THOOTHUKUDI: On the occasion of World Biological Diversity Day being observed on Monday, The New Indian Express takes a sneak peek into the concerns of environmental experts while prioritising seven irrigation tanks that are frequented by migratory birds in the Thamirabarani river basin.
In an effort to preserve the biodiversity in these tanks, the experts took a cue from the United Nation's theme for International Day for Biological Diversity 2023 -- 'From Agreement to Action: Build Back Biodiversity' -- in which, they picked out the tanks based on a research work published in the journal Wetland Ecology and Management. The research identified potential tanks for conservation based on the trends of waterbirds recorded in the Thamirabarani Waterbird Counts (TWC) over the past 12 years. The authors of the study include Patrick David, MB Prashanth, A Saravanan, S Thalavaipandi, P Maria Antony, M Mathivanan, and·T Ganesh -- all associated with Ashoka Trust for Research in Ecology and the Environment (ATREE).
The Thamirabarani River, known as the perennial river of Tamil Nadu, receives rain mostly during the Northeast monsoon that sets in during October and feeds over 1,059 irrigation tanks in Tirunelveli and Thoothukudi districts through an ancient network of large reservoirs, anaicuts, and canals. Most of the tanks are man-made wetlands that were created centuries ago. The unique feature of the basin is the presence of larger tanks towards its tail end that are filled with water until March-April, and slowly dry up by August-September.
Interestingly, these tanks are the final wintering site for most migrant birds flying down all the way from Eastern Europe, Siberian countries, Russia, Kazakhstan, Mongolia, and other places in central Asia. They are also a foraging and nesting site for resident birds from Gujarat, the Himalayan foothills, central India, and western Himalayan ranges. As per ATREE's records, over 4,39,356 individual birds belonging to 91 species have visited 125 tanks in the past 13 years.
The study, based on 12 annual bird surveys conducted in as many as 133 tanks every January since 2011, including four years of the lean period assessment done every September between 2019 and 2022, revealed that 89 out of 94 species were sighted at the 31 wetlands monitored over the past 9–12 years. As many as 52 species that made up 59% of the total bird population were resident birds, while the remaining 37 species were migratory birds.
The most abundant species that contributed to 29% of the total bird population were Common Coot Fulica atra, Cattle Egret Bubulcus ibis and Lesser Whistling Duck Dendrocygna javanica, whereas Little Cormorant Microcarbo niger, Cattle Egret, Indian Pond Heron Ardeola grayii and Little Egret Egretta garzetta were the most widespread species found in more than 70% of the tanks surveyed, the study says.
The 12 surveys conducted every year revealed that the bird species belonging to the families Anatidae, Ardeidae, and Scolopacidae, dominate the Thamirabarani river basin.
In Tamil Nadu, there is a widespread habit of hunting waterbirds to meet the demands in the food market, especially large water birds such as Spoonbills, Egrets, and Spot-billed Pelicans.
Commenting on the study, the lead author Dr Patrick David told TNIE that the migrant waterbirds visiting the Thamirabarani river basin have declined significantly (64%) over the years, whereas the resident waterbirds witnessed a decline of 27%. "However, the census has shown some recovery since 2020. Maintaining an optimum water level at these tanks that shelter a large population of waterbirds, will ensure their abundance in the coming years. At the time the wetlands are reducing in size and disappearing from the maps, the government should conserve these tanks and monitor them properly to protect its rich diversity," he pointed out. The water dynamics and its effect on the birds can be studied further, he added.
A comparison of trends before and after the 2016-2017 drought period, revealed that there was a drop in the population of 10 out of 50 species due to the failure of the northeast monsoon in 2016. However, some species, including the Kingfishers, Bar-headed Goose, and Black-tailed Godwit, showed a positive trend after the drought period.
One of the authors, Mathivanan told TNIE that the study determined the trends in species richness and abundance of waterbirds. "The tanks were categorised from high to low counts of migratory, resident species, feeding guild, declining families and other species, based on the bird census to arrive on the priority criteria for choosing those tanks," he said.
The waterbird congregations and richness during the winter season were witnessed in Arumugamangalam, Maanur, Kadambakulam, Vijayanarayanam, Prancheri Periyakulam, Tenkaraikulam, Nallur, Velur, Melpudukudi Sunai, Srivaikuntam Kaspa, and Peikulam, whereas in the summer season, the tanks such as Kadambakulam, Melpudukudi Sunai, Karungulam, Perungulam, Srivaikundam Kaspa, Tenkaraikulam, Velur, and Prancheri Periyakulam tanks recorded the highest bird population.
The experts prioritised the seven tanks that are common to both winter and summer species, including Arumugamangalam, Kadambakulam, Melpudukudi Sunai, Srivaikundam Kaspa, Vagaikulam, Prancheri Periyakulam, and Tenkaraikulam, for adequate conservation.
Mathivanan stated that the Wetlands (conservation and management) Rules, 2017 completely ignored irrigation tanks as potential sites for conservation. The landscape-level conservation planning for such wetlands is generally lacking, as the conservation potential of irrigation tanks has not been documented yet, said Mathivanan.
Stating that most of these tanks are in the Thoothukudi district, the experts said they have appealed to the district administration to prioritise the conservation of these tanks by forming Biodiversity Management Committees based on the Biological Diversity Act of 2002, so that the local communities who depend on these tanks for irrigation and drinking water, will will not be restricted by the conditions. If these tanks are not maintained and conserved, the waterbirds may shift to alternative habitats where conditions are more favourable, they cautioned.