Conservation insights for the Critically Endangered Black Softshell Turtle (Nilssonia nigricans) through landscape genetics approach in South Asia

The application of advanced molecular and ecological approaches has emerged as a globally recognized and robust framework for effective biodiversity conservation planning and management. Prior to this study, aside from taxonomic investigations, no comprehensive assessment integrating genetic and ecological data had been undertaken for the Critically Endangered Black Softshell Turtle (Nilssonia nigricans). The mitochondrial COI and CYTB gene data generated in this study clearly distinguish N. nigricans from its congeners, corroborating morphology-based species identification. Furthermore, the integration of four mitochondrial genes (COI, CYTB, ND4, and 12 S rRNA) and three nuclear genes (C-mos, ODC, and R35) through uncorrected p-distance analyses provides a robust understanding of population-level genetic divergence. The species distribution modelling identified only 12.81% (21,127 km²) of the area as suitable habitat for N. nigricans under current climatic conditions, while the connectivity analysis revealed four major corridors (Terai Landscape, Western Brahmaputra, Kaziranga–Dhansiri, and Eastern Brahmaputra) within its extended distribution range. Under future climate scenarios, the extent of suitable habitat is projected to expand remarkably by more than 64% across all future climatic scenarios and time periods. Notably, the connectivity is expected to decline within the identified corridors under all future projections. The Mantel analyses showed no significant association between genetic distance and landscape resistance, whereas significant isolation by distance was detected for some loci, indicating that geographic separation better explains genetic differentiation in N. nigricans. These patterns likely reflect historical demographic processes and possible human-mediated movements rather than contemporary landscape-driven connectivity. Furthermore, the delineation of priority conservation districts across eastern and northeastern India, northeastern Bangladesh, and eastern Nepal underscores the critical transboundary regions essential for sustaining the habitat requirements of the species. Overall, this study presents a novel approach to identify conservation priority areas and potential reintroduction sites for N. nigricans. The findings offer crucial insights for developing adaptive management strategies that incorporate both the present genetic status and the anticipated environmental changes influencing this species.