Invasive Rats Disrupt Coral Reef Food Webs by Decimating Seabirds

Invasive Rats and the Disrupted Seabird Nutrient Pipeline in the Indian Ocean

On remote tropical islands, invasive rats arrived as stowaways on ships hundreds of years ago, establishing populations that prey heavily on seabird eggs, chicks, and occasionally adult birds. This predation drives a massive wedge into marine-terrestrial ecosystems.

The ecological contrast between these two environments centers on seabird density. These massive colonies play a vital ecological transport role. Feeding in the open ocean and returning to land to roost, breed, and nest, seabirds accumulate and deposit vast quantities of nutrient-rich droppings known as guano.

When seasonal rains wash that guano off the islands and into the sea, it acts as a powerful natural fertilizer for the surrounding coral reef ecosystem. By wiping out the birds, invasive rats sever this critical lifeline, starving the adjacent waters of the nutrients they have evolved to rely on.

Cryptobenthic Fish Versus Invertebrates Around Chagos Archipelago Reefs

The loss of seabird guano ripples down to the smallest creatures inhabiting the seafloor. Marine scientists examined reef cryptofauna—tiny fish and invertebrates living in and around coral near the benthos. These organisms, often overlooked by casual observers, form the cornerstone of energy transfer on coral reefs.

Around rat-free islands where nutrients flow freely, reefs are dominated by tiny cryptobenthic fish species such as gobies and triplefins. The biomass of these small fish is more than five times greater than that of the small invertebrates sharing their habitat. But the script flips entirely near rat-infested islands. In those nutrient-deprived waters, small invertebrates like coral crabs, porcelain crabs, and snapping shrimps make up a much larger share of the reef community, bringing the biomass of cryptobenthic fish and invertebrates nearly equal.

Laura-Li Jeannot, lead author of the study and a PhD researcher at Lancaster University, noted that the ecological consequences of nutrient loss are nuanced. Contrary to our expectations, the low-nutrient conditions that rats create do not negatively affect all reef organisms, Jeannot said, adding that nutrient-loss impacts are not uniform. There are winners and losers.

How Nutrient Shifts Rewire Underwater Food Webs

The mechanisms driving this shift involve direct biological competition and metabolic demands. Cryptobenthic fish thrive in nutrient-rich environments because of specific physiological traits.

“Cryptobenthic reef fishes are characterised by extraordinarily high rates of growth and reproduction. As a result, they have high nutrient and energy requirements, and those can be matched in nutrient-rich environments. This allows these fish populations to flourish and expand in favourable conditions – such as seabird-fertilised reefs.”

Laura-Li Jeannot, PhD researcher at Lancaster University, via Indiatimes

As these fish populations expand on seabird-fertilized reefs, they colonize spaces that would otherwise belong to cryptic invertebrates. Jeannot explained that beyond displacing invertebrates from their habitats, the fish outcompete them for food resources, forcing invertebrates to seek alternate, less preferred sustenance.

Conversely, when rats cut off the seabird guano supply, cryptobenthic fish populations decline due to a lack of resources. This drop in fish abundance relieves competitive pressure on invertebrates. Furthermore, the lower metabolic rates of invertebrates give them an advantage in nutrient-poor waters, allowing them to maintain their biomass without requiring high energy inputs.

Broader Ecological Stakes and Conservation Prioritization

These microscopic shifts carry major implications for overall ocean health. These nearly invisible reef animals help keep larger marine food webs running by transferring energy upward to bigger fish and the wider ecosystem.

According to the research team, when seabird nutrients are added or removed, it causes a fundamental rewiring of how energy travels up food chains, reconfiguring larger fish communities. Because coral reefs support food systems, local livelihoods, and natural coastal resilience, weakened food webs leave already stressed ecosystems less able to recover from disturbances.

These findings offer conservation planners actionable intelligence. Protecting seabirds and restoring islands by removing invasive rats can help reinstate the natural nutrient flow that nearby reefs depend on. Rather than evaluating marine health in isolation, resource managers can use these insights to account for the deep interconnections between island wildlife, terrestrial nutrient cycles, and underwater biodiversity.