Seabird Droppings Help Tropical Coral Reefs Facing Climate Change Threat

Seabirds in the Indian Ocean. Credit: Cassandra Benkwitt

The scientists say these shorter recovery times could prove vital in aiding some reefs to bounce back in the face of a warming planet, where damaging bleaching events now occur much more frequently than in earlier decades.

“Our results clearly show that seabird-derived nutrients are directly driving faster coral growth rates and faster recovery rates in Acropora coral,” said Dr Casey Benkwitt, research fellow in coral reef ecology at Lancaster University and lead author of the study.

“This faster recovery may be critical as the average time between successive bleaching events was 5.9 years in 2016 – a reduction from 27 years in the 1980s. Even small reductions in recovery times during this window may be key to maintaining coral cover over the short-term,” she added.

The team’s study focused on a remote archipelago in the Indian Ocean. They compared reefs next to islands with thriving populations of seabirds, such as red-footed boobies, sooty terns and lesser noddies, against reefs next to islands with few seabirds.

The reefs in the study area suffered extensive coral bleaching and mortality following marine heatwaves in 2015-16, providing an opportunity to observe and compare how coral on different reefs recovered. The researchers surveyed the sites from one year before the bleaching event to six years after bleaching, and modelled the Acropora recovery for the years between surveys.

The results showed that seabird-derived nutrients taken up by corals next to ‘bird islands’ boosted coral growth rates – with the rate doubling for each unit of seabird nutrient increase. In contrast, corals near islands infested with rats, causing there to be fewer birds, had similar nutrient values to corals that live at a distance from islands. The additional supply of nutrients to the corals by the seabirds had been virtually cut off by the rats.

The scientists also undertook a coral transplantation experiment to check the results weren’t due to genetic differences in coral populations between different islands. They could confirm it was  indeed the presence of seabirds that caused the faster growth. They say their findings add further weight to the growing body of evidence showing ecological damage across ecosystems, on land and sea, from invasive rats on tropical islands.

Professor Nick Graham of Lancaster University and Principal Investigator of the study said: “Combined, these results suggest that eradicating rats and restoring seabird populations could play an important role in re-establishing the natural flows of seabird nutrients to the nearshore marine environment, bolstering rapid coral reef recovery which will be critical as we expect to see more frequent climate disturbances.”

The study, outlined in the paper ‘Seabirds boost coral reef resilience’, was supported by the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science.

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