Amphibian diversity in Sumatra

The information compiled here shows that our knowledge of the amphibians of Sumatra has improved over time (Fig. 1), from 61 species known in the early 20th century (van Kampen, Reference Van Kampen1923) to 135 species. Approximately 60% of this increase occurred in the past 2 decades (Supplementary Table 1), with a mean of three amphibian species being described annually. This increase is a result of greater recent survey effort compared to previous decades (Brown & Stuart, Reference Brown, Stuart, David, Kenneth, James, Brian, Lukas and Suzanne2012). The cumulative number of known species has not yet reached an asymptote, (Fig. 1), suggesting that additional species may yet be discovered. Greater survey effort has been a result of national and transnational collaborations: over the years analysed, multi-author publications have become more common than single-author publications describing new amphibian species in Sumatra. Only three of the 44 new species described or recorded during 2002–2022 were published by a single author (Supplementary Table 1).

Broad geographical surveys for both adults and larvae and the application of integrated taxonomy approaches (Arifin et al., Reference Arifin, Smart, Hertwig, Smith, Iskandar and Haas2018b) are key to accelerating species discovery, especially in a biodiversity-rich region such as Sumatra. Over the past 2 centuries, taxonomic studies of frogs have been exclusively based on morphological characteristics (Brown & Stuart, Reference Brown, Stuart, David, Kenneth, James, Brian, Lukas and Suzanne2012) and primarily examined mature individuals. This approach has limitations in the tropics, where large numbers of amphibians are cryptic (Elmer et al., Reference Elmer, Dávila and Lougheed2007; McLeod, Reference McLeod2010; Guarnizo et al., Reference Guarnizo, Paz, Munoz-Ortiz, Flechas, Mendez-Narvaez and Crawford2015). Dring (Reference Dring1983, Reference Dring1987) was one of the first proponents of integrative taxonomy in Southeast Asia. Describing species according to more than one data type (i.e. morphological, molecular, ecological, bioacoustical) is now common practice (Brown & Stuart, Reference Brown, Stuart, David, Kenneth, James, Brian, Lukas and Suzanne2012). As a result of this approach, the rate of discovery of newly described species in the region has accelerated (Joppa & Pfaff, Reference Joppa and Pfaff2011; Brown & Stuart, Reference Brown, Stuart, David, Kenneth, James, Brian, Lukas and Suzanne2012). Nevertheless, there is no general consensus regarding the minimum or maximum number of characteristics required to delineate a species. Taxa could vary in the number of combined characteristics adequate to define a species (Valdecasas et al., Reference Valdecasas, Williams and Wheeler2008; Padial & De la Riva, Reference Padial and De la Riva2010). Furthermore, it is advisable to combine characteristics of adults and larvae as both are equally important (Arifin et al., Reference Arifin, Smart, Hertwig, Smith, Iskandar and Haas2018b, Reference Arifin, Chan, Smart, Hertwig, Smith, Iskandar and Haas2021).

Knowledge gaps and challenges

Choice of survey methods could inadvertently lead to gaps in our knowledge of some groups of amphibians. The compiled data suggest that the frog family Rhacophoridae has the highest number of extant species in Sumatra (37), and the Ichthyophiidae (caecilians) has the lowest number (6). This difference is probably in part a result of the choice of survey methods. Amphibian inventories usually focus on riparian habitats and primarily use opportunistic methods or visual encounter surveys/transects (Heyer et al., Reference Heyer, Donnelly, Foster and Mcdiarmid1994). Caecilians live underground and are difficult to find, and therefore they are more effectively surveyed using pitfall traps, or observed when they come to the surface after heavy rains (U. Arifin, pers. obs., 2008). Using pitfall traps in combination with visual encounter surveys/transects has been recommended to ensure comprehensive species inventories (Ribeiro-Junior et al., Reference Ribeiro-Júnior, Rossi, Miranda and Ávila-Pires2011; Sung et al., Reference Sung, Karraker and Hau2011).

Several recently described species (e.g. Pulchrana fantastica and Sumaterana dabulescens) have been recorded in areas both previously explored and unexplored for amphibians (Arifin et al., Reference Arifin, Cahyadi, Smart, Jankowski and Haas2018a,Reference Arifin, Smart, Hertwig, Smith, Iskandar and Haasb). This suggests more amphibian surveys are required across the island, especially as the deforestation rate remains high (Margono et al., Reference Margono, Potapov, Turubanova, Stolle and Hansen2014; Global Forest Watch, 2022).

Further surveys are also required for clarifying the status of the 52 species endemic to Sumatra and of the 16 species categorized as Data Deficient, only two of which (Ichthyophis nigroflavus and Philautus petersi) are not endemic to Sumatra. Nineteen of the endemic species have only been recorded from their type locality. The endemic Pulchrana debussyi, for example, has been categorized as Data Deficient since 2014 (IUCN SSC Amphibian Specialist Group, 2018). Until a recent observation in Batang Gadis National Park (Kaprawi et al., Reference Kaprawi, Alhadi, Hamidy, Nopandry, Kirschey and Permana2020), the only information available for this species was from the original description (van Kampen, Reference Van Kampen1910).

Re-evaluation of conservation status is also required for Sumatran species in other IUCN Red List categories, including Least Concern. In tropical regions, so-called cryptic species (two or more species hidden under one species name) may be overlooked. For example, the Sumatran endemic species formerly known as Huia sumatrana was previously thought to be a single species with a Sumatra-wide distribution. Using integrated taxonomy approaches and thorough geographical surveys, it has been determined that this taxon comprises at least three distinct lineages (Arifin et al., Reference Arifin, Chan, Smart, Hertwig, Smith, Iskandar and Haas2021). In addition, it is important to document the biology of larvae in addition to that of adults (Arifin et al., Reference Arifin, Chan, Smart, Hertwig, Smith, Iskandar and Haas2021), as information on larvae and calls is available for only 53 and 56% of species in Sumatra, respectively. Incorporating bioacoustics data is crucial for species delimitation (Chan et al., Reference Chan, Abraham, Grismer and Brown2020).

Significance for conservation

Although 66% of the known amphibian species of Sumatra are categorized as Least Concern, many of the assessments are out of date. For example, Microhyla heymonsi, a widespread species in East and Southeast Asia (Frost, Reference Frost2022), was categorized as Least Concern in 2004 and this assessment needs updating (van Dijk et al., Reference Van Dijk, Iskandar, Kuangyang, Wenhao, Baorong, Ermi and Dutta2004). There has been subsequent research on frogs formerly known as M. heymonsi, leading to taxonomic changes for several populations. Populations currently recognized as M. heymonsi require re-evaluation, as the true M. heymonsi may only occur on the islands of Taiwan and Hainan and in southern China (Garg et al., Reference Garg, Sivaperuman, Gokulakrishnan, Chandramouli and Biju2022).

Approximately 30 species of amphibian in Indonesia are reported to be threatened (Summary Statistics Table 5 in IUCN, 2022). This number could, however, be higher as not all species have been assessed for the IUCN Red List and, as suggested by the absence of an asymptote in the cumulative number of species described over time in Sumatra (Fig. 1), more species may yet be discovered. Frost (Reference Frost2022) lists 8,485 amphibian species worldwide, which is c. 57% of an estimated 15,000 species (Chapman, Reference Chapman2009). Documenting amphibian diversity and distribution patterns (Inger et al., Reference Inger, Stuart and Iskandar2009; McLeod et al., Reference McLeod, Horner, Husted, Barley and Iskandar2011) is necessary for conservation. Given the current biodiversity crisis, accurate taxonomy and diversity estimates for amphibians are increasingly important (Brown & Stuart, Reference Brown, Stuart, David, Kenneth, James, Brian, Lukas and Suzanne2012) for planning conservation management strategies in Indonesia and elsewhere. Describing unknown species and rediscovering reportedly extinct species (Ladle et al., Reference Ladle, Jepson, Malhado, Jennings and Barua2011; Scheffers et al., Reference Scheffers, Yong, Harris, Giam and Sodhi2011; Roberts & Fisher, Reference Roberts and Fisher2020; Long & Rodriguez, Reference Long and Rodriguez2022) will help to highlight the remarkable biodiversity of Sumatra, improve knowledge of the amphibian communities on the island, and will be key for designing effective conservation strategies (Scheffers et al., Reference Scheffers, Joppa, Pimm and Laurence2012). Similarly, information on species diversity and distributions across geographical regions, including on species richness and endemism, can support the prioritization of areas for conservation (Azevedo-Ramos & Galatti, Reference Azevedo-Ramos and Galatti2002).

To conclude, I make five recommendations for improving knowledge of the amphibians of Sumatra: (1) increase survey effort in both explored and unexplored areas, for both adults and larvae; (2) improve survey efficiency, considering the biology of the targeted taxa and their preferred habitats in selecting the most appropriate methods; (3) apply integrative taxonomy approaches to confirm and validate the identity of species; (4) document associated data on calls, larvae and distributions; and (5) re-assess species with out-of-date Red List assessments.