Media ReleaseFrom: 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.