Island land snails’ extinction rates as high as 80% or more

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Extinction rates for island land snails commonly range from 30% to as high as 80% on some volcanic islands, according to a new review paper that confirms "devastation" is no exaggeration for the state of global snail biodiversity. Lead author Robert Cowie of the University of Hawai'i at Mānoa's School of Ocean and Earth Science and Technology (SOEST) and an international team of researchers published their findings inPhilosophical Transactions of the Royal Society B.

"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."

Tracking trends through 'shell bank'

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 noted. "The conservation status of many island land snails thus remains, at best, out of date.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, persisting in the soil for centuries, create a "shell bank" that allows researchers to identify species that went extinct before they could ever be officially recorded by modern science.

Extinctions and their causes

Most land snail extinctions have been caused primarily by habitat loss and the introduction of non-native species. Many high volcanic islands had 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 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.

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.