Land Snail Assemblages Research Articles

Exceptionally poor land snail fauna of central Yakutia (NE Russia): climatic and habitat determinants of species richness

There is an ongoing debate on the causes of the latitudinal diversity gradient, but diversity decline towards high latitudes is poorly documented for many invertebrate taxa. Therefore, we sampled land snail assemblages at 79 sites and in various habitat types in central Yakutia, a region with extremely continental, cool and dry climate. We tested whether habitats lacking suitable shelters for winter survival harbour less species than those with vegetation cover that softens climatic extremes. Both local species diversity and regional species diversity were extremely low: 13 species were recorded in total with an average of 1.4 species per site. While the majority of grassland sites were without snails (26 of 34 sites), forest sites supported at least one snail species in most cases (38 of 45 sites). Within grasslands, snail occurrences were associated with a higher herb-layer biomass. Numbers of snail species correlated with the amount of available calcium only in forests, in which species accumulation towards more favourable habitats was possible due to softening of climate harshness. As minute snails are known to be effective passive dispersers and the study area was not glaciated during the last glacial stage, there was certainly enough time for colonization of all favourable habitats. Our results suggest climatically driven limitations of both local and regional land snail diversity in central Yakutia. We conclude that the hypothesis of climate harshness remains the most probable explanation of a sharp drop in land snail diversity in high-latitude areas with cold climate.

SHELL TAPHONOMY AND FIDELITY OF LIVING, DEAD, HOLOCENE, AND PLEISTOCENE LAND SNAIL ASSEMBLAGES

Variations in the taxonomic composition of ancient land snail assemblages can potentially reflect changes in past ecosystems. The use of fossil associations as a paleoenvironmental-paleoecological proxy assumes that the original biological signature is retained, but postmortem processes can distort it. In this study, the fidelity of land snail assemblages was tested by comparing taphonomic and ecological variables recorded by live and dead, middle Holocene and Upper Pleistocene land snail shelly assemblages from San Salvador Island (Bahamas). Shells of living organisms were practically unaltered whereas dead and fossil shells were primarily affected by fragmentation, ornament loss, color loss, and carbonate coating. Taphonomic features fluctuated across space and time likely due to variable environmental conditions and/or time of exposure prior to shell burial. Live assemblages showed good taxonomic agreement with dead assemblages, although the later exhibited a higher number of taxa and individuals than the former. Assemblages that were moderately (dead and Holocene) and strongly (Pleistocene) taphonomically altered did not differ in species abundances, suggesting that the original biological signal was preserved. In contrast, unaltered (live and some dead) assemblages differed taxonomically from moderately and strongly damaged assemblages, likely as a consequence of different scales of time-averaging rather than variable shell-specific destruction rates. Taxonomic richness and simple dominance of time-averaged land snail assemblages were similar at various interglacial time periods (∼125 ka, ∼5–6 ka, and today). Such apparently equivalent snail richness may suggest that the climatic-environmental and/or ecological conditions at those times were comparable to the present.

The response of land snail assemblages below aspens to forest fire and clear-cutting in Fennoscandian boreal forests

Species can persist in a landscape with recurrent disturbances either through local survival or by dispersing to sites of a preferred successional stage. This study investigated in what extent forest floor dwelling land snails survived forest fires and clear-cutting. Snail fauna in LFH (litter, fermenting litter and humus) samples below retained aspen trees in disturbed areas were compared with samples under scattered aspens in adjacent forests by extracting snails from LFH samples below five aspens in several stands of each type (five forest fires, six clear-cuts, and seven undisturbed forests). LFH samples from burnt sites had a higher pH than from forests, but on average a lower abundance of individual snails (11 vs. 30 in 0.5 l LFH) and 50% lower species density (3 vs. 6 species). The abundances and species densities in the clear-cuts were less affected. There was generally a positive relationship between pH and both species density and abundance in all the stand types. Burning apparently depleted the snail fauna considerably and some species may be dependent on dispersal if they are to recover within the burnt area, while the snail assemblages at clear-cuts did not differ significantly in species composition from adjacent forests. The positive relationship between pH and snail prevalence on the burnt sites raises questions regarding the pre- and post-fire spatial variation in pH (and available minerals) within and among stands and how it relates to snail survival rates and their capacity to track suitable places after the disturbance. Retained aspens at clear-cuts seem to harbour a forest like land snail fauna.

Invasive rats alter assemblage characteristics of land snails in the Ogasawara Islands

Although a large number of studies have shown that rats have a serious effect on island ecosystems, questions remain regarding how their invasion alters native species diversity and assemblage patterns on islands. In particular, a long history of invasions makes it difficult to understand the impact of rat invasion. In the present study, I investigate how an increase in predation by black rats ( Rattus rattus) alters the assemblage characteristics of land snails on the Ogasawara Islands in the West Pacific. Radical declines in many land snail species have occurred since 2006 due to increasing predation by black rats. The frequency of shells with predation scars was greater on larger species and on those living on the trees and in a shallower position in the litter, suggesting greater predation pressure on these species. As a result of this selective predation, large species living on trees and in shallower parts of the litter declined. Accordingly, when black rats increased on the island, small species and species with a burrowing microhabitat became dominant in the land snail assemblage. The present findings can be used to aid in predicting the effects of rat eradication and in estimating the assemblage patterns and diversity prior to rat invasion.

Snail faunas in the Southern Ural forests and their relations to vegetation: an analogue of the Early Holocene assemblages of Central Europe?

A malacological study at 41 forest sites in the Southern Urals (Bashkortostan, Russia) conducted in 2007 gave the first quantitative data about land snail assemblages from this region. We explore the hypothesis that forests of this area are modern analogues of the Early Holocene forests of Central Europe. Snail species significantly accumulated towards more fertile, calcium-rich, and lowland sites; the richest faunas were in alluvial alder forests and mesic lime-maple-elm forests. Several features such as low snail species richness, predominance of generalist species with wide distributions, and broader realized niches of particular species in the Southern Ural forests relative to their niches elsewhere, corresponded to those of fossil assemblages from the Early Holocene deposits of Central Europe. Our data also suggest that the very limited species pool, results in species poor assemblages which are structured mainly by environmental filtering.

Structure and conservation of Sri Lankan land‐snail assemblages in fragmented lowland rainforest and village home gardens

Summary 1Modified habitats have a profound impact on the biota of fragmented tropical forests. We investigated the structure of land-snail assemblages in Sri Lankan lowland rainforest and village home gardens, a habitat of the surrounding matrix of modified habitats. We focused on four questions, (i) How do the land-snail faunas of lowland rainforest and home gardens differ and can forest species persist in gardens? (ii) Can environmental variables such as altitude, canopy density and soil pH explain significant variation in species composition independent of the effects of habitat type? (iii) What is the extent of geographical species turnover? (iv) How valuable are the two habitats for the conservation of native land snails? 2Sixty-nine standardized belt transects were sampled in 21 rainforest fragments and 12 gardens. Data were analysed using canonical and partial canonical correspondence analysis. 3Land-snail species composition varied significantly between gardens and forest. Lowland rainforest was dominated by native snails, many of which were restricted to forest. Gardens contained a distinct component of exotic and native synanthropic snails, as well as many native forest species. Altitude, canopy density and soil pH explained significant variation in species composition independent of the effects of habitat type but were closely correlated with variation in species composition between gardens and forest. Longitude and latitude explained significant variation in species composition across both gardens and forest. 4Synthesis and applications. Most rainforest snails were restricted to forest. This, together with the high level of geographical turnover and the scale of forest fragmentation, emphasizes the need to conserve all remaining rainforest cover. A substantial number of widespread and localized forest land-snail species can survive in gardens. Home gardens therefore offer great potential for restoring and increasing forest cover and connectivity.

The composition and richness of Danubian floodplain forest land snail faunas in relation to forest type and flood frequency

The species richness and composition of land snail assemblages in 42 floodplain forest sites along the Danube River in Slovakia were studied to find the main ecological gradients responsible for the variation in the faunas. We found just one, but steep, ecological gradient influencing the variation of snail species composition among different floodplain forest types, reflected in the first detrended correspondence analysis axis, which explained 29.6% of total variation. Site scores on this axis were significantly correlated with site humidity (rs ¼ 20.868; P , 0.001). Significant loading on the first axis was also found for flood frequency and several vegetation descriptors. Species composition mainly reflected differences between sites without floods and the others. Species richness as well as total abundances of live individuals were not significantly controlled by any explanatory variable, although some trends could be discerned. Considering vegetation classification, substantial differences were observed between wet softwood floodplain forests and the remaining types, drier softwood forests and different types of hardwood forests, which were impossible to distinguish based on land snail assemblages. The results are discussed in relation to earlier descriptive studies, and in terms of the conservation of these threatened habitats.

Land snail distribution patterns within a site: The role of different calcium sources

The aims of this study were to analyse whether land snail assemblage patterns reflect the gradient of calcium content on a very small scale within a site. We chose two sites differing in their calcium richness and source of the calcium. The “Tufa site” had abundant tufa in the soil, while the “Boulder site” was on chemically inert chert bedrock where calcium originated from vegetation. On each site a set of 20 quadrats from which snails were extracted was laid down in a line from the calcium rich patch to the calcium poorer surround matrix. At the both sites, available calcium contents of topsoil decreased significantly with samples' position from the patch centre. For the Tufa site, calcium content was a strong controller not only of species composition but also species richness, total abundance, and abundances of almost all species. At the Boulder site only species composition was significantly driven by calcium. Species composition was highly nested along calcium gradient at the Tufa site contrary to the Boulder site where the species had an almost random distribution. We conclude that the best predictor of species composition was in both cases content of carbonate calcium in topsoil. Topsoil pH was positively correlated with calcium content only at the Tufa site.

Terrestrial environments during MIS 11: evidence from the Palaeolithic site at West Stow, Suffolk, UK

Multidisciplinary investigations of the sequence at Beeches Pit, West Stow (Suffolk, UK), have a direct bearing the age of the Hoxnian Interglacial and its correlation with the continental Holsteinian and with the global marine record. At this site, glacial deposits (till and outwash gravels) referable to the Anglian Lowestoft Formation fill a subglacial channel cut in Chalk bedrock. Above these glacial deposits a series of interglacial sediments occurs, consisting of limnic, tufaceous and colluvial silts, lacking pollen but rich in shells, ostracods and vertebrates. Lower Palaeolithic flint artefacts of Acheulian character have also been recovered, including refitting examples. Charred material is abundant at certain horizons and many of the bones have been burned. Several discrete areas of burnt sediment are interpreted as hearths. The molluscan fauna comprises some 78 taxa and includes species of considerable zoogeographical and biostratigraphical importance. The land snail assemblage from the tufa consists of woodland taxa with no modern analogue, including species that are either extinct (e.g. Zonitoides sepultus) or which no longer live in Britain (e.g. Platyla polita, P. similis, Neniatlanta pauli). This is also the type locality of Retinella ( Lyrodiscus) skertchlyi, which belongs to a subgenus of zonitid land snail now living only on the Canary Islands. There are indications from this fauna (‘the Lyrodiscus biome’) that the climate was wetter and perhaps warmer than the present day. The vertebrate fauna is also noteworthy with species of open habitats, such as rabbit ( Oryctolagus cf . cuniculus), and of closed forest, such as squirrel ( Sciurus sp.) and garden dormouse ( Eliomys quercinus) present at different times. The occurrence of southern thermophiles, such as Aesculapian snake ( Zamenis longissimus), indicates temperatures warmer than those of eastern England today. The upper levels include much material reworked from the interglacial sediments, although there is clear faunal evidence for climatic deterioration. Both the molluscan and vertebrate faunas suggest correlation of the interglacial sediments with the Hoxnian. Uranium series dates from the tufa (∼455 ka BP), TL dates from burnt flints (414±30 ka BP) and a range of amino acid racemization data all support correlation of this interglacial with MIS 11. However, four OSL dates from sand beneath the interglacial sequence yield a mean age of 261±31 ka BP, far younger than all other age determinations and far younger than implied by the biostratigraphy. Archaeologically the site is unusual in showing prolonged human occupation within closed deciduous forest and evidence for controlled use of fire in a Lower Palaeolithic context. Biostratigraphical correlations with other Lower Palaeolithic sites support the suggestion that Acheulian and Clactonian industries both occurred in southern Britain during the same substage of the Hoxnian, although not necessarily at precisely the same time. The characteristics of the MIS 11 interglacial in Britain are discussed in the light of evidence from Beeches Pit and elsewhere.

The interspecific relationship between abundance and body size in central European land snail assemblages

Summary The distributions of abundance and body size and the interspecific relationship between abundance and body size in Central European assemblages of terrestrial snails were analysed to investigate whether the characteristics of land snails, especially their limited active dispersal abilities, result in differences from the macroecological patterns mainly found in analyses of vertebrates. The distributions of crude density (number of specimens divided by the sum of the sampled areas) and ecological density (number of specimens divided by the sum of the areas sampled in the habitat types where a given species had been recorded) and of body size are approximately log-normal. Controlling for phylogenetic relationships there was a significant positive correlation between the number of habitat types occupied by a species and its crude and ecological density. The negative relationship between the two measures of abundance and body size found in the across-species analyses could not be confirmed in the phylogenetic contrasts analyses. The slopes of the body size–abundance relations are shallower than predicted by the energetic equivalence rule, even if only the upper bound slopes are considered and irrespective of the regression method used. This indicates that large snail species control a greater proportion of available resources than smaller species. Most of the macroecological patterns found in land snails are similar to those of other animal groups. An exception is the approximately log-normal distribution of log-transformed body sizes which may be a consequence of the better passive dispersal ability of smaller snails.

A comparison of terrestrial snail faunas between strip-mined land and relatively undisturbed land in Ohio, USA – an evaluation of recovery potential and changing faunal assemblages

Terrestrial snail surveys were conducted at 1693 ha of reclaimed strip-mined land at The Wilds and at 3344 ha of woodland at the Shawnee Wilderness Area, both in Ohio, USA. These areas shared the same original physiography, vegetative cover, and soils prior to mining and are believed to have had the same land snail assemblages. The Wilds consisted of 37% woods and 63% open fields, both the result of reclamation activities. The Shawnee site was 3% fields and 97% woods. Fifty species were recorded from Shawnee and 60 from The Wilds. The older areas of woodlands and marginal new growth stands in The Wilds had recovered up to 55% of the presumed original woodland fauna based on shared assemblages with Shawnee. The fields at The Wilds were dominated by species that were rare at Shawnee. We hypothesize that these species were adapted to specialized but sparse microhabitats at Shawnee and were able to successfully colonize the disturbed areas (fields) at The Wilds when given the opportunity. As the fields at The Wilds yield to new growth woods these “field” species should return to their lower densities in the woodlands. The ranges of the original woodland species at The Wilds, now confined to the older growth woods, will expand as succession continues. If the woodlands are left to expand, then we expect the majority of the original land snail fauna will eventually return to The Wilds. However, the conversion of original woodlands at The Wilds to open fields during reclamation has complicated and slowed this recovery process. Fifty years after restoration began, The Wilds has not regained its original snail fauna.

Factors structuring land snail communities in South-Eastern France: a comparison of two estimation methods

Dans le but de révéler l’influence de la méthode d’échantillonnage sur la précision avec laquelle les facteurs structurant les communautés malacologiques sont estimés, les résultats de deux études sont comparés. Ces deux études, entreprises dans la même région du sud-est de la France, près de Marseille, diffèrent par leurs méthodes ainsi que par leurs objectifs initiaux. L’une utilise un échantillonnage stratifié et ne prend en compte que les grosses espèces récoltées à l’intérieur d’un carré de 25 m ² ; l’autre est fondée sur un échantillonnage systématique et prend en compte les espèces de toutes tailles à l’intérieur d’un carré de 400 m ² . Les résultats indiquent que trente échantillons (quelle que soit la technique) sont suffisants pour décrire les facteurs structurant les assemblages malacologiques à l’échelle d’une commune. De plus, la récolte des seules grosses espèces fournit une indication adéquate des principaux facteurs structurant ces assemblages. Il est suggéré que les quadrats malacologiques sont optimisés quand ils couvrent une surface limitée (ici 25 m ² ) et incluent la collecte de plusieurs prélèvements de sol pour la recherche des petites espèces. La comparaison des résultats des deux études révèle que les activités humaines sont le principal facteur structurant les communautés malacologiques à l’échelle de la commune. Elle souligne aussi que cette échelle, qui prend en compte la topographie et les activités humaines, est adéquate pour comprendre la distribution des mollusques terrestres au sein du paysage.

Effects of soil properties and land management on the structure of grassland snail assemblages in SW Germany

Quantitative analyses of the snail assemblages of four a priori defined categories of grassland sites, characterized by soil moisture, land use and management, were conducted in SW Germany. Topsoil pH and calcium content were determined for each of 39 sites. The objectives were to study the effects of these habitat characteristics on snail density, species richness and quantitative structure of the land snail assemblages. With one exception, no significant relationships between soil pH (as a proxy for Ca supply) and the total numbers of individuals and species at the four site categories were observed. Snail density and species richness on groundwater soils, represented by rarely mown sedge stands and regularly mown meadows, were clearly higher than on intermediate moisture soils (as defined by the climatic water balance) of regularly mown meadows, indicating that soil moisture was the most important factor accounting for these differences. Furthermore, we observed specific conditions associated with the presence of trees in the category of meadow orchards on intermediate moisture soils. There, numbers of individuals and species were significantly higher than in the meadows on intermediate moisture soils. A posteriori cluster analysis of quantitative similarity of all snail assemblages showed that soil moisture accounts for the major differences between the site categories with respect to snail density, species richness and quantitative composition of the grassland snail assemblages. As these results are in contrast to the findings of many woodland studies, we suggest that the occurrence or abundance of certain species in the site categories are explained by land management, especially by frequent disturbances at the soil surface caused by mowing.

Relationships between land snail assemblage patterns and soil properties in temperate‐humid forest ecosystems

AbstractAim The objectives were to (1) analyse the combined effects of soil pH, Ca content and soil moisture on total density and species richness of land snails in forest ecosystems, (2) explore relationships between the quantitative composition of snail assemblages and habitat characteristics, (3) investigate the relationships between soil pH and density of some of the most frequent species, and (4) compare the data with those from studies conducted in other temperate‐humid regions of Europe.Location Study sites were selected from 15 landscape types including different lithologies within the area of Baden‐Württemberg (35,000 km2), SW Germany.Methods Snails were recorded quantitatively from 83 study sites, with four plots representing a total of 0.25 m2per site. Topsoil samples from each site were analysed for pH, exchangeable Ca, and Ca content of carbonates. Three categories of soil moisture (dry, intermediate and wet) were established and defined according to the (climatic) water balance. Numbers of individuals and species were brought in relation to soil moisture and soil pH. Cluster analyses were conducted to identify groups of quantitatively similar snail species assemblages.Results Topsoil pH (2.7–7.5) and soil Ca contents were closely correlated. On dry soils, total snail density and species richness are generally low and do not change with pH, but clearly increase with increasing pH on intermediate moisture soils and on wet soils. On the latter, numbers of individuals and species are generally much higher compared with intermediate moisture sites at the same value of soil pH. Changes of density in relation to soil pH vary between species. Depending on the species, density increases only in the lower or only in the higher range of pH, is not related to pH, or decreases with increasing pH. Furthermore, these patterns vary within the same species depending on the region. This became evident from comparisons with other studies, particularly between sites in SW Germany and southern Scandinavia. From cluster analyses, subgroups of snail assemblages of high quantitative similarity were identified. Group formation is explained by soil pH to some extent, and one subgroup showed a connection with coniferous woodland sites on acidic soils. No further environmental factors available from our data could explain the clustering of snail assemblages more detailed.Main conclusions Soil moisture is the strongest determinant of snail density and species richness at undisturbed woodland sites, but effects of soil moisture and soil pH on these patterns are closely interrelated on intermediate moisture soils and wet soils. However, the quantitative species composition of the land snail assemblages is related to soil properties to a lower degree than snail density and species richness, and other habitat characteristics such as vegetation or litter quality, can be important for species dominance in addition.

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.

Molluscan successions from two Holocene tufas near Northampton, English Midlands

AimTo investigate the molluscan successions from two Holocene tufa sequences in the English Midlands to establish whether similar patterns of colonization occur and to document how they relate to other successions reported from southern Britain and elsewhere.LocationCourteenhall and Weston Favell near Northampton, English East Midlands.MethodsClose serial sampling and quantitative analyses have allowed detailed faunal successions to be reconstructed for each site. Radiocarbon dating of Cepaea shells by AMS has provided a chronology for each site.ResultsRemarkable similarities were found to exist between the two sites. The record from Courteenhall extended back to the end of the Devensian late‐glacial and continued until about 6000 yr BP. The sequence from Weston Favell was shorter, commencing at the beginning of the Holocene and ending about 7500 yr BP. Both successions show a progressive replacement of open country assemblages by those of woodland. At Courteenhall arctic‐alpine species, such as Columella columella (Martens) and Vertigo genesii (Gredler), were present in these early open‐ground communities. Both sites supported extremely rich (> 30 species) land snail assemblages during the mid Holocene. This richness cannot be matched at any single site in Britain today. These mid Holocene assemblages include species such as Spermodea lamellata (Jeffreys), Leiostyla anglica (Wood) and Vertigo alpestris Alder that are rare or absent today in southern Britain. Discus ruderatus (Férussac), which has a modern boreo–montane range, occurred at Courteenhall between about 9700 and 8750 yr BP but was also present in two consecutive samples just below a level dated at about 7000 yr BP, its latest known occurrence in Britain. These assemblages have no modern analogue because the ranges of several species (e.g. Discus ruderatus and Leiostyla anglica) never overlap today.General conclusionsDuring the Holocene land snails appear to have colonized southern Britain in a relatively ordered sequence. The history of some species is unpredictable and they may occur at different sites at different times. For others, the general pattern shows greater consistency, enabling the recognition of a series of mollusc zones applicable over large areas of southern Britain. These Northamptonshire sites also contribute to the debate surrounding the notion of a ‘late Holocene tufa decline’. Tufa formation began unusually early (c. 10,280 yr BP) at Courteenhall but it had ceased to form at both sites by 6000 year (maybe by 7500 yr BP at Weston Favell). There is no evidence for late Holocene tufa formation at either site, a pattern consistent with many other British sites. Rates of tufa growth from 2.7 to 7.0 cm 100 years–1, calculated from the radiocarbon dates from Courteenhall, are comparable with accumulation rates established from other Holocene tufas.

British late glacial and Holocene climatic history reconstructed from land snail assemblages

gations have enabled refinement of the stratigraphic scheme, as shells of land snails occur in quantity throughout the sequence (Preece, 1998). They provide a detailed mollusk succession from the beginning of the late glacial until the present day. The late-glacial assemblages are dominated by species of open country and marsh, but shadeintolerant species are replaced by a succession of woodland species during the early part of the Holocene. Following forest clearances during the Neolithic and Bronze Age (5000‐3000 yr B.P.), these closed forest communities show a reversion to grassland assemblages, with only minor indications of scrub (Fig. 2). A detailed chronology for this succession is provided by a series of

Late Pleistocene Climate Record in the Eustis Loess Section, Nebraska, Based on Land Snail Assemblages and Magnetic Susceptibility

Based on multivariate analysis of mollusk assemblages and magnetic susceptibility of the Eustis section in western Nebraska, the climate of the Peoria loess steppe was relatively mild and, in terms of annual precipitation, relatively moist during the advance stages of Woodfordian ice. It was cold and dry during ice retreat. The diversity and composition of mollusk species indicates that the extreme cold and dry climate was reached during the late phase of Peoria time, in mollusk zone MZ3. Although soil moisture during the growing season at that time was high, the growing season was short and annual precipitation totals were low. Low values of magnetic susceptibility imply relatively rapid dust deposition during this interval, Judging from the current ecological tolerance of the species, no closed forest existed in the area. Only scattered shrubs or isolated tree stands may have been present. The late Peoria MZ3 assemblage compares favorably with the Columella fauna of Europe. In contrast, the early Peoria mollusk assemblages MZ1 and MZ2 compare well with the European pleniglacial Pupilla associations. Based on available 14 C dates from the studied region and on assumed relation of magnetic susceptibility to sedimentation rates, the estimated age of the MZ1 and MZ2 zones is 24,000 to 16,000 yr, whereas that of MZ3 is 16,000 to 12,000 yr.

Fossil Evidence of Recent Human Impact on the Land Snail Fauna of Madeira

The Madeiran islands, located at 33?N in the eastern Atlantic, were colonized early in the 15th century, and perhaps up to a century earlier. The woodland and scrub cover was rapidly reduced and the amount of grassland and dis- turbed ground increased. The effect on the land snail popu- lation has been assessed by analysis of fossil assemblages from a deposit in eastern Madeira. Dates were obtained by measurement of amino acid epimer ratios (D-alloisoleucine/L- isoleucine) of individual shells, calibrated against radiocarbon- dated samples. Because this method allows dating of individual shells, it was possible to reconstruct the chronology of various species from mixed-age assemblages. It also per- mits dating of shells too young for radiocarbon dating (post- AD 1650). Fifteen land snail assemblages were analysed from deposits of post-settlement age (c. 420-50 yr BP). Of the thirty-four species of land snails present in the samples, nine have become extinct over the period. In contrast, only five species became extinct on the island during the previous c. 300,000 years. The majority of the extinctions have occurred within the last two centuries, as a result of habitat destruction. A major decline in the abundance of woodland species took place during the 20th century. The extinction of the endemic Caseolus bowdichianus, abundant in middle Holocene de- posits, occurred about a century after colonization of the island. Theba pisana, a snail of similar size and shape, was introduced around the time of colonization, and is now abun- dant in grassy areas. Two endemic grassland species, Discula polymorpha and Heterostoma paupercula, have also become more abundant and thus apparently benefited from the arrival of man.

The stratification and taphonomy of shells in calcareous soils: Implications for land snail analysis in archaeology

The ecological interpretation of land snails from buried soils has depended on the assumption that assemblages are age-stratified and not seriously distorted by differential destruction. The temporal and ecological resolutions of assemblages are determined by taphonomic processes controlling the incorporation, mixing and subsequent destruction of shells in the soil. This paper describes those soil processes affecting snail assemblages in Britain and attempts to quantify their effects. It is concluded that, except in certain restricted circumstances, the temporal and ecological resolutions of land snail assemblages in buried soils are poor.