Coral Reefs

The Impact of Black Rats on Coral Reefs

The British Indian Ocean Territory includes the Chagos Archipelago and 58 small islands. Some of which are home to black rats (Rattus rattus), while others luckily remained rat-free.

Researchers in the Bertarelli Programme in Marine Science studied both islands, with and without rats, to see what effect the presence of rodents has on the health of the surrounding coral reefs.

The presence of back rats on islands in BIOT results from human exploration of our seas. Many islands around the world have been completely taken over by black rats that we accidentally introduced through ships years ago. Upon arrival on these seabirds-filled islands rats began to eat their eggs, chicks and even full-grown adults. This had disastrous consequences on native seabird populations on these invaded islands, 90% of tropical islands saw their bird population crash.

Thankfully, a small number of islands were spared and could act as a haven for the remaining frigate birds, boobies, terns and shearwaters. In BIOT, 18 of the 58 islands remained rat free.

Professor Nick Graham commented: “The rat-less islands are full of noisy birds, the sky is full, and the smell is strong – because of the bird’s guano deposits on the island. If you visit an island with rats, there is almost no seabirds. “ 

On rat-free islands, seabirds, including boobies, frigate birds, noddies, shearwaters and terns, roam for hundreds of miles to feed out in the open ocean. Upon their return to the island, they deposit rich nutrients from the fish they eat. These nutrients then spread into the surrounding waters and affected the biology of reef systems.

The results, published in Nature, are clear: the fish on reefs adjacent to the rat-less islands grew faster and bigger than that of the fish living in islands infested by rats.

Therefore, rat-filled islands have an indirect impact on the already fragile reef, since seabirds provide nutrient rich guano to the surrounding reef.

Since coral reefs are constantly affected by changes in ocean temperature and with coral bleaching frequency increasing, this research suggests that the elimination of invasive species such as rodents could be an important way to increase the resilience of these critical habitats while also restoring native populations of seabirds.

To date 500 islands have been gradually de-ratted and, according to Dr. Graham, 2-3M$ is necessary to fully de-rat BIOT.