Lehigh Gorge State Park, PA Ken Hotopp

Land snail shells, for those species that have them, are made mostly of calcium carbonate with a protein outer coating. Many kinds of wildlife obtain the nutrient calcium by consuming live land snails or their empty shells.

Calcium plays a variety of roles in the body of land snails and slugs, including parts in fluid regulation, cell wall function, muscle contraction, and egg laying, and of course, the shelled animals use a large amount of calcium in forming their shell structure. The shell-building organ, the mantle, develops a pH gradient to create a small electric current to move calcium ions into place. A few slugs such as the non-native Limax maximus have a small, vestigial shell that can be found within the mantle.

Land snails obtain calcium from their environment in a variety of ways, depending upon their autecology. They eat live and decaying leaves and wood, fungi and algae on wood and rocks, sap, animal scats and carcasses, nematodes, and other snails. They can be found rasping old or occupied snails’ shells, bones and antlers, rock particles or larger stones and outcrops, and the soil that they regularly consume contains calcium. In captivity they consume lime and paper. Land snails also absorb calcium directly through the sole of their foot (Kado, 1960).

Calcium availability in forest environments generally is positively correlated with the number and species richness of land snails (Burch, 1955; Hotopp, 2002), and this is also the case in Pennsylvania forests (Hotopp, unpublished data). However, note that at the greatest levels of site calcium, snail numbers may actually be slightly reduced (Valovirta, 1968). Acid precipitation can reduce the amount of calcium in forest soils, and this in turn can depress snail numbers as much as 80% on sensitive sites (Wäreborn, 1992). Conversely, land snail numbers respond positively to the addition of calcium (Johannessen and Solhøy, 2001).

Calcium in forest soils can come from below, from the breakdown of bedrock and other parent material that contains calcium, but it also comes from above, in the form of plant debris, though plants have also obtained their calcium from soil and rock via roots. Plants use calcium in nutrient and water translocation, cell division and cell walls. The amount of calcium in tree leaves and other litter varies, so forest species composition influences soil calcium (Boettcher and Kalisz, 1990; Vesterdahl and Rauland-Rasmussen, 1998).

Soil profiles may exhibit declining or increasing calcium with depth, depending upon the calcium content of bedrock and leaf litter. For our purposes the important issue is the amount of calcium available to land snails at the upper soil horizons where they live, generally the O and A1 horizons. Soil horizon effects may also be interrupted, by calcium-rich limestone outcrops or limestone scree that can make large quantities of this nutrient available to land snails (and other animals). Or by decaying logs of certain species such as sugar maple (Acer saccharum) that appear to support large numbers of snails.

Although calcium-rich areas have many species, an interesting variety of land snails also exist on calcium-poor sites. Some shelled snail species persist on poor sites in refuge habitats such as deep leaf litter, logs, or around springs. Some shelled snail species appear to be specially-adapted to gleaning calcium from their surroundings through behavior, such as rasping old snail shells, or physiology. The button snails, Mesomphix spp., are an example of one genus that is common in the leaf litter of Pennsylvania’s relatively calcium-poor oak forests as well as rich woods.

Moving up the “food chain,” a variety of animals eat land snails. While there are a number of invertebrate predators of snails, such as beetles and fly larvae, the animals that consume the calcium-rich shell are mainly vertebrates. Circumstantially, these animals would appear to be those with a higher demand for calcium to build bone.

Snails are eaten by herptiles including turtles and salamanders; by mammals including shrews, mice, squirrels, and deer (probably accidentally); and by birds including thrushes, ruffed grouse and wild turkey (Martin et al., 1951). Snail availability may be critical to calcium provisioning for some of these animals. Changes in snail numbers can have ripple effects through an ecosystem, as demonstrated for the great tit (Parus major) in the Netherlands (Graveland, 1996; Graveland et al. 1994). There, reduction in soil calcium due to acid rain resulted in fewer snails, which caused eggshell thinning and reduced reproductive success for the birds. In North America, Hames et al. (2002) have found a correlation between acid rain and reduced numbers of wood thrush (Hylocichla mustelina), and are asking whether this is linked to reduced snail numbers.

However, the picture may not be that simple. Recent work suggests that forest soils in Northeastern North America may become more acid as stands age, overriding the influence of acid rain by a wide margin (Hamburg, et al. 2003). In Sweden a study of clearcutting in boreal forest found a long-term increase in calcium and the number of most land snails (Ström, 2004), despite an initial decline. An important outstanding question about soil calcium and forest age is the relative importance of tree species composition shifts with cutting. Other questions can be asked about how stands older than typical timber rotations behave, and how natural disturbances such as fire and windthrow influence soil chemistry and land snails.