What can Pacific islands teach us about invasive species?

Smack-bang in the center of the Pacific Ocean – slightly north of the equator and about 1,800 kilometers south of Hawaii – lies uninhabited Palmyra Atoll, a U.S. National Wildlife Refuge managed by The Nature Conservancy (TNC).

This year, a group of zoologists spent their Easter weekend there hunting for – and finding – Easter eggs of an unconventional kind: those laid by four much-watched pairs of sihek, or Guam kingfishers (Todiramphus cinnamominus).

“While they might not be as shiny or brightly colored, these tiny, unassuming-looking eggs are far more exciting and precious than any of their chocolate counterparts,” enthuses John Ewen of the Zoological Society of London (ZSL), who chairs its global Sihek Recovery Program Team.

“They are a remarkable milestone for the decades-long mission to rescue the sihek from the edge of extinction.”

His excitement was understandable: these eggs were the first laid by the species in the wild in almost 40 years.

Native to Guam, the sihek became extinct there in the 1980s after the accidental introduction of the brown tree snake (Boiga irregularis), a highly invasive species with a taste for birds and their eggs.

As the population plunged, a group of local biologists brought 29 of the birds into human care and began a decades-long conservation breeding program in zoos across the United States.

Finally, in September last year, four pairs of sihek were introduced to pest-free Palmyra Atoll. The new clutches of eggs are cause for hope that they will not only thrive there but one day – after the snakes are eradicated – return to Guam, too.

“The connection I feel with these birds is a kinship I hope for all to feel,” says Yolonda Topasña, a program coordinator at the Guam Department of Agriculture Division of Aquatic and Wildlife Resources (DAWR).

“I envision our people, the CHamoru, those who dwell in our beloved Guåhan [Guam] and those scattered across the vast oceans, to feel the same stirring of ancestral pride.”

Why are Pacific ecosystems so vulnerable to invasive species?

The story of the sihek resonates with that of screeds of Pacific Island species and communities. Before humans began moving plants and animals around, both deliberately and unwittingly, their dispersal across these islands was extremely slow and chancy.

For instance, new evidence shows that the forebears of Fiji’s native iguanas floated there around 31 to 34 million years ago from southwestern North America – more than 8,000 kilometers away.

It’s most likely that these desert-dwelling forebears were swept downstream during a flood and climbed onto ‘rafts’ of vegetation – probably snacking on them as they went. These natural vessels eventually carried them on ocean currents all the way to the archipelagos of present-day Fiji.

Given what it takes for terrestrial species to make it to islands, it’s not surprising that their ecosystems tend to see fewer species and higher levels of endemism than their continental counterparts. That also makes them particularly vulnerable to the impacts of invasive species.

For example, many Pacific islands evolved without any mammals apart from bats, which could fly there. Free of the mammalian predators common to other ecosystems, numerous bird species in the region evolved to be flightless.

This made them almost-literal ‘sitting ducks’ for predation from the rodents, mustelids, marsupials, cats and dogs that came later alongside human voyagers.

Because these ecosystems are so unique and delicately balanced, invasive plants and animals “can completely alter ecosystems and, consequently, the cultural and ecosystem services they provide,” say the co-authors of a recent study on the topic.

This, they note, threatens food and water security and reduces both environmental and human resilience to the climate crisis.

For instance, invasive Merremia vines smother both croplands and forest canopies on many Pacific islands. This reduces agricultural productivity and compromises key forest functions by reducing the production of flowers and fruits on which fruit bats and native birds rely.

How do invasive species make it to the Pacific islands?

The International Union for the Conservation of Nature (IUCN) has identified invasive species as a threat to 1,531 species in the Pacific Islands region to date. But how are these pesty plants and animals getting here, and is there still time to turn them back?

Many species were introduced deliberately and then went wild. Goats and pigs were initially brought as food sources; cats, dogs and even giant green iguanas came as pets.

Various ornamental plant species, like the crimson-red firespike shrub (Odontonema strictum) and the lilac-toned water hyacinth (Pontederia crassipes), were welcomed in by the region’s legions of proud gardeners, only to spread uncontrollably, choking coffee plantations and clogging up waterways.

Other species, particularly rodents, plants and insects, arrived accidentally, perhaps finding their ways onto ships or planes, getting stuck in shirt sleeves and pockets, or hiding in crates of food.

In a particularly ironic twist, a number of invasive species were introduced to the region to control otherspecies – and then ended up creating more problems of their own.

Cane toads (Rhinella marina), for instance, were brought to Fiji to eat the insects chewing on the country’s sugarcane plantations. Now, they teem thickly on many of its islands: some places have over 4,000 toads per square hectare, outcompeting native species such as the Fijian ground frog (Platymantis vitianus).

The small Indian mongoose (Herpestes javanicus) was introduced to Fiji and Hawaii for similar purposes – to eat the rats plaguing the sugarcane. It’s now responsible for the extinction of numerous native bird species and threatens many more.

Some native species can also become invasive if their natural predators are depleted, or if the environment changes in other ways.

Crown-of-thorns starfish (Acanthaster planci) populations, for instance, spike in the Cook Islands where there is overfishing and high nutrient runoff, destroying coral reefs and starving other animals that live there.

Inspiration from a no-longer-desert island

Yet despite these specific vulnerabilities and challenges, small isolated islands also have their advantages when it comes to invasive species.

Namely, it’s much easier to fully eradicate them from entire islands, and create relatively safe havens for endangered species, than on larger land masses.

Once the pests have gone, with monitoring and biosecurity put in place to stop them from returning, community members and conservationists frequently find that native populations can recover relatively quickly.

In many parts of the Pacific, they’re honing how to help these species return – and achieving some inspiring results.

Let’s revisit those native Fijian iguanas for an example. Monuriki is best known as the ‘desert island’ where Tom Hanks, as FedEx troubleshooter Chuck Noland in the 2000 film Cast Away, was stranded when his plane crashed in the South Pacific.

The island has its own genetically distinct population of the critically endangered Fijian crested iguana (Brachylophus vitiensis). But when Tom Hanks got washed up there, the forest was dying and the iguanas were almost gone: a 1998 survey estimated their population at less than 100.

By then, the islet’s dry broadleaf forests had been degraded over decades by a small but voracious contingent of feral goats and rats, who were also predating on the iguanas’ eggs and babies.

In 2011, Monuriki’s owners, the mataqali (clan) of Vunaivi, who live on the neighboring island of Yanuya, sought support from the National Trust of Fiji to remove goats and rats from the island and control landings there to protect against new incursions.

Once that was done, the vegetation began to rapidly recover. In 2015, they began introducing new iguanas from a captive breeding program.

Now, the community is supporting the iguanas’ continued survival by replanting the island’s forests with the iguanas’ preferred native fodder: four tree varieties with small fleshy fruit.

Once these new trees are established and fruiting on the island, the lizards will pass the seeds through their gut and distribute them further.

Thanks to the strategic removal of certain species and reintroduction of others, the iguana population and its forests are finally well-placed to grow back – together.