Developing an Original Theoretical Framework for Plant Community Assembly Across Nigeria’s Vegetation Gradient: Integrating Niche Theory, Neutral Theory, and Historical Biogeography

Plant community assembly theory has been dominated by the debate between niche-based determinism and neutral stochastic processes, yet neither framework alone adequately accounts for the diversity patterns observed across Nigeria's exceptional vegetation gradient from mangrove to Sahel. This study develops and empirically tests an original integrative framework for plant community assembly across Nigerian vegetation zones, combining niche filtering theory, neutral biodiversity theory, and historical biogeographic processes into a hierarchical assembly model. The framework proposes that the relative importance of niche versus neutral processes is predicted by a Climatic Heterogeneity-Historical Stability Index developed in this study, calculated from paleo-climate models and current-day environmental heterogeneity. Empirically, the framework was tested using a novel dataset of 1,840 georeferenced vegetation plots distributed across 7 Nigerian vegetation zones, compiled from herbarium records, published vegetation studies, and original field surveys in underrepresented zones. Null model analysis (SESMPD, SESNND), phylogenetic signal tests, and a randomisation-based Neutral-Niche Spectrum metric were used to assess assembly process prevalence at each vegetation zone. Results showed that niche filtering dominated in arid northern zones (Sahel, Sudan savanna), while evidence for neutral stochastic processes was stronger in the most species-rich rainforest zone, consistent with the framework's predictions from the Climatic Heterogeneity-Historical Stability Index. Transition zones showed intermediate signals. The integrative framework outperformed single-process models in predicting species richness patterns across zones. The study makes an original theoretical contribution to tropical plant community ecology and provides a quantitative foundation for predicting biodiversity responses to climate change across Nigerian vegetation.

Keywords: community assembly theory, niche filtering, neutral theory, vegetation zones, Nigeria