When invasive rats are removed from islands, the ecological benefits can ripple across both land and sea more quickly than scientists expected, according to recent research.
Scientists have long assumed that meaningful recovery after the predators are eradicated would take decades. However, researchers with the U.S.-based NGO Island Conservation conducted a rat-removal experiment on Ulong Island in Palau, which provides the first experimental evidence that ecosystems can rebound far more quickly than previously expected.
Until recently, rats, which are typically nocturnal, were so abundant on Ulong Island that they were regularly seen during the day. They were a nuisance to campers and deadly for wildlife.
As opportunistic omnivores, rats readily prey upon seabird eggs and chicks, devastating nesting colonies on tropical islands. As a result, there were “very few nesting seabirds that we would find,” Coral Wolf, the conservation science program manager at Island Conservation, told Mongabay in a video call.
To measure the effects of rat eradication, Wolf designed an experiment in which all the rats were removed from Ulong, while the rats on nearby Ngeruktabel Island remained, serving as a control site. Before the eradication, researchers collected baseline biodiversity data. On land, they recorded bird calls and took soil samples. In the surrounding water, they measured indicators like fish biomass and coral cover.
One year after rats were removed, the team repeated the survey and found a dramatic improvement in the biodiversity. Freed from rat predation, seabird activity on the island surged. Detections of bridled tern (Onychoprion anaethetus) calls rose by 286% while brown noddy (Anous stolidus) and white tern (Gygis sp.) calls increased by roughly 50%.
Seabirds are critical connector species in what scientists recently dubbed the “circular seabird economy.”
“They’re out foraging, they feed on fish [and] they bring those nutrients back to the island,” Wolf said. Such nutrients accumulate on land, improving soil quality, and are eventually washed back into the sea where they enrich surrounding marine ecosystems, she added.
Areas with large seabird populations are associated with more phytoplankton in the marine environment, higher fish biomass and better coral health, Wolf said.
On Ulong, researchers found fish biomass increased significantly once rats were removed. One location recorded a 183% increase. Increased nutrients in the water also appear to be supporting reef-building coral. In a statement shared with Mongabay, Island Conservation said early results suggest “seabird-derived nutrients [are] beginning to fuel reef productivity,” around the island.
“It’s powerful proof that terrestrial action spills over into benefits for surrounding reef communities, which people rely on for their livelihoods,” Nathaniel Hanna Holloway, marine ecologist at Scripps Oceanography, said in a statement.
Wolf said the team had expected such improvements to Ulong Island’s ecosystem would take decades. Seeing measurable gains after just a year, she said, “is pretty remarkable and gives us hope for the restoration of the Rock Islands across this island community.”
The study is currently being submitted for publication.
Banner image: A rainbow over Ulong Island. Image courtesy of Island Conservation.
This story first appeared on Mongabay
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