Snail Species Richness Research Articles

Coarse woody debris ameliorates chemical and biotic soil parameters of acidified broad-leaved forests

Base cation depletion due to soil acidification is a major environmental problem which can disrupt proper ecosystem functioning. We investigated nine mesic broad-leaved forests growing on Devonian sediments of the Rhenish Shield (Germany) and investigated the effect of coarse woody debris (CWD). The forests grow on soils originating from different parent material (graywacke, slates, dolostone) with upper soil pH-values ranging from 3.0 to 7.0. We analyzed soil chemistry, soil microbial parameters and the density of litter-dwelling detritivores (snails, slugs, woodlice and millipedes) to assess the influence of both acidification and CWD. The forests were classified according to their (Ca + Mg + K)/Al molar ratios into groups with ratios of either <1, 1–2 or ≫2. In general, microbial biomass and activity, as well as densities and species richness of snails and woodlice, decreased with decreasing (Ca + Mg + K)/Al molar ratios. Millipedes and slugs were not impaired by decreasing (Ca + Mg + K)/Al molar ratios. However, we found effects on species assemblages. Forward selection in canonical correspondence analyses revealed that species assemblages of millipedes and slugs were related to base cations and microbial parameters. Plots close to CWD exhibited improved soil quality including higher pH-values, greater Ca 2+ and Mg 2+ availability and an improved (Ca + Mg + K)/Al molar ratio. The presence of CWD also enhanced soil microbial biomass, the density of detritivores and species richness of snails, slugs and millipedes. Consequently, coarse woody debris influences nutrient fluxes and decreases the acidification-related risk of tree damage.

The Influence of Flowering Dogwood (Cornus florida) on Land Snail Diversity in a Southern Mixed Hardwood Forest

In many areas, soil calcium is an important limiting factor for land snail distributions. Some tree species are able to extract calcium from lower soil depths, concentrate it in their leaves and deliver it to the surface upon leaf fall. Flowering dogwood (Cornus florida) is an example of a “calcium-pumping” species and has been suggested to promote land snail diversity in woodland ecosystems. In this study, leaf tissue calcium, O- and B-horizon soil calcium and land snail diversity were compared for three common understory tree species: flowering dogwood, American beech (Fagus grandifolia) and blackgum (Nyssa sylvatica). Using a litter sieving technique, land snails were sampled underneath these three focal tree species. Land snail density, species richness and diversity were estimated for a total of 19 trees. As expected, B-horizon soil calcium was similar across all tree species, while leaf tissue calcium and O-horizon soil calcium were significantly higher for flowering dogwood. Land snail density, species richness and diversity (Shannon-Wiener Index) were also significantly higher for flowering dogwood. Calcium-pumping tree species are important ecosystem components in that they increase soil fertility and promote invertebrate diversity. Land snails are an important food and calcium source for a variety of woodland species including passerine birds, woodland salamanders, cychrine beetles, and wild turkeys. Flowering dogwood is currently seriously threatened in the southeastern United States by the parasitic dogwood anthracnose fungus (Discula destructiva). In light of the findings of this study, the implications of dogwood anthracnose may reach beyond the target species and impact multiple taxa.

SEQUENTIAL COLONIZATION AND DIVERSIFICATION OF GALAPAGOS ENDEMIC LAND SNAIL GENUS BULIMULUS (GASTROPODA, STYLOMMATOPHORA)

Species richness on island or islandlike systems is a function of colonization, within-island speciation, and extinction. Here we evaluate the relative importance of the first two of these processes as a function of the biogeographical and ecological attributes of islands using the Galápagos endemic land snails of the genus Bulimulus, the most species-rich radiation of these islands. Species in this clade have colonized almost all major islands and are found in five of the six described vegetation zones. We use molecular phylogenetics (based on COI and ITS 1 sequence data) to infer the diversification patterns of extant species of Bulimulus, and multiple regression to investigate the causes of variation among islands in species richness. Maximum-likelihood, Bayesian, and maximum-parsimony analyses yield well-resolved trees with similar topologies. The phylogeny obtained supports the progression rule hypothesis, with species found on older emerged islands connecting at deeper nodes. For all but two island species assemblages we find support for only one or two colonization events, indicating that within-island speciation has an important role in the formation of species on these islands. Even though speciation through colonization is not common, island insularity (distance to nearest major island) is a significant predictor of species richness resulting from interisland colonization alone. However, island insularity has no effect on the overall bulimulid species richness per island. Habitat diversity (measured as plant species diversity), island elevation, and island area, all of which are indirect measures of niche space, are strong predictors of overall bulimulid land snail species richness. Island age is also an important independent predictor of overall species richness, with older islands harboring more species than younger islands. Taken together, our results demonstrate that the diversification of Galápagos bulimulid land snails has been driven by a combination of geographic factors (island age, size, and location), which affect colonization patterns, and ecological factors, such as plant species diversity, that foster within-island speciation.

The effect of horse, cattle and sheep grazing on the diversity and abundance of land snails in nutrient-poor calcareous grasslands

Livestock grazing is a common management practise in semi-natural grasslands in Central Europe. Different types of livestock (horses, cattle, sheep) and grazing intensity are known to affect the richness and composition of plant species. However, knowledge of grazing-dependent effects on invertebrates is limited. We examined the influence of horse, cattle and sheep grazing on the richness, abundance and composition of land snail species in 21 calcareous nutrient-poor grassland areas in the northwestern Jura Mountains, Switzerland. Grazing by different livestock species did not affect the species richness, abundance and species composition of land snails. Furthermore, the number of open-land species and the ratio of large- to smal-sized snail species or individuals did not differ among the three pasture types. However, independent of livestock species, grazing intensity negatively influenced the snail fauna. Snail species richness, abundance and number of Red list species decreased with increasing grazing intensity. Grazing intensity also affected the occurrence of individual snail species (Truncatellina cylindrica, Cecilioides acicula, Candidula unifasciata and Trichia plebeia). To preserve the snail fauna in nutrient-poor grasslands, pastures can be stocked with horses, cattle or sheep. However, both maximum stocking rate (number of livestock units per hectare) and grazing duration (number of grazing days per year) must be carefully defined for the proper management of the pastures. (c) 2006 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved. Schneckenfauna im Nordwestschweizer Jura

Coarse woody debris, soil properties and snails (Mollusca: Gastropoda) in European primeval forests of different environmental conditions

We studied the snail fauna of four primeval forests of Central Slovakia and focused on the influence of forest type and coarse woody debris (CWD) on species assemblages. We found a total of 3281 individuals from 39 species. Species assemblages differed between the forests. In SW-facing oak forests, CWD enhanced the proportion of forest species such as the wood-grazing Cochlodina laminata and decreased the proportion of euryecious species like the litter-dwelling Punctum pygmaeum. In NE-facing beech forests, hygrophilous species such as Carychium tridentatum dominated the assemblages. This litter-dwelling snail was especially abundant close to CWD. The NE-facing beech forests harbored more species and individuals than the SW-facing oak forests did. CWD enhanced snail abundance, species richness and biodiversity, irrespective of forest type. We found positive correlations for abundance and species richness to the amount of leaf litter, C org, N tot, Ca 2+, and K +; snail abundance was also positively correlated to soil pH. All these soil chemical factors were higher close to CWD than distant from CWD.

SEQUENTIAL COLONIZATION AND DIVERSIFICATION OF GALÁPAGOS ENDEMIC LAND SNAIL GENUS BULIMULUS (GASTROPODA, STYLOMMATOPHORA)

Species richness on island or islandlike systems is a function of colonization, within-island speciation, and extinction. Here we evaluate the relative importance of the first two of these processes as a function of the biogeographical and ecological attributes of islands using the Galápagos endemic land snails of the genus Bulimulus, the most species-rich radiation of these islands. Species in this clade have colonized almost all major islands and are found in five of the six described vegetation zones. We use molecular phylogenetics (based on COI and ITS 1 sequence data) to infer the diversification patterns of extant species of Bulimulus, and multiple regression to investigate the causes of variation among islands in species richness. Maximum-likelihood, Bayesian, and maximum-parsimony analyses yield well-resolved trees with similar topologies. The phylogeny obtained supports the progression rule hypothesis, with species found on older emerged islands connecting at deeper nodes. For all but two island species assemblages we find support for only one or two colonization events, indicating that within-island speciation has an important role in the formation of species on these islands. Even though speciation through colonization is not common, island insularity (distance to nearest major island) is a significant predictor of species richness resulting from interisland colonization alone. However, island insularity has no effect on the overall bulimulid species richness per island. Habitat diversity (measured as plant species diversity), island elevation, and island area, all of which are indirect measures of niche space, are strong predictors of overall bulimulid land snail species richness. Island age is also an important independent predictor of overall species richness, with older islands harboring more species than younger islands. Taken together, our results demonstrate that the diversification of Galápagos bulimulid land snails has been driven by a combination of geographic factors (island age, size, and location), which affect colonization patterns, and ecological factors, such as plant species diversity, that foster within-island speciation.

Habitat and integration within indigenous communities of Xeropicta derbentina (Gastropoda: Hygromiidae) a recently introduced land snail in south‐eastern France

ABSTRACT Xeropicta derbentina is a recently introduced land snail in Provence, France. The present work aims at describing the habitats and the land snail communities that it has invaded. Also, its impact on the species richness and diversity of indigenous communities is assessed. Two areas have been studied in order to cover the range of conditions that X. derbentina is encountering within its current French distribution. Sixty‐five samples were analysed from the most Mediterranean site, Auriol, and 140 from a northern site, Craus de St‐Michel‐l’Observatoire. From these samples, X. derbentina is found to be a thermophilous open‐land species mostly inhabiting human‐impacted habitats. It occurs in large populations (more than 100 individuals collected in 15 min) in habitat characterizing the first stages of secondary successions, from cultivated crops to dense fallow lands. However, within a heterogeneous landscape and if enough time has allowed its dispersal, X. derbentina can be found in almost any kinds of habitat, such as garrigues and woodlands. Consequently, X. derbentina occurs in diverse land snail communities. At the local scale, its large populations could result in a significant decrease in species richness and diversity. At the regional scale, however, the introduction of X. derbentina seems to have increased land snail species richness.

Habitat factors for land snails in European beech forests with a special focus on coarse woody debris

Analyses of land snails and habitat factors in acid beech forests were conducted in southern Germany (northern Bavaria). The objectives were to study the effects of habitat characteristics on snail density and species richness. Habitat structures were determined for 37 plots in one big forest. We found a significant relationship between the number of snail species and individuals and the following set of habitat factors coverage of herbaceous layer, growing stock, mean diameter at breast height of the three largest trees (DBHmax), stand age, total dead wood volume per ha, and advanced decomposed dead wood volume per ha. We use maximally selected rank statistics to estimate cutpoints separating stands with low densities, from stands with high snail densities. Here, we define cutpoints for a significant higher snail density at a stand age of 187 years, 57 m3/ha dead wood, 40 m3/ha advanced decomposed dead wood, 63 cm DBHmax and more than 1% herbaceous layer. For species richness, cutpoints are estimated at 338 m3/ha stand volume, 170 years stand age, 50 m3/ha total dead wood amount, 15 m3/ha advanced decomposed dead wood and 56 cm DBHmax. The microhabitat analysis shows a higher pH value and a higher Calcium content at the bottom of large snags and under large lying dead wood pieces in comparison to litter, upper mineral soil and at the bottom of vital living trees. Snail species and individual density are significantly linked to these patterns of chemical parameters. The identified cutpoints are a good base for ecological management decisions in forest management.

Multi‐scale altitudinal patterns in species richness of land snail communities in south‐eastern France

AbstractAim Species richness is an important feature of communities that varies along elevational gradients. Different patterns of distribution have been described in the literature for various taxonomic groups. This study aims to distinguish between species density and species richness and to describe, for land snails in south‐eastern France, the altitudinal patterns of both at different spatial scales.Location The study was conducted on five calcareous mountains in south‐eastern France (Etoile, Sainte Baume, Sainte Victoire, Ventoux and Queyras).Methods Stratified sampling according to vegetation and altitude was undertaken on five mountains, forming a composite altitudinal gradient ranging from 100 to 3100 m. Visual searching and analysis of turf samples were undertaken to collect land snail species. Species density is defined as the number of species found within quadrats of 25 m2. Species richness is defined as the number of species found within an elevation zone. Different methods involving accumulation curves are used to describe the patterns in species richness. Elevation zones of different sizes are studied.Results Eighty‐seven species of land snails were recovered from 209 samples analysed during this study. Land snail species density, which can vary between 29 and 1 species per 25 m2, decreases logarithmically with increasing altitude along the full gradient. However, on each mountain separately, only a linear decrease is observable. The climatic altitudinal gradient can explain a large part of this pattern, but the great variability suggests that other factors, such as heterogeneity of ground cover, also exert an influence on species density. The altitudinal pattern of species richness varies depending on the spatial resolution of the study. At fine resolution (altitudinal zones of 100 m) land snail species richness forms a plateau at altitudes below 1000 m, before decreasing with increasing altitude. At coarse resolution (altitudinal zones of 500 and 1000 m) the relationship becomes linear.Main conclusions This study reveals that land snail species density and land snail species richness form two different altitudinal patterns. Species density exhibits strong variability between sites of comparable altitude. A large number of samples seem necessary to study altitudinal patterns of species density. Species density decreases logarithmically with increasing altitude. Above a critical altitudinal threshold, this decrease lessens below the rate seen in the first 1500 m. Different methods exist to scale‐up species density to species richness but these often produce different patterns. In this study, the use of accumulation curves has yielded a pattern of species richness showing a plateau at low altitude, whereas simple plotting of known altitudinal ranges from single mountains would have produced stronger mid‐altitudinal peaks. This study shows that not only factors such as temperatures and habitat heterogeneity, but also an ecotone effect, are responsible for the observed patterns.

RELATIVE STRENGTH OF TOP‐DOWN, BOTTOM‐UP, AND CONSUMER SPECIES RICHNESS EFFECTS ON POND ECOSYSTEMS

Theory and experiments demonstrate that the number of species in a local area can determine rates of ecosystem processes, but we know little about how the strength of that control compares with other influences or how it might vary across ecological gradients. Here I report results of a pond mesocosm experiment where consumer (snail) species richness, predation intensity, and nutrient availability were crossed in a full‐factorial design. This design allowed a direct comparison of the strength of the different factors on food web properties and ecosystem functioning (i.e., system‐level fluxes of energy or matter), and an evaluation of how the factors may interact.Systems with higher snail species richness had greater secondary production, consumer biomass, ecosystem respiration, and macrophyte stem growth, and lower epiphyton and periphyton biomass. However, snail species richness effects on periphyton and epiphyton were context dependent; predators reduced the effect of increasing snail richness on the biomass of attached algae. Species richness effects were most often the result of a biological mechanism (e.g., differential resource use), rather than being solely artifacts of the experimental design (e.g., sampling effects). The effects of nutrient enrichment and predation were mostly predictable from simple food chain models; increases in nutrient availability led to increased algal biomass, primary production, and snail biomass, while predators decreased snail biomass and indirectly augmented algae.Snail species richness effects on the biomass of many functional groups were as strong or stronger than those of a substantial nutrient enrichment or of the addition of a voracious predator (Belostoma flumineum). Nutrient enrichment had the most pronounced effects on several system‐level processes (e.g., primary production, sedimentation). Species richness had weaker effects on ecosystem properties than on particular functional groups, probably because of compensatory responses among different producer functional groups. In summary, this experiment suggests that the number of consumer species in a system can have large and meaningful effects on the distribution of biomass in a food web, and that these effects can depend on ecological context.

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.

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.

Changes in gastropod assemblages in freshwater habitats in the vicinity of Basel (Switzerland) over 87 years

The gastropod fauna in 18 freshwater habitats (streams, rivers and ponds) in the vicinity of Basel, first surveyed in 1906/1907, was re-examined in 1994. The freshwater snail fauna changed considerably over 87 years. One species (Aplexa hypnorum) recorded in 1906/1907 was not found in 1994, most probably it had become extinct in the period between the two surveys. Ten species were found in 1994, but not in 1906/1907. Two of them could not have been recorded in the first survey: the recently invading Potamopyrgus antipodarum and Ferrissia wautieri, a species first described in 1960. In contrast to these changes a part of the gastropod species appear to have a rather constant occurrence. All ten gastropod species found in both surveys showed similar percentage of occurrence (constancy) over 87 years. Species that were rare in 1906/1907 were also rare in 1994, and species common in the first survey were also common 87 years later. Interestingly, species considered as threatened throughout Switzerland towards the end of the 20th century (Red List species) were already rare at the beginning of the century in the habitats examined in the present study. The 1994 survey provides a baseline for monitoring future changes in snail species richness and abundance as riparian restoration actions are under-way or planned at several sites in the region of Basel.

LAND SNAILS IN AN AFFORESTED OIL-SHALE MINING AREA

The numbers and species richness of land snails in four (8, 13, 23, and 25 years old) neighbouring pine plantations in an afforested oil-shale mining area were studied. No snails were found in the eight-year-old plantation. In the 13-year-old plantation two species occurred, but the density of individuals was quite low. In the 25-year-old plantation nine snail species were registered. The degree of species richness in older than 20 years pine plantations of the former oil- shale mining area was the same as that in the most base-rich deciduous forests of West-Estonian large islands, while the density of individuals was even higher. Since land snails serve as a very important calcium source for many birds, former oil-shale mining areas may easily turn into far more bird-rich habitats than are most other Estonian pine forests.

Landscape spatial patterns in freshwater snail assemblages across Northern Highland catchments

1. Limnologists and landscape ecologists have illustrated how the spatial position of a lake in a landscape influences many of its properties, from the physical to the social. Taking a community ecology perspective, we investigated whether freshwater gastropod assemblages respond to lake landscape position.2. We determined: (a) whether there is any spatial pattern among lakes in either the species richness or composition of gastropod assemblages; (b) the form of any spatial pattern; and (c) if any explanatory variables (e.g. dispersal corridors and limiting local conditions) show a similar pattern.3. In three different hydrological catchments, snail species richness increased from isolated highland lakes to stream‐connected lowland lakes, probably reflecting increased colonization potential and less limiting local factors for lowland drainage lakes. Catchments appear to differ from one another with regard to relative species abundance, both in terms of macrophyte‐associated snail fauna and snails from all habitats aggregated. One or more historical events, such as chance dispersal, may have produced this pattern. Taken together, these results suggest that within‐catchment constraints produce repeated gradients in species richness, regardless of what species composition persists in the catchment.

History, island area and habitat availability determine land snail species richness of Aegean islands

Summary Aim We examined the species‐area relation of Aegean land snails, comparing different models to describe the relation. By examining those factors other than area that may also affect species richness, we tested whether the Aegean land snail fauna was more influenced by equilibrial migration and colonization processes, or rather is conservative and relictual. Location The Aegean archipelago (Greece). Methods Sixty‐five islands were examined. Data were taken from own collections and from literature sources. Multiple regression analysis was used to test the null hypothesis of no relationship between species richness and island area, elevation, distance to the next larger island, and the presence and extent of calcareous substrate. Results The single most important factor determining land snail species number was area. While colonization‐extinction dynamics have frequently been cited to explain this result, this conclusion was not tenable in this study as it was contradicted by species number not being related to the islands’ distances to neighbouring larger islands, after accounting for other factors affecting species number. We also found that habitat diversity affected species richness even after accounting for the effects of area: both increased elevation and greater extent of calcareous substrate on islands resulted in higher species number. This effect was most likely due to the fact that particular ecological conditions increased the probability that particular species could survive on an island.We compared the utility of the power and extreme‐value function models of the species‐area relation and found that both gave substantially the same results. However, fitting the power function model using nonlinear regression was of questionable utility. Main conclusions We conclude that the snail fauna of the Aegean is relictual, not equilibrial. The unusually high number of land snail species found on Crete is consistent with this conclusion. Crete is a currently united island which was separated into at least six smaller islands for 7–9 million years during the Neogene. Our results are consistent with the hypothesis that Crete still hosts a large number of endemic species of these paleoislands, resulting in a total number of species in excess of what would be expected based on area alone.

Predicting impact of freshwater exotic species on native biodiversity: Challenges in spatial scaling

Abstract Global homogenization of biota is underway through worldwide introduction and establishment of nonindigenous (exotic) species. Freshwater ecologists should devote more attention to exotic species for two reasons. First, exotics provide an opportunity to test hypotheses about what characteristics of species or habitats are related to successful establishment or invasibility, respectively. Second, predicting which species will cause large ecological change is an important challenge for natural resource managers. Rigorous statistical relationships linking species characteristics to probability of establishment or of causing ecological impacts are needed. In addition, it is important to know how reliable different sorts of experiments are in guiding predictions. We address this issue with different spatial scales of experiments testing the impact of two predators on native snail assemblages in northern Wisconsin USA lakes: an exotic crayfish, the rusty crayfish (Orconectes rusticus); and a native fish predator, the pumpkinseed sunfish (Lepomis gibossus). For the crayfish, laboratory experiments, a field cage experiment, and a snapshot survey of 21 lakes gave consistent results: the crayfish reduced abundance and species richness of native snails. Laboratory and field experiments suggested that pumpkinseed sunfish should have a similar impact, but the lake survey suggested little impact. Unfortunately, no algorithms exist to guide scaling up from small‐scale experiments to the whole‐lake, long‐term management scale. To protect native biodiversity, management of freshwater exotic species should be targeted on lakes or drainages that are both vulnerable to colonization by an exotic, and that harbour endemic species. Management should focus on preventing introduction because eradication after establishment is usually not possible.

Habitat Diversity or Area Per se? Species Richness of Woody Plants, Carabid Beetles and Land Snails on Islands

SUMMARY (1) We examine patterns of species richness of woody plants, carabid beetles and land snails, respectively, on seventeen undisturbed forested islands (area range: 0-6-75 ha) in Lake Malaren, Sweden, in relation to area, isolation, vegetation structure, habitat diversity, habitat heterogeneity, and other factors. (2) The slopes of the species-area relations (z-values in the power model) are 0 10 for woody plants, 0-16 for land snails, 0-36 for carabid beetles, and 0-62 for forest birds. The z-values differ significantly from zero and also differ between all organism groups except woody plants and land snails on one hand, and land snails and carabid beetles on the other. (3) Island area is the best single predictor of species richness in the organism groups examined. Since habitat variables and island area are uncorrelated on the studied islands, we can reject the habitat diversity hypothesis as an explanation for the species-area relations found. (4) Total densities of woody plants and land snails were not correlated with island area, while the total density of carabid beetles was positively correlated with island area. (5) Islands with a high proportion of wet forests have relatively higher species richness of carabid beetles, and the number of land snail species relates positively to the proportion of deciduous forest. The patterns found are discussed in relation to general species richness theories.