Oh, bird poop! Study shows invasive island predators can even disrupt life offshore

November 7, 2022

DeKalb, IL – A new study identifies the “circular economy” of seabirds linking land and sea and shows how invasive predators on an island can disrupt even what’s happening offshore beneath the waves.

A grey-faced petrel on Korapuki, the Mercury Islands, Aotearoa, New Zealand. Photo credit: © Steph Borrelle

What’s more, at the heart of it all is, well, bird poop.

Led by Professor Holly Jones of Northern Illinois University, the researchers studied four northern New Zealand islands in the same archipelago—two with histories of marauding invasive mammals such as rats, rabbits or cats and two that remain untouched by non-native predators.

Seabirds themselves are top hunters in the ocean, feeding on squid and fish. On the islands where they breed, the seabird guano, aka bird poop, is so rich in nitrogen and other nutrients that it’s sometimes called “white gold.” Along with the dead flesh of seabird prey, guano fertilizes island soil and runs off the islands into the sea, where it drives plant diversity and enriches nearshore aquatic ecosystems used by sea life as sources of habitat and protein.

The scientists hypothesized and demonstrated that the loss of island seabird populations would impact the composition and quality of nearshore seaweed, or macroalgae, which plays a vital role in fishery habitats, carbon sequestration and atmospheric oxygen production.

NIU Professor Holly Jones

“We found that nearly half of the variation in algae or seaweed-community composition in the nearshore ecosystem could actually be attributed to characteristics on the island—specifically seabird density, seabird-derived nutrients in soils and invasion history,” Jones said.

The new study was published recently online in the journal, Restoration Ecology. Jones is a faculty affiliate of the Northern Illinois Center for Community Sustainability and holds joint NIU appointments in biological sciences and environmental studies. Her coauthors are Dr. Lyndsay Rankin, a former NIU Ph.D. student who led a dive team in recovery of marine samples, and Dr. Stephanie Borrelle of Birdlife International.

An island country, New Zealand’s only native mammals are bats, so invasive predators have particularly strong impacts. Globally, 31 percent of seabirds are threatened, the highest percentage of any bird group. Over the last four centuries, most species (including seabirds, mammals and reptiles) driven to extinction have been island species, with most of those extinctions caused by invasive predators.

Lyndsay Rankin collects data on macroalgae community composition surrounding Korapuki, the Mercury Islands, Aotearoa, New Zealand. Photo credit: Evan Brown.

“By looking at the characteristics of certain seaweed communities, we could tell which islands had histories of invasive mammals, and the striking amount of variation confirms the importance of land-sea linkages that seabirds provide,” Jones said. “It was surprising to find that land variables are as important in driving the composition of seaweed communities as marine variables.”

Guano runoff from the islands fertilizes the nearshore ecosystem, impacting the abundance and types of seaweed species; it also enriches levels of nitrogen isotopes in the plants, making them more nutritious for sea creatures.

“Ocean herbivores need nutritious green stuff, and nutrient content in seaweed is passed up through trophic levels in the ocean,” Jones said.

“We found that nearshore ecosystem composition recovered more quickly than seaweed nutrient uptake,” she added. “On the island where invasive mammals were removed three decades ago, the composition of seaweed species was similar to the untouched islands. But nitrogen uptake levels in those species remained lower.”

Gannet backlit off the coast of Korapuki Island, the Mercury Islands, Aotearoa, New Zealand. Photo credit: © Steph Borrelle

Jones and her colleagues noted that some algae species are better indicators of seabird influence than others. On the two islands with predator invasion history, the researchers found four out of six species of seaweed studied showed depleted levels of seabird-derived nitrogen, indicating they had not fully recovered.

The studied islands are part of the Mercury Islands group, located eight kilometers off the east coast of the Coromandel, New Zealand. Burrowing seabirds that populate the islands include the fluttering shearwater, flesh-footed shearwater, little shearwater, Northern common diving petrel, Pycroft’s petrel, grey-faced petrel, white-faced storm petrel and little blue penguin.

Among many variables studied, the scientists quantified burrow density on the islands. On the newly eradicated island of Ahuahu, only one apparently unoccupied burrow was discovered. The average seabird burrow density on the island of Korapuki, where predators were eradicated 30 years ago, was still only 14% of the burrow density on never invaded islands.

Seabird burrows on Korapuki. Photo credit: © Steph Borrelle

The researchers also calculated the density of the sea urchins offshore that feed off seaweed. Previous studies have noted the importance of herbivory in driving macroalgae community composition.

“Seabird burrow density, soil nutrients and invasion history together explained more variation in macroalgae than sea urchins, the predominant seaweed herbivore,” Jones said.

Predator eradication can lead to recovery of seabird colonization and is critical to restoring populations. But to better inform policy and restoration efforts, it is important to understand whether removing predators is enough to precipitate recovery of land-sea linkages.

“Our study findings suggest that seabirds take a long time to recover from mammal invasions, and their influence in terrestrial and marine food webs, also takes a long time,” Jones said. “This work is unique because most researchers study impacts of invasive mammals and island recovery either on land or at sea, but very rarely do they connect the two together.”

Media Contact: Tom Parisi

About NIU

Northern Illinois University is a student-centered, nationally recognized public research university, with expertise that benefits its region and spans the globe in a wide variety of fields, including the sciences, humanities, arts, business, engineering, education, health and law. Through its main campus in DeKalb, Illinois, and education centers for students and working professionals in Chicago, Naperville, Oregon and Rockford, NIU offers more than 100 areas of study while serving a diverse and international student body.