Coral formations can appear quite formidable. They construct vast limestone edifices that might stretch for miles and persist for millennia. However, within these vibrant underwater metropolises, a fragile process is unfolding.
A recent investigation reveals that human activity has subtly eroded the very food webs sustaining coral reef life. The study’s findings were published in the journal Nature.
Energy that previously flowed freely from the smallest organisms to small fish and onward to larger predators now navigates a more truncated and simplified system. This simplification of complexity potentially increases the likelihood of reef collapse.
For years, scientists monitored coral bleaching, escalating ocean temperatures, and excessive fishing. Yet, they struggled to quantify what these reefs looked like prior to significant anthropogenic impact. Without this baseline information, grasping the true extent of the changes proved difficult.
To probe this question, researchers turned to an unlikely source: the ear bones of fish. These minute structures, known as otoliths, assist fish with hearing and balance. Composed of calcium carbonate, they fossilize remarkably well within oceanic sediments.
Over countless ages, fish feed, live their lives, and eventually their otoliths settle onto the seabed. Layer by layer, they form a historical record.
The research team, headed by Jessica Lüders-Dumon of Boston College, examined otoliths preserved in fossil reef deposits dating back approximately 7,000 years.
Working alongside collaborators, including Professor Sinchen (Tony) Wang, the team devised and employed a nitrogen isotope analysis technique to scrutinize the proteins embedded within these ancient ear stones and coral skeletons.
The group analyzed 136 fish otoliths and dozens of corals from fossil sites in Panama and the Dominican Republic.
These ancient ear bones were benchmarked against modern specimens collected from the same Caribbean locales. This region represents one of the world’s most degraded coral reef ecosystems, having seen stony coral cover drop by over 50 percent in recent decades.
By assessing nitrogen isotopes, the scientists could determine the trophic level, which pinpoints an organism’s position within the food chain. Higher values indicate that the fish consumed prey higher up that chain.
The outcomes were striking. Compared to the “pristine” coral reef ecosystems before widespread human influence, present-day Caribbean coral reefs host food chains that are 60 to 70 percent shorter, and fish communities exhibiting 20 to 70 percent less functional diversity.
This signifies fewer intermediate steps existing between the smallest prey items and the apex predators. It also suggests that fish communities now fulfill fewer distinct roles.
“We found that on healthier Caribbean reefs, fish communities exploited a wider array of food sources,” noted Lüders-Dumon. “On degraded reefs, the diet has become homogenized—different fish are increasingly subsisting on the same limited pool of resources. Previously, individual fish could afford to be selective; today, many must take what is available. It’s akin to a bustling district filled with diverse restaurants developing into just one, simplified menu.”
The research group focused on some of the most common small fish found in the paleontological archive, including gobies, silversides, and cardinalfish.
“These fish are crucial prey on reefs—they are essentially the ‘potato chips of the reef,'” Lüders-Dumon stated. “Over millennia, they were consumed, and their otoliths were deposited, accumulating in the sedimentary rock.”
The researchers had anticipated major shifts among apex predators, which are often targeted by fisheries. Instead, they detected substantial alterations even among fish situated lower on the food web.
“Because these isotopic signatures reflect the organism’s position in the food chain, analyzing multiple fish and coral groups from the same fossil reefs allows us to quantitatively reconstruct the reef’s food web structure before large-scale human impact began,” explained Wang. “Previously, this approach was limited by the minuscule amounts of protein preserved in fossils, but recent advancements in our methodologies allow us to apply it to fossil reef communities for the first time. It’s akin to ancient DNA, but instead of genes, we are using the chemical signatures encoded in ancient proteins.”
When reefs lose their complex three-dimensional structure, they offer less shelter and fewer feeding opportunities. This ripple effect can negatively impact every level of the ecosystem.
“These findings demonstrate that anthropogenic impacts—such as the loss of apex predators, reduced connectivity between different habitat types, the structural simplification of coral reefs, and other factors affecting modern reefs—have altered energy flow throughout the entire food web structure,” Lüders-Dumon concluded.
Coral reefs support the existence of at least a quarter of all marine species. They also shield coastlines from storms and provide sustenance for millions of people. As their food webs contract, so too does their resilience.
Conservation targets for coral reefs are often established by scientists based on what they observe in the present day. However, if reefs are already profoundly altered, these goals might prove unattainable.
“We can now observe what ecologically intact coral reef ecosystems looked like before human intervention,” said Lüders-Dumon. “Since our previous biodiversity preservation benchmarks were set by already degraded modern reefs, the ability to restore to ancient baseline metrics opens an entirely new perspective on what constitutes a healthy reef ecosystem, and how we might achieve its recovery.”
The past is literally etched in stone. And within these tiny ear bones, scientists have unearthed both a warning about what has been lost and a clearer objective for what restoration could look like.
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