A Comprehensive Assessment of Diversity Loss in a Well-Documented Tropical Insect Fauna: Almost Half of Singapore's Butterfly Species Extirpated in T 160 Years
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Biological Conservation 242 (2020) 108401 Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/biocon A comprehensive assessment of diversity loss in a well-documented tropical insect fauna: Almost half of Singapore's butterfly species extirpated in T 160 years Meryl Thenga, Wan F.A. Jusohb, Anuj Jainc, Blanca Huertasd, David J.X. Tana, Hui Zhen Tana, ⁎ Nadiah P. Kristensena, Rudolf Meiera, Ryan A. Chisholma, a Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, 117558, Singapore b Lee Kong Chian Natural History Museum, 2 Conservatory Dr, Singapore 117377, Singapore c BirdLife International (Asia), 354 Tanglin Road, #01-16/17, Singapore 247672, Singapore d Natural History Museum, Cromwell Road, London SW7 5BD, UK ARTICLE INFO ABSTRACT Keywords: Insects as a group are suffering rapid declines in many parts of the world but are also poorly studied relative to Butterflies vertebrate taxa. Comprehensive assessments of insect declines must account for both detected and undetected Extirpations species. We studied extirpations among butterflies, a particularly well-known insect group, in the highly de- Extinction debt veloped and biologically well-surveyed island city-state of Singapore. Building on existing butterfly species lists, Southeast Asia we collated museum and naturalist records over the last two centuries and used statistical models to estimate the total extirpation rate since the first major collections in 1854. In addition, we compiled a set of traits for each butterfly species and explored how they relate to species discovery and extirpation. With a database of 413 native species, 132 (32%) of which are recorded as extirpated in Singapore, we used a statistical model to infer that, in addition, 104 unknown species (95% CI 60–162) were likely extirpated before they were ever dis- covered, suggesting a total extirpation rate of 46% (41–51%). In the trait analyses, we found that butterfly species that were discovered later were weakly associated with rarer larval host plants and smaller wingspans, while species that persisted for longer were weakly associated with higher larval host plant abundance and lower forest-dependence. This exercise is one of the first to offer a holistic estimate of extirpations for a group of insects by accounting for undetected extirpations. It suggests that extirpations among insects, specifically in the tropics, may be higher than naïve estimates based only on known records. 1. Introduction Simmons et al., 2019; Thomas et al., 2019; Wagner, 2019). As such, comprehensive analyses that fulfil these criteria are invaluable because Insects make up an estimated 90% of the terrestrial world's animal they serve as case studies of insect population trends occurring across species and are thus essential to the overall state of biodiversity. Insects, the world. more so than vertebrates, may be appropriate indicators of global en- Among insects, butterflies are a well-known group and are known to vironmental impacts in the Anthropocene (Briggs, 2017). And yet, the be valuable indicators of their environment because of their high degree statuses of very few have been evaluated compared to vertebrate spe- of host-plant specialisation and vulnerability to habitat deterioration cies (Régnier et al., 2015). Assessments of insect declines and extinc- (Erhardt and Thomas, 1991; Koh et al., 2004; Jain et al., 2017). tions remain hampered by two main issues: the lack of baseline in- Moreover, it has been argued that butterflies are appropriate indicators formation and the lack of rigorous analyses to account for unobserved of change for many insect groups, as preliminary assessments have events. This was reflected in a call for more robust insect data and found similar rates of loss in other groups (Thomas, 2005; Thomas, rigorous analyses from all parts of the world following the highly 2016). Given this, and that butterflies are present in a broad range of publicised, and later criticised, review of global insect declines by habitats, investigating butterfly population declines and losses poten- Sánchez-Bayoa and Wyckhuys (2019) (Cardoso and Leather, 2019; tially offers an insightful metric of the decline of habitat quality and ⁎ Corresponding author. E-mail address: [email protected] (R.A. Chisholm). https://doi.org/10.1016/j.biocon.2019.108401 Received 8 August 2019; Received in revised form 18 December 2019; Accepted 26 December 2019 0006-3207/ © 2020 Elsevier Ltd. All rights reserved. M. Theng, et al. Biological Conservation 242 (2020) 108401 quantity and of biodiversity as a whole. 2. Methods The tropical island of Singapore (103°50′E, 1°20′N) has been used as a case study of tropical biodiversity loss (Corlett, 1992; Turner et al., 2.1. Compilation of butterfly records 1994). Singapore's small self-contained area (originally 540 km2; cur- rently 722 km2 due to land reclamation), its accessibility as a maritime Using the dataset from Jain et al. (2018) as a baseline, we refined port (as the main shipping channel between the Indian Ocean and South the list of butterfly species in Singapore and their respective record China Sea) and the enthusiasm of early natural historians have meant dates by collating individual records from specimens in museum col- that its fauna has been well documented relative to other tropical re- lections, available literature and contemporary sighting records (refer gions. The city-state has experienced massive forest loss since 1819 and to Supplementary information for full list of records and sources). Key only ≈2.0% of forest remains, most of it secondary (Yee et al., 2011). texts used include works from Corbet and Pendlebury (1956, 1978), Many species have been extirpated from Singapore during this process, and national checklists by Khew and Neo (1997) and Khew (2015). from large vertebrates such as tigers (Corlett, 1992) to small orchids Records of specimens were also acquired from available online data- (Turner et al., 1994; Chong et al., 2009). Most of these species were not bases, some of which are hosted on the Global Biodiversity Information endemic to Singapore and continue to persist elsewhere, and thus we Facility (GBIF) and the Naturalis Biodiversity Center in Leiden. refer to them as “extirpations” rather than “extinctions”. In addition, we physically searched through the butterfly collections The high quality of Singapore butterfly species data (Jain et al., (Main and Types Collections) at the Natural History Museum London 2018) provides an opportunity to conduct a comprehensive assessment (NHMUK). Over the period January–May 2018, we tracked down the of butterfly declines. A recent study found a 32% extirpation rate of oldest recorded specimens for each known butterfly species from Singapore butterflies since 1926 (Jain et al., 2018). This was based on a Singapore. The specimens were then digitally imaged and uploaded locally exhaustive documentation of species extirpations, discoveries online to the Biodiversity of Singapore digital reference collection and re-discoveries, resulting in a species list comprising 478 species (https://singapore.biodiversity.online/taxon/A-Arth-Hexa-Lepi). The from Singapore (301 resident, nine possibly extirpated, 144 extirpated, NHMUK has about 5 million butterfly collections and these specimens five migrants, 19 vagrants) belonging to six families (Hesperiidae, Ly- are often sorted based on taxonomy and biogeography—they are rarely caenidae, Nymphalidae, Papilionidae, Pieridae and Riodinidae) (Jain sorted according to the collector, country or time frame. Therefore et al., 2018). This list, though a comprehensive summary of present-day searching for the physical specimen record was a time-consuming knowledge and existing literature on local species declines, tells only process. Each species from the Singapore butterfly list was cross- part of the story because it does not account for undetected ex- checked against the NHMUK internal database system to locate cabinet tirpations—species that were extirpated before they could be dis- series, after which 800–1000 drawers were individually checked for covered (Tedesco et al., 2014; Lees and Pimm, 2014). This is a point potential Singapore specimens. Upon locating specimens, each spe- that Jain et al. (2018) also conceded, suggesting that there had been a cimen was examined to validate its record. In cases where there were poor-quality historic baseline for butterflies, as evidenced by the high typification errors or dubious specimen records, original species de- rates of recent (1990–2017) discoveries of species previously not seen scriptions were referred to before the specimen was accepted. Our in Singapore. highest priority was to find specimens with the type locality in To address the undetected extirpations question, a highly cited Singapore. As for non-type specimens known from Singapore, the study by Brook et al. (2003) made the key assumption that all lowland priority was to find those collected during the colonial period species found in Peninsular Malaysia originally also inhabited Singa- (1800–1900; e.g., within Alfred Russel Wallace's collection). pore in 1819, and came up with a strikingly high estimate of 73% ex- For modern records, we used Jain et al. (2018) as the main reference tirpations accounting for both detected and undetected species. How- (which comprised records from local bulletins and issues of the journals ever, the key assumption of the method of Brook et al. (2003) is not Raffles Bulletin