Land Snail Communities Research Articles

Large‐scale terrestrial gastropod community composition patterns in the Great Lakes region of North America

Abstract: Previous ordination studies of land snail community composition have been limited to four or fewer habitat types from sites separated by no more than 300 km. To investigate the nature of large‐scale patterns, North American land snail assemblages at 421 sites, representing 26 habitat types and covering a 1400 × 800 km area, were ordinated using global, nonmetric multi‐dimensional scaling (NMDS). These data were then subjected to model‐based cluster analysis and kmeans clustering to identify the main compositional groups and most important environmental covariables. Six primary compositional groups were identified. Three of these largely represent upland forest and rock outcrop sites, while the remaining largely represent either lowland forest, lowland grassland or upland grassland habitats. The geographical location and moisture level of sites also influences community composition. A strong compositional difference exists between sites having duff vs. turf soil surface layers. Only 8% of sites were improperly classified when soil surface architecture was used as the sole predictor variable. Fully 43% of taxa exhibited significant preferences towards one of these surface types, while only 15% of relatively common (10 + occurrence) taxa showed no preferences. Twelve groups of closely related taxa within the same genus had members that favoured different surface types, indicating that differential selection pressures have existed over evolutionary time scales. While turf faunas appeared unaffected by anthropogenic disturbance, duff faunas were strongly impacted, suggesting that their conservation will require protection of soil surface architecture.

Pastoral management vs. land abandonment in Mediterranean uplands: impact on land snail communities

AbstractThe aim of the study was to assess the impact of a pastoral management chosen to limit the recent expansion of woodland on a Mediterranean mountain on land snail diversity. An additional aim was to acquire quantitative data that could be used to identify pasture environments from Holocene molluscan assemblages. The work was undertaken at the Luberon mountain, Provence, south of France. We used a stratified quantitative sampling scheme according to altitude and vegetation structure. A total of 80 sites were studied. Large species were collected within a 5 × 5‐m plot. Small species were extracted from litter and surface soil. A standard procedure for site description was used based on 35 environmental variables. Grazing pressure was estimated according to the impact of grazing on the herb layer. Correspondence analysis and canonical correspondence analysis were performed using canoco 4.0 software. The distribution of land snails is related to altitude and grazing intensity. Large patches of grazed grassland harbour open country and mountain snail species. Thermophilic open ground species are located in grazed grasslands at lower altitude. Shade‐loving species are present in ungrazed scrublands or in small clearings on the upper slopes. The lowest species richness, diversity and equitability are associated with large patches of grazed grassland, the presence of a continuous cover of short grass reinforcing this negative impact on snail diversity. Our study is consistent with similar works on land snails or other invertebrates but discordant with vegetation studies. A homogeneous grazed herb layer significantly reduces snail diversity and abundance. Heterogeneity seems to favour snail diversity both at the local and landscape scales. However, sheep grazing contributes to the expansion of suitable habitats for rare snail species.

LAND SNAILS AND SOIL CALCIUM IN CENTRAL APPALACHIAN MOUNTAIN FOREST

Few studies have attempted to quantify the association between land snail communities and calcium (Ca) in upper soil horizons. If soil Ca is important to land snails, then land snail communities may be sensitive to reductions in soil Ca, including those caused by atmospheric acid deposition. In this study, snail density was estimated at ten 200m2 plots in mature forest using a litter sieving technique, and species richness was determined from litter sieving and timed searches. The most abundant snail species was the small spot, Punctum minutissimum (I. Lea), representing 34% of the specimens collected. Land snail density and species richness were positively correlated with extractable Ca, water soluble Ca, and pH in the Oe soil horizon and in the horizon below. Basal area of sugar maple, Acer saccharum Marsh., was positively associated with snail density and Ca in the Oe horizon, while basal area of red maple, Acer rubrum L., was negatively associated with snail density and Ca in the Oe horizon.

Gastropod communities, vegetation dynamics and landscape changes along an old-field succession in Provence, France

Studies about the relationships between land snail communities and landscape are scarce. The abandonment of many cultivated lands in Provence since the end of the last century gives us the opportunity of an analysis of the organisation pattern of gastropod communities in relation to the recent history and the present day structure of a Mediterranean landscape. The dynamics of a 40 ha landscape, mapped on a raster mode, was studied using cadaster data for seven periods between 1890 and 1990. These data were computed using MCA and PCA. Both floristic and malacological relevés were made at a scale of one grid cell. At the stand level, the patterns of gastropod communities are strongly correlated with the vegetation structure and composition. At the landscape level, the analysis of gastropod communities permits a very good perception of the present day landscape structure, as well as insight into the recent landscape history. However, even if the fits obtained are good, they should be improved in order to take into account scale dependences of snail communities.