Islands lost to the waves: how rising seas washed away part of Micronesia’s 19th-century history

File 20170824 6594 1ussrse.jpg?ixlib=rb 1.1 — Laiap, to the west of the site of the now-disappeared Nahlapenlohd.
Author provided

Patrick D. Nunn, University of the Sunshine Coast

At first glance it may not seem so, but the story of the now-vanished island of Nahlapenlohd, a couple of kilometres south of Pohnpei Island in Micronesia, holds some valuable lessons about recent climate change in the western Pacific.

In 1850, Nahlapenlohd was so large that not only did it support a sizeable coconut forest, but it was able to accommodate a memorable battle between the rival kingdoms of Kitti and Madolenihmw. The skirmish was the first in Pohnpeian history to involve the European sailor-mercenaries known as beachcombers and to be fought with imported weapons like cannons and muskets.

Today the island is no more. The oral histories tell that so much blood was spilled in this fierce battle that it stripped the island of all its vegetation, causing it to shrink and eventually disappear beneath the waves.

Like many oral tales, this one tries to explain island disappearance post-1850 by making reference to an historical event. But in light of what we know today, the more plausible cause of the island’s disappearance is the sea-level rise in the western Pacific since the early 19th century, which has accelerated significantly over the past few decades. The disappearance of islands in the Solomon Islands in the southwest Pacific has recently been attributed to sea level rise. Further north, the same is true of several reef islands off Pohnpei.

Pohnpei and its surrounding islands, both past and present.
CREDIT, Author provided

Surveys of 12 of these islands have shown that not only have some – like Nahlapenlohd – completely disappeared, but that most others have shrunk over the past decade. Islands such as Laiap and Ros, which have lost two-thirds of their land area over this time, are likely to disappear completely within the coming decade.

The island of Laiap has shrunk since 2007.
CREDIT, Author provided

Why are islands in the western Pacific becoming the earliest casualties of sea-level rise? Partly because sea levels in this region have risen at two to three times the global average over the past few decades.

In parts of Micronesia, sea level has risen by 10-12mm each year between 1993 and 2012, far outpacing the global average of 3.1mm a year. While this rate is unlikely to be sustained indefinitely, the current trend would raise sea levels by a further 30-40cm by mid-century if it were to continue.

What’s more, reef islands are particularly vulnerable to erosion by rising seas, being made almost entirely of sand and gravel. Whole islands – even some island nations with which we are familiar today – are likely to be rendered uninhabitable or even disappear within the next 30 years. These include islands in nations like Kiribati, the Marshall Islands, Tokelau and Tuvalu, as well as some in other island nations that comprise mostly larger islands, such as the Federated States of Micronesia, of which Pohnpei is one.

Armoured islands

Yet we should note that not all of Pohnpei’s reef islands are disappearing, at least not at the same rate, and some have fortuitously evolved protection that will likely help them outlive their neighbours.

The coasts of some islands – like Kehpara and Nahlap – are “armoured” by beaches of huge boulders left there by large storms, often along their most exposed coasts. Other reef islands off Pohnpei’s leeward coast, such as Dawahk, are becoming “skeletonized” as waves wash across the island removing the sand and leaving only rocks, held in place by a maze of giant mangrove roots.

Whether or not the islands themselves succumb or survive, sea-level rise is a clear threat to their habitability for humans. Short-term interventions – either natural fortifications such as boulder beaches, or human-built defences such as seawalls – are unlikely to change the long-term outcome.

This underscores the fact that low-lying reef islands are transient – most Pacific reef islands formed only in the past 4,000 years after sea levels fell and sediment began to pile up on exposed reef platforms. The sea will remove today’s islands, just as it has washed away countless others before.

But of course we cannot ignore the human dimension. While only a few dozen people today call the reef islands of Pohnpei home, they are similar to many larger reef islands in Micronesia from which people may well be involuntarily displaced during the next few decades. Where these people might go, and how they can be accommodated in ways that preserve their dignity as well as their unique cultures, are very real questions for community leaders.

People first reached the islands of Micronesia from the Philippines, about 3,500 years ago after an unbroken ocean crossing of 2,300km. It’s an extraordinary achievement when you consider that people in most other parts of the world at that time rarely sailed out of sight of land. To have survived on islands in the middle of the ocean for more than three millennia, Micronesians and other Pacific islanders must have developed considerable resilience.

On high islands in Micronesia, the evidence for this is manifest. Ancient stonework constructions line many parts of the coastline, testament to a long
history of resisting shoreline change, and sometimes of manipulating it for human advantage.

Perhaps nowhere is more evocative of this today than Nan Madol, a megalithic complex built 1,000 years ago on 93 artificial islands off southeast Pohnpei. There are many explanations about why Nan Madol was created. Perhaps the truth is that it is an expression of dogged human resilience – one of hundreds along Micronesian coasts – in the face of an unruly nature.

I thank my co-researchers on the project focused on Pohnpei’s reef islands, Augustine Kohler from the Department of National Archives, Culture and Historic Preservation of the Government of the Federated States of Micronesia, and my colleague Roselyn Kumar from the University of the Sunshine Coast’s Sustainability Research Centre.

Patrick D. Nunn, Professor of Geography, Sustainability Research Centre, University of the Sunshine Coast

This article was originally published on The Conversation. Read the original article.

Guam’s forests are being slowly killed off – by a snake

File 20170831 24267 mj7bts — Guam’s trees are struggling without the birds that spread their seeds.
Author provided

Elizabeth Wandrag, University of Canberra and Haldre Rogers, Iowa State University

Can a snake bring down a forest? If we’re talking about the Pacific island of Guam, the answer may well be yes.

Our research adds to mounting evidence that the killing of many of the island’s bird species by an invasive species of snake is having severe knock-on effects for Guam’s trees, which rely on the birds to spread their seeds.

Invasive predators are known to wreak havoc on native animal populations, but our study shows how the knock-on effects can be bad news for native forests too.

Globally, invasive predators have been implicated in the extinction of 142 bird, mammal and reptile species, with a further 596 species classed as vulnerable, endangered or critically endangered. But the indirect effects of these extinctions on entire ecosystems such as forests are much harder to study.

The brown tree snake was accidentally introduced to Guam in the mid-1940s and rapidly spread across the island. At the same time, bird populations on Guam mysteriously began to decline. For years, no one knew why.

In 1987 the US ecologist Julie Savidge provided conclusive evidence that the two were linked: the brown tree snake was eating the island’s birds. Today, 10 of Guam’s 12 original forest bird species have been lost. The remaining two are considered functionally extinct.

The brown tree snake has caused a cascade of problems.
Isaac Chellman, Author provided

But the ecological damage doesn’t stop there. The loss of native bird species has triggered some unexpected changes in Guam’s forests. Both the establishment of new trees and the diversity of those trees is falling. These changes show how an invasive predator can indirectly yet significantly alter an entire ecosystem.

Birds and trees

Birds are very important to trees. In the tropics, up to 90% of tree species rely on animals, often birds, to spread their seeds. Birds eat fruit from the trees and then defecate the undigested seeds far away from the parent tree’s canopy, where there are fewer predators and pathogens that specialise on that species, where competition for light, water and nutrients is less intense, and where seeds can take advantage of promising new real estate when old trees die.

Without birds, roughly 95% of seeds of two common tree species on Guam (Psychotria mariana and Premna serratifolia) land directly beneath their parent tree. Compare that with the nearby islands of Saipan, Tinian and Rota – none of which have brown tree snakes – where less than 40% of seeds land near their parent tree. On Saipan, seeds that escape their parent tree are five times more likely to survive.

Close neighbours, but very different situations.
Author provided

What’s more, passing through the gut of an animal can actually increase the likelihood that a seed will germinate. On Guam, seeds that had been eaten by birds were two to four times more likely to germinate than those that hadn’t.

Overall, for the roughly 70% of tree species on Guam that rely on birds to spread their seeds, research suggests that the bird deaths caused by the brown tree snake have reduced the establishment of new tree seedlings by 61-92%, depending on the species.

Forests’ future threatened

These numbers suggest that many tree species in Guam are under serious threat, which in turn threatens the species diversity of the island’s forests.

Our new research, published in Proceedings of the National Academy of Sciences, examined the number of seedling species growing in treefall gaps on Guam compared with Saipan and Rota, which still have their birds.

Treefall gaps appear when an adult tree dies, opening up the canopy and increasing the light that reaches the forest floor. Many species rely on this increased light for germination and early growth, so these gaps are hotspots for new seedlings.

Birds such as the Mariana fruit dove are a big help to the islands’ trees.
Lainie Berry, Author provided

We found that Saipan and Rota had roughly double the number of species of seedlings growing in these gaps, compared with Guam. What’s more, seedling species on Guam tended to be clumped together, as you might expect if more than 90% of seeds are falling beneath their parent trees.

We also found that birds are important in moving the seeds of certain types of species to gaps. In forests, “pioneer species” are those that rapidly colonise gaps, exploiting the increased light to grow fast and reproduce young. Crucially, we found pioneer species in all gaps on islands with birds, but in very few gaps on Guam, where these species could be at risk of being lost entirely.

Invasive predators are a reality for many ecosystems, particularly on islands, and the situation on Guam is particularly extreme. Perhaps nowhere else in the world has experienced such dramatic losses of native fauna as a result of invasion.

While these direct impacts of invasion are astounding, the indirect impacts cascading through the ecosystem are just starting to unfold, and may prove to be similarly catastrophic.

Elizabeth Wandrag, Postdoctoral Fellow, Ecology, University of Canberra and Haldre Rogers, Assistant Professor, Iowa State University

This article was originally published on The Conversation. Read the original article.