Killing rats can save coral reefs: Dal researcher

It’s well known that rat infestations are unhealthy for humans and now a new study has proven they’re harmful to coral reefs as well.

In a recently published paper in the journal Nature, Dalhousie University and Ocean Frontier Institute researcher Aaron MacNeil, along with the study’s lead author Prof. Nick Graham of Lancaster University in the United Kingdom, found that the reefs surrounding the northern atolls of the Chagos Archipelago, a set of tropical islands in the Indian Ocean, were much healthier near the islands that contained no rats.

According to MacNeil, the rats were introduced by European colonialists in late 18th and early 19th centuries to some islands in the archipelago, while some islands remained rat-free. On the unifested islands, local seabird populations continued to thrive, but on the islands with black rat populations, bird populations were decimated once the rats discovered that their eggs made for a tasty, nutritious treat.

A closed poop loop

Terns fly over a coral reef. Birds feed at sea, which brings new nutrients to the islands they roost upon. Rats don’t leave the islands, which means they don’t contribute fresh nutrients to the ecosystem. ( NICK GRAHAM)


MacNeil said Graham, who has spent more than a decade working in the region, noticed the lack of seabird life on the infested islands and he wondered how the introduction of an invasive species might affect surrounding ecosystems. The fact that the islands are in close proximity to one another, but some remain free of rats, provided a perfect setting for researchers.

As for the impact on surrounding coral reefs, it all comes down to poop. By examining soil samples, algae, and counting fish numbers close to the six rat-free and six rat-infested islands, scientists found the rats caused severe ecological harm by robbing the nearby coral reefs of essential nutrients, which is crucial as corals tend to exist in a very nutrient-poor environment.

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“About four years ago we started collecting some samples to actually look at this to see if we could see in the nitrogen flow that comes from the islands out onto the reefs and what we found out really is that there’s an enormous difference. The birds that feed way out in the open ocean fly back to these islands to roost and as a result, they release their excrement onto the islands and they fertilize the islands. That has a huge effect for the surrounding reef ecosystem,” MacNeil said.

“The rat poop does not come from the open ocean, the rat poop would come from the island itself so it would be a bit of a closed loop.”

MacNeil, who is a quantitative biologist, said even he was surprised at the size of the nutrient subsidy provided by the birds and the huge impact that it had on the reefs.

Where there’s a problem . . . 

Yellow and blue fusiliers shoal over a reef in the Chagos Archipelago. (NICK GRAHAM)


The good news in all of this? It’s easy and relatively cheap to remedy.

Modern eradication methods, which use an aerial bombardment of poison bait, have proven to be effective, MacNeil said, and do not negatively impact other wildlife.

“We estimate (it will cost) between $2 and $3 million for us to eradicate all the rats from all the islands in the Chagos Archipelago, which is a really tiny amount of money for the benefit,” he said.

Scientists are working the U.K., which has territorial authority over the islands, to fund the eradication program as well as trying to recruit philanthropists who may want to contribute to a conservation effort that can actually make a difference, MacNeil said.

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On a larger scale, this study is good news for coral reefs, as it proves that helping the engaged lifeform in a meaningful way might be easier than anticipated.

“Reefs are threatened, possibly existentially threatened by climate change and barring doing something about climate change a lot of research is going into how do we support reefs,” MacNeil said. “This study shows a very key result, which is that by eradicating rats there is a huge benefit to the ecosystem function through this nutrient subsidy and that’s a big deal because reefs are very complex. It’s hard to really pull out a real win and this is very low hanging fruit in terms of helping support reef ecosystems.”

Researchers have been collecting data on the Chagos Archipelago for four years. (GUY STEVENS / Manta Trust)


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