On some tropical islands, rats are a threat to coral ecosystems

ON some tropical islands where rats are rife, the fish that populate coral reefs are likely to run out of food. These coral ecosystems are nourished in part by seabird droppings. The problem is that rats present en masse eat the eggs of these birds. As a knock-on effect, the fish are likely to lack nutrients to survive, explains a recent study.

From global warming and overfishing to water pollution, mining and mass tourism, threats to coral reefs and their ecosystems are numerous.

Essential to biodiversity and human health, 99% of the world's corals are unlikely to resist climate change, according to UK research published in February 2022 in the PLOS Climate journal. But a previously underestimated threat could be added to the long list of dangers facing coral ecosystems: rats.

Surprising as it may seem, these rodents may represent an indirect risk to coral, at least on some tropical islands, as detailed in a recent study published in the journal, Nature Ecology and Evolution. Researchers studied rats in the Chagos Archipelago, located in the northern Indian Ocean and consisting of seven atolls comprising nearly 60 islands.

In total, 10 islands across three atolls were studied. Half of them were infested by rats, which arrived at the same time as humans.

During their research, the study authors found that the black rat (Rattus rattus) had impacted coral ecosystems by eating the eggs (and sometimes even the chicks) laid by seabirds. These birds produce droppings that are a source of nutrients for the organisms living in the coral ecosystems.

The fact that the rats eat the birds' young reduces the number of birds and droppings, which disrupts the food chain and deprives the coral reef fishes of part of their food.

Researchers found that seabird densities around rat-free islands were 760 times higher compared to rat-infested islands. The solution, they say, would be to eradicate these invasive rat species.

"Rat eradication as a conservation strategy therefore has the potential to restore species interactions via territoriality, which can scale up to influence populations and communities at higher ecological levels," the researchers conclude.