Coral reef food webs are more fragile than we thought

Coral reefs have long been seen as flexible and resilient – bustling underwater cities where species can shift diets, share space, and adapt to change. But a new study challenges that assumption.

The research reveals that many reef animals are far less adaptable than expected, relying on narrow energy pathways linked to specific food sources. In other words, these creatures may live side-by-side, but they aren’t eating from the same buffet.

Led by scientists at the University of Rhode Island’s Graduate School of Oceanography, the study focused on three common species of snapper in the Red Sea: Lutjanus kasmira, L. ehrenbergii, and L. fulviflamma.

All three were believed to be generalist predators, able to switch diets and thrive on a mix of prey. But when researchers tracked what these fish were really eating, the results told a different story.

Each fish has its own menu

The team used a high-resolution method called compound-specific stable isotope analysis of amino acids (CSIA-AA).

This technique lets scientists trace how carbon and nitrogen – key building blocks of life – move through an ecosystem over time. It’s not a short-term look at diet; it’s a long-term window into how energy flows.

What they found was striking: Lutjanus kasmira fed almost entirely on plankton drifting in the water column, L. ehrenbergii was tied to food chains rooted in macroalgae on the seafloor, and L. fulviflamma relied mainly on energy drawn from coral-based systems.

“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. We would never have guessed that each had carved out its own unique niche within these complex, biodiverse reef food webs,” said Professor Kelton McMahon.

These species aren’t opportunistic feeders hopping from one source to another. They’re loyal to specific food channels. And it’s not just the snappers – entire food webs are locked into these narrow lanes.

All or nothing coral food webs

In most ecosystems, we expect redundancy – multiple species doing similar jobs, drawing from shared resources. This overlap can make a system resilient. If one part fails, others can fill in.

But on these reefs, that overlap is missing. The study shows energy moves in what the team calls “vertical silos.” Each silo begins with a different primary producer – phytoplankton, macroalgae, or coral – and ends with specific predators.

“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,” said McMahon.

That rigid structure comes with risk. If one primary producer is lost – say, from bleaching, overfishing, or warming seas – it could disrupt the entire chain that depends on it.

Patterns in reef food webs

The research came from McMahon’s Ocean Ecogeochemistry Lab at URI, which studies how food moves through ecosystems and how animals’ diets reveal deeper environmental patterns.

The lab uses CSIA-AA to go beyond what an animal ate last week – it helps reveal what’s sustained them over time.

“People have used isotopes to understand food webs for nearly a century, where they turn an organism into a single isotope value,” said McMahon.

“I’ve spent my career developing knowledge and tools to isolate and analyze all individual compounds within complex organisms, unlocking a metabolic history of organisms in a way we have never done before.”

“That gives us the power to track where different sources of energy come from, and, in doing so, reveal patterns in the food web we couldn’t see before.”

New insights with advanced tools

The samples used in this study were collected over 15 years ago. But the analytical tools didn’t yet exist to make sense of them – until now.

“I collected these samples 15 years ago as a postdoc at King Abdullah University of Science and Technology,” shared McMahon.

“The data were there, but I didn’t yet have the tools or perspective to make sense of it in a meaningful way.”

“Sometimes, you have to let an idea sit until the knowledge and methods mature enough to be impactful. Finally, I feel prepared to handle this work in the way it deserves, and the outcome definitely rewarded that patience.”

Solving complex ecological problems

This research opens up a new way to look at ecosystems – not just reefs, but possibly kelp forests, deep-sea environments, and coastal systems too.

The team plans to apply this approach to other habitats and use DNA metabarcoding to better understand which prey are powering these energy silos.

“I get excited about opening up access to knowledge that people have been seeking for a long time,” said McMahon.

“In my lab, we don’t specialize on a system or species; we’ve worked in Antarctica with penguins, on ancient human diet in MesoAmerica, and locally with the burgeoning jonah crab fishery.”

“Our goal is to develop tools that solve vexing ecological problems, and make sure diverse people and approaches are effectively brought together.”

The full study was published in the journal Current Biology.

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