Exterminating rats on the islands of Chagos could conserve its coral reefs

BBC News reports 12th July 2018: A team working on the Chagos Islands in the Indian Ocean found that invasive rats on the islands are a “big problem” for coral reefs.

Rats decimate seabird populations, in turn decimating the volume of bird droppings — a natural reef fertiliser.

Scientists now advocate eradicating rats from all of the islands to protect these delicate marine habitats.

Photograph by Nick Graham

How do rats harm coral reefs?
The Chagos Archipelago provided a large-scale natural laboratory. Although the islands are uninhabited by humans, some of them are now home to invasive rats, brought by ships and shipwrecks. Other islands have remained rat-free.

“The islands with and without rats are like chalk and cheese,” said lead researcher Prof Nick Graham from Lancaster University. The islands with no rats are full of birds, they’re noisy, the sky is full and they smell – because the guano the birds are depositing back on the island is very pungent. If you step onto an island with rats, there are next to no seabirds.”

By killing seabirds, this study revealed, rats disrupt a healthy ecosystem that depends on the seabird droppings, which fertilise the reefs surrounding the island.

On rat-free islands, seabirds including boobies, frigatebirds, noddies, shearwaters and terns travel hundreds of kilometres to feed out in the ocean. When they return to the island, they deposit rich nutrients from the fish they feed on.

“These nutrients are leaching out onto the reef,” explained Prof Graham.

Seabirds like boobies nest far more abundantly on rat-free islands.
Photograph by Nick Graham

Prof. Graham and his team were able to track the source of those nutrients back to the fish that seabirds fed on by analysing algae and sponges growing on the reef.

“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,” Prof Graham explained.

There were also significantly more fish on rat-free reefs than on those around “ratty islands”.

Why does this matter?
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.”

Coral reef systems are at “crisis point” because of climate change
Photograph by Nick Graham

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.

In an accompanying commentary article Dr Nancy Knowlton, a marine scientist from the Smithsonian’s National Museum of Natural History in Washington DC, pointed out that while “adding rats to the list of dangers faced by reefs might seem discouraging, the discovery of these negative impacts points directly to a specific strategy” that could slow the ongoing degradation of reefs.

That strategy — rat eradication on islands throughout the world — is exactly what the researchers who carried out this study now advocate.

“Coral reef systems are at crisis point because of climate change,” said Prof Graham. “And we’re desperately trying to find ways to enhance the resilience of coral reefs and allow them to cope with climate change.

“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.”

Source: BBC News, 12th July 2018. For full details, see


Marinet observes: This is a modern version of Charles Darwin’s illustration of the dependency of life in his work On the Origin of Species.

There he cites how the abundance of red clover plants can easily be dependent on the local cat population. This arises because cats catch mice, mice destroy the nests of bumble bees, and the red clover plant is wholly dependent on the bumble bee for pollination. Thus the abundance of cats influences the abundance of red clover, although the immediate connection is not readily apparent.

Here in the Chagos Islands we have something very similar (albeit induced probably by man rather than nature). Here the rats kill the seabirds who produce guano (droppings) from eating the fish which live in the coral reef. This reduces the abundance of plant life on the coral reefs, due to a lack of fertiliser from the guano, with plant life being of course the food source for the fish which feed the birds which produce the guano. In short, feedback – which the rats are disrupting.

This perceptive study of what is happening on the Chagos Islands, based on the principles which Charles Darwin identified, is what is desperately need on a far broader basis for mankind’s management of its own activities in the ocean — from over-fishing to widespread plastic pollution and deeply dangerous ocean acidification (reduction in its alkalinity and the ability of plants and animals to take up and retain calcium carbonate for their bone structure and shells).

Marinet asks: Is anyone taking and responding to this larger, wider perspective? The clock is ticking, audibly.


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