Food chains in the coral reefs of the Caribbean are up to 70% shorter today than in the past, and fish living there have a much more limited selection of items to eat (Nature 2026; DOI: 10.1038/s41586-025-10077-z). These findings are based on stable isotope analysis to compare the ecosystems today with those 7,000 years ago.
Coral reefs are marine biodiversity hotspots and integral to ocean ecosystems. They comprise essential habitats for at least a quarter of marine species, and are also connected to the well-being, including storm protection and nutrition, of around 13% of the global human population. Yet coral reefs are under existential threat from human activity.
In the new study, researchers used a stable isotope analysis technique developed by Jessica Lueders-Dumont, a biogeochemist at Princeton University, that measures δ15N —the ratio of the two stable isotopes of nitrogen, 15N and 14N. The researchers used this technique on prehistoric and modern fossils to analyze 133 fish otoliths (inner-ear crystals) and co-occurring corals. The samples came from sediments in Panama and the Dominican Republic.
Organic nitrogen is present in the proteins of skeletal remains of organisms in the coral reef ecosystem. Sambuddha Misra, a chemical oceanographer from the Indian Institute of Science, Bengaluru, who wasn’t part of the study, says that nitrogen is a classical marker of how energy moves from one level of the food chain to another. “Lighter isotopes, like 14N—compared to 15N—will form a weaker bond,” he says, “and biology doesn’t want to spend energy to break bonds.” Thus, as molecules move up the food chain toward apex predators, they are progressively depleted in heavy isotopes. Using a nitrogen isotope marker, it is possible to pinpoint where a particular organism sits in the food chain.
By measuring nitrogen ratios, Lueders-Dumont says, the team found that at degraded reefs, food chains are shorter than they used to be in prehistoric times as there are fewer species still around. Reef fishes also have less diverse food options compared to prehistoric times. And that matters for the robustness of the ecosystem. “If there’s some disruption to the supply chain, then all of the fish will be affected, which makes the whole ecosystem much more vulnerable,” she says.
The researchers also noticed changes in fish behavior in response to their environment being changed. For example, anthropogenic activity has removed a lot of the top predators, like sharks, which means smaller prey fishes go out and forage.
As the structural complexity of the reef itself has declined, resulting in fewer microhabitats, and the diversity of habitats near and around reefs has also declined, “fish on modern coral reefs are foraging in such a way that they are all consuming from the same menu,” Lueders-Dumont says. “In the past, there was a greater complexity and diversity of habitats in which they would forage and become highly specialized.” The study observed a 20–70% reduction in dietary variation of reef fishes.
Misra calls the study robust and says the nitrogen isotope analysis toolbox is fascinating. He says that the quantitative demonstration of the change in dietary variation in coral reefs from before human intervention to after is a key finding: “This is something that one had suspected, but they are kind of showing it,” he says.
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