<b>Biodiversity mediates climate constraints on human population density across Africa</b>

Understanding how climate shapes human populations is central to sustainability science, yet most approaches assume that climatic conditions directly determine human distribution. Here we test an alternative hypothesis: that biodiversity and ecosystem functioning mediate the relationship between climate and human population density. Using a continental-scale structural equation model integrating climate, biodiversity, vegetation productivity and human population density across Africa, we show that biodiversity constitutes a critical intermediary linking environmental conditions to human systems. Climate variables strongly influenced species richness and phylogenetic diversity, which in turn enhanced vegetation productivity. Human population density was positively associated with ecosystem productivity and phylogenetic diversity but exhibited a negative independent relationship with species richness after accounting for shared environmental drivers, revealing hidden trade-offs between biodiversity and human concentration. Importantly, models that ignored biodiversity underestimated or misrepresented the pathways through which climate constrains human populations. Despite strong spatial structuring across all variables, the direction and significance of these relationships were robust, indicating that they reflect underlying ecological processes rather than geographic artefacts. Our findings demonstrate that biodiversity is not merely a consequence of environmental conditions, but a fundamental component of coupled human-environment systems that regulates how climate supports human populations. These results highlight the risk that biodiversity loss may disrupt the ecological pathways underpinning human well-being, with implications for climate adaptation, land-use planning and sustainable development across rapidly changing regions.