How Rats on Tropical Islands Are Quietly Dismantling Coral Reef Ecosystems

A Chain Reaction Spanning Land and Sea

It sounds like an unlikely connection: rats living on a remote tropical island dictating which fish thrive on a nearby coral reef. But a striking new study has revealed exactly that kind of long-distance ecological domino effect, tracing a nutrient pipeline from the open ocean, through seabirds, across island soil, and down into the reef below — and showing what happens when rats break that pipeline entirely.

Scientists from Lancaster University and the University of Texas examined reef cryptofauna — tiny fish and invertebrates that live in and around coral near the seafloor — in a recently published study in *Ecology* magazine. In the Chagos Archipelago, a remote chain of 60 islands in the Indian Ocean, the team compared reefs beside rat-free islands with reefs next to islands infested by invasive rats and found major differences between them.

Birds, Droppings, and the Nutrients That Feed a Reef

Seabirds travel out into the open ocean to feed and return to nest on islands, depositing nutrients through their droppings onto the land — and many of those nutrients are subsequently washed into the sea, fertilizing the surrounding coral reef ecosystems. Seabirds such as boobies, noddies, and shearwaters connect the islands to the open ocean, in some cases traveling hundreds of kilometers to feed on fish there, picking up a characteristic isotope of nitrogen along the way. Researchers traced that nitrogen's path to the islands, finding the isotope in soils, plants, algae, sponges, and fish living on the reef.

Invasive rats, which arrived as stowaways on ships hundreds of years ago, have decimated seabird populations on the islands by eating their eggs, chicks, and sometimes even adult seabirds. On islands without rats, the density of seabird populations is a staggering 760 times greater. That collapse in bird life translates almost directly into a collapse in reef nutrition. The result is a drop-off of nutrients in the seas surrounding rat-infested islands, with 251 times less nitrogen flowing onto the coral reefs around these islands.

Winners, Losers, and a Rewired Food Web

On reefs beside rat-free islands, tiny bottom-dwelling fish such as gobies and triplefins had more than five times the biomass of the small invertebrates sharing the same space. Around rat-infested islands, however, researchers found a much higher proportion of small invertebrates, such as coral crabs, porcelain crabs, and snapping shrimp, with cryptobenthic fish and invertebrate biomass nearly equal. That shift matters far more than it might seem.

Cryptobenthic fish are estimated to make up as much as 60% of all the biomass eaten on a coral reef, so a shift in which animals dominate ripples upward. Mixed carnivorous fish relied more heavily on cryptobenthic fish for food near seabird-rich islands, and fish-eating predators thrived alongside them. "Cryptobenthic fish are a nutrient-dense, protein-packed, highly digestible resource for predators. On the other hand, much of invertebrates' mass is represented by a tough outer shell," said co-investigator Simon Brandl of the University of Texas.

What This Means for Conservation

As one researcher noted, "the main mode of seabird nutrients moving up food webs is through fish, which feed both specialized fish-eating predators and more generalized carnivores" — and when seabird nutrients are added or taken away, there is a rewiring of how energy travels up food webs and a reconfiguration of larger fish communities. Coral reefs help support food systems, local livelihoods, and natural coastal resilience, so weakening reef food webs can leave already stressed ecosystems less able to recover.

These results have important conservation implications: safeguarding seabirds and their nutrient subsidies is also a way to safeguard the integrity of reef food webs. Research has shown that removing invasive rats and restoring native vegetation could help bring back hundreds of thousands of breeding pairs of seabirds lost to tropical islands. The message from this research is clear — protecting a reef may require starting on dry land, with something as unglamorous as a rat trap.