- Numbers of most species of migratory waterbirds in Bangladesh’s Tanguar Haor wetland, a key stopping point, have fallen over the past 15 years, a new study shows.
- The cause of the decline isn’t fully known yet, researchers say, but it’s clear that human activity has impacted the wetland, with 40% of the basin’s original area converted to farmland and settlements in just 30 years.
- The study recommends prioritizing conservation at two of the permanent waterbodies, or beels, in the Tanguar Haor complex, citing the high abundance and diversity of the birds that stop there.
- Tanguar Haor is the second Ramsar site in Bangladesh, after the Sundarbans mangrove forest, and accounted for nearly half of the more than 1.2 million waterbirds recorded in the country between 2008 and 2015.
One of Bangladesh’s two Ramsar sites, wetlands of international importance, is slowly losing some of the waterbirds that seek respite at this key migratory pit stop, a new study says.
Tanguar Haor, spanning 9,500 hectares (23,500 acres), sits in the floodplain of the Surma River and is home to more than 200 bird species, half of them migratory. And while birds continue to flock here in their hundreds of thousands on their annual migrations along the East Asian–Australasian Flyway, for many species the number of recorded individuals declined between 2008 and 2021, according to the study published in the journal Aviation Conservation & Ecology.
The researchers behind the study carried out censuses of 69 waterbird species during this period, 41 of them migratory. Counts for species like Baer’s pochard (Aythya baeri), the common pochard (Aythya ferina) and the tufted duck (Aythya fuligula) declined — the latter “severely” — as did most egrets and herons, while other species either increased or fluctuated. Overall, for migratory birds, “linear trend models of species count over time suggested the majority of species, 23 (59%), showed a declining trend, and 16 (41%) species showed an increasing trend,” the study says.
Study co-author A.B.M. Sarowar Alam, from the Bangladesh office of the IUCN, the global wildlife conservation authority, says the causes of the ongoing decline aren’t fully known yet. However, they’re likely connected to the deterioration of inland freshwater wetlands and the lack of appropriate nesting trees.
Human disturbances to the wetland
The entire Tanguar Haor wetland is submerged during the May-September rainy season, with average annual rainfall of about 5,000 millimeters (nearly 16.5 feet). During the dry season, October to April, water levels drop dramatically from 6-10 meters to just 2-6 m (20-33 ft to 6-20 ft). This results in the formation of several shallow waterbodies and channels, known as beels, and the emergence of reed beds, grassy patches and vegetation.
The land that emerges when the water recedes is rich in nutrients, making it a prime target for conversion to farmland. Over the past three decades, two-fifths of the original basin has been turned into low-lying farmland and settlements, according to the study.
“Other anthropogenic disturbances in the form of increased domestic duck rearing, cattle grazing, motorized and hand paddled boats, and fuelwood collection are some prominent threats to migratory waterfowl at Tanguar Haor,” it says. Another threat the researchers highlight is the illegal hunting of waterbirds using nets and poisoned bait.
The beels, meanwhile, are important for both the bird life and local fisheries during the dry season. The study was carried out at six of these beels, two of which it identified as priority areas for conservation because of their abundant bird life and diversity. Lechuamara beel was found to routinely support more than 20,000 waterbirds annually, while Chotainna beel hosted the same number at least once in recent years. Crucially, Chotainna was regularly visited by species considered threatened or near-threatened, while also recording the highest number of species (47 to Lechuamara’s 45).
“Therefore, we consider these two beels as the most important sites within the Tanguar Haor complex for the conservation of resident and migratory waterbirds,” the researchers wrote.
Bird diversity in Bangladesh
More than 700 bird species occur in Bangladesh, nearly half of which are migratory species that pass through from Mongolia, China and Russia during the Northern Hemisphere winter. Tanguar Haor accounted for nearly half of the more than 1.2 million waterbirds recorded in Bangladesh between 2008 and 2015.
The area was designated a Ramsar site in 2000, in recognition of its international importance as a wetland. The only other Ramsar site in Bangladesh is the Sundarbans, the world’s largest mangrove forest, which sits at the mouth of the Bay of Bengal.
Tanguar Haor’s plant and animal diversity is under serious threat due to natural resource depletion, habitat degradation, soil erosion, water pollution, forest degradation, and wildlife poaching. A 2012 publication by the IUCN says the swamp forests that were once common in this unique wetland have become rare today because of various human activities, including clearing and cutting. The biodiversity of the reed beds has also been greatly reduced due to continued overexploitation for fuelwood and conversion of land to cropland. This destruction has had a negative impact on fisheries resources, given that the swamp forests and reed beds provide shelter and food for fish.
Banner image: A northern pintail takes flight in the Tanguar Haor wetland. Image by Shimanto Dipu.
Alam, A. S., Ahmed, S., Azmiri, K. Z., Amin, R., van Toor, M. L., & Kumar, A. (2023). Population trends and effects of local environmental factors on waterbirds at Tanguar Haor freshwater wetland complex in northeast Bangladesh. Avian Conservation & Ecology, 18(1), 18. doi:10.5751/ACE-02405-180118