The Remarkable Healing Power of Seabird Poop for Climate-Stressed Coral Reefs

When marine ecologist Casey Benkwitt set out to study coral reefs, she never thought she’d spend so much time thinking about bird poop. Now, six years after she started exploring how seabird guano boosts these underwater biodiversity hotspots, she sees how important it is to look for unexpected links in ecology. “It's really intriguing and inspiring to think about how everything is connected,” she says. 

The Chagos Archipelago, a cluster of more than 50 islands in the Indian Ocean, is home to about a million seabirds, including 282,000 nesting pairs that produce a squawking cacophony and a strong scent of guano. But not all of the Chagos islands are equally raucous or smelly: More than half have few or no seabirds at all. Avian populations on these quieter coasts were decimated by invasive rats, which arrived with ships centuries ago and eat eggs and chicks. (Only one of the islands’ seabird species—Red-footed Boobies—can defend itself and its chicks.)

By comparing marine life near islands with and without seabirds, Benkwitt and her collaborators have shown how healthy seabird populations boost coral reef health. The birds’ guano offers an infusion of nutrients—helping corals grow faster and fish thrive.

The team’s findings are now informing efforts to protect and restore seabird habitat on Chagos and may add urgency to island conservation projects elsewhere in the tropics. “When we actually put our efforts into restoration on islands, it can have these cascading benefits,” says Benkwitt.

From the Pacific Ocean to the Caribbean, boosting reef resilience is crucial as climate change ramps up and marine heatwaves cause corals to bleach more frequently. Even if global warming is limited to 1.5°C, scientists project only 10 to 30 percent of reefs will persist—if warming rises to 2°C, only 1 percent of today’s reefs may survive this century. In either case, reefs where corals can more fully rebound between marine heatwaves have a better chance of survival.

To investigate the benefits of guano, Lancaster University's Nick Graham began by studying whether its nutrients were, in fact, washing out into Chagos’s reefs, where they could nourish coral, fertilize algae, and spur the growth of algae-eating fish. His study found that yes, they were—and fish further up the food chain benefitted, too. Overall fish biomass was 48 percent higher in reefs near seabirds than those without.

Then a disastrous global coral bleaching event triggered by extreme marine heat in 2015 and 2016 became the basis for another experiment. On the Chagos reefs, bleaching was widespread and many corals died. The researchers wondered: Would guano help reef ecosystems bounce back?

Tracking the most common type of corals over several years, Benkwitt and her colleagues found that corals near seabird-inhabited islands grew more than twice as quickly. The team even moved corals from reefs without nearby bird colonies to reefs near them, and those corals also grew faster. Overall, corals recovered 10 months sooner in reefs that received guano nutrients. “I've been just surprised over and over again at the clear patterns that we're seeing,” Benkwitt says.

The Chagos Conservation Trust is already acting on these results. In 2019, the nonprofit formed its Healthy Islands, Health Reefs program to restore seabird colonies. First they tested methods of eradicating invasive rats using poisoned bait, and an ecological impact assessment confirmed that no other animals would be harmed. Workers also began removing coconut palm trees, which are remnants of defunct colonial-era plantations. These trees form inhospitable conditions for nesting seabirds—a treacherous landscape of rolling coconuts and dead palm fronds that program manager Peter Carr calls “coconut chaos.” Once the trees and their detritus are gone, says Carr, native vegetation grows back on its own.

So far, the group has successfully eliminated rats from one island and removed palm trees from several areas. Next, they want to scale up the work across most of the island chain. If 25 islands can be made rat-free and their native vegetation restored, the researchers recently estimated that as many as 280,000 breeding pairs of the three most common seabird species could take up residence, approximately doubling the archipelago’s nesting pairs and producing up to 170 tons of guano each year. According to the 2024 study, reef fish biomass could jump by 52 percent, and the supercharged corals would grow an average of 90 percent faster. 

More broadly, Benkwitt suspects that seabird conservation could be an important boon for reefs throughout the tropics, particularly in places like the Chagos Archipelago where human communities are small or nonexistent and fish populations are relatively healthy. She and colleagues are now studying the effects of guano on reefs in the Seychelles. While the research is in progress, they are already seeing similar benefits there.

However, the guano effect may not hold for reefs near more populated coasts. Runoff from coastal communities, including fertilizer and sewage, damages reefs. Unlike guano, it does not have a well-balanced ratio of nitrogen to phosphorus. What’s more, reefs that are overfished may not see the same fish populations boosts—and without enough grazing fish, algae may grow unchecked.

Explaining how guano supports healthy marine ecosystems is a focus of biologist Juliana Coffey’s outreach to fishers in the Grenadines, a chain of nearly 100 islands, islets, rocks, and cays fringed by the eastern Caribbean’s largest reefs. “You need to protect your birds because you rely on them for fisheries,” she tells the fishers. Many of the islands in this sprawling chain remain isolated avian havens, while seabirds tend to avoid those with more development, rats, cats, and other introduced animals.

“Seabirds are difficult to study because they nest in these remote islands, and they live these secret lives,” says Coffey. To track birds and assess threats over the vast area, she trains fishers and other locals to monitor nesting areas as part of the Grenadines Seabird Guardians program.

Last August, Coffey and Vaughn Thomas, a fisher and nest monitor, were searching the craggy islands for seabirds from Thomas’s boat, wondering if they had survived the recent hurricane. Though the storm impacted nesting and nearby vegetation, they found many of the birds had made it back to feed and were perched on the rocky coasts. Below, white guano dripped down the dark volcanic rocks, adding nutrients that will hopefully help sustain a healthy coral reef ecosystem—and the fishers who depend on it. 

Hungry seabirds, of course, also need abundant fish, and they need corals, too. Reefs shelter shores from storm erosion, and on atolls like those in Chagos, coral skeletons form limestone platforms that undergird the islands themselves. In other words, as sea levels rise and hurricanes intensify, the benefits of seabird conservation may come full circle.