Rats on remote tropical islands could DESTROY coral reefs

Coral reefs could be destroyed by RATS: Rodents on tropical islands can cause devastating damage to marine life by killing birds and starving the environment of nutrients

  • Experts studied the Chagos Archipelago a group of 7 atolls in the Indian Ocean
  • Rats were introduced to some of the islands whereas others remained rat-free  
  • Experts found that found rat-free island had up to 760 times more bird life
  • This in turn boosts the health of reefs, with more algae, seaweed and fish 
  • They are able to flourish thanks to the fertiliser provided by the bird’s droppings
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Rats devastate bird populations on tropical islands, with the knock on effect of damaging nearby coral reefs, a new study suggests.

It has long been known that rats lay waste to bird colonies on islands, by eating their eggs and young.

Experts found that found rat-free islands in the Indian Ocean had up to 760 times more birds than those where rats run free. 

This positive effect spilled over into the sea – with nearly 50 per cent more fish living near islands fertilised by seabirds than those infested with rats.

This in turn boosts the health of reefs, with more algae, seaweed and fish able to flourish thanks to the natural fertiliser provided by the bird’s droppings.

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Tropical islands where rats have been killed off boost fish numbers on coral reefs nearby. It has long been known that rats devastate bird populations on islands – eating eggs and young. But the effect spilled over into the sea – with nearly 50 per cent more fish living near the islands

Seabirds that feed in the open ocean transport large quantities of nutrients onto nearby islands in the form of guano, or droppings.

Scientists found fish get the boost because the guano acts as a natural fertiliser which enriches the ocean and encourages seaweed and algae to grow, which in turn promotes fish life. 

Professor Nick Graham of Lancaster University and colleagues studied the northern atolls of the Chagos Archipelago in the Indian Ocean.

These islands are currently uninhabited by humans.

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Rats were introduced to some in the 18th and 19th centuries, whereas others remained rat-free.

Professor Graham 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 nutrients, nitrogen and phosphorus. Until now, we didn’t know to what extent this made a difference to adjacent coral reefs.’

On the atolls, which play host to boobies, frigatebirds, noddies, shearwaters, terns and tropicbirds, the authors found that seabird densities were 760 times higher without rats.


Coral reefs are under threat across the world. The findings highlight the importance of killing off rats to help conserve wildlife, the researchers said

HOW DO RATS DECIMATE TROPICAL ISLANDS AND THEIR REEFS?

It has long been known that rats lay waste to bird colonies on islands, by eating their eggs and young.

But seabirds that feed in the open ocean transport large quantities of nutrients onto nearby islands in the form of guano, or droppings.

This acts as a natural fertiliser which enriches the ocean and encourages seaweed and algae to grow, which in turn promotes fish life.

This in turn boosts the health of reefs, with more algae, seaweed and fish able to flourish thanks to the natural fertiliser provided by the bird’s droppings.

Experts found that found rat-free islands in the Indian Ocean had up to 760 times more birds than those where rats run free.

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.

The findings highlight the importance of killing off rats to help conserve wildlife, the researchers said.

One of the study authors, Associate Professor Aaron MacNeil from Dalhousie University, Canada, said: ‘These results show how conservation can sometimes be a bloody business, where doing right by the ecosystem means there is a time to kill. For these invasive rats, that time is now.’

With potential benefits for not only island ecosystems but also nearby coral reefs, eradicating island rat populations to restore seabird-sourced nutrients should be a priority, the authors conclude.

Coral reefs are under threat across the world.

Nancy Knowlton, a professor of Zoology at the Smithsonian Institution said the findings ‘added rats to the list of dangers to reefs’.

The findings are published in this week’s Nature. 


With potential benefits for not only island ecosystems but also nearby coral reefs, eradicating island rat populations to restore seabird-sourced nutrients should be a priority, the authors conclude (stock image) 

WHAT IS CORAL BLEACHING?

Corals have a symbiotic relationship with a tiny marine algae called ‘zooxanthellae’ that live inside and nourish them. 

When sea surface temperatures rise, corals expel the colourful algae. The loss of the algae causes them to bleach and turn white. 

This bleached states can last for up to six weeks, and while corals can recover if the temperature drops and the algae return, severely bleached corals die, and become covered by algae. 

In either case, this makes it hard to distinguish between healthy corals and dead corals from satellite images.

This bleaching recently killed up to 80 per cent of corals in some areas of the Great Barrier Reef.

Bleaching events of this nature are happening worldwide four times more frequently than they used to. 


An aerial view of Australia’s Great Barrier Reef. The corals of the Great Barrier Reef have undergone two successive bleaching events, in 2016 and earlier this year, raising experts’ concerns about the capacity for reefs to survive under global-warming

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