (Gekko hulk) as a candidate for reintroduction to Singapore.

Sankar, A., R. A. Chisholm. 2026. Where will they come from, when did they go? Squamate extirpations and recoveries in Singapore. Biological Conservation, 315:111721

New paper on spatial synchrony of forest tree population dynamics published in Proceedings of the Royal Society B

Light red meranti (Shorea parvifolia) is a canopy tree species native to tropical forests in Southeast Asia that can grow to over 65 m. In the two decades to 2005, the abundance of light red meranti decreased by 60% in a ForestGEO research plot in Pasoh, Malaysia. In a ForestGEO plot in Lambir, Malaysia, over 1000 km away, the same species decreased by 25% over a similar period. This example prompts the question of whether forest tree population dynamics are in general synchronised across space. In a paper just published in Proceedings of the Royal Society B, we found that the answer is yes.

We collated forest inventory data at three different scales—local, regional and global—and found signals of synchrony up to scales well beyond 100 km. For the Pasoh and Lambir forests, in particular, there was clear evidence of synchrony for larger trees (greater than 10 cm diameter) although not for smaller trees. A technical challenge in our analyses was developing statistical methods to produce an aggregate estimate of synchrony for each pair of forest sites while accounting for the noise inherent in datasets with large numbers of rare species. Although existing methods are available to estimate synchrony for individual species, these were not appropriate for our forest data because of the short duration of our datasets (a few decades at most) relative to the typical tree generation time. Our novel methods make use of copulas, which are used to model multivariate statistical phenomena in quantitative finance.

We attributed the observed synchrony in tree population dynamics to synchronised environmental drivers, such as climate, although we lacked the statistical power to identify particular drivers. The observed synchrony suggests greater risk of extinction for tree species that are more spatially constrained, especially those with range dimensions less than ~100 km. On the other hand, species with larger ranges may be buffered from extinction by the lack of synchrony in their population dynamics across space.

Forest tree population dynamics are synchronised across space, with synchrony detectable even beyond 100 km. Dark blue, light blue and green curves indicate the results of local, regional and global analyses. Line types indicate the spatial grain of each analysis: synchrony is lower for smaller spatial grains because of immigration.

Chisholm, R. A., T. Fung, K. J. Anderson-Teixeira, N. A. Bourg, W. Y. Brockelman, S. Bunyave- jchewin, C.-H. Chang-Yang, Y.-Y. Chen, G. B. Chuyong, R. Condit, H. S. Dattaraja, S. J. Davies, S. Ediriweera, C. E. N. Ewango, E. S. Fernando, I. A. U. N. Gunatilleke, C. V. S. Gunatilleke, Z. Hao, R. W. Howe, D. Kenfack, T. L. Yao, J.-R. Makana, S. M. McMahon, X. Mi, M. Bt. Mohamad, J. A. Myers, A. Nathalang, Á. J. Pérez, S. Phumsathan, N. Pongpattananurak, H. Ren, L. J. V. Rodriguez, R. Sukumar, I.-F. Sun, H. S. Suresh, D. W. Thomas, J. Thompson, M. Uriarte, R. Valencia, X. Wang, A. T. Wolf, and J. K. Zimmerman (2024) Assessing the spatial scale of synchrony in forest tree population dynamics. Proceeding of the Royal Society B 291: 20240486