Scientists from the United States discovered that small reef fish thrived near islands teeming with seabirds; where rats drove the birds away, the underwater food chain was forever changed

A group of scientists has found a surprising connection between seabirds, invasive rats and tiny reef fish.

The research, led by specialists from Lancaster University and the University of Texas at Austin, analyzed remote islands in the Chagos Archipelago in the Indian Ocean to compare reefs near islands with invasive rats to reefs beside rat-free islands.

The invisible role of seabirds

The key lies in nutrients. Seabirds feed in the open ocean and then return to the islands to rest, nest and breed. Their droppings, known as guano, transport nutrients from the ocean to the land. With rain and runoff, some of those nutrients flow back into the sea and fertilize nearby reefs.

But when invasive rats arrive, the cycle breaks down. The University of Texas explained that rats, historically introduced as stowaways on ships, eat eggs, chicks and even adult birds. On rat-free islands, seabird density is 760 times greater than on invaded islands.

This terrestrial change is reflected underwater. The researchers studied cryptofauna — the collection of tiny fish and invertebrates that live on the reef floor and are typically overlooked despite their importance in energy transfer.

On reefs near rat-free islands, small cryptobenthic fish dominated, such as gobies and triplefins. Their biomass was more than five times greater than that of small invertebrates. Around islands with rats, however, scientists found a higher proportion of coral crabs, porcelain crabs and pistol shrimp, with a more balanced biomass between tiny fish and invertebrates.

A food chain that takes a different path

Laura-Li Jeannot, the study's lead author, summarized the finding with the following statement: "The impacts of nutrient loss are not uniform. There are winners and losers."

The result is significant because these small fish are a key prey source. Simon Brandl, of the Marine Science Institute at the University of Texas, explained that cryptobenthic fish are a source that is "nutrient-rich, protein-packed and highly digestible" for predators; by comparison, much of the mass of many invertebrates is contained in hard, less nutritious shells.

A study published in 2018 in Nature had already demonstrated the scale of the phenomenon: on rat-free islands, reef fish communities had 48% greater total biomass, and functions such as grazing and bioerosion were several times higher.

Ultimately, rats do not only damage nests on land. By displacing seabirds, they cut off a flow of nutrients that reaches the reef and alters which species thrive. The ecological lesson is clear: protecting birds on remote islands can also be a way of protecting underwater life.

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