Tropical coral reefs, mesmerizing ecosystems, face an existential threat from our warming planet. Amidst this peril, a beacon of hope emerges in the form of seabirds, according to a groundbreaking study led by Lancaster University researchers. These avian allies, found on islands adjacent to tropical coral reefs, have been revealed to more than double coral growth rates, offering a lifeline for these vital ecosystems.
The study, featured in Science Advances, zeroes in on Acropora, a crucial coral type supporting fish populations, reef growth, and coastal protection. Notably, coral reefs near seabird colonies exhibit a remarkable ability to recover swiftly from bleaching events, where rising sea temperatures trigger mass coral die-offs. Acropora around islands with seabirds showed a recovery period approximately 10 months faster (around three years and eight months) compared to reefs distanced from seabird colonies (four years and six months).
As climate-related bleaching events intensify, shorter recovery times, facilitated by seabirds, become pivotal for the survival of these reefs. The key to the seabirds’ transformative impact lies in their droppings. Seabirds, after feeding on open-ocean fish, return to islands to roost, depositing nitrogen- and phosphorus-rich guano. Rain washes some of this guano into the surrounding seas, fertilizing corals and other marine species.
Dr. Casey Benkwitt, lead author and research fellow in coral reef ecology at Lancaster University, emphasizes the critical role of seabirds in driving faster coral growth rates and recovery. Given the accelerated pace of bleaching events, even minor reductions in recovery times are deemed essential for maintaining coral cover in the short term.
The study focused on an Indian Ocean archipelago, comparing reefs near islands with thriving seabird populations against those near islands with few seabirds plagued by invasive rats. The islands with thriving bird populations, notably free of rats, demonstrated a stark contrast to those infested by these destructive rodents.
During a period of extensive coral bleaching and mortality following marine heatwaves in 2015-16, the researchers observed and compared coral recovery on different reefs. Sampling nitrogen stable isotope values and measuring Acropora coral growth rates over three years, they found that seabird-derived nutrients near bird islands significantly boosted coral growth rates.
Conversely, coral near rat-infested islands lacked these vital nutrients, underscoring the impact of invasive species on nutrient supply. Experimental transplantation of Acropora corals confirmed that seabird presence directly caused nutrient enrichment, with coral colonies on seabird islands growing twice as fast as those on rat-infested islands.
Dr. Benkwitt highlights the exciting prospect of restoring seabird populations to enhance coral resilience, emphasizing the sustained, long-term benefits observed in their three-year experiment. The study underscores the broader ecological damage caused by invasive rats on tropical islands, supporting the case for eradicating rats and restoring seabird populations to rejuvenate marine environments.
Professor Nick Graham, Principal Investigator of the study, envisions a pivotal role for eradicating rats and revitalizing seabird populations in re-establishing natural nutrient flows to nearshore marine environments. Beyond coral recovery, seabird nutrients contribute to increased fish growth rates and a 50% greater fish biomass on reefs adjacent to islands with large seabird colonies, emphasizing the holistic benefits of this natural solution.
In conclusion, the study, supported by the Bertarelli Foundation, reveals a compelling link between seabirds and enhanced coral resilience, urging concerted efforts to preserve and restore seabird populations as a vital strategy in the face of escalating climate disturbances.