The Ecology of Terrestrial Gastropod Distribution in the Northern Hardwood Forest

Monica Skeldon

Brown University, Sc.B. May 2006.

The effects of reductions in soil Ca on land snail populations as a result of anthropogenic inputs such as acid rain have been of increasing concern. This study examines determinants of gastropod densities in, the northern hardwood forest, expanding on the number of variables previously examined to determine their relative importance in explaining gastropod densities. Differences in snail abundance and spatial variability of snail populations between two first order watersheds with differing calcium (Ca) availability were studied in Hubbard Brook Experimental Forest . Leaf litter calcium, tree species composition, slope, soil moisture, and sunlight data were quantified and analyzed in relation to overall snail density, and differences in spatial abundance of snails that exist within the two watersheds. Basal area of Fagus grandifolia (diameter breast height < 10cm) showed a negative relationship with snail density (R² = 0.324). Furthermore, higher available Ca in the leaf litter resulting from the addition of Wollastonite also produced higher snail numbers. A decrease in strontium isotope ratios post application of the Wollastonite suggests that the snails in the treated watershed received 20% of their Ca from the addition which is consistent with the increase in snail density. Therefore, Ca seems to be just one actor playing a role in the spatial abundance of snails. This study shows that the determinants of snail density are a complicated combination of forest composition, nutrients, and natural determinants.