Island Land Snails Face Devastating Extinction Losses

If you need a quiet yet brutally clear indication of how severely biodiversity is collapsing on islands, consider land snails.

A significant new review, led by Robert Cowie of the University of Hawai‘i at Mānoa, says the extent of the losses is not merely concerning – it is “devastating.”

On numerous high volcanic islands, land snail extinction rates typically sit between 30 and 80 percent, placing them among the animal groups most severely affected anywhere on Earth.

Although the paper assesses the issue globally, it concentrates on Hawai‘i and other Pacific islands for one straightforward reason: nowhere else has seen the disappearance of so many land snail species.

The researchers emphasise that this is not simply a case of little-known animals vanishing without notice. Rather, it is a collapse taking place in ecosystems that were once worldwide centres of evolutionary experimentation.

The decline of these snails provides a severe warning of what can occur when isolated biodiversity meets human expansion.

Islands as extinction hotspots

A key difficulty in documenting invertebrate extinctions is that many species disappear before anyone has even discovered that they exist. Land snails, however, possess an inbuilt record that most small animals lack: their shells.

After a snail dies, its shell may remain in the soil for decades and, in some cases, centuries. As the years pass, these dead shells accumulate into what scientists describe as a “shell bank.”

Such a shell bank functions as a time capsule, preserving signs of species that may have vanished long before modern researchers documented them.

The review refers to a well-known example from the Gambier Islands in French Polynesia, where shells uncovered a whole flourishing of snail diversity that would otherwise have remained unknown.

The authors point out that many islands are distant and that land snails receive limited attention within global biodiversity conservation, meaning the conservation assessments for numerous species are out of date.

Consequently, snails are not only being lost – they are frequently disappearing without being properly counted.

Ancient losses written into the landscape

Snail extinctions are not all modern. According to the review, climatic and sea-level changes during and after the last Ice Age contributed to the creation of “fossilized” sand dunes, which entombed many species.

On O‘ahu, for instance, some of these extinct snails remain visible in exposed deposits beside the trail to Ka‘ena Point from the Wai‘anae side.

Those losses occurred gradually over thousands of years, driven by natural environmental shifts.

People reshape island ecosystems

Human activity, however, is the dominant force behind present-day losses. Habitat destruction delivers the first major blow: forests are cleared, watersheds altered, and the moist, stable microhabitats required by snails are broken up. On many islands, invasive species then deliver the decisive blow.

High volcanic islands commonly developed snail communities that were exceptionally diverse as well as highly endemic.

Even relatively small islands could support 50 to 100 endemic species, as Rapa in the Austral Islands did. Hawai‘i was in an entirely different category.

“The Hawaiian Islands, especially, were home to at least 750 known species,” said Cowie, who is a research professor with the Pacific Biosciences Research Center.

“All but a tiny handful of which are found nowhere else on Earth. Estimates have suggested that only 10 to 35 percent of this spectacular diversity, including some of the well-known and beautiful Hawaiian tree snails, still survive, a mere fraction of the unique native Hawaiian natural heritage.”

Losing a Hawaiian snail species does not mean losing a local form of a widespread animal. It means the loss of a unique evolutionary experiment.

When people arrive, snails disappear

The review identifies a recurring pattern across islands: extinctions begin shortly after people arrive, often as a result of deforestation and altered habitats.

Following Western colonisation, the situation worsened – land was transformed more extensively, trade increased, and many more non-native predators were introduced.

Several of the most damaging offenders are familiar. Rats, for example, are highly destructive on islands. Yet the snail story also includes intentional introductions that became ecological catastrophes, particularly predators brought in to control other snails.

Two infamous examples repeatedly appear: the rosy wolf snail (Euglandina) and the New Guinea flatworm (Platydemus manokwari), both of which are effective snail predators.

“These have probably been the ultimate cause of extinction following the devastating habitat loss that initiated the extinction process,” Cowie said.

The pressures on island snails

Other, less conspicuous pressures also exist. Although land snails are not routinely eaten by most island communities, collecting shells – particularly attractive or striking specimens – can be significant when populations are already declining.

The review additionally mentions decorative uses, including using shells to adorn lei or hats. Such harvesting may not ignite the problem, but it can hinder recovery and make it more difficult for endangered species to rebound.

The review argues that climate change has not yet caused many recorded island snail extinctions – though it is likely to develop into a serious threat.

Species occupying cool mountain habitats encounter a firm boundary: as temperatures increase, they cannot move “up” indefinitely, and their climatic zone will ultimately vanish.

What’s keeping island snails alive

Despite the bleak figures, the paper is not wholly without hope. Conservation programmes are working actively to protect what remains, concentrating much of their effort on areas including Hawai‘i and the Society Islands.

Comparable work is taking place in Japan’s Ogasawara Islands, alongside Bermuda, Madeira’s Desertas Islands, and the Mascarene Islands in the Indian Ocean.

Safeguarding snails in the wild is difficult once invasive predators have become established within an ecosystem. Many initiatives therefore also depend on captive breeding.

This is not intended as a permanent answer, but as a means of stopping ancient lineages from disappearing altogether while habitat protection and predator-control measures gain traction.

Put another way: shells reveal what has already been lost. Captive programmes are among the limited tools preventing the “what’s left” column from contracting even more quickly.

The review appears in the Philosophical Transactions of the Royal Society B.