Oh rats! Seabird-eating rodents are hurting coral reefs near tropical islands

Rats are decimating bird populations on tropical islands, but coral reefs need bird guano to be healthy.
Invasive rats have caused seabirds to abandon nesting sites on many tropical islands because the rats will eat the birds eggs and chicks. (Paula O Sullivan )
Listen7:36

Coral reefs around the world are under threat from pollution, overfishing and climate change, which is warming oceans and driving mass coral bleaching. 

Scientists have now identified a new threat, but one that can be relatively easily eliminated: rats.

 A team of scientists including Aaron MacNeil, associate professor and Canada Research Chair in Fisheries Ecology in the department of biology at Dalhousie University, has shown the indirect effect invasive rats have on adjacent coral reefs. The unknown link was guano — bird poop.

"Birds forage out on the open ocean and they eat fish and they bring that energy back to the Island and release it in their guano. Their poo is like a fertilizer and those nutrients then get washed out by rainfall and waves out onto the coral reef," MacNeil said.

What rats do is interrupt the flow of energy between the open ocean and coral reefs. 

Invasive rats on tropical islands will eat eggs and chicks of nesting seabirds. This booby chick lives on an island in the Chagos archipelago without rats. (Nick Graham)

"It's really that the birds just stop using those islands and so the flow stops because birds know that if they roost and try to raise young on those islands they'll be just decimated by the rats," he said.

A  natural laboratory

The Chagos Archipelago is in the middle of the Indian Ocean and the surrounding reefs are pristine.

Nick Graham, the lead researcher on this study was working on the coral reefs and while visiting the islands, noticed some were loud, full of birds and smelled terrible while others were quiet, smelled great, and were bird free, said MacNeil.

90% of the world's temperate and tropical islands have had their bird populations decimated by rats. Humans brought rats to the Chagos Archipelago in the late 18th century, where they wreaked havoc on the islands' native birds which included red-footed boobies and terns.

But before this study it wasn't known how far-reaching the rats' impact was.

Yellow and blue fusiliers shoal over a healthy reef in the Chagos Archipelago (Nick Graham)

The Chagos provided a very rare natural laboratory said MacNeil. The researchers compared corals reefs surrounding six islands infested with rats with the coral reefs surrounding six islands without rats.

"He designed a study and used some chemical tracers to work out the nuts and bolts of where the energy was being sourced from and to show that yes it was coming from the open ocean and landing on the islands and then getting washed out and incorporated into the reef ecosystem," said MacNeil.

Underwater proof

MacNeil said the degree of impact of rats on surrounding reefs was astounding. The team found that fish biomass in the reefs surrounding rat-free islands was 50 percent greater than fish near reefs with rats. 

"The fish that are on islands that had rats grew at a much slower rate than the fish that were living next to the regular bird adjacent islands," he added.

"The bird poo actually helps things grow faster we see more of them, the ecosystem is more productive and as a result the jobs that these fish play, the functional roles that they have are enhanced."

Healthy reef ecosystems benefit from birds on nearby islands because the birds' guano contributes nutrients to the reef organisms. (Nick Graham)

For example, they found this effect on grazing, which is an important ecosystem service that fish provide by cropping down algae that grows on reefs.

"Reefs are sort of in an arms race for space with algae and that the reason the coral is able to persist over algae is because of this function 'herbivory', this eating of algae, and it's the reef fish that play this role in many ecosystems," said MacNeil.

Some fish also consume dead coral (bioerosion) and poop out sand which provides a stable base for new coral growth. Reefs are much more resilient to threats like bleaching if they're well grazed.

An easy fix

Rats have been successfully eliminated from tropical Islands in the past.

It's also a cost effective solution said  MacNeil. There are about 35 islands on Chagos that still have rats and the estimated cost is $2-3 million dollars to eradicate them.

"It's very hard to know what to do to help reefs in the interim before we actually get a handle on climate change," he said.

"And so this study is really important because it's a very clear cut case of how we can intervene and actually have a really beneficial effect on the resilience of those reefs."