Rat removal helps seabirds thrive on restored islands

Seabird restoration on remote island archipelagos involves removing invasive rats and reviving native vegetation, which could attract hundreds of thousands of breeding seabirds.

A recent study reveals that the surrounding seas contain enough fish to support these increased seabird populations, a crucial factor often overlooked in previous restoration efforts.

Seabirds and invasive rats

Seabird populations decline drastically due to invasive rats, which prey on seabird eggs, chicks, and even adult birds. Rats, introduced to islands by human activities, pose a severe threat to seabirds, especially on islands where seabirds evolved without such predators.

Seabirds typically nest on the ground or in burrows, making their nests highly accessible to rats. These invasive rodents consume eggs and young birds, disrupting the breeding success of seabird colonies.

With fewer chicks surviving to adulthood, seabird populations cannot sustain themselves and begin to dwindle. This predation pressure leads to significant declines in seabird numbers, affecting the entire island ecosystem.

Seabirds play crucial roles in nutrient cycling and supporting other wildlife, so their decline impacts biodiversity and ecological health. Effective restoration efforts, such as rat eradication and habitat restoration, are essential to reviving seabird populations and restoring island ecosystems.

Restoring seabird populations

The first-of-its-kind study, led by marine scientists from Lancaster University, highlights the significant impact of restoring seabird populations on island ecosystems.

“We know that invasive species, such as rats, have devastating impacts on native seabird populations – they eat the eggs, chicks, and even sometimes adult birds,” explained Dr. Dunn, the lead author.

“Restoration projects that remove invasive species, such as rats, are effective. However, it is also important to know if restored seabird populations will have enough fish to hunt and eat, especially as threats like overfishing and climate change make fish populations more uncertain.”

Energy requirements and fish availability

The researchers assessed the energy needs of restored seabird populations and the quantity of prey fish using available data.

The focus was on the Chagos Archipelago in the Indian Ocean, which includes nature reserves where fishing is restricted. These protected areas may influence the availability of prey fish for seabirds.

“Our study was the first to factor in this important consideration, and encouragingly, we found that there is enough fish in the sea for restored populations of seabirds,” Dr. Dunn noted.

Potential of seabird restoration efforts

The team modeled three scenarios for 25 islands: complete rat eradication, partial restoration of native vegetation, and extensive vegetation restoration. The results were promising:

• Rat eradication alone could result in nearly 24,000 breeding pairs of lesser noddies, sooty terns, and red-footed boobies.
• Partial vegetation restoration (half of the island’s surface) could increase this number to 83,000 pairs.
• Extensive vegetation restoration (three-quarters of the island’s surface) could boost the breeding pairs to over 280,000.

Benefits to coral reefs

The positive effects of seabird restoration extend beyond the islands themselves. Seabird droppings, known as guano, are rich in nitrogen and phosphorus, which fertilize surrounding coral reef environments.

Increased seabird numbers could boost nitrogen input from 78 tons to 170 tons annually, leading to a 52% increase in fish biomass on the reefs, approximately 50,000 tons more reef fish.

Enhancing coral reef resilience

Restoration would also significantly increase grazing fish populations, such as parrotfish, which play crucial roles in maintaining healthy reefs by eating algae and removing dead coral. This helps reefs recover from disturbances like storms and bleaching events.

“These findings underscore the substantial role island restoration can have not just in bolstering vulnerable seabird populations, but also in enhancing the resilience of adjacent coral reefs to the impacts of climate change,” said Professor Graham, a co-author of the study.

Seabird restoration and rat species

The study emphasizes the importance of considering the broader ecological context when planning island restoration projects.

By modeling the impacts of different levels of vegetation restoration and available food sources, such projects can maximize benefits for nature.

In conclusion, restoring rat-free islands holds immense potential for reviving seabird populations and bolstering the health and resilience of coral reef ecosystems. The findings serve as a vital guide for future restoration efforts worldwide.

The study is published in the journal Conservation Biology.

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