PATTERNS OF LAND SNAIL ASSEMBLAGES ALONG A FINE-SCALE MOISTURE GRADIENT

We studied changes of terrestrial snail assemblages over a gradient of soil moisture using 60 sampling plots in the White Carpathian Mountains of the south-eastern Czech Republic. We used within-site design to control for confounding effects of site characteristics other than humidity, and we directly measured soil moisture along nine transects at distinct locations. Each transect had from 4 to 16 plots, and it was laid down from wet calcium-rich spring fen habitats to semi-dry meadows in the fen surroundings. We observed a sharp moisture gradient along each transect, with the measured soil moisture varying from 97% in fen plots to 19% in semi-dry grassland plots. Altogether 29 land snail species and 4,213 live individuals were collected. Species richness of land snails varied from 2 to 11 species per plot. However, we did not observe any significant linear or unimodal response of species richness or total abundances to measured soil moisture. In contrast, sharp compositional changes along studied transects were found, suggesting differences in species preference to soil moisture conditions at fine, within-site scale. Among 21 species with the frequency higher than five, 10 (after a Bonferroni correction) showed a significant response to soil moisture. Three species expressed significant affinity to drier plots, five species were more abundant in moister plots and only two species preferred middle values. In several previous studies, both linear and hump-shaped relation between soil moisture and number of land snail species were documented. This raised questions about general response of land snails to soil moisture and the importance of possible bias caused either by using only estimated values of site moisture or sampling in distinct sites differing also in other environmental factors that might potentially overtopped importance of soil moisture for land snail distribution.