Credit: Nick Graham

Rat-free islands help keep coral reefs healthy

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Rat infestations on coral atolls are cutting bird numbers – limiting the nutrients from bird poop reaching the surrounding ocean, according to an international study. Using uninhabited atolls in the Chagos Archipelago, the researchers compared the nitrogen levels on the atolls in the plants in the surrounding ocean for both rat-infested and rat-free islands. They found certain birds – including boobies, noddies and shearwaters – on rat-free islands deposit about 250 times more nitrogen onto the land than on rat-infested ones. The coral reefs near rat-free islands are much healthier, say the researchers, allowing plant-eating damselfish to grow bigger.

Journal/conference: Nature

Organisation/s: James Cook University, The University of Western Australia, ARC Centre of Excellence for Coral Reef Studies, Lancaster University, UK

Media Release

From: Springer Nature

Bird droppings help to fertilize islands and nearby coral reefs — but this nutrient flow can be disrupted by rat infestations, reports a paper published in this week’s Nature. This finding highlights how reefs connect to surrounding ecosystems.

Seabirds that feed in the open ocean transport large quantities of nutrients onto nearby islands in the form of guano, or droppings. This fuels faunal and floral productivity. Whether these nutrients leach back into the ocean, however, has been unknown.

Nicholas Graham and colleagues studied the northern atolls of the Chagos Archipelago in the Indian Ocean. These islands are currently uninhabited by humans; rats were introduced to some, however, in the 18th and 19th centuries, whereas others remained rat-free. Rat predation devastates bird populations, thus reducing the rates of bird droppings. On the atolls — which play host to boobies, frigatebirds, noddies, shearwaters, terns and tropicbirds — the authors found that seabird densities were 760 times higher without rats. Based on bird abundances, estimated defecation rates and the nitrogen content of droppings, the authors calculate that the birds on rat-free islands deposit about 250 times more nitrogen onto the land than on rat-infested ones.

By measuring the ratios of nitrogen isotope in soils and shrubs, the authors could distinguish locally sourced nitrogen from bird-transported nitrogen, which comes from more productive parts of the ocean. They found that rat-free islands had substantially more dropping-derived nitrogen. Higher values of seabird-sourced nitrogen were also found in seaweed, filter-feeding sponges, turf algae and fish on coral reefs adjacent to rat-free islands. Plant-eating damselfish on these reefs grew faster; rates of two critical ecosystem functions, bioerosion and grazing activity, increased by 3.2 and 3.8 times, respectively; and fish communities exhibited a 48% overall increase in biomass when near rat-free, more-fertilized islands.

With potential benefits for not only island ecosystems but also nearby coral reefs, eradicating island rat populations to restore seabird-sourced nutrients should be a priority, the authors conclude.

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    Credit: Nature
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    Terns on a rat-free island in the Chagos Archipelago. Credit: Jon Slayer
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    Seabirds flying over a rat-free island in the Chagos Archipelago. Credit: Jon Slayer

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  • Killing rats could preserve more coral

    Lancaster University video summary of the study

    File Size: 35.1 MB

    Attribution: Lancaster University (UK)

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    Last Modified: 12 Jul 2018 3:02am

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  • Prof Nick Graham explaining the study

    Lead author of the study, Prof Nick Graham, describing the research whilst on a rat-free island surrounded by seabirds in the Chagos Archipelago. Note: The download file of this video is 950MB

    File Size: 72.9 MB

    Attribution: Casey Benkwitt

    Permission Category: © - Only use with this story

    Last Modified: 12 Jul 2018 3:02am

    Note: High resolution video files are only available for download here by registered journalists who are logged in.

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