Rat cull urged on tropical islands where they are destroying coral reefs

‘Rat eradication should be a high conservation priority on islands. It could tip the balance of survival for reefs’

RATS must be wiped out on tropical islands to stop them killing off coral reefs, scientists have said.

A study has found that black rats introduced to tropical islands in the past 300 years have caused devastating damage to reefs.

Scientists assessed the impact of the rodents by comparing neighbouring islands in the Indian Ocean that were infested with those which remained free of rats.

Their findings, published in the journal Nature, revealed that the ecological effects of rats destroying populations of sea birds extended beyond island shores to the surrounding reefs.

Rat-free islands had much 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 of that around islands that were rat-free, the researchers found.

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

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

“This guano is rich in 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.

“Getting rid of the rats would be likely to benefit terrestrial ecosystems and enhance coral reef productivity and functioning.

“It could tip the balance for the future survival of these coral 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.