The other side of conservation: How mangrove extension is threatening birds' habitats
Expansion of mangrove distribution has led to a conservation dilemma in some areas, particularly along the East Asian-Australasian flyways
The reflection of the setting sun glittered on the tidal waves of the Meghna estuary.
And the distant Domar Char, with multiple green layers — a thin yellow-green brackish marsh and a comparatively thicker deep green layer of mangroves over it — became visible as our motorised boat approached from Bandartila Ghat of Nijhum Dwip.
As we were reaching close to the mudflat, little crowns of young Keora trees rose from the water. Interestingly, they were lined up like fort guards, protecting the intertidal island from erosion.
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Pointing to the submerged trees, a Forest Department official said, "Look, how beautiful the sight (coastal afforestation) is!"
Forest officials may be satisfied with the expansion of mangroves along the coast, but conservationists disagree upon taking a closer look.
The coastal afforestation, actually transformation of the mudflats into mangrove forests, takes away habitats of the waterbirds. Because of the abundance of mangroves, the birds cannot forage there even if it ebbs and the tidal mudflats are exposed.
Finding a gap, the boatman intruded into Domar Char through a creek. Unsurprisingly, we spotted a few wader birds foraging, scattered along the shallow water flow. Our presence certainly disturbed them.
A flock of shorebirds suddenly flew away too. A minute before, perhaps, they might have roosted on the marsh.
Located in the southern part of Hatia Island in Noakhali district, Nijhum Dwip and its surrounding islands, Domar Char and Char Kalam — collectively known as Nijhum Dwip National Park — are identified as a crucial stopover in the Central Asian and East Asian-Australasian flyways for the migratory shorebirds and a Flyway Network Site.
With a wide variety of wetland habitats like intertidal mudflats, brackish marshes, sand dunes, planted mangroves, sandbars, lagoons and beaches, every year, the park offers shelter to tens of thousands birds of more than 50 species, including 13 species that are classified as globally threatened or near-threatened.
However, expansion of mangrove distribution has led to a conservation dilemma in some areas, particularly along the East Asian-Australasian flyways, which encompasses 45.6% of the global mangrove extent and has the highest proportion (23% or 46 species) of threatened migratory waterbirds, a 2022 study, titled "An Emerging Coastal Wetland Management Dilemma between Mangrove Expansion and Shorebird Conservation", says.
In the 1960s, the Forest Department started forestation on the newly accredited coastal land in Bangladesh.
Following decades-long coastal afforestation along the 710-km-long coastline of the country, the department created around 2.5 lakh hectares of coastal forests, mostly of Keora (Sonneratia apetala), Gewa (Excoecaria agallocha) and Baen (Avicennia officinalis) trees, in Noakhali, Lakshimpur, Bhola, Patuakhali, Barguna, Pirojpur, Chattogram, and Cox's Bazar districts.
How does mangrove extension impact shorebird habitats?
When mangrove vegetation systems invade a bare tidal flat, dense canopies are gradually formed and they outcompete other types of vegetation. The process transforms mudflats, tidal creeks, and seagrass beds into homogeneous mangrove forests.
Coastal shorebirds primarily forage at low tide when tidal mudflats are most exposed and they roost on the brackish marshes after high tides inundate the mudflats. Shorebirds rarely use densely vegetated areas such as mangroves and saltmarsh, a 2017 study titled, "Migratory Shorebird Monitoring – Understanding Ecological Impact", shows.
Continuous seeding and expansion of mangroves change the structure and functions of an estuarine wetland ecosystem like that which exists at Nijhum Dwip. As a result, migratory shorebirds that rely on mudflats for feeding and resting can lose suitable habitats.
Take mangrove extension in northern Taiwan as an example. Monospecific extension of mangrove Kandelia (Kandelia obovata) has converted intertidal mudflats and other habitats into mangrove forests, thus reducing estuarine biodiversity in the Danshuei River estuary.
Some researchers at universities in China, Australia, and Taiwan have examined several case studies that illustrate the trade-off between mangrove restoration and bare tidal flat maintenance.
They observed mangrove extensions (natural dispersal or artificial afforestation) and interventions like removal of seeds or matured trees and their consequences in the Firth of Thames on the North Island of New Zealand, Hunter Valley in Eastern Australia, Leizhou Peninsula and Shenzhen Bay in southern China, and Guandu on Taiwan Island--the sites connected to the East Asian-Australasian flyways.
These are the habitats of critically endangered Black Stilt (Himantopus novaezelandiae), Spoon-billed Sandpiper (Calidris pygmaea), and endangered Eastern Curlew (Numenius madagascariensis).
The researchers found that there is a potential conflict between mangrove expansion and shorebird conservation.
Professor Dr Mohammad Firoj Jaman, a faculty of the Zoology Department at Dhaka University, said that the dilemma between mangrove extension and loss of shorebird habitat is related to site selection for coastal afforestation.
"If coastal afforestation continues at a place where occurrence of shorebirds is high, the birds will definitely lose their habitat," Jaman said.
For more than a decade, conservation biologist Sayam U Chowdhury and a team of other experts have been researching migratory birds.
His 2020 study titled, "Globally Threatened Shorebirds of Nijhum Dwip National Park and Management Implications", finds an increasing trend of waterbirds' occurrence at Nijhum Dwip in recent years; it also traces some emerging threats.
Currently a PhD researcher at University of Cambridge, Sayam too said impacts on specific shorebirds varies from locations of mangrove extension, hence the process needs careful planning.
Broadly, the bird species that prefer bare mudflats will lose their foraging and roosting habitats when mangrove trees are planted, he said.
"For Nijhum Dwip, I think the following species may be impacted: Black-tailed Godwit, Whimbrel, Eurasian Curlew, Common Redshank, Terek Sandpiper, Lesser and Greater Sand Plovers and Indian Skimmer," Sayam said.
Among the bird species, Indian Skimmers are assessed as endangered while Black-tailed Godwit and Eurasian Curlew are near-threatened.
The Forest Department has now decided to go ahead with afforestation in a more planned manner to save shorebirds' habitats.
Mohammad Harun Ur Rashid Khan, the Conservator of Forest for Coastal Circle under the Forest Department, said, "Shorebirds roost and forage at some specific locations. We are now avoiding afforestation in the places where their occurrence is high.
