Long-term Population Dynamics of Large Wading Birds in Gulf of Mannar Lagoons: Potential Influence of Changing Landscapes with Expanding Agriculture

Large wading birds are key indicators of wetland ecosystem health and productivity. Despite supporting significant populations, lagoon systems along the coast of the Gulf of Mannar Biosphere Reserve have received limited studies. The present study analyses long-term (1985-2024) discontinuous population trends (1985-1989, 2005-2007, 2018-2019, 2021-2024) of large wading birds recorded in coastal lagoons at Pillaimadam, Valinokkam, and Dhanushkodi. Bird count data obtained from monthly systematic field surveys were compiled; seasonal peak count per year was taken, and principal coordinates analysis (PCoA), Mann–Kendall test, and beta diversity partitioning were employed to assess community assemblage and temporal changes. A total of 15 large wading species belonging to three families were recorded, where egrets constituted the dominant guild. Species richness and diversity substantially increased after 2018. Beta diversity indicated moderate temporal dissimilarity, driven primarily by nestedness, while turnover contributed to a lesser extent. A substantial change in community composition and dominance from Little Egret in earlier years (1985-88 and 2005-07) to Intermediate Egret in the recent period (2018-2024) was recorded. The recent period has also recorded several new species: Black-headed Ibis, Glossy Ibis, Painted Stork and Asian Openbill. Over the last few decades, the observed increase in cultivated land area in Ramanathapuram may have influenced resource availability, thereby altering species composition relative to the earlier study period, but causality remains untested. These findings highlight the ecological importance of lagoon-agriculture mosaics and the need for integrated conservation strategies.