The phylum Mollusca is one of the most diverse groups of animals in the world. It includes familiar organisms, like the snail in our gardens, and not-so-familiar organisms, like the recently discovered Colossal Squid that cam grow to up to 33 ft long and weight 1,100 lbs! This group is comprised of three major classes the Gastropods (slugs and snails), the Cephalopods (squid, octopus, cuttlefish, nautilus), and the Bivalves (clams, mussels, oysters, scallops). Several more smaller groups exist as well. In total, all these groups combined account for about 200,000 living species! Ecologically, molluscs interact in many complex and important ways from decomposers to predators to prey.
Of the 200,000 species of molluscs, most are marine, but as we all know from everyday lives there are plenty of terrestrial molluscs as well. At least for now. Terrestrial molluscs have been experiencing dramatically high rates of extinction in the past half century. The International Union for the Conservation of Nature (IUCN) publishes a Red List which identifies the conservation status of all species every five to ten years. The IUCN has identified about 800 in the last 500 years, and of those about 300 are molluscs. That means that about 40% of all extinctions belong to this one group, and some people have estimated the number of molluscs we have lost to be as much as double the IUCN estimate.
Most of these extinctions have taken place on small oceanic islands which is not surprising. Ecosystems on oceanic islands are notoriously delicate, and extinctions often occur in response to ecological disturbances such as the introduction of some new predator, the clearing of land for agriculture, or competition with other newly introduced species. Their fragility makes oceanic islands the canaries in the coal mines giving us early warnings of what might befall continents if we do not stop, or at least slow, the current rates of ecosystem disturbance. On the Gambier Islands, for example, there were 46 species of terrestrial snail. Of those 46 species, 43 are now extinct. Many of them have not even been given names.
An interesting footnote is that while terrestrial molluscs are disappearing disturbingly quickly, marine molluscs are not. Is there something about the marine environment that makes species that live there less prone to extinction? Is it just that there have been fewer introduced pests and predators to oceans and to land (since land is where humans spend most of their time)?
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