From environmental heterogeneity to stability: beta-diversity stabilises biomass across space

Understanding how biodiversity influences ecosystem stability across heterogeneous landscapes is a central challenge in ecology. The spatial insurance hypothesis proposes that turnover in species composition stabilises ecosystem processes in heterogeneous environments by allowing species to occupy the habitats where they perform best. Yet, empirical evidence remains mixed, partly due to confounding factors and overlooked causal mechanisms. We combined metacommunity simulations and a field experiment with marine benthic communities to test how β-diversity (species turnover) affects the spatial stability of biomass. The simulations, which varied the strength of selection via niche overlap, revealed that turnover enhanced spatial biomass stability in scenarios with low to medium niche overlap, but only when environmental heterogeneity was statistically accounted for. This stabilising effect weakened as niches overlapped more. In our field experiment, high turnover similarly mitigated the negative impact of environmental heterogeneity on spatial biomass stability. Together, these results provide strong support for the spatial insurance hypothesis, demonstrating that species turnover can stabilise ecosystem processes such as biomass production across heterogeneous landscapes. Crucially, failing to account for environmental heterogeneity can mask this effect, leading to the mistaken conclusion that turnover is neutral or destabilising. Our study highlights the importance of a causal framework that disentangles the effects of environmental heterogeneity and β-diversity, helping to resolve previous inconsistencies and advance a mechanistic understanding of biodiversity–stability relationships at landscape scales.