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Science has Spoken: Coral Reefs Thrive by Rat-free Islands

New research identifies the direct benefits to coral reef ecosystems and communities after invasive rat removal on islands.

By: Emily Heber

Islands only make up approximately 5% of the earth’s land mass but they are home to 20% of the world’s biodiversity and 41% of the world’s Critically Endangered and Endangered species. Invasive species are one of the biggest threats facing our world’s seabird populations today including invasive rats, feral cats, and goats. Invasive rats have historically been one of the leading causes of seabird extinction on islands and continue to threaten species around the world by predating on eggs, chicks, and adult seabirds.

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Invasive rats predate upon native seabird eggs, chicks, and adults. Credit: Island Conservation

Past studies have shown that removing invasive rats from islands has a profound positive impact on native seabird populations. Following removal of invasive species, seabirds and other native species have been downlisted on the IUCN red-list or have avoided listing on the Endangered Species List. We now have concrete data that terrestrial and seabird species are not the only ones that benefit. A new study published in Nature quantifies the proven benefit to marine ecosystems, particularly coral reefs, on rat-free islands.

Researchers identified 12 islands in the Chagos Archipelago, half of which have invasive rats present and half of which do not in order to study the effects invasive rat removal can have on near-shore marine ecosystems. What they found confirmed what many conservationists already suspected – marine species benefit from removing invasive rodents from islands.

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Previous research on Palmyra Atoll has shown the important linkages between island and marine conservation, but this new study identifies invasive rat removal as a global conservation priority that can protect coral reefs and marine ecosystems. Credit: Aurora Alifano/Island Conservation

The result – coral reefs blossom

Seabirds spend the majority of their time in the open ocean, feeding on fish and only return to islands to breed. In doing so, they bring with them nutrients including nitrogen and phosphorus which fertilize native plants. Excess nutrients then trickle off into the ocean where it feeds the marine ecosystem.

The balanced, natural influx of nitrogen and phosphorus to the marine environment adjacent to rat-free islands had astounding results. The coral reefs near invasive-free islands showed higher fish production and 48% greater biomass than their invasive rat-infested counterparts. Researchers found that Parrotfish, an abundant herbivorous fish which cycle nutrients through algal grazing, have been shown to graze algal ridges 9 times per year as opposed to 2.8 times per year near rat-infested islands.

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Coral reefs of Palmyra Atoll. Credit: Abram Fleishman/Island Conservation

The Human Aspect

This new research shows that removing rats from islands is a win-win-WIN scenario with cascading benefits from native seabirds to coral reefs to human beings. By removing invasive rodents from islands, seabirds are given a chance to rebound, which leads to an influx of nutrients to near-shore marine ecosystems. Healthy coral reefs mean greater resilience against climate change and are vital to sustainable fishing and responsible ecotourism. But the benefits extend beyond just the island they surround – when fish have a healthy coral to protect them, they are able to grow large before venturing out to the sea.

The science is in. Invasive rat removal must be a global conservation priority.

Featured photo: An underwater view of Palmyra Atoll’s thriving coral reef ecosystem. Credit: Andrew Wright
This research was also featured on WIRED.

About Emily Heber

Emily is a recent graduate from UC Santa Barbara with a BS in Zoology. As a student, she discovered that she had a passion for the conservation of endangered species and their ecosystems. Her background in informal education has allowed her the opportunity to share her passion for animals with others, something she seeks to continue doing while working with the communication team. In her spare time, Emily enjoys exploring the amazing hiking trails found in Santa Cruz and tries to SCUBA dive whenever possible. Emily is excited to join the Island Conservation team and to help share the amazing work that is being done here.

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