Jubb M.R., Wilkin T.A. & Gosler A.G. (2006) Eggshell-pigmentation, soil calcium and the local abundance, distribution and diversity of woodland snails (Mollusca). ARDEA 94 (1): 59-70

Recent studies of Great Tit Parus major eggs in Wytham Woods, UK, indicate that the eggshell’s pigmentation is related to the eggshell’s thickness and so also to calcium availability (indicated by soil analysis) near to the nest. Previous work elsewhere has shown that snails are the principal source of calcium for eggshell formation in the Great Tit. Although these observations suggest that snail abundance and/or size (mass) is related to local soil calcium availability, this has never been demonstrated at the fine spatial scale required to support the observations on eggshell characteristics. We therefore examined the effect of soil calcium concentration on snail distribution and size across a series of sites of similar habitat, along a soil calcium gradient in Wytham Woods, UK. Soil and litter samples were collected from 30 sites and any snails present extracted by intensive sorting. The abundance, biometrics and dry mass of shells were recorded, and modelled against log10 soil calcium using General Linear Modelling. Our analyses confirmed the existence of strong correlations between snail abundance and size, and soil calcium at this fine spatial scale, presumably due to the snails’ requirement for calcium for shell formation and growth, general health and successful reproduction. Snail diversity was also correlated significantly with soil calcium, implying that calcium scarcity limits the abundance of particular snail species. We believe that these observations provide an important brick in the wall of our understanding of calcium limitation in breeding birds. They are also important to understanding the ecology of snails.