Coral Reefs Need Bird Poop, But Rat Infestations Threaten Seabird Population

Save the coral reefs by saving the birds by eradicating rats, scientists say in a study that exposes how rat infestation impacts deteriorating reefs.

This newly released research demonstrates the spectacular links between life on land, in the air, and at sea — and how devastating their effects on each other could ultimately be.

Rats In The Isles

In the study published in the journal Nature, researchers observed 12 islands in the Chagos Archipelago, half of which are rat-free and the other half infested with black rats that's believed to be introduced in the late 1700s and early 1800s. The team discovered serious ecological damage extending from land to sea in the rat-infested islands by analyzing the soil, algae, and fish populations.

The rat-free islands were found to have more seabirds, whose waste supplies the soil with abundant nitrogen. The nitrogen then makes its way to the sea, where the researchers observed it benefits marine life such as algae, filter-feeding sponges, and fish.

"Seabirds are crucial to these kinds of islands because they are able to fly to highly productive areas of open ocean to feed," lead author Professor Nick Graham from the Lancaster University in United Kingdom explains in a statement. "They then return to their island homes where they roost and breed, depositing guano — or bird droppings — on the soil. This guano is rich in the nutrients, nitrogen and phosphorus."

Unfortunately, invasive rats feed on bird eggs, chicks, and adults birds. Scientists estimate that rats have actually decimated seabird populations in 90 percent of the islands in the world.

"Until now, we didn't know to what extent this made a difference to adjacent coral reefs," Graham continues.

How The Corals Are Affected

The team found that the effects of the seabird population on the corals are massive. Consequently, rats slaughtering birds in certain islands spell doom for coral reefs, as well.

There are 760 times more seabirds in the rat-free islands — and fish life also thrived with the mass of fishes in the reefs surrounding these islands nearly 50 percent greater.

The researchers traced the nitrogen from the birds, finding that the sponges and seaweed off the shores of the rat-free islands contain more nitrogen. Damselfish grew faster with the extra nitrogen.

With the significant links found between rat presence and coral reef health, the team who spearheaded the study is pushing for the prioritization of eradicating rats from islands.

"It could tip the balance for the future survival of these reefs and their ecosystems," Graham says.