Coastal and inland wetlands in southern India are declining. A study by a team of researchers in Kerala has found that the population of wading birds, key indicators of wetland health, have declined in the decade from 2010-2019.
A decline in primary productivity and fish stock availability affected large waders, the authors wrote. They were adversely affected by long-term, anthropogenically driven, environmental changes.
The waders consequently shifted to agroecosystems and estuaries as their alternate foraging habitats, the authors noted.
The researchers selected 27 sites comprising of estuaries, sand beaches, mudflats, mangroves and agro-ecosystems along the coast of Kerala to carry out the study from 2010 to 2019.
The sites were Azheekkal estuary, Ayikkara beach, Iringal Estuary, Kollavi beach, Thikkodi beach, Kappad beach, Korappuzha beach, Elathur beach, Kallayi beach, Kappakkal beach, Gotheeswaram beach, Pulimudu beach, Chooloor wetland, Mavoor Palliyol wetland, Manthalakkadavu wetland, Vadakkumpad wetland, Vazhakkad Agroecosystem, Kallampara inland mangroves, KVCR mangroves and Mudflats, Ariyallur beach, Chettuva beach, Puthuvypu beach, Changaram wetland, Purakkad beach, Thottappalli beach and Kayamkulam estuary.
“Large waders including herons and egrets are important ecological wetland health indicators since these species are sensitive to a complex set of environmental factors. Anthropogenic factors are driving major changes in coastal and inland wetlands. We investigated the relationship between environmental variables, productivity, fish diversity and large waders in southwestern India,” the researchers wrote.
The variables covered climatic variables (rainfall), sediment variables (salinity, pH, organic carbon, nitrogen, phosphorus), water variables (Salinity, pH, temperature, nitrate, phosphate), primary productivity (Chlorophyll-a) and prey availability (mean fish stock availability).
The researchers monitored six of the most commonly available wader species: Great Egret (Ardea alba), Median Egret (Ardea intermedia), Little Egret (Egretta garzetta), Indian Pond Heron (Ardeola grayii), Western Reef Egret (Egretta gularis) and, Black Headed Ibis (Threskiornis melanocephalus).
They conducted bird counts at all 27 sites during low tide from established fixed scanning points at the estuaries, mudflats, mangroves and agro-ecosystems.
They found that densities of all species decreased significantly from 2010 to 2019, and it also varied significantly over seasons and habitats.
For instance, the densities of Little Egret, Median Egret, Great Egret and Indian Pond Heron were highest in agroecosystems and estuarine habitats but lowest in mudflats.
The highest densities of Western Reef Heron and Oriental White Ibis were observed in beaches and agroecosystems respectively.
All six species showed highest densities during post-monsoon seasons and lowest densities during the monsoon seasons.
“Structural equational modelling revealed that rainfall and organic carbon positively affected the primary productivity, whereas water variables (nitrate, salinity and phosphate) positively affected the fish stock availability. This in turn positively affected the density of large wading birds,” the researchers reasoned.
They added that the entire western coastline appears to be undergoing major changes in nutrient discharge, salinity, pH and temperature. Phosphorus, salinity and pH in water and in sediments increased steadily whereas organic carbon and nitrogen in the sediment decreased significantly.
While salinity, water temperature, pH, and phosphate showed a significant increase, water nitrate, and chlorophyll-a decreased over the period of study.
“Significant decline in fish stock availability and altered environmental variables resulted in long term decline in the abundance of wading birds. Thus, large waders have adapted to these long-term, anthropogenically driven, environmental changes by shifting to agroecosystems and estuaries as their alternate foraging habitats,” the paper noted.
The researchers warned that the population declines indicate consistent decline in ecosystem health of wetlands. “Strategies need to be formulated to mitigate declines and restore ecosystem health,” they urged.
The paper, titled Assessing environmental change and population declines of large wading birds in southwestern India, has been published in the journal Environmental and Sustainability Indicators.
Its authors are K M Aarif, Aymen Nefla, K A Rubeena, Yanjie Xu, Zakher Bouragaoui, M Nasser, C T Shifa, T R Athira, K Jishnu, Jasmine Anand, Seerangan Manokaran, P P Moosa, Anu Gopinath, Omer R. Reshi, K M Rajaneesh, H Byju, Thadickal V Joydas, Karuppasamy P Manikandan, Mohd Irfan Naikoo, Christian Sonne, and Sabir Bin Muzaffar.