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beautiful shell colours and patterns of the Cuban snail Polymita picta. International trade of this species is prohibited by the Convention on International Trade in Endangered Species.
Photo Credit: B. Reyes-Tur.
Scientific Frontline: Extended "At a Glance" Summary
The Core Concept: A comprehensive scientific review quantifying the catastrophic loss of biodiversity among Pacific Island land snails, revealing that extinction rates on high volcanic islands range from 30% to 80% of total species.
Key Distinction/Mechanism: Unlike standard biodiversity assessments that rely on living populations, this research utilizes the "shell bank"—shells preserved in the soil for centuries. This mechanism allows scientists to identify and catalog "silent extinctions" of species that vanished before they could be formally described by modern science.
Origin/History: Published in Philosophical Transactions of the Royal Society B, the study traces the timeline of these extinctions to two primary waves: the initial arrival of humans on the islands and the subsequent, more extensive impact of Western colonization.
Major Frameworks/Components:
- The Shell Bank: A fossil-like record of calcium carbonate shells used to reconstruct pre-human biodiversity baselines.
- Invasive Predation: Identification of key biological drivers of extinction, specifically rats, the rosy wolf snail (Euglandina), and the New Guinea flatworm (Platydemus manokwari).
- Habitat Alteration: The correlation between deforestation/land-use change and the collapse of endemic populations.
- Extinction Trajectories: A model distinguishing between "natural" background extinction (e.g., via fossilized dunes) and the accelerated anthropogenic rates observed recently.
- Branch of Science: Conservation Biology, Malacology (the study of mollusks), and Island Biogeography.
Future Application: Data from this review supports the development of urgent captive breeding programs ("buying time") and argues for a revision of global conservation agendas to prioritize non-charismatic invertebrates often overlooked in biodiversity crises.
Why It Matters: This research corrects the historical record, demonstrating that global extinction estimates are likely severe underestimates. By documenting species that were lost before they were found, it highlights the extreme vulnerability of island ecosystems to invasive species and human activity.
A comprehensive new review paper reveals the staggering loss of biodiversity among Pacific Island land snails. Lead author Robert Cowie of the University of Hawai‘i at Mānoa’s School of Ocean and Earth Science and Technology (SOEST) and co-authors note that ‘devastation’ is not a hyperbolic term, pointing out that extinction rates on high volcanic islands commonly range from 30% to as high as 80%. The review was published recently in Philosophical Transactions of the Royal Society B.
Tracking trends through the ‘shell bank’
While the review is global in scope, Cowie, along with Philippe Bouchet and Benoît Fontaine of the Muséum national d’Histoire naturelle in Paris, placed an emphasis on Hawai‘i and other Pacific Islands because this region has experienced the highest numbers of land snail species extinctions.
“Many islands are remote and the level of interest in land snails as a component of the global biodiversity conservation agenda is low,” the authors write. “The conservation status of many island land snails thus remains at best out of date.”
However, land snails have an asset that other animal groups do not—their shells, which can remain in the soil for many tens or several hundreds of years after the death of the animal. These shells can persist in the soil for centuries, creating a “shell bank” that allows researchers to identify species that went extinct before they could ever be officially recorded by modern science. A classic example is the discovery of a remarkable radiation of land snails in the Gambier Islands of French Polynesia. Without their shells, we would never have known they existed.
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The predatory flatworm Platydemus manokwari in the Ogasawara Islands, head on right, length ca 8 cm. Along with Euglandina and rats these invasive flatworms have had a devastating impact on land snail populations across the Pacific and Indian Oceans.
Photo Credit: S. Sugiura
Extinctions and their causes
During and after the last Ice Age, climate change and sea-level fluctuations led to the formation of so-called ‘fossilized’ sand dunes that buried numerous species; some of these extinct species can now be seen, for instance, in exposed deposits along the trail to Ka‘ena Point from the Wai‘anae side of O‘ahu. But most extinctions have been anthropogenic, caused primarily by habitat loss and the introduction of non-native species.
Many high volcanic islands had extraordinarily diverse and highly endemic land snail faunas, with 50–100 endemic species on even very small islands such as Rapa in the Austral Islands.
“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 in SOEST. “All but a tiny handful of which are found nowhere else on earth. Estimates have suggested that only 10-35% 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.”
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Necklace bought in 2003 in the airport on Rurutu (Austral Islands, French Polynesia), where these were openly sold to tourists despite legal protection; the white shells are Partula hyalina
Photo Credit: B. Fontaine
Extinction trajectories
The research team identified a recurring pattern of extinction that follows human arrival: deforestation and the indirect impacts of invasive species began with the initial arrival of people and became even more extensive and catastrophic following Western colonization.
Direct impacts of invasive species on island land snails are exemplified by rats and deliberately introduced predators such as the rosy wolf snail (Euglandina) and the New Guinea flatworm (Platydemus manokwari), both snail predators.
“These have probably been the ultimate cause of extinction following the devastating habitat loss that initiated the extinction process,” said Cowie.
Lastly, although few island people eat snails, collecting shells and the use of the shells of pretty species for decorating lei or hats, and other ornamental uses may have had an impact on snail populations. The authors note that although climate change has not yet done so, it may lead to extinction of island land snail species, especially species in mountainous regions, as their cool habitat vanishes with a warming climate.
“On a positive note, significant efforts to conserve what’s left of these unique and diverse faunas are being undertaken, notably in Hawai‘i and the Society Islands, as well as in the Ogasawara Islands of Japan, Bermuda, the Desertas Islands in the Madeiran Archipelago, and the Mascarene Islands in the Indian Ocean” said Cowie.
While conservation of snails in their natural environments is difficult because of the presence of invasive predators, captive breeding programs are “buying time” for these ancient lineages.
Published in journal: Philosophical Transactions of the Royal Society B
Title: Devastation of island biodiversity: a land snail perspective
Authors: Robert H Cowie, Philippe Bouchet, and Benoit Fontaine
Source/Credit: School of Ocean and Earth Science and Technology | Marcie Grabowski
Reference Number: cons012926_01