Voracious rats putting coral reefs in danger

Christopher Davidson
July 14, 2018

"These results not only show the dramatic effect that rats can have on the composition of biological communities, but also on the way these vulnerable ecosystems function (or operate)", said co-author Dr Andrew Hoey from the ARC Centre of Excellence for Coral Reef Studies, Australia.

Bird droppings are naturally rich in nitrogen and they keep the reefs productive.

Rat control should be considered an urgent conservation priority on many remote tropical islands to protect vulnerable coral reefs, according to an global team of scientists.

Scientists now advocate eradicating rats from all of the islands as a way to minimise the threat.

The larger number of fish - the study showed almost 50 percent more fish biomass near the rat-free islands - nibbled on the algae and the dead corals within the adjacent reefs.

An extraordinary set of remote tropical islands in the central Indian Ocean, the Chagos islands provided a flawless "laboratory" setting as some of the islands are rat-free, while others are infested with black rats - thought to have been introduced in the late 1700s and early 1800s. "They then return to their island homes where they roost and breed, depositing guano - or bird droppings - on the soil".

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.

Researchers said that rat control should be considered a conservation priority to protect these coral reefs.

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"If you step onto an island with rats, there are next to no seabirds". Fewer birds on the invaded islands means less guano, the researchers write, which leads to less nitrogen seeping into the surrounding ocean to feed organisms such as plankton that form the basis of the food web.

Based on bird abundances, estimated defecation rates and the nitrogen content of droppings, the authors calculate that the birds on rat-free islands deposit about 250 times more nitrogen onto the land than on rat-infested ones.

Coral reefs cover less than 0.1% of the ocean's area, but house about one third of ocean biodiversity.

"Coral reefs are also hugely threatened", said Prof Graham. "So anyone who cares about extinctions and biodiversity needs to care about the future of coral reefs".

The reefs and their abundance of marine life provide livelihoods for millions of people around the world, so the decline in coral reefs is poised to become a humanitarian crisis.

Professor Graham said: "We also found that fish on the reefs adjacent to islands with seabirds were growing faster and were larger for their age than the fish on reefs next to rat-infested islands".

"This is one of the clearest examples so far, where eradicating rats will lead to increased numbers of seabirds and this will bolster the coral reef".

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