Conserving migratory waterbirds and the coastal zone: the future of South-east Asia's intertidal wetlands

Abstract

South-east Asia's diverse coastal wetlands, which span natural mudflats and mangroves to man-made salt pans, offer critical habitat for many migratory waterbird species in the East Asian–Australasian Flyway. Species dependent on these wetlands include nearly the entire population of the Critically Endangered spoon-billed sandpiper Calidris pygmaea and the Endangered spotted greenshank Tringa guttifer, and significant populations of several other globally threatened and declining species. Presently, more than 50 coastal Important Bird and Biodiversity Areas (IBAs) in the region (7.4% of all South-east Asian IBAs) support at least one threatened migratory species. However, recent studies continue to reveal major knowledge gaps on the distribution of migratory waterbirds and important wetland sites along South-east Asia's vast coastline, including undiscovered and potential IBAs. Alongside this, there are critical gaps in the representation of coastal wetlands across the protected area networks of many countries in this region (e.g. Viet Nam, Indonesia, Malaysia), hindering effective conservation. Although a better understanding of the value of coastal wetlands to people and their importance to migratory species is necessary, governments and other stakeholders need to do more to strengthen the conservation of these ecosystems by improving protected area coverage, habitat restoration, and coastal governance and management. This must be underpinned by the judicious use of evidence-based approaches, including satellite-tracking of migratory birds, ecological research and ground surveys.

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, 2008; Donato et al., 2011), disaster risk mitigation (de Groot et al., 2012; Kuenzer & Renaud, 2012) and livelihoods to human communities through supporting key fisheries (Brander et al., 2012; Murata et al., 2016). These dynamic ecosystems form global centres of diversity for taxa such as mangroves, seagrasses and horseshoe crabs (Chou, 1994; Ooi et al., 2011; Obst et al., 2012, Richards & Friess, 2016), and provide critical habitats for migratory waterbird species (Zöckler et al., 2018a,b; Chan et al., 2019) and nesting sea turtles (MacKinnon et al., 2012). Amidst a broad focus on conservation of tropical forests in South-east Asia (e.g. Sodhi et al., 2010), less species-rich ecosystems in this region, such as coastal wetlands, have been less studied (e.g. Friess et al., 2012) and are often underrepresented in conservation discourse (Coleman et al., 2019).

With a better understanding of the importance of coastal wetlands to migratory waterbirds in the East Asian–Australasian Flyway (Yang et al., 2011; Murray et al., 2014; Duan et al., 2020) and the ecosystem services they provide, there has been renewed urgency to strengthen the conservation of networks of coastal wetland sites (Bamford et al., 2008). Recent efforts by China and South Korea to restore coastal wetlands, limit coastal reclamation (Kim, 2018; Zhou, 2018) 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, 1994; MacKinnon et al., 2012; Ma et al., 2014; Richards & Friess, 2016).

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., 2019) and mangroves (Richards & Friess, 2016) 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., 2016; Chan et al., 2019). This is accentuated by the region's long and poorly surveyed coastlines (e.g. Borneo, Wallacea, Philippines, New Guinea; Chou, 1994). Additionally, many wetlands in South-east Asia, including even protected sites, are being increasingly impacted by encroachment and aquaculture expansion (Eng et al., 1989; Round, 2006; Yasue & Dearden, 2009), sand mining and other forms of unsustainable coastal development (Hilton & Manning, 1995; Yeap et al., 2007) before they can be studied. Migratory waterbird populations also under significant hunting pressure in many parts of the Flyway (Turrin & Watts, 2016), particularly in South-east Asia where there have been well-documented instances of unsustainable offtake (Gallo-Caijiao et al., 2020).

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., 2012): 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., 2006; Li et al., 2006; Iqbal et al., 2010), with large areas of coastal wetlands on both sides of the straits recognized as IBAs. In the 1990s, Verheugt et al. (1992) 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., 2010). New sites continue to be discovered (Putra et al., 2017; Putra et al., 2019), 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, 2006). Its representation of coastal wetland types includes artificial supratidal wetlands such as salt pans, and intertidal flats and mangroves (Round, 2006; Yasue & Dearden, 2009; Yu et al., 2020), and supports globally significant congregations of shorebirds, with an estimated 30,000–40,000 individuals in midwinter (Sripanomyom et al., 2011). 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., 2014; Aung et al., 2020).

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., 2014; Aung et al., 2020) and spotted greenshank Tringa guttifer (Sripanomyom et al., 2011), have global wintering strongholds concentrated in South-east Asia (Zöckler et al., 2018a; Yu et al., 2020). 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., 2006; Bakewell et al., 2017; 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., 2019; Nguyen et al., 2020; Yu et al., 2020). Using satellite trackers on migrating great knot, Chan et al. (2019) 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, 2006). Recent studies (Yasue & Dearden, 2009; Sripanomyom et al., 2011; Jackson et al., 2020), 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, 2006; Lim & Posa, 2014; Lai et al., 2015; Jensen, 2018). 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, 1998; Richards & Friess, 2016; Rahman, 2017; Jensen, 2018), leading to a loss of habitat for migratory waterbird species (Pedersen & Nielsen, 1998; Buckton & Safford, 2004; Yasue & Dearden, 2009). 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, 2011; Kuenzer & Renaud, 2012; Marchesiello et al., 2019).

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., 2015); from 1922 to 1993 intertidal sand/mudflat extent declined by > 75%, to 8 km² (Hilton & Manning, 1995). In addition to extensive habitat contraction in this region, migratory waterbird populations are probably also affected by unsustainable hunting pressure (Turrin & Watts, 2016; Gallo-Caijiao et al., 2020).

Although wetland restoration has slowed the decline of mangroves in some countries in the region (e.g. Philippines, Thailand; Erftemeijer & Lewis, 1999; Friess et al., 2020), 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, 1999), which in some instances has further contributed to the loss of habitat for migratory waterbirds (Jensen, 2018).

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., 2012; Nguyen et al., 2020). 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, 2006; Yu et al., 2020). Viet Nam has a well-established network of protected areas but coastal wetlands are poorly represented (Rambaldi et al., 2001), 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., 2007; Jensen, 2018).

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., 2010; Yong et al., 2018), 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., 2012). The low-lying coastal belt across much of South-east Asia is densely populated and human populations continue to grow (e.g. Chou, 1994). 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., 2014; Piersma et al., 2016; Studds et al., 2017). 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., 2011; Friess et al., 2020). 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., 2019), and to complement ground-based field surveys (Putra et al., 2019; Nguyen et al., 2020). 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., 2004), 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., 2017). In instances where migratory waterbirds are directly benefiting from man-made wetlands (e.g. Inner Gulf of Thailand; Round, 2006; Yasue & Dearden, 2009; Sripanomyom et al., 2011; Jackson et al., 2020; Nguyen et al., 2020), 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., 2004), 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., 2018; Zöckler et al., 2018b; Jearwattanakanok & Yong, 2019), and elsewhere in the world, such as the Wadden and Yellow Sea (Boere & Piersma, 2012; 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.