Document Repository and Sectoral Scope Summary

Abstract

Grassland ecosystems sustain biodiversity, ecosystem services, and rural livelihoods but are increasingly threatened by hydrocarbon contamination. The Ugwueme Guinea savanna of southeastern Nigeria, characterized by natural oil seepages and tar sand deposits, provides a unique setting to evaluate the ecological consequences of chronic hydrocarbon exposure. This study assessed changes in floristic diversity, soil quality, and community livelihoods using vegetation surveys across three hydrocarbon exposure zones, soil physicochemical analyses, and structured questionnaires administered to 150 households. A total of 90 plant species belonging to 72 genera and 23 families were recorded. Species richness declined from 85 species in the reference zone to 54 in the moderate-exposure zone and 38 in the high-exposure zone, while the Shannon–Wiener diversity index decreased significantly from 3.45 ± 0.21 to 2.56 ± 0.18 and 1.65 ± 0.14, respectively. Surface-soil total petroleum hydrocarbon (TPH) concentrations reached 937.33 mg kg⁻¹ in the highly impacted zone, approximately 18.7 times above the Department of Petroleum Resources intervention limit for agricultural soils. Hydrocarbon contamination simplified plant community structure by favouring a few disturbance-tolerant species while reducing the richness of key grassland families. Consequently, 86.7% of households reported declining crop productivity, 75.3% reduced access to medicinal plants, and 65.3% reduced livestock forage. These findings demonstrate that chronic hydrocarbon seepage drives biodiversity loss, soil degradation, and declining ecosystem services, underscoring the need for ecological restoration and community-based conservation in oil-affected grassland landscapes.

Keywords: Grassland biodiversity, Hydrocarbon contamination Floristic diversity, Species richness, Total petroleum hydrocarbons, Ecosystem services, Southeastern Nigeria