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Rat Infestation In Oceanic Islands Deprives Coral Reefs Of Nutrients From Bird Poop

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Invasive rats that decimate the population of island seabirds have unwanted consequences on the health of coral reefs.

Researchers said that rat control should be considered a conservation priority to protect these coral reefs.

Birds In Rat-Free And Rat-Infested Islands

In a new study published in the journal Nature, researchers studied islands in the Chagos Archipelago in the Indian Ocean.

The rat-free islands had an average 1,243 birds per hectare. Rat-infested islands, on the other hand, only had about two birds per hectare.

Bird Poop Provides Nutrients To Coral Reefs

Study researcher Nick Graham, from Lancaster University in the UK, and colleagues found that the rodentless islands tend to have healthier coral reef ecosystems, which they said is due to a secret ingredient: bird poop.

Bird droppings are naturally rich in nitrogen and they keep the reefs productive. The researchers tested for heavy nitrogen isotopes on 12 islands, half of which had no rats and the other half was rat infested, and in nearby coral reefs.

They found more amounts of heavy nitrogen in the soil of rat-free island where the bird populations still thrive, as well as in the sponges, algae, and fish in reefs surrounding those islands.

The findings suggest that the nutrient-rich guano from birds can seep beyond the islands, and into the sea and the surrounding corrals.

"We found that seabird densities and nitrogen deposition rates are 760 and 251 times higher, respectively, on islands where humans have not introduced rats," Graham and colleagues wrote in their study.

"Consequently, rat-free islands had substantially higher nitrogen stable isotope (δ15N) values in soils and shrubs, reflecting pelagic nutrient sources.

The researchers also found that fishes adjacent to the rat-free islands were more abundant and the mass of the fish is estimated to be 50 percent greater than that in the rat-infested islands.

The grazing of algae, which provides a stable base for new coral growth as fish consumes algae and dead coral, was 3.2 times greater adjacent to rat-free islands.

Rat Eradication As A Conservation Strategy

Graham said that the results of the study show that rat eradication needs to 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," Graham said. "It could tip the balance for the future survival of these reefs and their ecosystems."

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