Regional range restriction, not national endemism, predicts plant extinction risk: operationalising Conservation Imperatives for under-assessed tropical floras

Blaise Binama 

Abstract
Aim: Tropical African flora is potentially threatened, yet species-level conservation priority-setting is rare in under-assessed floras. We tested whether range restriction drives extinction risk and operationalised the Conservation Imperatives at species level.
Location: Rwanda, Albertine Rift biodiversity hotspot, eastern Africa.
Methods: We applied this workflow to 367 IUCN-assessed vascular plant species. The approach combined: (i) Random Forest, partial proportional-odds models, propensity-score matching with Rosenbaum-bounds sensitivity; (ii) a three-tier classification linking formal Protected Area (PA), remnant forest patch and exposed habitat to distinct actions; (iii) species distribution modelling across three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP5-8.5) × two horizons (2041–2060, 2061–2080) with EDGE2 phylogenetic-priority scoring; and (iv) cross-validation against an independent field inventory.
Results: Regional range restriction dominated risk: 68% of 86 Albertine Rift near-endemic species were threatened, against 16% of 266 wider-ranging and 14% of 15 strict Rwandan endemics. The matched effect was robust but modest (OR = 2.74, 95% CI 1.46–5.14; Rosenbaum-bounds Γ ≈ 1.5). Twenty-five of 105 threatened species occupied exposed-habitat tier; only six gained protections from a feasible 2.5 km expansion occurred in the exposed-habitat tier; only six gained protections from a feasible 2.5 km boundary expansion. EDGE2 and threat-based priority were weakly correlated (ρ = 0.162, p = 0.098), indicating partly independent priorities. Projected habitat loss increased monotonically with emissions and time; 41 species (22%) exceeded the IUCN Criterion A2 threshold (≥ 80% loss) under SSP5-8.5 by 2061–2080, and threatened-lineage phylogenetic diversity declined nationally by 22.5%, concentrated in Nyungwe National Park. Cross-validation recovered Capparis lucens from a harmonisation artefact and identified further threatened species absent from global datasets, including Pterygota mildbraedii.
Main conclusions: Within Rwanda's vascular flora, regional range restriction is the dominant driver of extinction risk and is independent of, and compounded by, projected climate loss. The Conservation Imperatives workflow supports Kunming-Montreal Global Biodiversity Framework (KMGBF) Target 3 reporting in under-assessed floras.