Hendersonia occulta

Last year, Aydin over at Snail's Tales posted a note about an interesting land snail, Hendersonia occulta. This particular snail is one of the few operculate land snails in North America and represents an important evolutionary link between terrestrial and marine snails. It also is an important snail when one thinks about climate.

In the late Pleistocene (a mere 20,000 years ago), this snail roamed over a large portion of the interior of North America. Once the climate started warming, this snail started restricting its range, eventually becoming isolated in a few key regions today. One is in my field area - the Driftless Area of Wisconsin, Minnesota, Iowa, and Illinois. Another place was in eastern Wisconsin, where the dolomite bedrock is close to the surface. And finally, in the western Appalachian region including Pennsylvania.

Distribution map of H. occulta showing live (solid dots) and fossil (yellow dots) sites where it has been found.

What this tells us about climate is important when we think in terms of how organisms responded to past climate change, and how they might deal with future changes. In the late Pleistocene (i.e. Ice Age), much of North America had suitable habitat for this snail. They like relatively cool summers, but not intense winters. They often live near standing water, or places with plenty of moisture. A source of calcium is important (that makes sense, since they have pretty robust shells - with opercula - for land snails), and they need food. These snails generally live in dense leaf litter, probably feeding on Birch, Maple and other deciduous leaf material. So we can infer that many places in the Midwest had these conditions. As the climate warmed, however, the available moisture, temperature, and food/shelter sources disappeared throughout much of its previous habitat. Now, it is left in places that still have these conditions - refugia.

Suitable refugia exist in several locations - but what about future change? What would happen if these refugia disappeared? This snail has adjusted its range to stay in suitable areas, but now that we have drastically altered the landscape, there might not be a path for these snails to find refuge. A sad thought indeed.

So when people poo-poo climate change as just a "natural cycle" and that we shouldn't worry about it, they are ignoring an important point. We have created an artificial obstacle course that will interfere with organisms trying to adjust their ranges to match changes in climate. We may not be the sole cause of global warming, but we are actively placing many organisms at peril because of our existence. Food for thought...

Some H. occulta information:
MNFI (Michigan) zoological abstract

L. Watrous images of H. occulta and other land snails in Missouri