Coral reefs damaged by ecological effects of rat invasion

Black rats introduced to tropical islands in the past 300 years have caused devastating damage to coral reefs, a study has found.

Scientists assessed the impact of the rodents by comparing neighbouring islands in the Indian Ocean that were either infested or remained rat free.

They showed that the ecological knock-on effects of rats destroying sea bird populations extended beyond island shores to the surrounding reefs.

Rat-free islands had significantly more bird life, resulting in greater amounts of nitrogen from bird droppings entering the soil and ending up in the sea.

The nitrogen fuelled the growth of micro-algae, which in turn benefited filter-feeding sponges and fish further up the food chain.

Fish biomass near rat-infested islands was half what it was around those that were rat-free.

Lead scientist Professor Nick Graham, from the University of Lancaster, said: “Seabirds are crucial to these kinds of islands because they are able to fly to highly productive areas of open ocean to feed.

“They then return to their island homes where they roost and breed, depositing guano, or bird droppings, on the soil.

“This guano is rich in the nutrients, nitrogen and phosphorus.

“Until now, we didn’t know to what extent this made a difference to adjacent coral reefs.

“The results of this study are clear.

“Rat eradication should be a high conservation priority on oceanic islands.

“Getting rid of the rats would be likely to benefit terrestrial ecosystems and enhance coral reef productivity and functioning by restoring seabird derived nutrient subsidies from large areas of ocean.

“It could tip the balance for the future survival of these reefs and their ecosystems.”

The remote Chagos islands in the central Indian Ocean acted as a “natural laboratory” where the ecological impact of invasive rats could be studied, said the researchers.

Some of the islands were rat free while others were infested with rodents thought to have been introduced by visiting sailing ships in the late 1700s and early 1800s.

As a result, the scientists were able to make direct comparisons between neighbouring reef ecosystems.

The study, published in the journal Nature, found that algae grazing, an important role played by fish providing a stable base for new coral growth, was 3.2 times higher off rat-free islands.

Rats are voracious bird predators, feeding on eggs, chicks and even adult birds.

They are estimated to have decimated seabird populations on 90% of the world’s temperate and tropical island groups, said the scientists.

However, until now the extent of the damage they have caused to coral reefs was not known.