Intrageneric Diversity and Stenoendemism in Garcinia, Humboldtia and Symplocos in the Agasthyamala Biosphere Reserve, India

Abstract

This study examines the high intrageneric diversity and significant stenoendemic representation (i.e., taxa with extremely narrow and restricted geographic distributions) within Garcinia (Clusiaceae), Humboldtia (Fabaceae-Caesalpinioideae), and Symplocos (Symplocaceae) in the Agasthyamala Biosphere Reserve (ABR), India. Taxonomic revisions reveal that the ABR supports a disproportionately high concentration of species in these genera, compared to the broader Western Ghats. In Garcinia, the ABR harbours 64% of the regional diversity, including three stenoendemic taxa. Similarly, nearly 55% of Indian diversity of Humboldtia is confined to the ABR, comprising four stenoendemic taxa. In Symplocos, the ABR harbours 42% of regional diversity, including nine stenoendemic taxa. These distributional patterns indicate that the ABR functions as an island-like phytogeographic unit, with the Shencottah Gap acting as a significant geographical barrier. The regions unique topography and stable ever-wet microclimate have facilitated its role as both an ecological refugium and an evolutionary ecotone, fostering the preservation of ancient lineages and allopatric speciation. These findings underscore the ABR’s significance as a global centre of stenoendemism and emphasise the urgent need for targeted conservation measures.