Global Change Is Reshaping Coral Communities and Threatening Reef Physical Functionality.

Coral reefs are undergoing profound compositional changes driven by climate change, disease, and chronic human disturbance. Coral cover is the primary metric used to track reef condition, but it often fails to capture the contributions of different coral species to reef carbonate production, structural complexity, and associated biodiversity. Here, I review evidence showing that global change disproportionately removes competitively dominant and architecturally complex corals, driving non-random changes and coral community homogenisation. These changes erode reef functionality, even when overall cover is maintained. Species-specific contributions to calcification and structural complexity span orders of magnitude, such that targeted losses of framework-building taxa collapse carbonate budgets and habitat architecture in ways not predictable from cover trends. These structural changes cascade through associated communities, undermining functional diversity and the ecosystem processes on which reef resilience depends. Critically, rebounds in coral cover frequently reflect the colonisation by fast-growing opportunistic species, which yield slow or absent recovery of physical functionality, while reassembly and the re-establishment of large, structurally complex colonies (and the physical functionality they provide) might lag by decades. Furthermore, as mass disturbance events occur at ever-shorter intervals, recovery windows close before reef-building potential and complexity can be fully restored. Monitoring, management, and restoration should therefore focus on trait- and species-explicit assessments that prioritise protection and restoration of keystone reef-builders, target sites with high functional potential, and reduce stressors at appropriate scales. Integrating functional metrics into policy and restoration planning is essential to safeguard reef geomorphic and ecological services under continued global change.