Rat eradication is beneficial for seabirds and coral reefs

Rat eradication on tropical islands can benefit the surrounding coral reefs in a major way, according to a new study from Lancaster University. The researchers found that in the absence of rats, valuable nutrients in the form of seabird poop are able to reach the reefs. 

The experts report that seabird nutrient flows can bolster the health of delicate coral reef ecosystems, and may improve their chances of recovering climate disturbance events.

Seabirds make critically important contributions to nutrient distribution on tropical islands. After feeding on fish far out in the open ocean, the birds return to roost and deposit nitrogen-rich nutrients in their poop, or guano.

When it rains, the guano is washed off into the surrounding ocean where it fertilizes corals, algae, and other marine species. 

Rats have been accidentally introduced to many tropical islands over the last few centuries through human settlement and sailing between islands. The invasive animals can completely alter ecosystems and are particularly threatening to seabirds, eating eggs, chicks and even some adults. 

In previous work, the Lancaster team showed that islands with rat infestations had much smaller seabird populations than islands without rats. As a result, there was much less nitrogen on the islands and in the surrounding marine environments. The lack of nutrient runoff was linked to smaller reef fish and slower coral growth on adjacent reefs.

The current study was focused on 20 islands in the central and western Indian Ocean, including the Chagos Archipelago and the Scattered islands.

The team compared islands with ongoing infestations to those where rats have been eradicated, as well as with islands that never had rats. 

The greatest number of seabirds was found on islands where rats have never been introduced. Islands with existing rat infestations had the fewest seabirds, as well as seabird-derived nutrients. 

Seabird populations are steadily increasing on two sites within the Scattered Islands – Île du Lys and Tromelin.

“Rats were eradicated on Tromelin in 2005,” said study co-author Matthieu Le Corre. “Since then there has been an eight-fold increase in seabirds, and six species that were locally extinct because of the rats have restarted to breed after rat eradication. On Île du Lys, where rats were eradicated in 2003, surveys show there has been a ten-fold increase in seabirds since.”

By testing fish and algae, the researchers determined that there were higher levels of seabird-derived nitrogen surrounding these islands.

The study showed that – even around islands that have been infested for hundreds of years – seabird nutrients begin flowing to coral reefs within 16 years of rat eradication.

“We know that rats are devastating for island seabird populations, and that the loss of these birds from islands is harmful to the surrounding coral reef ecosystems. However, we needed to test whether removing the rats from islands could help these important nutrient cycles return, and if so, how quickly and how far the benefits would spread onto fragile coral reefs,” explained study lead author Dr. Casey Benkwitt.

“Our study shows the first evidence that rat eradication programs can indeed be an important tool in helping restore these vital seabird-led nutrient cycles not just on tropical islands themselves, but also in their surrounding seas – and that they can recover within relatively short timeframes, which is very encouraging.”

The researchers believe that by combining rat eradication programs with other restoration strategies, it could be possible to speed up the recovery of seabird populations, and their benefits to surrounding marine environments.

“This study adds to the weight of evidence suggesting rat eradication can have substantial conservation benefits to tropical island and adjacent marine ecosystems,” said Professor Nick Graham.

“The nutrient cycles that returning seabirds bring can bolster coral and fish assemblages. With climate impacts severely impacting coral reefs, management actions to boost the ecosystem are incredibly important.”

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer