Diversity persists while function shifts: post-heatwave reorganization of reef fish communities in Baja California.

Background: Marine heatwaves are increasing in frequency and intensity, driving persistent changes in kelp forests worldwide. Along the Pacific coast of Baja California, the 2014-2016 marine heatwave produced contrasting post-disturbance states, from persistent urchin barrens to understory-dominated reefs and declining canopy kelp. Kelp forests support reef-fish assemblages that sustain key ecosystem functions, yet how these functions track habitat reorganization after extreme warming remains unclear. Because no pre-heatwave baseline exists for the region, we examined post-disturbance trajectories from 2017 onward without assuming a common starting state across subregions. Methods: We analyzed underwater visual census data from 2017 to 2024 across 12 sites spanning three biogeographic subregions (North, Middle, South) along ∼600 km of coastline. We quantified temporal changes in algae, invertebrates, and reef fishes, and assessed fish assemblages using a trait-based framework built on nine ecological traits, including body size, trophic group, growth performance, and maximum fecundity. Taxonomic and functional α-diversity were estimated as biomass-weighted Hill numbers, and β-diversity as temporal β-decay (dissimilarity over time since 2017). Species and traits driving compositional change were identified Results: Reefs followed divergent post-heatwave trajectories across subregions. Fish species richness and functional α-diversity remained stable everywhere, yet this stability masked contrasting regional dynamics in composition. In the North, assemblages showed minimal temporal change, consistent with a persistent urchin-barren configuration. In the Middle, functional β-decay increased over time, driven by shifts in trophic, growth, and body-size traits. The South showed the broadest reorganization, with significant increases in both taxonomic and functional β-decay and the strongest trait signal in maximum fecundity. Macroinvertivore biomass declined significantly in the North and showed a pronounced, though non-significant, decline in the South, with no compensatory increase in P/B among other functional groups. Our findings reveal a decoupling between biodiversity and ecosystem functioning in post-heatwave kelp-forest fish assemblages. Stable diversity metrics can conceal substantial reorganization in how assemblages produce and sustain biomass, and the biomass-dominant groups that underpin ecosystem processes are declining without compensation. Safeguarding these processes will require shifting from tracking diversity alone to directly monitoring and managing functional groups and biomass dynamics.