Tropical islands are often imagined as idyllic places: lush stands of coconut palms, white sand beaches, turquoise waters, and vibrant coral reefs. But few people realize that many of these islands were — and still are — profoundly shaped by seabirds.
Why Are Seabirds Important?
Seabirds — like boobies, frigatebirds, petrels, and terns — spend most of their lives feeding at sea, only returning to land to nest and raise their chicks. They often gather in dense colonies on isolated tropical islands and forage for protein-rich prey such as fish, squid, and zooplankton. They then transport these ocean-derived nutrients back to land in the form of their droppings, rich in nitrogen, phosphorus, and other trace elements.
For this reason, seabirds are often called “connector” species: they shuttle nutrients between the sea and the land. On islands with large seabird populations, these inputs fertilize soils, stimulate vegetation growth, and sustain the productivity of the entire terrestrial ecosystem.
Transforming Seabird Droppings into Guano
While fresh seabird droppings are often called guano, the term technically refers to older, mineral-rich deposits that have dried and accumulated over long periods. These deposits are chemically stable and nutrient-dense — essentially a natural fertilizer formed by time and the heat of the tropics.
The Incas recognized this value, mining guano from offshore islands and using it to fertilize nutrient-poor Andean soils. So vital was guano to their agriculture that disturbing nesting seabirds could be punishable by death.
By the 1800s, guano was in global demand and mined extensively on islands across the Pacific and Atlantic. The practice peaked in the mid-20th century on islands such as Nauru, which exported phosphate-rich guano for use in synthetically enhanced phosphate fertilizers.
Seabirds Nourish the Sea, Too
These nutrients don’t just enrich the land — rain and groundwater carry them into the surrounding ocean. This slow release fertilizes nearby coral reefs, seagrass beds, and algal mats in ways that differ from harmful, human-sourced runoff.
Seabird-derived nutrients support organisms like parrotfish and queen conch, and nourish the symbiotic algae within coral tissues. This enhances coral growth and resilience, helping reefs recover from bleaching events and hurricane damage. Healthy corals in turn support diverse, productive reef ecosystems.
When Seabirds Disappear, Nutrient Connections Are Broken
Seabirds thrive on islands free from mammalian predators and with intact native vegetation for nesting. Unfortunately, invasive species such as mice, rats and cats — introduced by humans — have devastated colonies around the world.
As seabird numbers fall, the nutrient flow from ocean to land and back to ocean again collapses. Research shows nitrogen inputs to rat-free islands can be orders of magnitude higher than nearby rat-infested islands.
The Good News? Restoration Works — If Done Holistically
Organizations like Island Conservation and the Chagos Conservation Trust have shown that tropical islands can recover through coordinated “ridge-to-reef” restoration. Eradicating invasive species and restoring native vegetation helps seabirds return and nutrient flows resume.
As seabird populations grow, more nutrients enrich island soils and leak into the sea — boosting coral and seagrass recovery. This also supports active restoration efforts, like transplanting corals and seagrasses to degraded areas.
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Further Reading
Benkwitt, C. E., et al. (2023). Seabirds boost coral reef resilience. Science Advances, 9(49). https://doi.org/10.1126/sciadv.adj0390
De La Peña-Lastra, S. (2021). Seabird droppings: Effects on a global and local level. Sci. Total Environ., 754, 142148. https://doi.org/10.1016/j.scitotenv.2020.142148
Dunn, R. E., et al. (2025). Island restoration to rebuild seabird populations and amplify coral reef functioning. Conserv. Biol., 39(1), e14313. https://doi.org/10.1111/cobi.14313
Graham, N. A. J., et al. (2018). Seabirds enhance coral reef productivity and functioning in the absence of invasive rats. Nature, 559(7713), 250–253. https://doi.org/10.1038/s41586-018-0202-3
Grant, M. L., et al. (2022). The influence of seabirds on their breeding, roosting and nesting grounds: A systematic review and meta-analysis. J. Anim. Ecol., 91(6), 1266–1289. https://doi.org/10.1111/1365-2656.13699
Linhares, B. d. A. & Bugoni, L. (2023). Seabirds subsidize terrestrial food webs and coral reefs in a tropical rat-invaded archipelago. Ecol. Appl., 33(2), e2733. https://doi.org/10.1002/eap.2733
Savage, C. (2019). Seabird nutrients are assimilated by corals and enhance coral growth rates. Sci. Rep., 9, 4284. https://doi.org/10.1038/s41598-019-41030-6
Schnug, E., et al. (2018). Guano: The White Gold of the Seabirds. In: H. Mikkola (Ed.), Seabirds. InTech. https://doi.org/10.5772/intechopen.79501
