Rat populations causing severe damage to coral reefs, study finds

Coral reefs around islands with rat populations are significantly less healthy than those near islands where no rats live, a scientific study has found.
The research by Australian and British universities, published in the Nature journal on Wednesday, was conducted on 12 uninhabited atolls in the Chagos Archipelago in the Indian Ocean.
Rats have been introduced to some coral islands as a result of human explorations, while others remain rat-free.
The animals are known to prey on bird eggs, chicks, and sometimes adults, decimating populations where they have been introduced. This in turn reduces the amount of bird droppings, which contain vital nutrients needed by fish and reefs in the surrounding ocean, the study said.
The islands in the Chagos Archipelago are populated by birds such as boobies, noddies, terns and shear waters.
Levels of nitrogen — a key plant food contained in bird droppings — were 251 times lower per hectare on the six islands with rat populations than on the six rat-free islands, said the study, led by Nick Graham, a scientist at UK’s Lancaster University.
The study found reefs near rat-infested islands were a lot less healthy, with fewer nutrients and fewer, smaller fish, including plant-eating fish that help protect corals from algae.
The total biomass of the fish population was 48 per cent higher around rat-free islands than around those with rat populations.
The study said there were 760 times fewer nesting seabirds per hectare on islands with rats than on those without.
The research for the study was conducted in 2015. In 2016, a mass coral bleaching event reduced the size of the coral reefs in the Chagos Archipelago by about 75 per cent.
The researchers said the corals near the rat-free islands would probably have been more resilient to bleaching — caused by unusually warm water — because bird droppings are rich in phosphorus, which has been shown to improve coral’s tolerance for heat.
“This work has immediate practical implications, particularly because reefs are under grave threat around the world,” said Nancy Knowlton,a zoologist with the Smithsonian Institute, in a supplement article published in Nature.
“Now, however, one of the major causes for concern over reef survival is the impact of climate change and the ability of reefs to recover from disturbances due to oceanic warming, which is causing mass coral bleaching and death affecting even remote and protected reefs.”
The authors of the study said eradicating island rat populations to restore seabird-sourced nutrients should be a priority for island ecosystems and nearby coral reefs. — dpa