One year after invasive rats were removed from Ulong Island in Palau during 2024, a nearby reef site showed a 183% increase in fish biomass and roughly an 80% rise in leaf nitrogen. The results offer the first experimental evidence that removing a single ecological threat—invasive rodents—can trigger measurable recovery across both terrestrial and marine ecosystems within months, not years.
Researchers from Island Conservation and the Scripps Institution of Oceanography at the University of California San Diego conducted before-and-after monitoring of Ulong Island, comparing results with nearby Ngeruktabel, where invasive rats remain. The work was part of the Island-Ocean Connection Challenge, or IOCC, and tracked seabirds, nutrients, reef fish, and other marine life across both sites.
How Seabirds Restore Nutrients
The mechanism behind the reef recovery is straightforward biology. Seabirds feed in the ocean but nest on land. Their droppings, known as guano, carry nutrients from the sea to islands. Rain washes some of those nutrients into nearby coastal waters, where they support marine life. Before the rat removal, invasive rodents preyed on bird eggs and chicks, preventing seabird populations from recovering and cutting off this nutrient flow entirely.
After the rats were removed, seabird populations rebounded sharply. Calls of Bridled Terns rose by 286%, while Brown Noddy and White Tern calls increased by around 50% compared with the control island. The endangered Palau Ground Dove was detected more often after restoration work. Each returning bird represented restored nutrient cycling—a free ecosystem service that no government program could replicate.
Coral Wolf, Conservation Impact Program Manager at Island Conservation, noted the speed of change: "Seeing measurable ecological change just one year after restoration is extraordinary. It demonstrates the power of local leadership and science working together to heal island ecosystems from ridge to reef."
Market and Livelihood Implications
The fish biomass increase carries direct economic weight. Nathaniel Hanna Holloway, a marine ecologist at Scripps, emphasized the human dimension: "For the first time in Palau, we're seeing measurable evidence of seabird-driven nutrient flow returning to the land and sea. It's powerful proof that terrestrial action spills over into benefits for surrounding reef communities, which people rely on for their livelihoods."
Palau's economy depends heavily on reef health. Fish stocks feed local populations and support tourism. The 183% biomass increase at one reef site suggests that targeted, science-based interventions—removing a specific invasive species—can restore productivity faster than waiting for natural recovery or relying on broad regulatory restrictions that burden fishing communities.
Researchers also tracked nitrogen signatures in island soil and surrounding waters. Decline in nitrogen happened more slowly on Ulong than at other sites, matching what scientists expected if seabird guano was beginning to add nutrients back into the soil and nearby marine environment. Fish populations responded earlier than expected, with the significant increase in biomass suggesting returning nutrients were already supporting reef productivity.
The study's strength lies in its experimental design. Previous observational studies had suggested a link between land restoration and marine recovery, but the Ulong project is the first to demonstrate the process through before-and-after monitoring and direct comparison with a control island where rats remained. This rigorous approach eliminates guesswork and provides policymakers with concrete evidence of cause and effect.
Why This Matters:
The Ulong Island results demonstrate that targeted, science-backed environmental interventions can deliver measurable economic and ecological returns within a single year—without requiring sweeping regulations or government expansion. The rat removal required local action, private research partnerships, and clear baseline measurements. The results show how removing a specific threat to a system allows natural market mechanisms (nutrient cycling, fish reproduction, seabird nesting) to restore productivity on their own. For island nations like Palau, where reef health directly affects food security and economic activity, this evidence suggests that focused conservation efforts deliver faster returns than broad restrictions on human activity. The findings also underscore why invasive species control ranks among the highest-impact environmental investments available—the cost of removal produces cascading benefits across ecosystems and human communities that depend on them.