Biogeographic Patterns and Endemism in the Vascular Flora of the Cross River Rainforest Belt, Nigeria

The Cross River rainforest belt in South East Nigeria represents one of the most species-rich and biodiverse forest ecosystems in Africa, yet systematic quantitative analysis of biogeographic patterns and endemism in its vascular flora has not been conducted using contemporary analytical frameworks. This study analysed biogeographic distribution patterns and levels of endemism in the vascular flora of the Cross River rainforest belt using specimen data from National Herbarium Nigeria, University of Calabar Herbarium, and the Kew Herbarium Catalogue database, comprising 8,430 georeferenced specimen records. Species occurrence data were compiled for an area of approximately 15,000 km2, and species range types were classified following the White African phytogeographic framework. Endemism was assessed at three scales: local endemic (Cross River only), regional endemic (Lower Guinea endemic), and African endemic. Hotspot analysis was conducted using kernel density estimation. A total of 2,340 vascular plant species were compiled for the study area. Regional Lower Guinea endemics constituted 18.3% of the flora, substantially higher than comparable West African forest areas. Local endemics numbered 84 confirmed species and 32 suspected endemics pending final taxonomic confirmation. Hotspot analysis identified the Obudu Plateau and the Boshi-Okwangwo corridor as the two highest-priority endemic hotspot zones. Families Rubiaceae, Orchidaceae, and Annonaceae contained the highest numbers of endemic species. The study provides the first quantitative biogeographic analysis of Cross River endemic flora and generates an evidence-based priority species list for the Nigerian National Biodiversity Strategy and Action Plan revision.

Keywords: biogeography, endemism, Cross River rainforest, vascular flora, biodiversity hotspot