There’s A Secret Ingredient That Could Save Tropical Reefs But It's Not Good News For Rats


Katy Evans

Katy is Managing Editor at IFLScience where she oversees editorial content from News articles to Features, and even occasionally writes some.

Managing Editor

That secret ingredient turns out to be guano - droppings - from the local bird population, like this red-footed booby found on the Chagos Archipelago. Jele/Shutterstock

As the state of the world’s coral reefs appear to be getting worse with every passing year, an international team of scientists studying the reefs of tropical islands have come up with an unusual idea for how to help them: Kill all the nearby rats.

Studying the Chagos Archipelago in the Indian Ocean, the researchers found a surprising link between invasive rats on the island, the local seabird populations, and the health of the nearby reefs. They realized that the issue of introduced rats decimating the local bird populations was having an unforeseen effect on the health of the reefs.


It is estimated that invasive predators like rats, which feed on bird eggs and chicks, have hugely impacted bird populations on about 90 percent of the world’s tropical islands, but this is the first time rats have been identified as the enemies of reefs too.  

“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,” Professor Nick Graham of Lancaster Universtiy explained in a statement.

“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 Chagos Islands were the perfect place to carry out this study as 18 of the 55 islands are miraculously rat-free while the others have been overrun with black rats. This meant they could directly compare the effects of the rodents on both ecosystems and draw substantial conclusions, publishing the dramatic results in Nature.


By examining soils, algae, and fish numbers from six infested islands and six rat-free ones they found that the devastation caused by the rats didn’t just stay on the islands but spread out into the surrounding seas too.

The rat-free islands had more abundant bird life, and the nutrients from their guano made the soil rich in nitrogen that filtered its way into the sea, providing food for the abundance of life – from sponges to algae to fish – that live on the coral reefs. In fact, the team estimated that bird population densities are 800 times higher and fish populations 50 percent higher on these islands.

They also found that the grazing of algae on reefs of the rat-free islands was much higher, which is vital to helping clear the dead coral from the reef and prove a stable base for new coral to grow on.

On the rat-infested islands, the reefs and their inhabitants didn’t fare so well. “The rats feed on bird eggs and chicks, decimating seabird numbers. Seabirds consequently learn to avoid islands infested by rats altogether,” Graham told Newsweek. No birds, no bird poop.


The researchers hope that by identifying this unusual chain of events, it may help in the fight for coral reefs around the world. They estimate the cost of exterminating the infested islands of the Chagos Archipelago at around $2-3 million, but as we haven’t found a way yet of protecting the world's reefs against the devastating effects of climate change, this is action that can be carried out here and now, which makes that cost worth it.

"It could tip the balance for the future survival of these reefs and their ecosystems,” Graham said.


  • tag
  • conservation,

  • coral reefs,

  • introduced species,

  • rat infestations