Környezeti változók hatása szárazföldi csiga (Gastropoda, Mollusca) populációkra és közösségekre

Abstract

The geographical distribution of the European gastropod fauna is well documented and provides a habitat-level representation of species occurences and associations with habitat types and environmental factors. However, the study of microhabitat requirements is an underrepresented research area (Cameron 2013). Previous investigations regarding micro-scale ecological needs of land snails were mainly focused on the exhaustive study of smaller areas or focused on a single type of structural element (eg. dead wood). Our first research goal was to examine the impact of environmental factors on snail species richness and abundance in a micro-scale analysis with well replicated natural experiment using a balanced sampling design. We studied four distinct microhabitat types (litter, live trees, dead wood, rock) in 16 dolinas of the Alsó-hegy (Gömör-Tornai Karst, Aggtelek National Park) considering also topographic factors. This complex dataset allowed simultaneous analysis of habitat requirements of individual species and investigation of the effect of biases of the applied sampling methods (time restricted direct search and soil plus litter sampling) on the results of ecological inferences. Former studies showed that the main difference of the above mentioned sampling methods derives from the body- and shell sizes of the molluscs effectively collected by the methods. In addition, our present study found further differences significantly affecting the results. The proportion of broken shells was significantly higher in the samples collected by soil plus litter sampling than by time-restricted direct search. The proportions of dead shells also varied according to their taxonomic group. Helicoids tended to accumulate more (i.e., disintegrate less) than other groups. This can be attributed to their relatively large, thick shells compared to zonitids or other smaller sized and thinner shelled taxa. In the case of Clausiliids, although their shells are also durable, we could not find significant shell accumulation. The reason might be that this species group is associated with the coarse woody debris, where intensive microbial (decaying) processes are taking place. In accordance with these findings, the analysis of the proportion of broken shells among microhabitats showed that the rate of shell accumulation was less in the case of dead wood and highest in the rock microhabitat. Thus the observed difference of shell accumulation rate in relation to shell size and taxa also might account for microhabitat preference.