Experimental test of the intermediate disturbance hypothesis: frequency effects of emersion on fouling communities

The validity of predictions derived from the intermediate disturbance hypothesis (IDH) was tested in situ by manipulating mussel dominated Western Baltic fouling communities. Assemblages of two different successional stages, 3 and 12 months old, underwent a 3-month period of disturbance treatment in terms of various frequencies of emersion. Emersion frequency levels ranged from 1×15 to 48×15 min emersion day⁻¹. The study on the 3-month-old communities was repeated in 2 subsequent study years. Species richness, evenness and diversity (Shannon index) were recorded to measure the effects of frequency treatments on community structure.The IDH was confirmed in the first year, when diversity was found to be a unimodal function of the applied emersion frequency gradient. Diversity-disturbance relationships were inverse unimodal or non-significant in the second year, which was true for both successional stages. This ambiguous picture partially confirms the validity of the mechanisms proposed by the IDH, but also shows that their forcing can be masked by fluctuations in environmental parameters, such as climatic conditions. Diversity increased again under severe disturbance conditions, due to a disturbance-induced change in community structure, namely the shift from mussel to algal dominance. This is a new aspect in the discussion concerning disturbance-diversity relationships.     

Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis–Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.     

环境因子和干扰强度对高寒草甸植物多样性空间分异的影响

采用样带与样地结合的方法在三江源自然保护区的核心区沿海拔梯度在阴坡、阳坡分别进行草本植被调查,通过因子分析和偏相关分析研究丰富度指数、多样性指数与环境梯度（包括海拔梯度、裸斑面积、坡度、土壤总碳、总氮含量、土壤pH值、土壤总可溶性盐含量）和干扰强度（鼠类干扰强度、放牧强度）之间的关系。研究结果表明：杂类草丰富度指数（DMa杂）与总物种丰富度指数（DMa总）极显著相关（P<0.01）;阳坡DMa杂和DMa总均呈现“中海拔膨胀”现象,阴坡DMa杂和DMa总与海拔梯度呈正相关,莎草科和禾本科的丰富度指数（DMa莎和DMa禾）随海拔升高并无明显规律;通过主成分分析,及偏相关分析,第一主成分（裸斑面积、鼠类干扰和放牧强度）与除莎草科Margalef丰富度指数、禾本科Simpson指数和禾本科Pielou均匀度指数外的其他草地多样性指数均显著相关,是影响阳坡草地植物多样性的主要因子,土壤总碳、总氮含量对阳坡禾本科类群的多样性指数和均匀度指数有极显著影响,土壤pH值、TDS含量和坡度对阳坡莎草科类群的丰富度有显著影响;海拔梯度、土壤总碳、总氮以及pH值对阴坡草本植物群落的多样性影响较大。研究结论认为,植物群落生物多样性的空间分异特征是地理环境、土壤环境以及干扰强度等因素综合作用的结果。无干扰或干扰较弱时,物种多样性主要受土壤环境状况所影响;而在强干扰存在条件下,干扰强度对物种丰富度和多样性的影响比环境因子更显著;遏制高寒草甸植物多样性降低应首先控制放牧及鼠类等强干扰活动。     

Relationship between light availability and species richness during fen grassland succession

The complex relation between standing crop and species richness in herbaceous plant communities is still one of the most intensively discussed topics in vegetation ecology. In this study, we focus on the extent to which light availability is able to explain this relationship in fen grasslands. We analyse changes of light availability (measured as relative irradiance, RI), standing crop and species richness during fen grassland succession. Our study include communities representing differences in drainage intensity and nutrient status (‘land use intensification sere’) and successional stages developing after abandonment (‘abandonment seres’). Both in the land use intensification sere and in the abandonment seres, we recorded an increase in standing crop reaching highest values of approx. 800 g m^‐2. RI, species richness and number of small‐growing species declined in the successional seres. RI was highest in stands of low‐productive communities, where light compensation points (5% RI) were already attained at soil surface and light saturation points (30% RI) at soil surface or at 15 cm height. Horizontal heterogeneity of RI at soil surface and at 30 cm height decreased in the abandonment seres, but not at 30 cm height in the land use intensification sere. Furthermore, we detected a signiñcant negative relation between standing crop and RI. Species richness declined with increasing standing crop and thus consequently also with decreasing RI. This result points out that light competition might be of importance for this pattern. The number of small‐growing species decreased exponentially with an increase in standing crop (and decrease in RI). It can be assumed that low light availability negatively affects small‐growing species at standing crop values higher than 400–500 g m². As the maintenance of small‐growing species and the improvement of habitat conditions for their establishment are important aims of nature conservation in fen grasslands, management strategies should be orientated at maintaining standing crops that are not higher than these values.     

Neutral communities may lead to decreasing diversity-disturbance relationships: insights from a generic simulation model

Many attempts have been made to confirm or reject the unimodal relationship between disturbance and diversity stated by the intermediate disturbance hypothesis (IDH). However, the reasons why the predictions of the IDH apply or fail in particular systems are not always obvious. Here, we use a spatially explicit, individual-based community model that simulates species coexistence in a landscape subjected to disturbances to compare diversity-disturbance curves of communities with different coexistence mechanisms: neutrality, trade-off mechanism and intraspecific density dependence. We show that the shape of diversity-disturbance curves differs considerably depending on the type of coexistence mechanism assumed: (1) Neutral communities generally show decreasing diversity-disturbance curves with maximum diversity at zero disturbance rates contradicting the IDH, whereas trade-off communities generally show unimodal relationships confirming the IDH and (2) density-dependent mechanisms do increase the diversity of both neutral and trade-off communities. Finally, we discuss how these mechanisms determine diversity in disturbed landscapes.     

What determines disturbance‐productivity‐diversity relationships? The effect of scale,species and environment on richness patterns in an Australian woodland

Much of the observed variation in relationships between diversity and disturbance or productivity may be attributed to scale, species characteristics, or environment. We used exclusion fences to create gradients of grazing (by native and introduced herbivores), cover, and standing crop in temperate Eucalypt woodlands. We investigated patterns of native, exotic, and total plant species richness at two scales (1 m² and 625 m²). Richness patterns were similar at both scales, though species richness at 1m² was more strongly affected by our grazing treatments. Season and rainfall explained more variation in richness than did surrogate measures of productivity or disturbance by herbivores. The richness-herbivory relationship depended strongly on rainfall, season, and species origin, and altering these factors produced the entire range of observed diversity-disturbance relationships. Richness-biomass and richness-cover relationships were consistently hump-shaped, and related to species origin with native richness negatively related and exotic richness positively related. The ability of weedy annuals to pre-empt space after death may have contributed to the observed unimodal responses.     

Relationships between spatial environmental heterogeneity and plant species diversity on a limestone pavement

No empirical studies have examined the relationship between diversity and spatial heterogeneity across unimodal species richness gradients. We determined the relationships between diversity and environmental factors for 144 0.18 m² plots in a limestone pavement alvar in southern Ontario, Canada, including within-plot spatial heterogeneity in soil depth, microtopography and microsite composition. Species richness was unimodally related to mean soil depth and relative elevation. Microsite heterogeneity and soil depth heterogeneity were positively correlated with species richness, and the richness peaks of the unimodal gradients correspond to the maximally spatially heterogeneous plots. The best predictive models of species richness and evenness, however, showed that other factors, such as ramet density and flooding, are the major determinants of diversity in this system. The findings that soil depth heterogeneity had effects on diversity when the effects of mean soil depth were factored out, and that unimodal richness peaks were associated with high spatial heterogeneity in environmental factors represent significant contributions to our understanding of how spatial heterogeneity might contribute to diversity maintenance in plant communities.     

Influence of grazing and fire on breeding birds and perennial plants in Cyprus scrub and forest systems

Grazing and browsing by sheep and goats has been an important anthropogenic influence on ecosystems in the Mediterranean region for centuries. This influence has changed significantly in recent decades, with a general shift from range grazing to the penning of animals. The intermediate disturbance hypothesis (IDH) proposes that perturbation - including anthropogenic disturbance - is the norm for Mediterranean ecosystems, and that higher species diversity is found under conditions of continuous, moderate disturbance. Here we test the IDH as it relates to grazing of scrub and open forest habitats in Cyprus, while also testing for the effects of fire. We carried out surveys of breeding birds and vegetation at 48 study sites in scrub and open woodland across Cyprus. We estimated relative grazing pressure (past and present) and fire history at these sites, and tested for associations between these factors, breeding bird species richness and perennial vegetation diversity. We found moderate current grazing reduces density of lower and middle level scrub, and a higher level of diversity of perennial vegetation at moderately over intensively grazed sites. We found that moderate historical grazing pressure had a positive influence on richness of breeding bird species in lower scrub habitat such as phrygana, but that this effect was weaker the taller and denser the habitat. By contrast, current grazing pressure had a negative effect on species richness in lower scrub, but species richness in grazed habitats was higher in denser, taller scrub habitat such as garrigue and maquis. Our study suggests grazing plays an important role in maintaining habitat heterogeneity but the impact on avian species richness depends on the density and height of the vegetation.     

Microtopographic heterogeneity constrains alpine plant diversity, Glacier National Park, MT

Theoretical and empirical evidence exists for a positive relationship between environmental heterogeneity and species diversity. Alpine plant communities can exhibit exceptional diversity at a fine scale, which niche theory would suggest is the result of fine scale spatial heterogeneity of the environment. To test if species diversity of alpine plants is driven by environmental heterogeneity, we sampled vascular plant species composition, microtopography, and ground cover within 1?m² plots with and without solifluction forms in Glacier National Park, MT. We analyzed the relationship between microtopographic heterogeneity and species richness at the plot and sub-plot scale with linear and quantile regression, respectively. Species richness does not differ between the plots varying in cover type. Species richness is negatively related to the fractal dimension (D) of the ground surface and non-vegetated ground cover within 1?m² plots. At a finer scale, the standard deviation of elevation and slope appear to impose a limit on species richness such that more variable sub-plots have lower species richness. Contrary to our expectations, microtopographic heterogeneity does not promote the diversity of alpine plants. The negative relationship between topographic heterogeneity and species richness is contrary to the theoretical prediction that environmental heterogeneity generally results in greater species diversity. It is possible that microtopographic variability represents a measure of soil disturbance, which would be expected to have a negative effect on species diversity in alpine tundra due to its low productivity.     

Plant species richness in continental southern Siberia: effects of pH and climate in the context of the species pool hypothesis

Aim Many high‐latitude floras contain more calcicole than calcifuge vascular plant species. The species pool hypothesis explains this pattern through an historical abundance of high‐pH soils in the Pleistocene and an associated opportunity for the evolutionary accumulation of calcicoles. To obtain insights into the history of calcicole/calcifuge patterns, we studied species richness–pH–climate relationships across a climatic gradient, which included cool and dry landscapes resembling the Pleistocene environments of northern Eurasia. Location Western Sayan Mountains, southern Siberia. Methods Vegetation and environmental variables were sampled at steppe, forest and tundra sites varying in climate and soil pH, which ranged from 3.7 to 8.6. Species richness was related to pH and other variables using linear models and regression trees. Results Species richness is higher in areas with warmer winters and at medium altitudes that are warmer than the mountains and wetter than the lowlands. In treeless vegetation, the species richness–pH relationship is unimodal. In tundra vegetation, which occurs on low‐pH soils, richness increases with pH, but it decreases in steppes, which have high‐pH soils. In forests, where soils are more acidic than in the open landscape, the species richness–pH relationship is monotonic positive. Most species occur on soils with a pH of 6–7. Main conclusions Soil pH in continental southern Siberia is strongly negatively correlated with precipitation, and species richness is determined by the opposite effects of these two variables. Species richness increases with pH until the soil is very dry. In dry soils, pH is high but species richness decreases due to drought stress. Thus, the species richness–pH relationship is unimodal in treeless vegetation. Trees do not grow on the driest soils, which results in a positive species richness–pH relationship in forests. If modern species richness resulted mainly from the species pool effects, it would suggest that historically common habitats had moderate precipitation and slightly acidic to neutral soils.     

Inter-relationships between standing crop, biodiversity and trait attributes of hydrophytic vegetation in artificial waterways

1. The extensive British canal system potentially provides a favourable habitat for aquatic plants and, because of its uniformity, it is ideal for studying relationships between disturbance intensity (largely from boat traffic) and communities of aquatic macrophytes. 2. The standing crop and species composition of aquatic plants were measured in summer at 396 sites distributed randomly over the canal network. We also quantified the number of rare species and ‘attribute groups’ (groups of species sharing similar suites of biological traits). These data were analysed in relation to standing crop, assumed to indicate a disturbance gradient. 3. Consistent with unimodal models and the Intermediate Disturbance Hypothesis, maximum hydrophyte species richness occurred at an intermediate biomass (50–200 g DWm^?2). This corresponded to a low frequency of low magnitude disturbance (light boat traffic) on navigable sections, or occasional high magnitude disturbance (the post‐interventionist phase) on sections currently unnavigable and subject to active vegetation management. The frequency of rare species was also the greatest under these conditions, reflecting the availability of regeneration niches. 4. Sorting of species into attribute groups revealed that the overall relationship between species richness and standing crop comprised of closely overlapping unimodal responses of nine attribute groups, superimposed on a core vegetation of Potamogeton pectinatus, together with greater representation of filamentous algae, lemnids and elodeids with increasing standing crop (i.e. assumed low disturbance). High species richness was associated with the overlap of functionally different groups of species, rather than with disturbance‐mediated coexistence of functionally similar plants. 5. The analysis of a matrix of sites and the representation of plant traits weighted by the biomass of species that displayed them, in relation to different aspects of disturbance, highlighted a shift from traits associated with resilience (turions, unanchored floating or submerged leaves, low body flexibility, budding, small body size), or competitiveness (entire leaves, low reproductive output, high biomass density, large body size) at high standing crop, through to attributes more associated with resistance to disturbance (rhizomes, tubers, streamlining of foliage, low biomass density) at low standing crop. 6. Comparison with a stochastic null model of change in species number along a constrained gradient, after correction for variation in sampling effort, indicated that sites towards the tails of the gradient (excluding those with extremely low biomass) supported more species than might be expected from chance alone, while the most species‐rich sites in mid‐gradient generally supported many fewer species than expected. 7. We suggest that a disturbance regime that maintains intermediate standing crops would be appropriate for the conservation of species‐rich aquatic vegetation. Precise definition of this regime, under a range of circumstances, requires the study of temporal change at representative sites.     

Is high species richness at intermediate disturbance level valuable for phylogenetic conservation? A test on bird species in South‐East Botswana

The intermediate disturbance hypothesis (IDH) is one of the most debated theories in ecology. However, even when evidence is provided to support the hypothesis, its relevance for phylogenetic conservation has rarely been tested. Here, we investigated this question on birds in the South‐East district of Botswana along a disturbance gradient across three types of landscapes. We first reconstructed the phylogeny for all species recorded. Next, we assessed the relationship between dissimilarity measures and habitat types using the permutational MANOVA. Finally, we tested the IDH by fitting a generalized linear mixed effect model to account for errors due to spatial pseudo‐replications of our collection design. We found that, although species richness and phylogenetic diversity (PD) follow the prediction of the IDH, the evolutionary component of PD (i.e. PSV, phylogenetic species variability) contributes little to the prediction, suggesting that the correlation between PD and disturbance level is driven by the richness component of PD (i.e. PSR, phylogenetic species richness). However, the increased richness at the intermediate disturbance level does not result in phylogenetically diverse bird communities, indicating that the IDH contributes little to phylogenetic diversity. Our study adds to the body of literature questioning the relevance of IDH in ecology.     

Deer density affects ground-layer vegetation differently in conifer plantations and hardwood forests on the Boso Peninsula, Japan

Deer overabundance reduces forest ground-layer vegetation and can cause cascading impacts on a forest ecosystem. To predict these effects, we must elucidate the relationship between deer density and the status of ground-layer vegetation. This relationship was studied in the Boso Peninsula, where the deer population density exhibits a clear geographical gradient. We examined species richness and cover of ground-layer vegetation at several cedar plantations and hardwood forests in the Boso Peninsula. We also examined whether deer impacts were altered by light condition, soil water content and forest type (cedar and hardwood). Species richness of ground-layer vegetation was maximized at an intermediate level of deer density, suggesting an intermediate disturbance effect. This phenomenon was compatible with the observation that evergreen species, which were competitive dominants, decreased with increasing deer density, whereas less competitive deciduous species increased until herbivory was intermediate. As deer density increased, cover of ground-layer vegetation gradually decreased, but species unpalatable to deer increased in abundance, suggesting indirect positive effects of deer for unpalatable species. Cedar plantations tended to have greater species richness and ground-layer cover than hardwood forests with similar deer levels. Canopy openness, an indicator of light conditions, increased species richness of hardwood forests and ground-layer cover of cedar plantations, even under deer herbivory. Topographic wetness index, an indicator of soil water content, significantly increased the ground-layer cover of cedar plantations under deer herbivory. These results emphasize the importance of environmental productivity and forest type in the management of ground-layer vegetation experiencing deer overabundance.     

Influence of experimental soil disturbances on the diversity of plants in agricultural grasslands

Aims and Methods Disturbance is supposed to play an important role for biodiversity and ecosystem stability as described by the intermediate disturbance hypothesis (IDH), which predicts highest species richness at intermediate levels of disturbances. In this study, we tested the effects of artificial soil disturbances on diversity of annual and perennial vascular plants and bryophytes in a field experiment in 86 agricultural grasslands differing in land use in two regions of Germany. On each grassland, we implemented four treatments: three treatments differing in application time of soil disturbances and one control. One year after experimental disturbance, we recorded vegetation and measured biomass productivity and bare ground. We analysed the disturbance response taking effects of region and land-use-accompanied disturbance regimes into account.Important findings Region and land-use type strongly determined plant species richness. Experimental disturbances had small positive effects on the species richness of annuals, but none on perennials or bryophytes. Bare ground was positively related to species richness of bryophytes. However, exceeding the creation of 12% bare ground further disturbance had a detrimental effect on bryophyte species richness, which corresponds to the IDH. As biomass productivity was unaffected by disturbance our results indicate that the disturbance effect on species richness of annuals was not due to decreased overall productivity, but rather due to short-term lowered inter- and intraspecific competition at the newly created microsites. Generally, our results highlight the importance of soil disturbances for species richness of annual plants and bryophytes in agricultural grasslands. However, most grasslands were disturbed naturally or by land-use practices and our additional experimental soil disturbances only had a small short-term effect. Overall, total plant diversity in grasslands seemed to be more limited by the availability of propagules rather than by suitable microsites for germination. Thus, nature conservation efforts to increase grassland diversity should focus on overcoming propagule limitation, for instance by additional sowing of seeds, while the creation of additional open patches by disturbance might only be appropriate where natural disturbances are scarce.     

Introduced and native ground beetle assemblages (Coleoptera: Carabidae) along a successional gradient in an urban landscape

According to the intermediate disturbance hypothesis (IDH), species diversity should be higher at sites with intermediate levels of disturbance. We tested this hypothesis using ground beetles (Coleoptera: Carabidae) collected in pitfall traps from sites that varied in time since last disturbance. This successional gradient was embedded in an urban landscape near Montreal, Quebec. We predicted that diversity in young forests and old fields would be higher than in agricultural fields and old forests. Fifty-five species (2932 individuals) were found in 2003 and 46 species (2207 individuals) in 2004. In both years, species richness was highest from traps placed in agricultural fields. We collected nine introduced species; these had higher catch rates than the native species in both years (64.8% of total catch). When introduced species were removed from the Nonmetric Multidimensional Scaling ordination analysis, the assemblages from agricultural fields were less distinct compared to those of the other habitats, suggesting the introduced fauna is important in structuring carabid assemblages from the agricultural fields. Introduced species may play a significant role in the community composition of ground beetles in urban landscapes, and their influence may be the cause of the lack of support found for the IDH.     

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.     

Effects of disturbance frequency,intensity, and area on assemblages of stream macroinvertebrates

Disturbance frequency, intensity, and areal extent may influence the effects of disturbance on biological communities. Furthermore, these three factors may have interacting effects on biological diversity. We manipulated the frequency, intensity, and area of disturbance in a full-factorial design on artificial substrates and measured responses of benthic macroinvertebrates in a northern Vermont stream. Macroinvertebrate abundance was lower in all disturbance treatments than in the undisturbed control. As in most other studies in streams, species density (number of species/sample) was lower in disturbed treatments than in undisturbed controls. However, species density is very sensitive to total abundance of a sample, which is usually reduced by disturbance. We used a rarefaction method to compare species richness based on an equivalent number of individuals. In rarefied samples, species richness was higher in all eight disturbed treatments than in the undisturbed control, with significant increases in species richness for larger areas and greater intensities of disturbance. Increases in species richness in response to disturbance were consistent within patches, among patches with similar disturbance histories, and among patches with differing disturbance histories. These results provide some support for Huston’s dynamic-equilibrium model but do not support the intermediate-disturbance hypothesis. Our analyses demonstrate that species richness and species density can generate opposite patterns of community response to disturbance. The interplay of abundance, species richness, and species density has been neglected in previous tests of disturbance models. Received: 20 July 1999 / Accepted: 26 January 2000     

Plant diversity changes and succession along resource availability and disturbance gradients in Kamchatka

Changes in plant species richness across environmental and temporal gradients have often been explained by the intermediate disturbance hypothesis and a unimodal diversity–productivity relationship. We tested these predictions using two sets of mountain plant communities assembled along postglacial successional and snow depth (disturbance and stress) gradients in maritime Kamchatka. In each community, we counted the number of species in plots of increasing sizes (0.0025–100 m²) and analyzed them using species–area curves fitted by the Arrhenius power function and the Gleason logarithmic function. A comparison of successional communities along a 270-year-old moraine chronosequence behind the receding Koryto Glacier—representing gradients of increasing productivity and resource competition—confirmed the unimodal species richness pattern. The plant diversity peaked in a 60–80-year-old Salix–Alnus stand where light availability was sufficient to sustain a rich understory combining pioneer and late successional herbs. The closed Alnus canopy on older moraines caused a pronounced decrease in species richness for all plot sizes (interactive stage 80–120 years since deglaciation). A slight increase in species richness in the oldest assortative stages (120–270 years), when Alnus stands are mature, was found only at the smaller spatial scales. This reflects (i) the consolidation of clonal understory dominants and (ii) the absence of other woody species such as Betula ermanii whose invasion would eliminate Alnus and increase diversity at larger spatial scales. A comparative study of major mountain plant communities distributed above the Koryto Glacier foreland did not confirm the highest species richness at intermediate levels of disturbance and stress. Contrary to our expectation, the species richness was highest in alpine tundra and snowbed communities, which are subjected to severe winter frost and a short summer season, while less disturbed communities of subalpine meadows, heaths, and Betula ermanii woods were less species-rich. We attribute this pattern to differences in habitat area and species pool size.     

Using richness of native and non-native aquatic species along a climatic gradient to test the intermediate disturbance hypothesis

The intermediate disturbance hypothesis (IDH) has been thoroughly investigated, but much controversy has been found for supporting its assumptions, which rely largely on the nature of the disturbance, spatial scale, and biological predictors tested. In this paper, richness of native and non-native species along a suite of Neotropical aquatic ecosystems across a broad latitudinal and geographical range was used to test the IDH predictions. An extensive literature survey was performed to compile native species richness and the occurrence of several taxonomic groups listed as non-native for twenty-four coastal rivers and bays evenly distributed into three climatic zones (tropical, transitional, and subtropical). The climatic gradient was confirmed by NMDS and PERMANOVA, but IDH predictions were only significantly supported for native and total species richness in the coastal bays. The distribution patterns of non-native marine species showed a linear instead unimodal pattern of increase with latitudinal climatic gradient, but the responses are complex and dependent of many non-exclusive factors, such as the sampling effort per ecosystem and the potential interference of other disturbance gradients that should be further addressed to unravel the role of IDH for non-native species distribution.

     

Variation in species composition and species richness within Phragmites australis dominated riparian zones

In riparian wetlands total standing crop often fails to account for a significant part of the observed variation in species richness and species composition within communities. In this study, we used abundance of the dominant species instead of total standing crop as the biotic predictor variable and investigated its relationships with species composition and species richness in communities dominated by Phragmites australis (Cav.) Trin. ex Steudel. This was done by measuring soil organic matter content, litter cover and elevation, Phragmites abundance (standing crop and stem density) and species composition in 78 relevés. In addition, we tried to identify the environmental boundaries of Phragmites communities by sampling relevés in neighbouring communities.Two gradients were related to a decline in Phragmites abundance: one gradient, perpendicular to the shoreline, was mainly related to increased elevation and the second gradient ran parallel to the shoreline and was related to increased amounts of soil organic matter. Within the relevés dominated by Phragmites, stem density of Phragmites and litter cover were the only factors significantly related to species composition in the RDA solution. Litter cover and standing crop of the dominant accounted for 64% of the variation in species richness within the Phragmites-dominated community. These results show that dead and living biomass of the dominant species may account for a substantial part of the variation in species composition and species richness within a single community.