A new study led by associate professor Kelton McMahon at the University of Rhode Island Graduate School of Oceanography has found that food webs on tropical reefs are more fragile than once thought.

Instead of being part of a highly connected system where species can easily switch food sources, many reef creatures in these incredibly biodiverse ecosystems rely on surprisingly narrow, specialized energy pathways that link specific species to distinct sources of primary production.

Using compound-specific stable isotope analysis of amino acids — a cutting-edge technique McMahon helped pioneer that allows scientists to follow the path of nutrients as they flow through ecosystems — the team from his Ocean Ecogeochemistry Lab investigated three common reef-dwelling snapper species: Lutjanus kasmira, L. ehrenbergii, and L. fulviflamma. Though previously considered opportunistic predators, these fish, and the food webs supporting them, turned out to be remarkably specialized.

Lutjanus kasmira fed almost exclusively within a food web based on water column phytoplankton. L. ehrenbergii was tied to a macroalgae-based food web on the seafloor. L. fulviflamma primarily fed within a coral-based food web.

“When you dive on these beautiful Red Sea reefs, one of the first things that you’ll notice is these snapper species schooling together in perfect synchrony,” McMahon said. “We would never have guessed that each had carved out its own unique niche within these complex, biodiverse reef food webs.”

As abundant predators, snappers were long assumed to be generalists, roaming the reef together and feeding broadly on whatever prey was available.

But the study’s findings reveal a different story: the flow of energy from primary producers low on the food chain, such as coral, macroalgae, and phytoplankton, to predators is highly compartmentalized. Basically, each species relies on a distinct “silo” of production, forming self-contained food chains within specific microhabitats on the reef.

“It’s one thing to see a species or two specializing on a specific food item, but to see entire food chains of potentially dozens or even hundreds of species form tight relationships connected to a single primary producer (e.g., macroalgae) when equally tasty coral is just inches away fundamentally reshapes how we think about biodiversity of coral reefs,” McMahon said.

This kind of separation within reef ecosystems challenges long-held assumptions that coral reefs are naturally resilient because they have so many species serving similar, interconnected roles. In systems where multiple species can perform the same role, the loss of one part doesn’t necessarily collapse the whole. But in siloed reef food webs, if a single primary producer is disturbed by climate change, overfishing, or bleaching, it can fracture an entire food chain.

That makes coral reef food webs more structured and more fragile than we thought, according to McMahon.