From environmental heterogeneity to stability: Beta-diversity stabilizes 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 stabilizes ecosystem processes in heterogeneous environments if species dominate in the habitats to which they are best adapted. 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 (its invariability among sites at a single point in time). The simulations, which varied the strength of selection via niche overlap, revealed that turnover enhanced spatial stability of biomass in scenarios with low to medium niche overlap, but only when environmental heterogeneity was statistically accounted for. This stabilizing effect weakened when species' niches were broader. In our field experiment, high turnover similarly mitigated the negative impact of environmental heterogeneity on spatial stability. Together, these results provide strong support for the spatial insurance hypothesis, demonstrating that species turnover can stabilize ecosystem functioning across heterogeneous landscapes. Crucially, failing to account for environmental heterogeneity can mask this effect, leading to the mistaken conclusion that turnover has a neutral or destabilizing effect. 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.