From BBC Future Planet, 8 October 2020 by Sophie Hardach
Palmyra had been an isolated and tranquil Pacific atoll, until a 20th-Century invasion of black rats arrived, setting the whole atoll’s ecology hurtling down a different path.
By the end of the century, the rats and palms had transformed the atoll’s entire ecosystem. Eight seabird species roaming the wider area were ominously missing – according to conservationists, possibly because the rats had driven them into local extinction. Some crab species were dwindling, or had even completely disappeared from sight. On other tropical islands, evidence emerged that rodent invasions were affecting species as ostensibly far-removed as coral reefs, by disrupting their supply of nutrient-rich seabird droppings.
The coconut palms also damaged the delicate chain of nutrients that sustained life on and around Palmyra. They took over half the atoll. Seabirds avoided nesting in the palms, preferring sturdy native trees with branches. As the supply of bird droppings declined, the impact rippled through the ecosystem. On islets with palm forests, the soil was poorer in nutrients than on those with native forests, as was the water running off them. The plankton along the palm forest coastlines was less abundant, and there were fewer manta rays, which feed on plankton, than along native forest coastlines.
The cascading effects show how an intricate ecological web can be broken by a single invasive species. But there is now also evidence that this web can be repaired.
“Islands offer us this opportunity for hope and for restoration, because you can remove invasive species from islands and see a dramatic recovery,” said David Will, a programme manager for Island Conservation, a non-profit organisation that specialises in invasive species removal.
Islands play an outsized role in our planet’s biodiversity. They make up only 5% of our planet’s land area, but are home to an estimated 19% of its bird species and 17% of flowering plants. Tropical atolls like Palmyra also present us with a compelling ecological mystery: they are lush and teeming with life, yet exist in very nutrient-poor environments.
“When you start thinking about atolls, they’re in very, very remote places where there’s very, very little input of nutrients to the environment,” says Rebecca Vega Thurber, a microbiologist at Oregon State University and expert in marine ecosystems.
Seabirds act as a kind of nutrient delivery service for these remote places. They nest in native trees, sheltered by branches and foliage, and fly far out to sea to catch fish. When they return, their droppings, called guano, fertilise the soil and run off into the water, nourishing plankton and algae, and the fish that feed on them. The nutrient mix in guano, particularly the ratio of nitrogen to phosphorus, is thought to be ideal for corals, as well as the beneficial algae that live in them.
But when rats arrive, all that changes. Rats can destroy an island’s seabird population, and thus its nutrient supply. Rat droppings only recycle what’s already on an island, as they don’t add nutrients from further afield.
“Once you lose the seabirds, you are no longer getting these nutrient links from the open ocean to islands and reefs,” says Casey Benkwitt, a biologist and coral reef specialist at Lancaster University. “So you completely lose this nutrient subsidy going onto reefs.”
A 2018 study in the Chagos Archipelago in the Indian Ocean found that islands without rats housed much larger seabird communities and had substantially higher nutrient levels in their coral reefs, compared to islands where rats had been introduced.
That nutrient boost may make reefs more resilient. Benkwitt and her colleagues studied coral bleaching around the Chagos Archipelago, a stress reaction to warming water during which corals expel their symbiotic algae and turn white. They found that the corals around rat-inhabited and rat-free islands were equally affected by such bleaching. But near the rat-free islands that had plenty of seabirds, a pink, crusty algae known as crustose coralline algae grew in the aftermath of bleaching. This algae attracts baby corals, which then settle on it and grow, forming the basis for replenishing the stressed out reef. There were also more herbivorous reef fish around rat-free islands, which graze on seaweed that might otherwise crowd out corals.
It became clear that by inadvertently introducing one invasive rodent species, an entire tropical island ecosystem could be pushed to the brink.
Rats in the canopy
“When I first got to Palmyra, rats were present everywhere,” recalls Alex Wegmann, director of science for Palmyra at The Nature Conservancy, a non-profit environmental organisation that bought the atoll from private owners in 2000. The atoll is now a National Wildlife Refuge managed by the US Fish and Wildlife Service, which includes a nature preserve managed by The Nature Conservancy. Wegmann has been studying Palmyra’s ecosystem since 2004, when rats dominated the island. “They were in the canopy, they were on the ground, they were under the ground.” They even chewed their way into his tent.
About 20,000 rats lived on Palmyra, with a density about 10 times higher than in cooler climates, thanks to the tropical environment. Rats in the equatorial tropics breed all year round, because it is always warm and there is plenty of food. The plan was to get rid of the rats, and then the palms, which were likely to spread further without the rats keeping them in check.
For an eradication to be successful, all of the rats on an island have to be killed, otherwise the population will rebound. According to James Russell, a conservation biologist at the University of Auckland, the rule of thumb is that if a single pregnant rat is left on a 1,000-hectare (3.9 square miles) island, the place will be overrun by rats within two years – and even sooner in the tropics.
Such complete removal is difficult anywhere, but especially in the tropics. A 2015 study found that 16% of rat eradications on tropical islands failed, compared to about 6% outside the tropics. Land crabs are one common problem. They are immune to the poison in the bait but enjoy eating it, and can gobble it up before the rats ever get to it.
“I’ve seen them have one bait under each of their eight legs, one in their mouth and one in each arm,” Russell says. “They roll one under each leg and then they stand over them – mine, mine, mine, mine. And they get all snappy at you when you come near them.”
The solution lies in diligent preparation, according to Araceli Samaniego, a rodent ecologist at Landcare Research, a research institute in New Zealand. She has worked on eradication projects around the world but started out in Mexico, where her team removed rats from 15 tropical islands, with a success rate of 100%. Their method involved spending months and even years studying each ecosystem, camping on uninhabited islands for weeks at a time to understand its seasons, features, plants and animals.
“Ninety percent of the work is before you get there with the bait,” Samaniego says. On one island, she and her team even learned to work around a thriving local crocodile population. “By year three, you are totally chilled with them, and they all have names. It’s just one more thing you need to watch out for, but it’s fine.” Another time, they tracked a rat with a radio collar, only to find the signal led them to a rattle snake – which had eaten the rat.
Such preparation meant that on the day of the eradication, the team faced no surprises and knew exactly where, how and when to place the bait for maximum effect. In flooded mangrove forests, for example, they attached bait blocks to the mangrove branches.
On Palmyra, where Samaniego also worked alongside scientists from all over the world, the rats were eradicated in 2011, in a joint project with the US Fish and Wildlife Service and Island Conservation. The team chose a time when most of the migrating birds were away. They temporarily captured as many of the remaining ones as they could. A helicopter dropped the poisoned bait so that some of it landed in the tree crowns, where it could be reached by the rats but not the crabs, while some would fall to the ground. Since then, Palmyra has been rat-free.
Still, the eradication came at a price. Twelve birds and 47 mullets were later found dead with rat poison residues in their bodies.
Coral Wolf, a biologist who works for Island Conservation, travelled to Palmyra a month after the eradication. Before leaving, she checked her gear for seeds and bugs, and froze her clothes to kill off any unwanted passengers that might cling to them. Then she boarded a small plane from Hawaii to the atoll, which lies 1,000 miles to the south. On arrival, several sets of never-worn clothes went straight into a freezer at the research station on the main island. Every time Wolf set off for the other islets in the atoll, she slipped into these frozen clothes to prevent any cross-island contamination. “That’s always refreshing to put on at the beginning of the work day,” she says.
Wolf was looking for signs of a recovery in native plants such as the Pisonia tree, which is ideal for nesting seabirds. She hoped to spot a few sprouts. Instead, she found a “whole carpet of Pisonia seedlings”. Previously, the rats had kept them in check. A year later, the Pisonia seedlings were knee-high. After two more years, they measured a couple of metres. By 2016, they were “way overhead”, Wolf recalled. Other native tree species also gained ground. The local forest was bouncing back.
The Nature Conservancy’s Wegmann also noticed innumerable changes once the rats were gone. Two new land crab species were observed on the islands. Presumably, they had been around before, but the rats had kept their numbers too low to be spotted. Once, he walked along the shore and saw 50 rock crabs running along. Before, he had only ever seen them in groups of two or three: “There were all of these super cool observations of life rebounding.”
Palm leaves everywhere
Not all of that rebounding life was native to Palmyra. Invasive coconut palms from abandoned plantations were spreading, more vigorously than they had before. This had been expected, given that the rats were no longer eating the seedlings, but was still striking to observe.
“All of a sudden, you could not walk anywhere,” Wolf remembers. “Your face was in all these palm leaves.”
Like rats, palms can damage the land-sea link, because seabirds avoid nesting in their swaying, exposed crowns. Getting rid of the palms required other interventions, including injecting the trunks with herbicide, and is still ongoing. The other challenge was to coax back the eight missing seabird species to the atoll.
One ongoing effort to tempt them back involves what Wegmann calls “our seabird discotheque”: electronic speakers that emit the calls of four seabird species, in a kind of public broadcasting message to passing birds. They are meant to attract the Phoenix petrel, the tropical shearwater, the wedge-tailed shearwater and the white-throated storm-petrel. While the efforts are ongoing, the researchers have yet to see first signs of any of the missing eight seabird species returning.
To take it up a notch, next the team plan to place wooden replicas of two more species, the grey-backed tern and blue noddy, around the atoll in clusters that resemble nesting colonies. The idea, Wegmann says, is that a seabird flying over might see them “and think, oh wow, there’s 30 of my relatives sitting there, maybe there’s something going on there, I should go check it out”.
‘Beautiful and restorable’
Encouraged by evidence that restoration is possible on atolls like Palmyra, conservationists are turning to other tropical islands. A team of specialists including, Landcare Research’s Samaniego, are planning to eradicate rats from islands in the Chagos Archipelago, using drones to drop the bait, a practical solution for remote islands.
On the Tetiaroa Atoll in French Polynesia, north of Tahiti, a project is underway that is similar to that on Palmyra. Tetiaroa is uninhabited except for a research station and a luxury resort. It presents specific challenges, such as the co-existence of two rat species: the Pacific rat, thought to have been introduced by Polynesian voyagers centuries ago, and the black rat, which probably arrived in the 20th Century. Rats swim back and forth between some of the 12 islands, meaning that if you remove them from only one, they may come back from another.
Nevertheless, one of the Tetiaroa islets, Reiono, was freed from rats in 2018, and two more followed this year. The goal is to eradicate rats from the entire atoll.
“It’s a beautiful atoll, and it’s a restorable atoll,” says the University of Auckland’s Russell. He first studied the rats on Tetiaroa with a colleague in 2009, kayaking between the islets and sleeping in tents. From a scientific perspective, the 12 small islands are like “micro-laboratories”, he said, each subtly different.
“We’re at a time where we have more capacity to do these, what I call ‘super ecosystem experiments’, where you can track multiple things by getting collaborators from all over the world to do their part and then integrate the data,” says Oregon State University’s Rebecca Vega Thurber. She is overseeing a new marine ecology research project at Tetiaroa, and plans to do similar research at Palmyra, unpicking the interactions between land conservation and marine life.
Looking back on Palmyra’s recent history, Wegmann sees it as a source of hope. Tropical islands may seem fragile, but they can turn out to be surprisingly robust. The conservation story of Palmyra should remind us of two things, he says: “One, nature is resilient. And two, we as humans can engineer ourselves out of some of the major problems that we’ve caused.”