Introduction
South-east Asia's coastal wetlands are among the region's most overlooked ecosystems for biodiversity conservation in spite of their diverse roles in provisioning ecosystem services such as carbon sequestration (Suratman, Reference Suratman2008; Donato et al., Reference Donato, Kauffman, Murdiyarso, Kurniato, Stidham and Kanninen2011), disaster risk mitigation (de Groot et al., Reference de Groot, Brander, van der Ploeg, Costanza, Bernard, Braat and Hussain2012; Kuenzer & Renaud, Reference Kuenzer and Renaud2012) and livelihoods to human communities through supporting key fisheries (Brander et al., Reference Brander, Wagtendonk, Hussain, Mcvittie, Verburg, de Groot and van der Ploeg2012; Murata et al., Reference Murata, Merriman, Le and Nguyen2016). These dynamic ecosystems form global centres of diversity for taxa such as mangroves, seagrasses and horseshoe crabs (Chou, Reference Chou1994; Ooi et al., Reference Ooi, Kendrick, van Niel and Affendi2011; Obst et al., Reference Obst, Faurby, Bussarawit and Funch2012, Richards & Friess, Reference Richards and Friess2016), and provide critical habitats for migratory waterbird species (Zöckler et al., Reference Zöckler, Li, Chowdhury, Iqbal and Yu2018a,Reference Zöckler, Aung, Grindley and Aungb; Chan et al., Reference Chan, Tibbitts, Lok, Hassell, Peng and Ma2019) and nesting sea turtles (MacKinnon et al., Reference Mackinnon, Verkuil and Murray2012). Amidst a broad focus on conservation of tropical forests in South-east Asia (e.g. Sodhi et al., Reference Sodhi, Posa, Lee, Bickford, Koh and Brook2010), less species-rich ecosystems in this region, such as coastal wetlands, have been less studied (e.g. Friess et al., Reference Friess, Phelps, Leong, Lee, Wee, Sivasothi and Webb2012) and are often underrepresented in conservation discourse (Coleman et al., Reference Coleman, Ascher, Bickford, Buchori and Cabanban2019).
With a better understanding of the importance of coastal wetlands to migratory waterbirds in the East Asian–Australasian Flyway (Yang et al., Reference Yang, Chen, Barter, Piersma, Zhou, Li and Zhang2011; Murray et al., Reference Murray, Clemens, Phinn, Possingham and Fuller2014; Duan et al., Reference Duan, Xia, Jackson, Zhao, Liu and Teng2020) and the ecosystem services they provide, there has been renewed urgency to strengthen the conservation of networks of coastal wetland sites (Bamford et al., Reference Bamford, Watkins, Bancroft, Tischler and Wahl2008). Recent efforts by China and South Korea to restore coastal wetlands, limit coastal reclamation (Kim, Reference Kim2018; Zhou, Reference Zhou2018) and successfully promote coastal wetland sites for World Heritage Site listing (UNESCO WHC, 2019), and by Myanmar to expand its Ramsar Sites (Myat Moe Aung, 2020), offer renewed momentum for wetland conservation in the region. However, the loss and conversion of coastal wetlands throughout Asia continues because of the economic pressures on the low-lying coastal zone, ranging from urban, industrial and infrastructure development to rice and oil palm monoculture, and fish farming (Chou, Reference Chou1994; MacKinnon et al., Reference Mackinnon, Verkuil and Murray2012; Ma et al., Reference Ma, Melville, Liu, Chen, Yang and Ren2014; Richards & Friess, Reference Richards and Friess2016).
Recent analyses of the distribution of coastal wetlands show that the South-east Asian region (e.g. Indonesia, Myanmar, Viet Nam) holds some of the largest areas of mudflats (Murray et al., Reference Murray, Phinn, Dewitt, Ferrari, Johnston and Lyons2019) and mangroves (Richards & Friess, Reference Richards and Friess2016) globally. Yet, there are significant knowledge gaps in the state of South-east Asia's wetland ecosystems, and how wetland sites are connected by migratory waterbird populations (e.g. Choi et al., Reference Choi, Rogers, Gan, Clemens, Bai and Lilleyman2016; Chan et al., Reference Chan, Tibbitts, Lok, Hassell, Peng and Ma2019). This is accentuated by the region's long and poorly surveyed coastlines (e.g. Borneo, Wallacea, Philippines, New Guinea; Chou, Reference Chou1994). Additionally, many wetlands in South-east Asia, including even protected sites, are being increasingly impacted by encroachment and aquaculture expansion (Eng et al., Reference Eng, Paw and Guarin1989; Round, Reference Round2006; Yasue & Dearden, Reference Yasue and Dearden2009), sand mining and other forms of unsustainable coastal development (Hilton & Manning, Reference Hilton and Manning1995; Yeap et al., Reference Yeap, Sebastian and Davison2007) before they can be studied. Migratory waterbird populations also under significant hunting pressure in many parts of the Flyway (Turrin & Watts, Reference Turrin and Watts2016), particularly in South-east Asia where there have been well-documented instances of unsustainable offtake (Gallo-Caijiao et al., Reference Gallo-Cajiao, Morrison, Woodworth, Lees, Naves and Yong2020).
Here, we synthesize recent developments in migratory waterbird conservation in South-east Asia (herein defined to include all Association of South-east Asian Nations states, and Timor-Leste), and discuss knowledge gaps in the conservation of coastal wetlands and migratory waterbirds, with a focus on coastal Important Bird and Biodiversity Areas (IBAs) in this region (BirdLife International, 2004, 2020a; see Table 1 for definitions). We used a definition of coastal wetlands following the wetland classification scheme adopted by the Ramsar Convention Secretariat (2016); in this region coastal wetlands largely consist of intertidal mud and sand flats, estuarine waters, sandy shores and mangroves, and associated supratidal areas such as salt pans and aquaculture ponds. We then review anthropogenic threats faced by these wetlands and argue that a combination of better science and political commitment is needed to secure the future of South-east Asia's valuable coastal wetlands.
Table 1 Definitions of key terms.
Importance of coastal wetlands
Key sites for migratory waterbirds
Of 672 IBAs identified within South-east Asia, 181 (26.9%) hold some form of coastal wetland cover, of which 52 are identified here to support at least one globally threatened migratory waterbird (Fig. 1). Presently, Indonesia holds the largest number and area of IBAs with coastal wetlands (86 sites totalling c. 84,533 km2), followed by Myanmar (three sites totalling c. 12,646 km2) and Malaysia (15 sites totalling c. 4,316 km2; Fig. 1). At least seven key coastal areas in the region have been identified that are crucial for migratory shorebirds, all comprising IBAs or clusters of IBAs (MacKinnon et al., Reference Mackinnon, Verkuil and Murray2012): the Inner Gulf of Thailand, the Gulf of Mottama (Myanmar), the Mekong and Red River deltas (Viet Nam), the west coast of Peninsular Malaysia, the Sarawak coastline (Malaysia), and the southeast coast of Sumatra (Indonesia).
Fig. 1 Total number and area of coastal Important Bird and Biodiversity Areas (IBAs) and total number of coastal IBAs with ≥ 1 threatened bird species in South-east Asia that overlap with coastal wetlands and support threatened migratory species.
The Malacca Strait coastlines of Peninsular Malaysia and Sumatra collectively hold the most extensive areas of intertidal flats, mangroves and associated wetlands in South-east Asia for significant congregations of migratory waterbirds (Crossland et al., Reference Crossland, Sinambela, Sitorus and Sitorus2006; Li et al., Reference Li, Yeap, Lim, Kumar, Lim, Yang and Choy2006; Iqbal et al., Reference Iqbal, Giyanto and Abdillah2010), with large areas of coastal wetlands on both sides of the straits recognized as IBAs. In the 1990s, Verheugt et al. (Reference Verheugt, Skov and Danielsen1992) estimated 500,000 migratory shorebirds on the coast of South Sumatra province alone, which extrapolated to > 1 million shorebirds staging or wintering along the Sumatran coast. Surveys post-2010 continue to demonstrate the importance of Sumatra's east coast for wintering shorebirds, with many species meeting the 1% threshold of their flyway population, a key criterion for Ramsar site designation (Iqbal et al., Reference Iqbal, Giyanto and Abdillah2010). New sites continue to be discovered (Putra et al., Reference Putra, Perwitasari-Farajallah and Mulyani2017; Putra et al., Reference Putra, Hikmatullah, Zöckler, Syroechkovskiy and Hughes2019), reflecting the substantial gaps in knowledge.
Further north, the Inner Gulf of Thailand is one of South-east Asia's best-surveyed coastlines for migratory waterbirds (e.g. Round, Reference Round2006). Its representation of coastal wetland types includes artificial supratidal wetlands such as salt pans, and intertidal flats and mangroves (Round, Reference Round2006; Yasue & Dearden, Reference Yasue and Dearden2009; Yu et al., Reference Yu, Ngoprasert, Savini, Round and Gale2020), and supports globally significant congregations of shorebirds, with an estimated 30,000–40,000 individuals in midwinter (Sripanomyom et al., Reference Sripanomyom, Round, Savini, Trisurat and Gale2011). In Myanmar, surveys of the spoon-billed sandpiper Calidris pygmaea on the vast intertidal wetlands along the poorly surveyed Gulf of Mottama have revealed these coastlines to be internationally important for migratory waterbirds, supporting an estimated total of 150,000 individuals of 80 species (Zöckler et al., Reference Zöckler, Zaw Naing, Moses, Nou Soe and Htin Hla2014; Aung et al., Reference Aung, Moses, Clark, Anderson, Hilton and Buchanan2020).
Threatened waterbirds
Field surveys, citizen science monitoring and satellite tracking data collectively confirm South-east Asia's coastal wetlands to be important for at least 12 globally threatened waterbird species, of which seven are migratory (Table 2) and five resident (e.g. the Philippine duck Anas luzonica and milky stork Mycteria cinerea). A further 13 species are Near Threatened. Two of the most imperilled species on the East Asian–Australasian Flyway, the spoon-billed sandpiper (Zöckler et al., Reference Zöckler, Zaw Naing, Moses, Nou Soe and Htin Hla2014; Aung et al., Reference Aung, Moses, Clark, Anderson, Hilton and Buchanan2020) and spotted greenshank Tringa guttifer (Sripanomyom et al., Reference Sripanomyom, Round, Savini, Trisurat and Gale2011), have global wintering strongholds concentrated in South-east Asia (Zöckler et al., Reference Zöckler, Li, Chowdhury, Iqbal and Yu2018a; Yu et al., Reference Yu, Ngoprasert, Savini, Round and Gale2020). The region supports nearly the entire Chinese egret Egretta eulophotes population in winter (BirdLife International, 2020b), and globally significant wintering congregations at various South-east Asian sites (> 1% of flyway populations; Ramsar Convention Secretariat, 2016) of the black-faced spoonbill Platalea minor (Xuan Thuy, Viet Nam), great knot Calidris tenuirostris (many sites) and far eastern curlew Numenius madagascariensis (Bako-Buntal Bay, Sarawak) (Li et al., Reference Li, Yeap, Lim, Kumar, Lim, Yang and Choy2006; Bakewell et al., Reference Bakewell, Wong, Kong and Au2017; BirdLife International, 2020a).
Table 2 Globally threatened migratory waterbirds dependant on coastal wetlands in South-east Asia, and countries with significant staging and/or wintering populations.
1 VU, Vulnerable; EN, Endangered; CR, Critically Endangered (IUCN, 2021).
2 N, north; S, south; E, east; W, west; C, central.
Although there is now substantial knowledge of the status and distributions of many waterbirds in this region, there remain large gaps in our understanding of their distribution, ecology and population concentrations in South-east Asia's coastal wetland sites. For example, despite the high survey effort for the spoon-billed sandpiper and spotted greenshank, previously unknown wintering sites continue to be discovered for both species (Putra et al., Reference Putra, Hikmatullah, Zöckler, Syroechkovskiy and Hughes2019; Nguyen et al., Reference Nguyen, Cao, Bui and Yong2020; Yu et al., Reference Yu, Ngoprasert, Savini, Round and Gale2020). Using satellite trackers on migrating great knot, Chan et al. (Reference Chan, Tibbitts, Lok, Hassell, Peng and Ma2019) found that 63% of stopover sites used by the species were previously unknown, with the majority of such sites in South-east Asia, and particularly in Borneo, the Philippines, Wallacea and New Guinea. Much of the New Guinea coastline has not been surveyed for migratory shorebirds in recent years despite having vast, little-disturbed areas of estuarine wetlands, and further fieldwork is expected to uncover new sites for species such as the far eastern curlew (Bishop, Reference Bishop2006). Recent studies (Yasue & Dearden, Reference Yasue and Dearden2009; Sripanomyom et al., Reference Sripanomyom, Round, Savini, Trisurat and Gale2011; Jackson et al., Reference Jackson, Choi, Amano, Estrella, Lei and Moores2020), have demonstrated the importance of supratidal artificial wetlands (e.g. salt pans, ash ponds) in South-east Asia to threatened shorebirds such as the spotted greenshank and far eastern curlew, complementing the role of natural tidal flats.
Key challenges for conserving coastal wetlands
Many important wetlands remain largely unprotected and at the mercy of the development agendas of various countries. Land-use change as a result of aquaculture, energy, infrastructure and residential development, sand mining and land reclamation are the leading drivers of wetland conversion and loss in South-east Asia (Round, Reference Round2006; Lim & Posa, Reference Lim and Posa2014; Lai et al., Reference Lai, Loke, Hilton, Bouma and Todd2015; Jensen, Reference Jensen2018). Aquaculture intensification for fish and prawn farming across coastal South-east Asia, and the expansion of wet paddy cultivation and mega coastal development projects for ports and residential areas have contributed to the widespread reclamation and degradation of mangroves, salt marshes, seagrass beds and mudflats (de Graaf & Xuan, Reference de Graaf and Xuan1998; Richards & Friess, Reference Richards and Friess2016; Rahman, Reference Rahman2017; Jensen, Reference Jensen2018), leading to a loss of habitat for migratory waterbird species (Pedersen & Nielsen, Reference Pedersen and Nielsen1998; Buckton & Safford, Reference Buckton and Safford2004; Yasue & Dearden, Reference Yasue and Dearden2009). Over time, coastal erosion, exacerbated by anthropogenic alteration of the sediment regimes of rivers and sea level rise poses a threat to coastal wetlands, especially in the Mekong Delta and the Gulf of Thailand (Duriyapong & Nakhapakorn, Reference Duriyapong and Nakhapakorn2011; Kuenzer & Renaud, Reference Kuenzer and Renaud2012; Marchesiello et al., Reference Marcheciello, Nguyen, Gratiot, Loisel, Anthony and San Dinh2019).
Presently, at least seven coastal IBAs in South-east Asia are already considered to be IBAs in Danger (BirdLife International, 2020a). Coastal wetlands in Singapore have suffered the greatest relative decline of any South-east Asian country as a result of urban expansion and coastal reclamation (Lai et al., Reference Lai, Loke, Hilton, Bouma and Todd2015); from 1922 to 1993 intertidal sand/mudflat extent declined by > 75%, to 8 km2 (Hilton & Manning, Reference Hilton and Manning1995). In addition to extensive habitat contraction in this region, migratory waterbird populations are probably also affected by unsustainable hunting pressure (Turrin & Watts, Reference Turrin and Watts2016; Gallo-Caijiao et al., Reference Gallo-Cajiao, Morrison, Woodworth, Lees, Naves and Yong2020).
Although wetland restoration has slowed the decline of mangroves in some countries in the region (e.g. Philippines, Thailand; Erftemeijer & Lewis, Reference Erftemeijer and Lewis1999; Friess et al., Reference Friess, Yando, Abuchahla, Adams, Cannicci and Canty2020), the longer-term outlook for the conservation of South-east Asia's coastal wetlands remains poor. Across the region, a pervasive problem is poorly planned mangrove planting on natural mudflats under the pretext of habitat rehabilitation (Erftemeijer & Lewis, Reference Erftemeijer and Lewis1999), which in some instances has further contributed to the loss of habitat for migratory waterbirds (Jensen, Reference Jensen2018).
Many coastal wetland sites of conservation significance in this region receive little or no legal protection and are unrepresented in the national protected area estate (MacKinnon et al., Reference Mackinnon, Verkuil and Murray2012; Nguyen et al., Reference Nguyen, Cao, Bui and Yong2020). In the Inner Gulf of Thailand, few coastal sites important to migratory shorebirds have formal protection under Thai legislation or are actively managed (e.g. Round, Reference Round2006; Yu et al., Reference Yu, Ngoprasert, Savini, Round and Gale2020). Viet Nam has a well-established network of protected areas but coastal wetlands are poorly represented (Rambaldi et al., Reference Rambaldi, Bugna and Geiger2001), and in the Philippines and Malaysia many of the most important coastal wetlands for migratory shorebirds (e.g. large tracts of Manila Bay, coastline of mainland Penang) lack formal protection (Yeap et al., Reference Yeap, Sebastian and Davison2007; Jensen, Reference Jensen2018).
The conservation of coastal wetland ecosystems in South-east Asia is impeded by a number of fundamental institutional challenges. Firstly, throughout the region, conservation prioritization has emphasized species-rich tropical forests, and charismatic terrestrial and endemic megafauna (e.g. Sodhi et al., Reference Sodhi, Posa, Lee, Bickford, Koh and Brook2010; Yong et al., Reference Yong, Jain, Liu, Iqbal, Choi and Crockford2018), often overlooking coastal wetlands and other ecosystems with a lower profile. Secondly, the management of coastal resources, including intertidal wetlands and mangrove forests, often slips into the ambiguities and gaps in governance between agencies overseeing terrestrial and marine resources (Mackinnon et al., Reference Mackinnon, Verkuil and Murray2012). The low-lying coastal belt across much of South-east Asia is densely populated and human populations continue to grow (e.g. Chou, Reference Chou1994). This threatens coastal wetlands with intensive development, land reclamation and expansion of aquaculture, whilst remaining mud/sand flats and mangrove forests are under pressure from shellfisheries, fisheries and fuelwood harvesting. Any conservation intervention needs to negotiate a complex entanglement of local communities and their livelihood needs, coastal development agendas, and the various hierarchies of government.
Discussion
The extent and underlying causes of the decline of many migratory waterbird populations in the East Asian–Australasian Flyway are increasingly understood (Murray et al., Reference Murray, Clemens, Phinn, Possingham and Fuller2014; Piersma et al., Reference Piersma, Lok, Chen, Hassell, Yang and Boyle2016; Studds et al., Reference Studds, Kendall, Murray, Wilson, Rogers, Clemens and Milton2017). The importance of coastal wetlands for carbon storage, disaster risk reduction under climate change scenarios, and ecosystem service provisioning is now relatively well acknowledged and increasingly mainstreamed into major policy frameworks and initiatives (e.g. McLeod et al., Reference Mcleod, Chmura, Bouillon, Salm, Bjork and Duarte2011; Friess et al., Reference Friess, Yando, Abuchahla, Adams, Cannicci and Canty2020). One persistent challenge, however, is that important gaps remain in knowledge of the distribution and ecology of threatened migratory species in the region. Recent satellite tracking research is only beginning to address this (Chan et al., Reference Chan, Tibbitts, Lok, Hassell, Peng and Ma2019), and to complement ground-based field surveys (Putra et al., Reference Putra, Hikmatullah, Zöckler, Syroechkovskiy and Hughes2019; Nguyen et al., Reference Nguyen, Cao, Bui and Yong2020). There remains an urgent need for continued field-based research to determine (1) where the key migratory bird congregations are, (2) species use of the various wetland types, and (3) migratory routes and the migratory connectivity of waterbird populations, which can guide the identification of priority sites such as IBAs and the planning of protected areas.
All countries in South-east Asia are party to the Convention on Biological Diversity, and all but Brunei, Singapore and Timor-Leste to the Ramsar Convention. In 2018 these Conventions called for the establishment of a multi-stakeholder global coastal forum to facilitate coastal wetland conservation (Ramsar Convention Resolution XIII.20, Convention on Biological Diversity Decision 14/30). All South-east Asian countries except Laos, Brunei and Timor-Leste are also partners in the East Asian–Australasian Flyway Partnership. Partner countries can designate Flyway Network Sites that similarly meet the criteria for Ramsar Sites (Ramsar Convention Secretariat, 2016; EAAFP, 2018), and therefore directly drive work to improve the protection and management of priority wetland sites. Several countries have also established frameworks and initiatives at the national level to address the conservation of wetlands and migratory birds (e.g. Myanmar, Thailand, Indonesia).
However, these frameworks on their own are unlikely to conserve South-east Asia's coastal wetlands if there is no sustained commitment from governments and local stakeholders. Fulfilling these commitments would require substantial effort to improve the representation of coastal wetlands in national protected area networks and community conservation areas (Borrini-Feyerabend et al., Reference Borrini-Feyerabend, Kothari and Oviedo2004), to promote integrated coastal zone management, and to better define the remit of various agencies in relation to the management of the coastal zone. In particular, there is a need to implement better zoning of coastlines to plan for sustainable coastal land use and to facilitate managed retreat and coastal restoration under projected climate change scenarios (Hino et al., Reference Hino, Field and Mach2017). In instances where migratory waterbirds are directly benefiting from man-made wetlands (e.g. Inner Gulf of Thailand; Round, Reference Round2006; Yasue & Dearden, Reference Yasue and Dearden2009; Sripanomyom et al., Reference Sripanomyom, Round, Savini, Trisurat and Gale2011; Jackson et al., Reference Jackson, Choi, Amano, Estrella, Lei and Moores2020; Nguyen et al., Reference Nguyen, Cao, Bui and Yong2020), there is a window of opportunity for authorities to encourage low-intensity, sustainable uses of the coastal zone, such as through artisanal fisheries and salt farms, an approach already promoted under the Ramsar Convention.
To strengthen the conservation of coastal wetlands in the region, there is evidently a critical role for conservation organizations to work more closely with local people, academia and governments to understand the socio-economics of these communities, promote local custodianship of wetland sites and put livelihoods on a more sustainable and equitable footing (e.g. Borrini-Feyerabend et al., Reference Borrini-Feyerabend, Kothari and Oviedo2004), in collaboration with regional frameworks such as the Association of Southeast Asian Nations. Examples exist of successful engagement with local communities on coastal wetland conservation in this region, particularly in Myanmar's Gulf of Mottama, parts of the Gulf of Thailand (e.g. community participation in the establishment and management of a new nature reserve), the Philippines (Denyer et al., Reference Denyer, Akoijam, Kenza Ali, Khurelbatar, Oviedo and Young2018; Zöckler et al., Reference Zöckler, Aung, Grindley and Aung2018b; Jearwattanakanok & Yong, Reference Jearwattanakanok and Yong2019), and elsewhere in the world, such as the Wadden and Yellow Sea (Boere & Piersma, Reference Boere and Piersma2012; UNESCO WHC, 2019). These offer good lessons and scalable models of engagement, and demonstrate that the challenges faced in South-east Asia are not insurmountable.
Acknowledgements
We thank Eduardo Gallo-Caijiao for useful discussions on the hunting of shorebirds in South-east Asia; K. David Bishop, Batrisyia Teepol, David Li, Arne Jensen, Taej Mundkur, Vinayagan Dharmarajah and Christoph Zöckler for discussions on the conservation status of specific shorebird species in South-east Asia; and two anonymous reviewers and the Editor for their constructive feedback. We dedicate this article to our colleague, the late Balu Perumal, a passionate conservationist whose pioneering work on wetland conservation and environmental education in Malaysia and across the region, was tragically cut short by his untimely passing in August 2021.
Author contributions
Study design: DLY; data analysis: DLY, A Jain, JYK; writing: DLY, PPA, A Jain, C-AY, NJA, A Jearwattanakanok, KKL, YTY, VWKF, PI-C, SC, MC, NJC, YS.
Conflicts of interest
None.
Ethical standards
This research abided by the Oryx guidelines on ethical standards.
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