Coral Reefs Are Being Plagued By Rats — And It’s All Connected To Bird Poop


Rats on tropical islands are wreaking havoc on nearby coral reefs, reveals a new study published yesterday in the journal Nature. The rodents have invaded some of the islands of the Chagos Archipelago in the Indian Ocean, producing a surprisingly negative impact on the neighboring reefs.

According to the paper, the rodents — black rats believed to have been introduced on these islands by humans in the late 1700s and early 1800s, notes — are preying on the region’s seabird population, triggering an entire chain of effects that has permanently changed the ecosystem.

“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,” says study lead author Prof. Nick Graham, a marine ecologist at Lancaster University in the U.K.

But the rats that have taken the islands by storm are decimating seabird populations, attacking their nests and feeding on both their eggs and on the birds themselves.

Therefore, there’s little wonder that seabirds have learned to avoid these islands and no longer nest here, depriving the ecosystem of the rich nutrients found in guano, Graham told Newsweek.

The research examined a dozen islands in the Chagos Archipelago — six of them rat-infested and the other six rat-free — and compared the ecological effects that these rodents have not only on island wildlife, but also on the marine ecosystem surrounding the islands.


The unique conditions of this remote group of tropical islands — half of them swarmed by rats and half still pristine — allowed the scientists to make a series of unprecedented observations that ended up leading to an unexpected conclusion.

In order to protect both the seabirds and marine wildlife, including the vulnerable coral reefs in the area, the rat populations on these islands need to be curtailed

“Rat eradication should be a high conservation priority on oceanic islands. Getting rid of the rats would be likely to benefit terrestrial ecosystems and enhance coral reef productivity and functioning by restoring seabird derived nutrient subsidies from large areas of ocean,” said Graham.

Here’s Why The Rats Need To Go

In what may seem like an ironic twist of events, conservationists are advocating for the ending of an animal’s life.

Study co-author Prof. Aaron MacNeil, from Dalhousie University in Canada, explains why that is.

“These results show how conservation can sometimes be a bloody business, where doing right by the ecosystem means there is a time to kill. For these invasive rats, that time is now.”

Here’s why the rats that infested these tropical islands need to be disposed of, according to the team. First off, the study uncovered that, in the absence of rats, seabird populations were thriving and helping nourish the island soil with nitrogen and phosphorus found aplenty in their droppings.

This large supply of nitrogen made its way into the sea, to the benefit of a wide array of marine creatures, including macroalgae, filter-feeding sponges, turf algae, and fish on adjacent coral reefs.

The reefs themselves were doing much better, as a balanced input of nitrogen and phosphorous is known to help these organisms grow faster and better cope with heat stress, Graham told Wired.


The total tally showed that rat-free islands were home to 750 times more seabirds, which deposited nitrogen at a much higher rate. The element got soaked into the soil and trailed into the sea water, feeding both land and marine vegetation, as well as the fish population in the vicinity of the islands.

The larger number of fish — the study showed nearly 50 percent more fish biomass near the rat-free islands — nibbled on the algae and the dead corals within the adjacent reefs.

These two processes, known as grazing and bioerosion, are vital for the health of the reef ecosystem and help corals reproduce. Near the islands where there weren’t any rats, grazing and bioerosion of the coral reefs were 3.2 and 3.8 times higher, respectively.

“These results not only show the dramatic effect that rats can have on the composition of biological communities but also on the way these vulnerable ecosystems function (or operate),” said study co-author Dr. Andrew Hoey, from the ARC Centre of Excellence for Coral Reef Studies in Australia.