The vulnerability paradox in Atlantic marine caves: A multiscale mechanistic explanation from wave exposure to cave gradients

High-diversity ecosystems are often presumed to be resilient. Yet, marine caves, along with other high-diversity systems, have proved highly sensitive to both climate change and anthropogenic stressors. This study seeks to resolve the underlying causes of this apparent paradox by investigating the functional resilience of marine caves ecosystems. We investigated nine Atlantic marine caves to identify key environmental drivers at landscape, local, and within-cave scales of sessile community structure and assess their functional resilience. Using hierarchical sampling (414 photoquadrats, 126 taxa), biological trait analysis (six traits), and statistical modelling (RLQ-Fourth Corner, GLMs, GAMMs), we assessed trait-environment relationships. Wave energy was the dominant driver among seascape and local-scale factors, explaining 34.5% of trait-environment associations, followed by depth range (7.7%). Four functional syndromes reflected major ecological trade-offs, such as autotrophs vs. active sponge feeders. Despite high regional taxonomic diversity (Shannon H' = 3.6) and functional richness (1.93), 57% of functional entities were represented by single taxa revealing low functional redundancy and thus low resilience potential. Functional vulnerability decreased and redundancy increased with spatial scale, with semi-dark zones being the most vulnerable cave sector (81%). Overall, wave exposure and depth shaped functional structure and resilience potential at a seascape scale in marine caves. The decoupling between taxonomic diversity and functional redundancy suggests that this vulnerability paradox may be a common pattern in high-diversity systems, as demonstrated in coral reefs and tropical forests, partially explaining their unexpected fragility under global change.