Original Contribution to the Ethnobotanical Theory of Biocultural Diversity Erosion: Evidence from a 30-Year Longitudinal Study of Medicinal Plant Knowledge in Niger State

Biocultural diversity, the co-evolution of cultural knowledge and biological diversity, is eroding globally, yet longitudinal empirical studies that track the simultaneous decline of plant species diversity and associated traditional knowledge over multi-decade periods are exceedingly rare in the African literature. This study makes an original contribution to biocultural diversity erosion theory using a unique 30-year longitudinal dataset from Niger State, North Central Nigeria, comparing medicinal plant knowledge documented by an original ethnobotanical survey conducted in 1993 with a restudy of the same communities conducted in 2023. The 1993 survey documented medicinal plant uses by 94 healers and community members in 12 communities across three Local Government Areas. The 2023 restudy replicated the exact methodology with a new generation of 94 informants from the same communities. Quantitative ethnobotany indices (species utility value, cultural importance index, informant consensus factor, and intergenerational knowledge transfer coefficient) were computed for both datasets and compared. Parallel vegetation surveys at both time points assessed changes in plant species availability. Results showed a 38% reduction in the number of medicinal plant species cited, a 44% decline in total use reports, and a 52% decline in cultural importance index scores over 30 years. The intergenerational knowledge transfer coefficient declined from 0.76 to 0.41, reflecting severe intergenerational knowledge erosion. Cross-referencing with vegetation data revealed that 23% of the knowledge decline was attributable to species habitat loss, while 77% reflected sociocultural knowledge transmission failure. An original Biocultural Erosion Index is proposed, integrating plant species availability and knowledge depth into a single composite metric. The study constitutes one of very few documented 30-year longitudinal ethnobotanical comparisons from sub-Saharan Africa.

Keywords: biocultural diversity, longitudinal ethnobotany, medicinal plant knowledge erosion, Niger State, intergenerational transfer