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Tropical islands without rats have healthier coral reefs

An international team of scientists led by Lancaster University is reporting that rats must be eliminated on many tropical islands in an effort to protect coral reefs. Invasive rats are killing off seabirds in large numbers, and the researchers have identified previously undetermined consequences for the coral reefs that surround and protect the islands.

While it has been documented that invasive predators such as rats have annihilated seabird populations across most tropical islands, the impact that this was having on coral reefs was not known prior to this study.

The team investigated the health of tropical ecosystems in the northern reefs of the Chagos Archipelago.

“Seabirds are crucial to these kinds of islands because they are able to fly to highly productive areas of open ocean to feed,” said study lead author Professor Nick Graham.

“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.”

Located in the central region of the Indian Ocean, the Chagos islands were ideal for this study because some of the islands are infested with black rats, while others are completely free of rats. The researchers compared the ecosystems surrounding six islands with rats and six rat-free islands.

The study revealed severe ecological damage caused by the rats that extended beyond the islands and into the sea. The experts found that islands without rats had substantially more seabirds as well as nitrogen in the soil. In the ocean, the increased nitrogen benefited macroalgae, filter-feeding sponges, turf algae, and fish on the adjacent coral reefs.

Fish were found to be about 50 percent more abundant in the waters adjacent to rat-free islands. In addition, the consumption of algae and dead coral, a replenishing process known as grazing, was over three times higher in the sea adjacent to islands that were free of rats.

“The results of this study are clear. Rat eradication should be a high conservation priority on oceanic islands,” said Professor Graham.

“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 study is published in the journal Nature.

By Chrissy Sexton, Staff Writer

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