Winners and losers of reef flattening: An assessment of coral reef fish species and traits

Anthropogenic stressors are causing widespread coral mortality, leading to loss of coral cover and decreased structural complexity that threatens reef biodiversity, functioning, and ecosystem services. Reef fishes are intimately linked to coral reef complexity, but we lack a generic understanding of which species are particularly affected by reef flattening and what traits make them susceptible. We used extensive species- and trait-based analyses to build a framework for western Atlantic fish association with both structural complexity and coral cover to better understand the implications of reef degradation. These analyses also investigated the relative importance of live coral versus the value of the structure it provides to reef fishes. We modeled how 25 biophysical and anthropogenic factors correlated with the densities of 109 fish species across 3292 Floridian reef sites. The importance of a metric of structural complexity and coral cover to the abundance of each species was then isolated. Species with positive associations were categorized as likely future 'losers' and negative associations as 'winners'. We showed that structure loss was more critical than loss of coral cover, as 53% of species were predicted as losers on low-relief reefs, while only 11% were losers with decreased coral cover. We found morphological, behavioral, and ecological traits mediate species' responses to reef degradation, and shared evolutionary history is unlikely to be a strong driver of trait relationships. Eight traits explained 79.7% of variation in species' associations with relief and six traits explained 27.8% of associations with coral cover. Smaller, streamlined, habitat and trophic generalists are more likely winners on flattened reefs and large-bodied predators and species with deeper bodies and intermediate caudal fin shapes are likely losers. Identifying these important traits provides insight into mechanisms that may link fish and complex habitats, which allows us to better predict assemblage-wide responses to future reef flattening.