Mollusc community patterns and species response curves along a mineral richness gradient: a case study in fens

Aim The goals of this study were to: (1) compare water conductivity and pH as proxy measures of mineral richness in relation to mollusc assemblages in fens, (2) examine the patterns of mollusc species richness along the gradient of mineral richness based on these factors, (3) model species–response curves and analyse calcicole–calcifuge behaviour of molluscs, and (4) compare the results with those from other studies concerning non‐marine mollusc ecology. Location Altogether, 135 treeless spring fen sites were sampled within the area of the Western Carpathians (east Czech Republic, north‐west Slovakia and south Poland; overall extent of study area was 12,000 km²). Methods Mollusc communities were recorded quantitatively from a homogeneous area of 16 m². Water conductivity and pH were measured in the field. The patterns of local species diversity along selected gradients, and species–response curves, were modelled using generalized linear models (GLM) and generalized additive models (GAM), both using the Poisson distribution. Results When the most acid sites (practically free of molluscs) were excluded, conductivity expressed the sites’ mineral richness and base saturation within the entire gradient, in contrast to pH. In the base‐rich sites, pH did not correlate with mineral richness. A unimodal response of local species diversity to mineral richness (expressed as conductivity) was found. In the extremely mineral‐rich, tufa‐forming sites (conductivity > 600 μS cm^?1) a decrease in species diversity was encountered. Response curves of the most common species showed clear differentiation of their niches. Significant models of either unimodal or monotonic form were fitted for 18 of the 30 species analysed. Species showed five types of calcicole–calcifuge behaviour: (1) a decreasing monotonic response curve and a preference for the really acid sites; (2) a skewed unimodal response curve with the optimum shifted towards the slightly acid sites; (3) a symmetrical unimodal model response curve with the optimum in the base‐rich sites, with no or slight tufa precipitation; (4) a skewed unimodal response curve but with the optimum shifted to the more mineral‐rich sites; and (5) an increasingly monotonic response curve, the optimum in the extremely base‐rich sites with strong tufa precipitation. Main conclusions Conductivity is the only reliable proxy measure of mineral richness across the entire gradient, within the confines of this study. This information is of great ecological significance in studies of fen mollusc communities. Species richness does not increase with increasing mineral richness along the entire gradient: only a few species are able to dwell in the extremely base‐rich sites. The five types of calcicole–calcifuge behaviour seen in species living in fens have a wider application: data published so far suggest they are also applicable to mollusc communities in other habitats.     

Testing the species pool hypothesis for mire vegetation: exploring the influence of pH specialists and habitat history

The Evolutionary species pool hypothesis (ESPH) predicts that historically more common habitats will be richer in species because they have had greater opportunity for the evolution of suitably adapted species. We explored the relationship between mire species richness and pH, an important environmental variable in mires, in two regions that differ in habitat pH distribution: the West Carpathians and Bulgaria. Mire habitats in both the West Carpathians and Bulgaria demonstrate support for the ESPH prediction that habitats with more common pH values host more species. We also explored the influence of habitat history by examining the distribution of generalists and specialists along gradients of habitat pH, using extensive community-level vegetation data from European mires in these two regions. We found a striking pattern with the distribution of pH-specialists having three distinct peaks in both regions, whereas the total species pool peaked in near neutral pH habitats in both regions. Because the peaks in specialist richness do not correspond to regional pH distribution patterns, we hypothesize that historical explanations may be important, and that habitats currently rich in pH-specialists may have historically acted as pleniglacial refugia for many mire species. Our findings support the general predictions of the ESPH, but further suggest that historical processes such as patterns of glacial refugia, may significantly influence contemporary species distributions and the diversity of plant species in mire habitats.     

Patterns of bryophyte and vascular plant richness in European subalpine springs

The diversity of spring habitats can be determined not only by local environmental conditions, but also by large-scale biogeographical effects. The effects can differ across various groups of organisms. We compared α-, β- and γ-diversity patterns of bryophytes and vascular plants of (sub)alpine springs in three contrasting mountain ranges: Alps (Switzerland), Balkans (Bulgaria), Western Carpathians (Slovakia, Poland). We used univariate and multivariate statistics to test for the effects of pH, conductivity, altitude, slope, mean annual temperature and annual precipitation on diversity patterns of both taxonomic groups and compared diversity patterns among the regions for particular pH and conductivity classes. We identified acidophyte and basiphyte, calcifuge and calcicole species using species response modelling. All regions displayed significant relationship between conductivity and α-diversity of vascular plants. Bulgaria showed the highest α-diversity of vascular plants for the middle part of the conductivity gradient. For both taxonomic groups, the β-diversity in the middle part of gradient was highest in Swiss Alps. The total species pool was lowest in Bulgaria. The percentage of basiphyte and calcicole species was highest in the Alps. In (sub)alpine springs, mineral richness was a better determinant of vascular plant α-diversity than pH, and the extent of the alpine area did not coincide with α-diversity. Observed inter-regional differences in diversity patterns could be explained by the different proportion of limestone bedrock and different biogeographic history. The differences in α-diversity between both taxonomic groups are presumably result of the different rates of adaptation processes.     

Imputation of environmental variables for vegetation plots based on compositional similarity

Question: Large databases contain many plots, but few subsets with measured environmental data. To obtain broader datasets, researchers use expert‐based indicator values as surrogates; alternatively, these can be estimated by imputation. Does imputation provide more exact approximations than indicator values? Location: West Carpathians (Slovakia, Poland, Czech Republic) and Bulgaria. Methods: We developed a simple imputation method based on plot similarity that estimates missing environmental variables for plots –MOSS (mean of similar samples). This was tested for pH and conductivity, important environmental factors influencing vegetation composition and structure within wetlands, on two datasets of 485 (West Carpathians) and 118 (Bulgaria) plots for which directly measured values were available. The West Carpathian dataset was used for calibration. Imputation was based on calculating mean of the measured factor from a group of most similar plots. Using pre‐defined similarity criteria, we selected subsets of both datasets and compared estimated and measured values. Using root mean‐squared error of prediction, we compared predictive power of MOSS with Ellenberg indicator values and other recent methods. Results: Within one study region, MOSS predicts sample pH and conductivity more precisely than Ellenberg and similar calibration methods. Predictive power slightly decreased when MOSS was transferred to a distant region. Conclusions: Imputation using MOSS appears to accurately predict pH and conductivity from existing composition data within a single geographical region, and thus increases number of replicates. MOSS does not require expert‐based indicator values, which may be imprecise. We provide examples where MOSS can be utilised without risk of circular reasoning or introducing pseudo‐replications.     

Mollusc diversity patterns in Central European fens: hotspots and conservation priorities

Aim To assess mollusc species composition and diversity patterns of treeless fen sites and to find simple environmental parameters that characterize diversity hotspots and priority sites for conservation. Location Western Carpathian Mountains, Europe. Methods Mollusc communities were sampled quantitatively from a homogeneous area of 16 m² in the central part of each of 145 treeless fen sites. Water conductivity and pH, geographical coordinates, altitude and habitat size of the sites studied, mean annual rainfall, mean annual temperature and mean January temperature were compiled for each plot. Nestedness in species composition was tested using the binmatnest program to confirm the ‘nested habitat‐quality hypothesis’. Patterns in species diversity were analysed using the regression trees method to isolate the main predictors of species diversity. Results Nested subset patterns of species composition were found along the gradient of mineral richness. Species distribution was highly nested (T_obs = 11.21, P << 0.001) in mineral‐poor sites (with water conductivity < 300 μS cm^?1, n = 42) and was highly correlated with the site’s mineral richness (r_s = 0.76, P << 0.001). By contrast, species distribution and richness of mineral‐rich sites (c.≥ 300 μS cm^?1, n = 103) were not controlled by mineral richness. Variation in species richness was further explained by January temperature, landscape geomorphology, and total habitat area. The southern mineral‐rich low altitude fens were the most species rich, especially those of larger total area (23 species on average). These 24 sites (17% of all sites) harboured 90% of all recorded species, including all highly endangered ones. Mineral‐rich fens in montane valleys were the second most important group because they hosted the majority of populations of two rare glacial relict species (Vertigo geyeri and Pupilla alpicola). Main conclusions The significant nestedness raises the possibility of conserving the whole fen‐mollusc species pool within the most species‐rich sites. Thus, to select the conservation priority sites, easily available site characteristics for the prediction of species richness are needed. This knowledge can help us maintain fen biodiversity, which has become closely dependent on conservation management practices after the cessation of traditional mowing of fens for haymaking.     

Species Richness, Community Specialization and Soil-Vegetation Relationships of Managed Grasslands in a Geologically Heterogeneous Landscape

The increasing importance of the conservation value of managed grasslands has led to many studies exploring edaphic determinants of grassland biodiversity. Most studies, however, come either from very large areas, where biogeographical factors such as dispersal limitation may play a role, or from small, but ecologically rather uniform, regions. In addition, few studies further distinguish between plant specialists and generalists in the interpretation of the observed patterns. Here we studied species richness in semi-natural, managed grasslands in the Strá?ovské vrchy Mountains in the West Carpathians, Slovakia, where there is a matrix of different bedrocks (crystalline, sandstone, claystone, limestone) on a steep altitudinal gradient. In 89 vegetation plots we sampled the species composition of vascular plants and bryophytes and measured soil chemistry, slope angle, heat index, altitude and soil depth. We further applied Ellenberg indicator values and classified species into community specialists or generalists based on the analysis of a large phytosociological database. Using cluster analysis, we delimited five vegetation types that clearly differed in response to soil characteristics. Species richness varied between 19 and 64 species per 16?m². The main compositional gradient correlated with measured soil pH and calcium, but species richness was not significantly correlated with these factors. Soil available phosphorus was not associated with species composition as has been found elsewhere, but it did correlate negatively with species richness and the richness of specialists. Overall, species richness was largely driven by the number of specialists in the plot and particular vegetation types differed conspicuously in their number. We further found significant effects of iron, potassium and sodium on species richness, species composition and the representation of specialists and generalists. Our results provide new insights into the determinants of diversity in managed grasslands as well as to the theoretical species pool concept, explaining species richness variation along a pH gradient.     

Diversity of wetland vegetation in the Bulgarian high mountains, main gradients and context-dependence of the pH role

We fill a gap in understanding wetland vegetation diversity and relationship with environmental determinants in Bulgarian high mountains. A total of 615 phytosociological samples were taken from springs, mires, wet meadows and tall-forb habitats throughout Bulgaria, of which 234 relevés are from mire and spring vegetation above timberline. The vegetation was classified by TWINSPAN and the resulting vegetation types were reproduced by the formal definitions using the combination of Cocktail species groups based on phi-coefficient of joint co-occurrence of the species. Nine vegetation types of springs and fens have been clearly delimited above the timberline. All vegetation types include Balkan endemic species, the representation of which varies. Fens generally harbour more Balkan endemics than do springs, with the exception of species-poor high-altitude Drepanocladetum exannulati. The gradient structure of the vegetation was revealed by DCA and by CCA with forward selection of environmental factors. The major determinants of vegetation variation strongly differ above and below the timberline and likewise between springs and fens. The base-richness gradient controls the floristic variation of Bulgarian submontane fens, whereas the complete data set including both submontane and subalpine fens is governed by the altitude gradient from lowland and basin fens to subalpine fens rich in Balkan endemics. When focusing on sites above the timberline only, the first DCA axis separates fens from springs without organic matter. The major species turnover in springs follows the variation in water pH and mineral content in water, whereas fen vegetation variation is primarily controlled by succession gradient of peat accumulation. Altitude remains an important factor in all cases. Weak correlation between water pH and conductivity was found. This correlation was even statistically insignificant in fens above the timberline. Water pH is not influenced by mineral richness in Bulgarian high mountains, since it is buffered by decomposition of organic matter in fens. In springs, pH reaches maximum values due to strong aeration caused by water flow. The plant species richness decreases significantly with increasing altitude. The increase of species richness towards circumneutral pH, often found in mires, was not confirmed in Bulgarian high mountains. The correlation between species richness and pH was significant only when arctic-alpine species and allied European high-mountain species were considered separately. The richness of boreal species was independent on pH. Some of them had their optima shifted to more acidic fens as compared to regions below the timberline. Our results suggest that subalpine spring and fen vegetation should be analysed separately with respect to vegetation-environment correlations. Separate analysis of fens below and above timberline is quite appropriate.     

The functional trait space of tree species is influenced by the species richness of the canopy and the type of forest

Analysing how species modify their trait expression along a diversity gradient brings insight about the role that intraspecific variability plays over species interactions, e.g. competition versus complementarity. Here, we evaluated the functional trait space of nine tree species dominant in three types of European forests (a continental‐Mediterranean, a mountainous mixed temperate and a boreal) growing in communities with different species richness in the canopy, including pure stands. We compiled whole‐plant and leaf traits in 1719 individuals, and used them to quantify species trait hypervolumes in communities with different tree species richness. We investigated changes along the species richness gradient to disentangle species responses to the neighbouring environment, in terms of hypervolume size (trait variance), shape (trait relative importance) and centroid translation (shifts of mean trait values) using null models. Our main results showed differences in trait variance and shifts of mean values along the tree diversity gradient, with shorter trees but with larger crowns in mixed stands. We found constrained functional spaces (trait convergence) in pure stands, suggesting an important intraspecific competition, and expanded functional spaces (trait divergence) in two‐species admixtures, suggesting competition release due to interspecific complementarity. Nevertheless, further responses to increasing species richness were different for each forest type, waning species complementarity in sites with limiting conditions for growth. Our results demonstrate that tree species phenotypes respond to the species richness in the canopy in European forests, boosting species complementarity at low level of canopy diversity and with a site‐specific pattern at greater level of species richness. These outcomes evidence the limitation of functional diversity measures based only on traits from pure stands or general trait database values.     

Elevational gradients of reptile richness in the southern Western Ghats of India: Evaluating spatial and bioclimatic drivers

Exploring elevational patterns in species richness and their underlying mechanisms is a major goal in biogeography and community ecology. Reptiles can be powerful model organisms to examine biogeographical patterns. In this study, we examine the elevational patterns of reptile species richness and test a series of hypotheses that may explain them. We sampled reptile communities along a tropical elevational gradient (100–1,500 m a.s.l.) in the Western Ghats of India using time‐constrained visual encounter surveys at each 100‐m elevation zone for 3 years. First, we investigated species richness patterns across elevation and the support of mid‐domain effect and Rapoport's rule. Second, we tested whether a series of bioclimatic (temperature and tree density) and spatial (mid‐domain effect and area) hypotheses explained species richness. We used linear regression and AICc to compare competing models for all reptiles, and each of the subgroups: snakes, lizards, and Western Ghats’ endemics. Overall reptile richness and lizard richness both displayed linear declines with elevation, which was best explained by temperature. Snake richness and endemic species richness did not systematically vary across elevation, and none of the potential hypotheses explained variation in them. This is the first standardized sampling of reptiles along an elevational gradient in the Western Ghats, and our results agree with the global view that temperature is the primary driver of ectotherm species richness. By establishing strong reptile diversity–temperature associations across elevation, our study also has implications for the impact of future climate change on range‐restricted species in the Western Ghats.     

Can tissue element concentration patterns at the individual species level indicate the factors underlying vegetation gradients in wetlands?

Question: Do tissue element concentrations at the individual species level vary along major vegetation gradients in wetlands, and can they indicate environmental conditions? Location: West Carpathians. Methods: Total plant species composition was recorded in plots distributed along a poor to rich gradient within spring fens and along the gradient from fens to wet meadows. Eriophorum angustifolium (Cyperaceae) and three broadleaf dicotyledonous herb species were collected from the vegetation plots. Tissue N, P, K, Ca and Fe concentrations, N:P and N:K ratios of the species were determined. Each variable was correlated with the sample scores along the first two axes of the DCA ordination, which represented the two main vegetation gradients. Results: K and Ca concentrations in a particular species correlated well with the vegetation gradients, thus indicating changes in the element availability to the species. The trends were sometimes contradictory to known patterns at the community level, but the differences could be ecologically interpreted. Contrary to Ca and K, patterns in N, P and Fe concentration appeared to be more species‐specific. E. angustifolium had a lower K and Ca concentration than the broadleaf herbs. Conclusions: Compared to community‐level measurements, element concentrations in individual species correlated less with observed vegetation gradients. Trends found at the species level may indicate changes in ecological conditions affecting the species, although they need not correspond with trends found at the community level. We conclude that the species‐level approach cannot substitute, but can advance, the community‐level approach in searching for mechanisms underlying vegetation gradients within wetlands.     

Vascular plant species richness and rarity across a minerotrophic gradient in wetlands of St. Lawrence County,New York,USA

Thirteen wetlands in St. Lawrence County, NY were sampled to examine the effect of a minerotrophic gradient on vascular plant species richness and rarity. Wetlands ranged from organic soil based poor fens (average conductivity 46.40 microsemens, average Ca 3.55 ppm) to mineral soil based rich fens (average conductivity 342.10 microsemens, Ca 23.00 ppm). Vascular plant species richness was sampled during 1990 in randomly located 1.0 m² quadrats. Specific conductivity, presence or absence of hummocks, and water depth predicted 62% of the variation in richness. Richness increased as conductivity increased until 413 microsemens at which a down trend became obvious. The negative curvilinear relation between conductivity and richness is in accordance with the hump-backed model of Grime but occurs at high rather than intermediate conductivity values. State-listed rare species were found in species-rich wetlands only and had a mean associated richness value of 14.50 species m^-2. This relationship should be taken into consideration when selecting wetlands for protection or managing wetlands for maximum plant diversity.     

Climate stability is more important than water–energy variables in shaping the elevational variation in species richness

Changes in climate variables have an important impact on the prediction and protection of elevational biodiversity. Gaps exist in our understanding of the elevational distribution patterns in seed plant species richness. Our study examines the importance of climate variables in shaping the elevational variation in species richness. The importance of boundary constraint was also taken into account. Model selection based on Akaike's information criterion was used to select the best explaining climate models. Variation partitioning was used to assess the independent and joint effects of water–energy, physiological tolerance, and environmental stability variables on species richness. Our results revealed that: (a) Both raw (boundary constraint unreduced) and estimated (boundary constraint reduced) species richness showed large elevational variation, with the peak species richness seen at midelevations. The environmental variables were better at explaining the distribution pattern of species richness along the elevation, when the effect of boundary constraint was reduced; (b) the physiological tolerance and environmental stability variables explained more variation in raw and estimated species richness compared with the water–energy variables. Estimated species richness was better explained (98.6%) by the environmental variables than raw species richness (94%); (c) the water‐related variables generally had the highest independent effect on raw and estimated species richness and were dominant in shaping the elevational variation in species richness. Our findings quantify the influence of boundary constraint on the distribution pattern of species along an altitudinal gradient and compare the relative contributions of environmental stability and water–energy in explaining the altitude gradient distribution pattern of plant seed species.     

Patterns of species richness and turnover along the pH gradient in deciduous forests: testing the continuum hypothesis

Question: (i) How do species richness and species turnover change along a pH gradient? (ii) What are possible driving factors behind these patterns? (iii) Can the observed patterns be explained by an individualistic continuum concept that postulates independence of species responses and constant turnover rates? Location: Semi‐natural, deciduous hardwood forests in NW Germany (558 plots). Methods: The instantaneous rate of compositional turnover is measured by the sum of slope angles of modelled response curves (119 understorey species) at any point along the pH gradient. Total turnover rate, positive turnover rate (species increasing in probability of occurrence) and negative turnover rate (species decreasing in probability of occurrence) are calculated separately. Species richness is modelled using Poisson regression and by calculating the sum of predicted probabilities at any gradient point. Turnover rates are compared with those calculated from a null model based on a Gaussian community model. Soil chemical analyses of 49 plots are used to interpret biodiversity patterns. Results: Species richness shows a hump‐shaped relation to pH(CaCl₂) with a minor decline at approximately pH>5.0. The decline is possibly due to the confounding influence of water regime and local species pool effects. Increasing richness from pH 2.5 to 4.7 can be traced back to positive turnover exceeding negative turnover. Peaks in turnover rates, dominated by positive turnover, are located at pH 3.7 and 2.8, where turnover rates considerably exceed rates derived from the null model. The turnover pattern can be related to soil chemical conditions, e.g. decreasing base saturation, Al and H⁺ toxicity and the occurrence of mor. Conclusions: The high turnover rates and the massive imbalance in positive and negative turnover rates found in deciduous forests cannot be explained by the individualistic continuum concept. Physiological constraints at the gradient limits and species pool effects could be responsible for this. Their role should be considered more explicitly in vegetation concepts dealing with the continuum‐discontinuum controversy. The presented approach can be regarded as a comprehensive analytical tool for a better understanding of biodiversity patterns along environmental gradients by linking species richness, turnover and response curve types.     

Diversity of the Western Carpathian flysch grasslands: Do extremely species-rich plant communities coincide with a high diversity of snails?

Upland fringes of the White Carpathians (Czech Republic) are known to support grasslands with the world’s highest local plant species richness. We investigated whether this unusually high plant richness has a parallel in snail communities, whether patterns of species composition of snail and plant communities in grasslands co-vary and how they are affected by local environment and landscape history. We compared plant and snail communities of dry to mesic grasslands in three neigh bouring regions: (1) hilly lowland of the Central Moravian Carpathians, (2) upland fringes and (3) upland of the White Carpathians. Both snail and plant communities exhibited a strong gradient in species composition associated with altitude, annual temperature and precipitation, soil calcium and pH. However, there was no correlation between local species richness of plants and snails in individual plots. The upland fringes of the White Carpathians were richest in snail species, probably due to intermediate environmental conditions, supporting the occurrence of species with contrasting environmental requirements. The highest local numbers of plant species were also recorded there, although differences among regions were not significant. The regional species richness of plants was also highest in the upland fringes, whereas that of snails was highest in the hilly lowland. Similarities in the diversity patterns of plants and snails among regions suggest the importance of regional factors for local richness, although local abiotic factors, which are partly correlated with the three regions, also influence local species composition and richness.     

The evolutionary species pool hypothesis and patterns of freshwater diatom diversity along a pH gradient

Aim To interpret the unimodal relationship between diatom species richness and lake pH within the context of the evolutionary species pool hypothesis (SPH). We test the following primary prediction arising from the SPH: the size of the potential species pool (PSP) will increase along a gradient representing the historical commonness of different pH environments (pH commonness). To do this we assume that the present‐day spatial dominance of near‐neutral pH conditions compared with acidic and alkaline conditions reliably mimics the relative spatial availabilities of historical pH conditions among freshwater lakes. We also determine whether local richness represents a constant proportion of PSP size along the pH commonness gradient. Location Two hundred and thirty‐four lakes distributed over a 405,000 km² region of the north‐eastern United States of America. Methods Sediment diatom morphospecies lists and pH data were acquired from the US Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP) website. Using 248 morphospecies that occurred in at least 10 of the 234 lakes, four different measures of PSPs were calculated along the pH gradient. Local species richness was equated with the number of species occurring within the lake. Alpha diversity was equated with the average species richness of lakes with similar pH values. A combination of statistical methods were employed, including correlations, quadratic regression and piecewise regression. Results PSP size increased significantly with pH commonness for all four measures of PSP size, thus supporting the primary prediction of the evolutionary SPH. Local richness comprised a larger proportion of the PSP within acidic lakes than within circumneutral lakes. Alpha diversity and lake species richness both increased significantly with pH commonness, but the former did so in a two‐step fashion. We test and reject several alternative contemporary time‐scale explanations for our findings. Main Conclusions Our findings are consistent with the hypothesis that diatom taxonomic richness is presently lower within acidic and highly alkaline lakes than in circumneutral lakes owing to the limited opportunity in space and/or time for the evolution of suitably adapted species. Whereas ecological processes can explain why certain species are excluded from particular habitats, e.g. acidic lakes, they cannot account for why so few species are adapted to those habitats in the first place.     

Causes of the large variation in bryophyte species richness and composition among boreal streamside forests

Questions: Boreal forests along small streams are bryophyte diversity hotspots because they are moist, productive and relatively high pH. Do these factors also explain the large differences in species richness and species composition found among streamside sites? Do the species of species‐poor sites represent nested subsets of the species of more species‐rich sites? How do the results apply to conservation? Location: Forests along small streams in mid‐boreal Sweden. Methods: Survey of the flora of liverworts and mosses and habitat properties, including calculation of a pH‐index based on species indicator values, in 37 sites (1000‐m² plots). Results: The number of bryophyte species per plot ranged from 34 to 125. Neither soil moisture nor basal area of trees (a proxy for productivity) correlated significantly with species richness and composition, whereas pH‐index and cover of boulders did. Species richness and composition were more strongly correlated with pH‐index for mosses than for liverworts. The richness and composition of bryophyte species most frequently found on moist ground, stream channel margins and, most unexpected, woody debris were all more strongly associated with the pH‐index than with other habitat properties. Although species composition was significantly nested, there was still some turnover of species along the first ordination axis. Conclusions To attain high numbers of species, streamside forests need to have boulders and at least pockets with higher soil and stream‐water pH. The number of Red list species was weakly correlated with total species richness and the most species‐rich sites contained many species found more in non‐forest habitats. Hence, bryophyte conservation in streamside forests should not focus on species‐rich sites but on the quality and quantity of substrate available for assemblages of forest species that are strongly disfavoured by forestry.     

Bryophyte and vascular plant responses to base-richness and water level gradients in Western Carpathian<Emphasis Type="Italic">Sphagnum</Emphasis>-rich mires

We investigated the importance of water chemistry and water regime for vascular plant and bryophyte species distribution in Western Carpathian mires dominated bySphagnum. Seventy-seven small circle plots distributed across a wide geographical area, a wide range of mineral richness and all possible microtopographical features were sampled in terms of species composition, physical-chemical water properties and water regime during one growing season. Both water chemistry and water regime were found to be important factors for vegetation composition. Bryophytes reflected only one clear gradient, connected to base-richness (pH, conductivity) and maximal water-level, whereas three different environmental gradients determined the occurrence of vascular plants: water-level amplitude, base-richness and an indistinct gradient presumably connected to peat layer thickness. When the entire data set was subjected to DCA ordination, the first resulting axis was governed by the bryophyte subset, whereas the second one was governed by the vascular plant subset. The species density of vascular plants was positively correlated with pH and conductivity. On the contrary, bryophyte species density showed no relationship to environmental factors. We further compared the pH values measured in groundwater and in water squeezed from bryophytes from the same plot; these plots were distributed along the base-richness gradient. Only in the acidic mires did the use of squeezed-water chemistry in the analyses give results similar to the use of groundwater pH. Further, we found thatSphagnum species with a similar response to the base-richness gradient had differentiated niches with respect to the water level gradient and vice versa.Sphagnum contortum andS. warnstorfii exhibiting the same demands for groundwater pH were segregated along the gradient of maximum water level. An analogous pattern was detected for acidophilous speciesSphagnum magellanicum andS. papillosum.     

Constraints on trait combinations explain climatic drivers of biodiversity: the importance of trait covariance in community assembly

Trade‐offs maintain diversity and structure communities along environmental gradients. Theory indicates that if covariance among functional traits sets a limit on the number of viable trait combinations in a given environment, then communities with strong multidimensional trait constraints should exhibit low species diversity. We tested this prediction in winter annual plant assemblages along an aridity gradient using multilevel structural equation modelling. Univariate and multivariate functional diversity measures were poorly explained by aridity, and were surprisingly poor predictors of community richness. By contrast, the covariance between maximum height and seed mass strengthened along the aridity gradient, and was strongly associated with richness declines. Community richness had a positive effect on local neighbourhood richness, indicating that climate effects on trait covariance indirectly influence diversity at local scales. We present clear empirical evidence that declines in species richness along gradients of environmental stress can be due to increasing constraints on multidimensional phenotypes.     

敦煌西湖荒漠-湿地生态系统优势物种生态位研究

水文情势改变会引起土壤盐分变化,直接影响到荒漠-湿地生态系统植被的分布与演替。基于对57个样地、171个样方植物物种分布影响较大的土壤pH值和土壤电导率两个环境因子,将其划分为6个梯度等级,测度分析了敦煌西湖植被群落中15个主要优势种的生态位特征,了解不同物种利用资源和占据生态空间的能力,对维持和科学保育植物群落的多样性具有重要意义。结果表明：（1）在土壤pH值和电导率梯度两个资源维上,多枝柽柳和芦苇的重要值和生态位宽度均较大,说明这两个物种适应能力强能够较好地利用环境资源,分布范围大且均匀。它们作为敦煌西湖植被群落中的广域种,具有重要的生态地位和作用。其次生态位较宽的疏叶骆驼刺、胡杨和苏枸杞对环境因子也具有较强的适应能力。（2）两个土壤因子梯度下植物种群生态位宽度相似,但也存在差异。如泡泡刺、蒙古沙枣在土壤pH值梯度资源维上的生态位宽度值远大于在土壤电导率梯度资源维上,但尖叶盐爪爪和甘蒙柽柳在土壤电导率梯度资源维上生态位宽度较大,表现出较强的耐盐能力,从而说明这些物种对不同土壤因子的利用能力和适应性不完全相同。（3）在两个资源维上优势物种间的生态位重叠值小于0.5的种对均为61对,占总种对的58.10%,因此生态位重叠值整体保持在较低水平,说明物种在土壤pH值和土壤电导率两个环境梯度上生态位分化明显。（4）敦煌西湖优势物种间总体表现为不显著的负关联,表明物种之间处于竞争关系,但竞争强度不大且群落结构稳定性较弱。     

Pattern of local plant species richness along a gradient of landscape topographical heterogeneity: result of spatial mass effect or environmental shift?

Several processes are hypothesised to mediate the relationship between local (microsite) plant species richness and the topographical heterogeneity of the surrounding landscape. In a topographically heterogeneous landscape with various habitats occurring close to each other, local species richness may be enriched by species from surrounding habitats due to the spatial mass effect (sink‐source dynamics). In contrast, increased habitat fragmentation due to spatial heterogeneity may have a negative effect on local species richness. The spatial mass effect is thought to be more pronounced in communities with a higher ratio of generalists, as generalists are more likely to establish viable populations in sink habitats. To reveal the pattern of local species richness along a gradient of landscape topographical heterogeneity at middle altitudes of the Bohemian Massif, we used 2551 forest vegetation plots stored in the Czech National Phytosociological Database. We developed an analytical approach relating the pattern of local species richness of vegetation types to the gradient of landscape topographical heterogeneity. An increase or decrease in species richness with increasing landscape heterogeneity was related to changes in the generalist/specialist ratio, and also to changes in soil reaction and productivity estimated through Ellenberg indicator values. Local species richness along a gradient of increasing landscape heterogeneity increased in nutrient‐poor vegetation and decreased in nutrient‐rich vegetation. Nutrient‐poor vegetation types, such as thermophilous and acidophilous oak forests, also had a high proportion of habitat generalists, supporting the hypothesis that increased richness in heterogeneous landscapes may result from the spatial mass effect. However, the same pattern may be explained by a shift in environmental conditions along the landscape heterogeneity gradient, such as increasing productivity of nutrient‐rich vegetation types or increasing soil reaction of most vegetation types in more heterogeneous landscapes. We discuss available evidence and conclude that these two explanations need not be mutually exclusive.