Highly siloed nutrient pathways fuel meso-predator fishes on coral reefs.

Ecologists have long sought mechanisms to explain the productivity and diversity of coral reef communities while simultaneously seeking to predict the vulnerability and resilience of said communities to environmental change. We used compound-specific stable isotope analysis to examine how different sources of primary production support coral reef food webs. We found multiple lines of evidence for distinct end-member use among three Lutjanid snapper species that are typically considered "generalist" predators and thought to feed on multiple production sources on reefs. Instead, we found that Lutjanus kasmira foraged predominantly in a water column-based phytoplankton food web (74% of carbon contribution; 95% credible interval (Crl): 62%-85%), whereas L. ehrenbergii (58% [Crl: 42%-73%]) and L. fulviflamma (55% [Crl: 44%-67%]) partitioned resource use between benthic macroalgal and coral sources, respectively. These results indicate the existence of highly siloed carbon pathways in reef food webs across at least three trophic levels, with little mixing of primary producers among species. This siloing necessitates that the intermediary (primary and secondary) consumers in these food webs forage within the same tight energy silos, likely indicating the existence of strongly maintained microhabitats on the reefs that expose consumers within these compartments to prey items linked to different primary producers through isolated food web interactions. This work identifies important resource dimensions along which species separate, providing a compelling explanation for the remarkable diversity of coral reef fishes while simultaneously raising questions about the vulnerability of food web compartments to disturbances that threaten large-scale shifts in benthic community composition.