<p>Understanding the relationship between biodiversity and both the functioning and stability of ecosystems has been a central focus of ecologists for decades. A step-change in our understanding of the biodiversity–ecosystem functioning relationship was enabled by explicit measurement of the additional functioning provided by biodiversity through comparing expected and observed yields in multi-species communities. However, we lack an equivalent measure for stability. Here, we quantify the net biodiversity effect on stability using model simulations and a microcosm experiment that exposes different phytoplankton species and their combinations to temperature increases and fluctuations. As an emergent property of communities, stability frequently exceeds the expected stability of the combined component species, leading to a net biodiversity effect on stability analogous to the effect on functioning. In our simulations, these effects depend on the strength of competitive interactions as well as species composition and their thermal optima. Experimentally, the stabilising effect of biodiversity is, however, non-linear, greatest for two-species combinations, and varies with both community composition and disturbance regime. Quantifying the net biodiversity effect on ecological stability advances our mechanistic understanding of the biodiversity–stability relationship, and provides crucial information to support ecosystem management and conservation.</p>