Weak trophic position-body mass relationships undermine simple size spectrum models for coral reefs

Unravelling food web dynamics across biological communities is a central goal of ecology. In size-structured ecosystems, the shape of trophic pyramids is often inferred from the distribution of biomass across body-mass classes, i.e. its size spectrum. Size-spectrum analysis has become a popular tool to study the ecosystem functioning of aquatic ecosystems, including highly diverse coral reefs. However, the key assumption behind size spectra, i.e. that body size directly and positively correlates with trophic position, has rarely been directly evaluated in these systems. Here, we test this assumption by quantifying body mass, population densities and estimating trophic position from stable isotopes for 325 fish species across four Indo-Pacific locations. Consistent with prior studies, we found a positive relationship between biomass and body mass. However, weak and variable relationships between body mass and trophic position led higher biomass in primary consumers than in predators, as expected in traditional bottom-heavy or diamond-shaped trophic structures. Our findings thus challenge previous reports of coral reef fish biomass prevalence in higher trophic levels (e.g., inverted biomass pyramids), suggesting that simple size spectrum models do not adequately represent the trophic structure of reef fish communities.