As anthropogenic disturbances intensify across the world, it is becoming critical to understand communities’ intrinsic capacities to withstand change. High biodiversity has been thought to bolster communities against disturbances, leading to disproportionally higher levels of ecosystem functioning. The biodiversity-ecosystem function relationship, or BEF, has been extensively tested across nearly every ecosystem on Earth; yet it is still unclear under which circumstances the BEF is positive, negative, or neutral. We used 7,685 individual growth curves across 1,480 species to generate a process-based estimate of biomass production and assess the global BEF across marine fishes. Here, we found that Hill diversity emphasising abundance effects outpaced the effects of species richness and community evenness. Despite a positive global BEF relationship, we found a clear shift in the relationship between abundance and per-capita productivity: low-diversity communities were characterised by lower abundances but higher per-capita productivity; high-diversity communities the opposite (i.e. higher abundances and lower per-capita productivity). Whole-community productivity remained relatively stable across diversity gradients; trade-offs in abundances with per-capita productivity were presumably driven by metabolic constraints on growth and body size imposed by warmer temperatures. It appears that biodiversity can only bolster ecosystem functioning to a limited extent; stressors will likely limit the function delivery of marine fishes globally.