A comprehensive new review paper reveals the staggering loss of biodiversity among island land snails globally. 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 "The conservation status of many island land snails thus remains at best out of date."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, especially invertebrates, 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.
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."
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.
