Bounded ecological memory in coral reefs: divergent demographic pathways under recurrent heatwaves and collapse under extreme thermal stress

Anthropogenic warming is causing marine heatwaves (MHWs) to affect coral reefs, leading to variable responses to similar thermal stress levels. Researchers have been investigating whether reduced bleaching indicates acclimatization-like persistence or mortality-driven filtering resulting in demographically depleted coral assemblages.

By surveying individual corals across the Florida Reef Tract from 2014 to 2023, they found that bleaching sensitivity had decreased over time, suggesting an ecological memory of prior exposure.

The study used a spatial matched-event framework to assess bleaching severity and adult colony density, revealing two different pathways that could explain the reduced bleaching. The first pathway involves demographic persistence, where coral densities remain stable or increase. The second pathway is mortality-filtered tolerance, where coral densities decrease.

Persistence was more likely when thermal exposure was predictable, characterized by lower interannual variability. This persistence was driven by weedy species like Porites replacing the historical coral framework builders, meaning stability results from reassembly rather than uniform increases in thermal tolerance.

However, the unprecedented severity of the 2023 MHW broke down this acclimatization-like buffering, causing widespread sensitization and collapse of previously persistent coral networks. The findings support a bounded ecological memory framework, where prior exposure reduces bleaching sensitivity under predictable thermal conditions but fails under extreme heat stress.

Therefore, modern refugia should be defined by adult standing stock retention, representing systems undergoing selective ecological reassembly rather than full functional recovery.