What have rats got to do with the number of fish on coral reefs? Quite a lot, according to new research.

The black ship rat — Rattus rattus — wreaks havoc on island seabirds globally, including throughout the Great Barrier Reef where it has been a pest for more than 200 years.

The animals eat the eggs of nesting birds and prey on the young, suppressing bird numbers on the islands they inhabit.

A paper in Nature today demonstrates how this triggers a series of events that not only damages surrounding coral reefs, but impacts fish and other marine life as well.

It's all about poo. Bird poo.

Researchers found fish biomass was much higher on reefs around rat-free islands. ( Supplied: Nick Graham )

An ideal test

On the Indian Ocean's Chagos Archipelago near the Maldives, islands are split between those with introduced rats and those without.

This provided researchers with a unique opportunity to study their impact on island ecosystems, according to James Cook University researcher and one of the paper's authors, Dr Andrew Hoey.

"It's really a natural, quite a pristine setting where you've got six islands that have rats on them and then six right next door that don't have the rats," he said.

"So you couldn't really design the experiment much better."

Dr Hoey was part of a team that compared the abundance of seabirds and nitrogen levels in the soil between the rat-infested and rat-free islands.

Where rats were absent, they found seabird abundance was 760 times greater than on islands with rats.

As a consequence, nitrogen-deposition rates from the birds' poo, known as guano, was 251 times higher.

Rats' impact stretches to outer reefs

Birds are a major source of new nutrients to isolated islands.

"The key thing for seabirds is that they're travelling a fair way offshore and bringing in these oceanic sources of nutrient onto the islands," Dr Hoey explained.

Reduced nutrient levels were found to have an effect all the way out at the reef crests. ( Supplied: Nick Graham )

The researchers specifically looked at nitrogen-delta-15, which is present in seabird droppings and can be traced to their oceanic diet.

"That guano that gets there then leaches out through rainfall etcetera, then into the surrounding reef environment," Dr Hoey said.

Rats eat eggs and young birds. ( Supplied: Nick Graham )

Although it has been previously understood that nitrogen from bird poo enters reef lagoons, Dr Selina Ward from the University of Queensland, who wasn't involved in the study, said it was "incredible" to see how far it permeates through the reef.

Macroalgae, filter-feeding sponges, turf-eaters and fish on adjacent reefs all showed elevated nitrogen levels where rats were absent.

Herbivorous damselfish were found to grow significantly faster, and fish populations had almost 50 percent greater biomass on reefs where nitrogen levels were elevated.

And the difference continued into the structure of the reef itself.

Rat-affected reefs carried less parrotfish, which feed on things like algae.

"One of the unique things with parrotfish is that you can go down and measure their feeding range, you can measure their bite sizes, and you can...get an estimate of how much of the reef they're grazing and how much of the reef carbonates they're removing," Dr Hoey said.

"The capacity of the fish communities to remove that algae and keep it in a cropped state and prevent those larger algae developing is critical."

Removing rats a win for conservation

Black ship rats have wiped out numerous bird species on Lord Howe Island. ( Getty: Michael Leach )

Although black ship rats are believed to have arrived in Australia on board the First Fleet, there's still some mystery about their origin, according to Professor Peter Banks from the University of Sydney.

"They've been found in gun barrels of sunken ships in Western Australia on Dutch ships," he said.

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Regardless, after 1788 they spread rapidly across the mainland and nearshore islands, including throughout the Great Barrier Reef.

Their impact was exaggerated on islands that were not used to predators, Professor Banks said.

"We've got more than 60 species of native rodent here so our fauna in Australia are not so naive," he added.

"But Lord Howe Island is distinct in that they never had any mammals there."

On Lord Howe Island, 600 kilometres east of Port Macquarie off the New South Wales coast, the damage wrought by introduced rats was witnessed first hand.

After an American supply ship ran aground in 1918, a few rats scurried ashore and wiped out five endemic birds and 13 species of invertebrates.

A planned rat-baiting program on Lord Howe Island has divided opinions. ( Supplied: Sarah Oxenham )

In 1964, rock climber and scientist David Roots discovered a Lord Howe stick insect, thought to be extinct, on a rock outcrop about 30 kilometres from the island.

Now, a controversial program to rid the island of rats and reintroduce the stick insect, known as the Lord Howe tree lobster, is edging closer to a start date.

The plan would see tonnes of poison-laced pellets dropped from helicopter and by hand across the mostly pristine island.

Bird droppings or guano provide high levels of nitrogen to island ecosystems. ( Supplied: Guy Stevens )

It has proved controversial among locals who are concerned that the poison may impact other native wildlife.

Researchers will be watching the program closely. If it works, it could serve as a model for similar ventures on other Australian islands.

In the case of uninhabited islands, getting rid of rats should be a focus of conservation in the near future, according to Dr Hoey.

"There's growing evidence, like down on Macquarie Island right down near Antarctica, where they've had successful eradication of things like rats and foxes as well," he said.