Terrestrial gastropod responses to ecosystem-level calcium manipulation a northern hardwood forest

The effects of acid deposition on soil calcium (Ca), and in turn on land snail populations, have been of heightened concern for several decades. We compiled a 10 year record (1997-2006) of gastropod abundance on two small watersheds at the Hubbard Brook Experimental Forest, one of which was treated with a Ca addition in 1999. In years 3-7 post Ca addition, snail abundance in the treated watershed was 73% higher than in the reference area (p < 0.001); there was no significant difference in the 3 years prior to treatment, and no significant difference in slug abundance in any year. We analyzed relationships between snail density and microsite spatial variation in leaf-litter Ca concentration, litter-layer thickness, tree species composition, slope, dead wood, and forest-floor light level. We found that snail abundance was significantly correlated with litter Ca concentration (p < 0.001) and negatively correlated with the importance value of American beech (p = 0.05). Isotopic-tracer analysis indicated that, on average, 76% of Ca in snail shells 5 years post treatment was derived from the added Ca. However, interannual variation in snail numbers indicates that other factors beyond available Ca have a strong influence on snail abundance.