Dominance–diversity relationships in ant communities differ with invasion

The relationship between levels of dominance and species richness is highly contentious, especially in ant communities. The dominance‐impoverishment rule states that high levels of dominance only occur in species‐poor communities, but there appear to be many cases of high levels of dominance in highly diverse communities. The extent to which dominant species limit local richness through competitive exclusion remains unclear, but such exclusion appears more apparent for non‐native rather than native dominant species. Here we perform the first global analysis of the relationship between behavioral dominance and species richness. We used data from 1,293 local assemblages of ground‐dwelling ants distributed across five continents to document the generality of the dominance‐impoverishment rule, and to identify the biotic and abiotic conditions under which it does and does not apply. We found that the behavioral dominance–diversity relationship varies greatly, and depends on whether dominant species are native or non‐native, whether dominance is considered as occurrence or relative abundance, and on variation in mean annual temperature. There were declines in diversity with increasing dominance in invaded communities, but diversity increased with increasing dominance in native communities. These patterns occur along the global temperature gradient. However, positive and negative relationships are strongest in the hottest sites. We also found that climate regulates the degree of behavioral dominance, but differently from how it shapes species richness. Our findings imply that, despite strong competitive interactions among ants, competitive exclusion is not a major driver of local richness in native ant communities. Although the dominance‐impoverishment rule applies to invaded communities, we propose an alternative dominance‐diversification rule for native communities.     

Environmental Productivity and Biodiversity Effects on Invertebrate Community Invasibility

Productivity influences the availability of resources for colonizing species. Biodiversity may also influence invasibility of communities because of more complete use of resource types with increasing species richness. We hypothesized that communities with higher environmental productivity and lower species richness should be more invasible by a competitor than those where productivity is low or where richness is high. We experimentally examined the invasion resistance of herbivorous meiofauna of Jamaican rock pools by a competitor crustacean (Ostracoda: Potamocypris sp. (Brady)) by contrasting three levels of nutrient input and four levels of species richness. Although relative abundance (dominance) of the invasive was largely unaffected by resource availability, increasing resources did increase the success rate of establishment. Effects of species richness on dominance were more pronounced with a trend towards the lowest species richness treatment of 2 resident species being more invasible than those with 4, 6, or 7 species. These results can be attributed to a ‘sampling effect associated with the introduction of Alona davidii (Richard) into the higher biodiversity treatments. Alona dominated the communities where it established and precluded dominance by the introduced ostracod. Our experimental study supports the idea that niche availability and community interactions define community invasibility and does not support the application of a neutral community model for local food web management where predictions of exotic species impacts are needed.     

Richness, structure and functioning in metazoan parasite communities

Ecosystem functioning, characterized by components such as productivity and stability, has been extensively linked with diversity in recent years, mainly in plant ecology. The aim of our study was thus to quantify general relationships between diversity, community structure and ecosystem functions in metazoan parasite communities. We used data on parasite communities from 15 species of marine fish hosts from coastal Chile. The volumetric abundance (volume of all parasite species per individual host, in mm³) was used as a surrogate for productivity. Species diversity was measured using both species richness and evenness, while community structure was estimated using the co‐occurrence indices V‐ratio, C‐score and a new C‐score_s index standardized for the number of host replicates. After correcting for fish size, 47% of host species show no relationship, 13% show a hump shaped curve and 40% show positive monotonic relationships between productivity and parasite richness across all host individuals in a sample. We obtained a logarithmically decreasing relationship between evenness and productivity for all fish species, and propose a ‘dominance‐resistance’ hypothesis based on immunity to explain this pattern. The stability of the parasite community, measured as the coefficient of variation in productivity among individual hosts, was strongly and positively related to mean species richness across the 15 host species. The C‐score_s index, based on the number of checkerboard units in the host‐parasite presence/absence matrix, increases linearly with mean productivity across the 15 host species, suggesting that parasite communities tend to be more structured when they are more productive. This is the likely reason why linear relationships between richness and productivity were not observed consistently in all fish species. Parasite communities provide some clear patterns for the diversity–ecosystem functioning debate in ecology, although other factors, such as the history of community assembly, may also influence these patterns.     

Relationship between dominance and richness of local species: an analysis of the underlying reasons with arboreal and avian communities of the West Caucasus as an example

Dominance level is traditionally expressed as a ratio between the number of individuals belonging to the most abundant species and the total number of individuals in a biological community. It is known that local species richness is usually higher in biological communities with high dominance level than in communities with low one. Taking into account a complex nature of the dominance phenomenon, the underlying reasons (or mechanisms) may be diverse: 1. Dominance level may be determined by bioecological traits of the most abundant species as well as stochastic impacts. The more abundant is dominant species, the fewer amount of resources goes to concomitant species and, therefore, the lower is community species richness. 2. The part of community resources used by the dominant species may be not a special case but can be a reflection of general pattern of resources distribution among species under specific environmental conditions. Correspondingly, in communities with higher dominance level there might be more "strict" distribution of resources among concomitant species, which, in turn, might influence community species richness. 3. The relationship between dominance level and community species richness may be caused by their dependence on the third variable, namely regional species pool. In the present paper we tackle the problem using arboreal and insectivorous bird communities of the West Caucasus as a case study. The data were collected in different altitudinal belts on both macroslopes of the western part of the Main Caucasian Ridge. The number of tree species and individual trees was counted within homogenous patches of arboreal phytocenoses 300 m2 in area. Species richness and numbers of insectivorous birds were estimated in course of route surveys with a route length being about 5 km. An analysis of empirical data was carried out using univariate and multiple correlation-regression techniques. The results indicate that the relationship between dominance and local species richness is determined to a large extent (by 50-60%) by a dominant taking over greater or lesser amount of the resources (mechanism 1). The role of two other mechanisms (2 and 3) is not so prominent--together, they are responsible for 25-40% of the relationship power. Relative contribution of different mechanisms to the relationship under consideration depends on conformity of species abundance rank structure with the geometric series model. At those sites where this conformity is manifested, the relationship between dominance level and species richness is due mainly to mechanisms 1 and 2, i.e., is determined by local processes. At other sites, where the conformity of species abundance rank structure with the geometric series model is not so good, a certain role belongs to the size of regional species pool (mechanism 3).     

取样尺度对亚高寒草甸物种多样性与生产力关系的影响

植物物种多样性与生产力之间的关系是群落生态学的一个热点问题, 目前仍存在着很多争议。为探究自然群落中二者之间的关系, 对青藏高原亚高寒草甸3个样地的自然植物群落分别进行了不同取样面积的抽样调查。结果显示, 取样样地和取样尺度均对物种丰富度有显著影响, 取样样地而非取样尺度对群落地上生物量有显著性影响。在某一时刻对某一样地进行取样, 其单位面积生产力并不因取样面积的增加而提高, 而是保持恒定的, 尽管物种数随取样面积的增加而有明显增多。物种多样性与生产力之间的回归关系因样地与取样尺度不同而不同, 有U型、单峰型、正线性相关和无相关性, 其中无相关性出现的最多。据此推测, 亚高寒草甸群落物种多样性与生产力之间不存在某种确定性关系, 或者说, 亚高寒草甸物种多样性和生产力之间不存在必然的因果联系。     

Diversity and species distribution of ectomycorrhizal fungi along productivity gradients of a southern boreal forest

Coniferous forests with diverse ectomycorrhizal fungus (EMF) communities are associated with nutrient-poor, acidic soils but there is some debate whether EMF can be equally adapted to more productive, nitrogen-rich sites. We compared EMF species distribution and diversity along a replicated productivity gradient in a southern boreal forest of British Columbia (Canada). Roots from subalpine fir (Abies lasiocarpa) saplings of the understory were sampled and EMF species were identified by morphotypes supplemented with ITS rDNA analysis. There were significant changes in the distribution and abundance of 74 EMF species along the productivity gradient, with as little as 24% community similarity among contrasting sites. Species richness per plot increased asymptotically with foliar nitrogen concentrations of subalpine fir, demonstrating that many EMF species were well suited to soils with high rates of nitrogen mineralization. EMF species abundance in relation to site productivity included parabolic, negative linear, and positive exponential curves. Both multi-site and more narrowly distributed EMF were documented, and a diverse mix of mantle exploration types was present across the entire productivity gradient. The results demonstrate strong associations of EMF fungal species with edaphic characteristics, especially nitrogen availability, and a specialization in EMF communities that may contribute to the successful exploitation of such contrasting extremes in soil fertility by a single tree host.     

Zooplankton species richness–productivity relationship: Confronting monotonic positive and hump‐shaped models from a local perspective

This study examined two models that are most frequently used to describe the relationship between species richness and productivity (SPR): monotonic positive and hump‐shaped models. We assessed zooplankton community diversity in response to algal productivity. The relationship between net primary productivity (NPP) and rarefied species richness was examined by fitting the data to two models and comparing them using the Akaike information criterion (AICc). Macrophyte banks with the highest net primary productivity had the highest zooplankton abundance. Our results pointed to a hump‐shaped model as the best fit to describe the relationship between zooplankton species richness and primary productivity (ΔAICc > 4). Thus, the diversity was lower at the extremes of productivity and higher at intermediate levels of productivity. We suggest that this relationship might occur because when the resource supply rates are low, environmental conditions are stressful, whereas a high availability of resources enhances competitive exclusion. Two observations supported this statement: (i) the total abundance of the community positively correlated with NPP (P < 0.05), indicating that less productive sites had few consumers and the raised productivity tended to favour the total abundance; (ii) NPP was negatively correlated with evenness (P < 0.05), indicating that productivity increased the dominance of certain species in the communities. Therefore, we challenged two of the models most frequently used to explain SPR, and discuss some mechanisms underlying a hump‐shaped SPR.     

Impacts of species interactions on grass community productivity under contrasting management regimes

Productivity–diversity relationships are routinely described mainly in terms of species richness. However, these relationships can be affected by the functional strategy and physiological plasticity characterizing each species as they respond to environment and management changes. This study, therefore, aimed to analyze species interactions in grass communities presenting the same number of species (n = 6) but different growth strategies, and the impact on community productivity across several forms of field management (two different fertilizer application rates, i.e. 120 and 360 kg N ha⁻¹ year⁻¹, and two cutting frequencies, i.e. 3 and 6 cuts per year). For this purpose, we applied the tripartite partitioning method introduced for the analysis of biodiversity effects (BE). Grass species were cultivated on small plots (4.2 m²) in both mixtures and monocultures. Different management regimes altered both net BE and its component effects: dominance and potential for complementarity. A higher cutting frequency significantly reduced net BE, via a reduction in dominance effect. We found that increased N supply could either increase or decrease complementary effect according to grass mixture composition, i.e. species strategy. Regardless of management intensity, net BE was in general significantly positive especially when including individual species-specific plasticity effects. We conclude that a combination of different grasses has a positive effect on community biomass. Furthermore, both the functional strategy and the functional plasticity of component species play an important role in the intensity of BE. Therefore, biological mechanisms leading to enhanced biomass in six-grass communities are as effective for productivity as management conditions.     

井冈山自然保护区蛾类多样性及人为干扰的影响

对井冈山自然保护区蛾类群落的多样性及其受人为干扰的影响进行了研究。结果显示：（1）人为干扰导致井冈山自然保护区蛾类群落构成有较大差别,严重干扰导致蛾类科数明显降低;物种数和个体数呈现出随着人为干扰加强而降低的趋势;蛾类各优势科在不同人为干扰样点的相对多度存在差异。（2）人为干扰导致蛾类主要科的物种数和个体数变化不同：尺蛾科、夜蛾科、毒蛾科和舟蛾科的物种数和个体数在人为干扰下降低;轻度和中度干扰导致螟蛾科和天蛾科的物种数和个体数升高,重度干扰下物种数和个体数降低;人为干扰导致灯蛾科的物种数和苔蛾科的个体数升高。（3）人为干扰导致蛾类群落Shannon多样性指数、Pielou均匀度指数降低,Berger-Parker优势度指数升高。（4）人为干扰对蛾类群落的物种数和个体数的时间动态影响较大,对Shannon指数和Pielou均匀度指数时间动态影响较小,导致Berger-Parker优势度指数时间波动幅度增大。（5）人为干扰导致样点间蛾类群落相似性较低。     

Experimental Manipulation of Grassland Plant Diversity Induces Complex Shifts in Aboveground Arthropod Diversity

Changes in producer diversity cause multiple changes in consumer communities through various mechanisms. However, past analyses investigating the relationship between plant diversity and arthropod consumers focused only on few aspects of arthropod diversity, e.g. species richness and abundance. Yet, shifts in understudied facets of arthropod diversity like relative abundances or species dominance may have strong effects on arthropod-mediated ecosystem functions. Here we analyze the relationship between plant species richness and arthropod diversity using four complementary diversity indices, namely: abundance, species richness, evenness (equitability of the abundance distribution) and dominance (relative abundance of the dominant species). Along an experimental gradient of plant species richness (1, 2, 4, 8, 16 and 60 plant species), we sampled herbivorous and carnivorous arthropods using pitfall traps and suction sampling during a whole vegetation period. We tested whether plant species richness affects consumer diversity directly (i), or indirectly through increased productivity (ii). Further, we tested the impact of plant community composition on arthropod diversity by testing for the effects of plant functional groups (iii). Abundance and species richness of both herbivores and carnivores increased with increasing plant species richness, but the underlying mechanisms differed between the two trophic groups. While higher species richness in herbivores was caused by an increase in resource diversity, carnivore richness was driven by plant productivity. Evenness of herbivore communities did not change along the gradient in plant species richness, whereas evenness of carnivores declined. The abundance of dominant herbivore species showed no response to changes in plant species richness, but the dominant carnivores were more abundant in species-rich plant communities. The functional composition of plant communities had small impacts on herbivore communities, whereas carnivore communities were affected by forbs of small stature, grasses and legumes. Contrasting patterns in the abundance of dominant species imply different levels of resource specialization for dominant herbivores (narrow food spectrum) and carnivores (broad food spectrum). That in turn could heavily affect ecosystem functions mediated by herbivorous and carnivorous arthropods, such as herbivory or biological pest control.     

Diversity–productivity relationships in two ecologically realistic rarity–extinction scenarios

To develop a better understanding of how biodiversity loss and productivity are related, we need to consider ecologically realistic rarity (i.e. reduced evenness and increased dominance) and extinction (i.e. reduced richness) scenarios. Furthermore, we need to identify and better understand the factors that influence species and community yielding behaviors because the general conditions for overyielding are the same as those for coexistence. We established experimental tallgrass prairie plots in Iowa to determine how two ecologically realistic rarity–extinction scenarios influenced aboveground net primary productivity (ANPP) and disassembly. Equal‐mass seedlings of six tallgrass prairie species were transplanted into field plots to establish realistic declining species evenness (high, medium, low) and richness (4, 1) treatments. Across declining evenness treatments, the relative abundance of the ubiquitous tall species Andropogon gerardii increased, the relative abundance of the tall species Salvia azurea was constant, and the relative abundance of two short (dissimilar height scenario) or two tall species (tall scenario) decreased. Monocultures of Andropogon represented a continuation of this trend until there was complete dominance by Andropogon and extinction of all other species. Our treatments also allowed us to test if variation in plant height contributes to the complementarity effect. Niche partitioning in plant height was not positively related to complementarity. The effects of declining species evenness and richness on the diversity–productivity relationship were different for these two ecologically realistic rarity–extinction scenarios. Specifically, as diversity declined across treatments, ANPP and the selection effects decreased in tall communities, but not in dissimilar communities. Additionally, differences between these two scenarios revealed that decreased species yielding behavior is associated with two tallgrass prairie extinction risk factors, rarity and short height. The differences between these scenarios demonstrate the importance of incorporating the known patterns of diversity declines into future studies.     

Dominance determines fish community biomass in a temperate seagrass ecosystem

Biodiversity and ecosystem function are often correlated, but there are multiple hypotheses about the mechanisms underlying this relationship. Ecosystem functions such as primary or secondary production may be maximized by species richness, evenness in species abundances, or the presence or dominance of species with certain traits. Here, we combine surveys of natural fish communities (conducted in July and August 2016) with morphological trait data to examine relationships between biodiversity and ecosystem function (quantified as fish community biomass) across 14 subtidal eelgrass meadows in the Northeast Pacific (54°N, 130°W). We employ both taxonomic and functional trait measures of diversity to investigate whether ecosystem function is best predicted by species diversity (complementarity hypothesis) or by the presence or dominance of species with particular trait values (selection or dominance hypotheses). After controlling for environmental variation, we find that fish community biomass is maximized when taxonomic richness and functional evenness are low, and in communities dominated by species with particular trait values, specifically those associated with benthic habitats and prey capture. While previous work on fish communities has found that species richness is often positively correlated with ecosystem function, our results instead highlight the capacity for regionally prevalent and locally dominant species to drive ecosystem function in moderately diverse communities. We discuss these alternate links between community composition and ecosystem function and consider their divergent implications for ecosystem valuation and conservation prioritization.     

Shrub invasion into subalpine vegetation: implications for restoration of the native ecosystem

The ability of plant communities to recover after non-native species invasion will depend upon the nature of their soil seed bank and seed rain characteristics. This study assessed changes in the soil seed bank and seed rain associated with the invasion of the non-native shrub Cytisus scoparius in subalpine vegetation. Soil seed bank and seed rain composition, density and richness were investigated at three areas of different stages of invasion: (i) recent (8–10 years), (ii) mature (15–16 years) and (iii) long-term (25 years). There were few changes in seed bank composition or richness regardless of invasion stage. By contrast, the seed rain composition, richness and density was substantially different within long-invaded areas. Very few seeds were able to colonise the dense barrier characteristic of larger, more mature C. scoparius stands. Some prominent herbs from the native vegetation were under-represented or absent from the seed bank, both in invaded and uninvaded areas. Laboratory germination experiments demonstrated that most native species germinate easily, which may imply a transient seed bank, rather than a persistent one. The majority of herbaceous and shrub species were capable of resprouting vegetatively. Therefore, regeneration appeared more reliant on the bud and tuber bank than a persistent soil seed bank. The dominance of graminoid species and C. scoparius rather than other herbaceous, shrub or tree species suggests that the regenerating vegetation will be dominated by grass species and/or C. scoparius. Hence, in areas where long-invaded C.␣scoparius stands are present the recovery of native subalpine vegetation maybe difficult. Recovery may only be possible through wind dispersal from the surrounding intact vegetation or through actively reseeding the area. This study highlights the importance of early intervention in invasive species management.     

Succession May Maintain High Leaf Area: Sapwood Ratios and Productivity in OldSubalpine Forests

Old forests are generally believed to exhibit low net primary productivity (NPP) and therefore to be insignificant carbon sinks. This relationship between age and NPP is based, in part, on the hypothesis that the biomass of respiratory tissues such as sapwood increases with age to a point where all photosynthate is required just to maintain existing tissue. However, this theoretical connection between respiration:assimilation ratios and forest productivity is based on age-dependent trends in the sapwood:leaf ratios of individual trees and even-aged stands; it does not take into account such processes in natural forests as disproportional increases in shade-tolerant species over time and multiple-age cohorts. Ignoring succession and structural complexity may lead to large underestimates of the productivity of old forests and inaccurate estimates of the ages at which forest productivity declines. To address this problem, we compared biomass allocation and productivity between whitebark pine, a shade-intolerant, early-successional tree species, and subalpine fir, a shade-tolerant, late-successional species, by harvesting 14 whitebark pines and nine subalpine firs that varied widely in dbh and calculating regression models for dbh vs annual productivity and biomass allocation to leaves, sapwood, and heartwood. Late-successional subalpine fir allocated almost twice as much biomass to leaves as early-successional whitebark pine. Subalpine firs also had a much lower allocation to sapwood and higher growth rates across all tree sizes. We then modeled biomass allocation and productivity for 12 natural stands in western Montana that were dominated by subalpine fir and whitebark pine varying in age from 67 to 458 years by applying the regressions to all trees in each stand. Whole-stand sapwood:leaf ratios and stand productivity increased asymptotically with age. Sapwood:leaf ratios and productivity of whitebark pine in these stands increased for approximately 200–300 years and then decreased slowly over the next 200 years. In contrast, sapwood:leaf ratios of all sizes of subalpine fir were lower than those of pine and productivity was higher. As stands shifted in dominance from pine to fir with age, subalpine fir appeared to maintain gradually increasing rates of whole-forest productivity until stands were approximately 400 years old. These results suggest that forests such as these may continue to sequester carbon for centuries. If shade-tolerant species that predominate late in succession maintain high assimilation-to-respiration ratios in other forests, we may be underestimating production in old forests, and current models may underestimate the importance of mature forests as carbon sinks for atmospheric CO₂ in the global carbon cycle. Received 16 February 1999; accepted 24 November 1999.     

The effects of species pool,dispersal and competition on the diversity–productivity relationship

Aim The diversity–productivity relationship is a controversial issue in ecology. Diversity is sometimes seen to increase with productivity but a unimodal relationship has often been reported. Competitive exclusion was cited initially to account for the decrease of diversity at high productivity. Subsequently, the roles of evolutionary history (species pool size) and dispersal rate have been acknowledged. We explore how the effects of species pool, dispersal and competition combine to produce different diversity–productivity relationships. Methods We use a series of simulations with a spatially explicit, individual‐based model. Following empirical expectations, we used four scenarios to characterize species pool size along the productivity gradient (uniformly low and high, linear increase and unimodal). Similarly, the dispersal rate varied along the productivity gradient (uniformly low and high, and unimodal). We considered both neutral communities and communities with competitive exclusion. Results and main conclusions Our model predicts that competitive interactions will result in unimodal diversity–productivity relationships. The model often predicts unimodal patterns in neutral communities as well, although the decline in richness at high productivity is less than in competing communities. A positive diversity–productivity relationship is simulated for neutral communities when the species pool size increases with productivity and the dispersal rate is high. This scenario is probably more widespread in nature than the others since positive diversity–productivity relationships have been observed more frequently than previously expected, especially in the tropics and for woody species. Our simulated effects of species pool, dispersal and competition on diversity patterns can be linked to empirical observations to uncover mechanisms behind the diversity–productivity relationship.     

广西马尾松林植物功能多样性与生产力的关系

探索植物多样性与生产力的关系可为森林经营与管理提供科学基础。本研究以广西4个地区的马尾松(Pinus massoniana)人工林群落为研究对象, 通过计算物种多样性、功能多样性和功能优势值, 运用相关分析、自动线性建模和方差划分等方法, 分析了多样性与生产力的关系。研究发现, 生产力与物种丰富度、Shannon指数、功能丰富度、功能均匀度极显著正相关(P < 0.01), 与物种均匀度、功能多样性、功能离散度、功能团个数、坡向显著正相关(P < 0.05), 与林龄极显著负相关(P < 0.01), 4个功能多样性参数和4个物种多样性参数两两之间皆为显著正相关; 未发现初始生物量制约生产力的提高; 方差划分最优模型中, 功能多样性参数效应、功能优势值效应和林龄效应分别解释生产力方差的56%、43%和33%, 功能多样性参数效应和功能优势值效应重叠部分高达27%; 生态位互补效应主要由功能丰富度和功能均匀度产生, 选择效应主要由生长型优势值产生; 生长型优势值为灌木的样地生产力较高, 次优种或过渡种对生态系统功能也有重要作用。以生产力为响应变量的自动线性建模最佳子集包括重要性由大到小的5个因素: 林龄、生长型优势值、功能丰富度、功能均匀度、功能团个数。建议维护森林功能多样性, 加强林下叶层植物保护, 用好功能重要的物种, 通过林下叶层的补偿性光合作用和生长竞争, 有效地提高生产力和生物多样性。     

Is the productivity of vegetation plots higher or lower when there are more species? Variable predictions from interaction of the ‘sampling effect’ and ‘competitive dominance effect’ on the habitat templet

Using a habitat templet model, we predict that the productivity (total biomass) of plots within a plant community may be positively, negatively or not at all related to variation in the number of species per plot, depending on successional stage (time since major disturbance) and habitat carrying capacity (reflecting the total resource supplying power of the habitat). For plots of a given size, a positive relationship between productivity and species richness is predicted in recently disturbed habitats because local neighbourhoods here will have been assembled largely stochastically, usually from a pool of available species with a right‐skewed size frequency distribution. Hence, in the earliest stages of succession, plots will have relatively high total biomass only if they contain at least some of the relatively uncommon larger species which will, in turn, be more likely in those neighbourhoods that contain more species (the sampling effect). Among these will also be some of the more common smaller species; hence, these high biomass, species‐rich plots should have relatively low species evenness, in contrast to what is predicted under effects involving species complementarity. In late succession, the plots with high total biomass will still be those that contain relatively large species but these plots will now contain relatively few species owing to increased competitive exclusion over time (the competitive dominance effect). In intermediate stages of succession, no relationship between plot productivity and species richness is predicted because the opposing sampling and competitive dominance effects cancel each other out. We predict that the intensity of both the sampling and competitive dominance effects on the productivity/species richness relationship will decrease with decreasing habitat carrying capacity (e.g. decreasing substrate fertility) owing to the inherently lower variance in between‐plot productivity that is predicted for more resource‐impoverished habitats.     

DOES INTERFERENCE CAUSE NICHE DIFFERENTIATION? EVIDENCE FROM SUBALPINE PLANT COMMUNITIES

The effects of interference on community structure of subalpine meadows were investigated. Adults greatly reduced seedling survival in the greenhouse, and natural seedling survival was low. In fell fields, nurse plant effects were common, while survival was confined to gaps in productive meadows. A greenhouse experiment demonstrated that community dominants were strong competitors able to suppress subordinates. Simulated grazing of Festuca, a dominant, reduced its yield relative to most subordinates. Interference intensity appears to be a function of productivity and proximity. The proportion of negative species associations increased as productivity increased. Morphological similarity between species is least in the most productive community, but greatest where productivity is only moderate. Interference may permit only relatively dissimilar species to coexist unless it is ameliorated by factors such as grazing or heterogeneity. Spacing patterns suggest that minimal contact between dominants and other species is a characteristic of communities with intense interference. The evidence points to this hypothesis: interference acts contemporaneously to limit niche width in productive communities, but evolutionary changes are unlikely where complete dominance is possible and specialization is not a viable option. In contrast, species in unproductive communities, where abiotic stress is likely to have been an evolutionary force, appear genetically more niche differentiated. Where contemporary interference is moderate, evolutionary effects are possible because species may use resources not preempted by the dominants.     

Variation in structure and diversity in Mediterranean grasslands related to trophic status and grazing intensity

Abstract. The spatial organization of Mediterranean grassland in Spain is described, based on samples from 71 sites covering the existing variation in slope exposition and inclination.The whole set can be regarded as representing a trophic gradient, along which gradual variation in soil, species composition, biomass, and coverage were quantified. Corresponding to other studies from varying habitats, maximal species richness, diversity, and heterogeneity were observed on moderately infertile sites. Maximum species richness, over 60 species, occurred on sites with biomass values from 150–350 g / m². Species richness values are much higher and biomass values are much lower than those found in temperate grasslands. The decrease in diversity towards the mosteutrophic communities is stronger than expected, but can be easily explained by the high grazing pressure here. The variation in diversity observed runs parallel with that in heterogeneity. Zones with a high species richness also have a high heterogeneity, meaning a low amount of dominance. Grazing is understood as abiotic form of disturbance. Differences in grazing pressure may modify the relation between richness and fertility. While the greatest grazing pressure coincides with the most eutrophic communities, decreasing progressively towards the oligotrophic ones, the trend predicted by the resource availability theory is maintained; species diversity will be maximal at intermediate levels of disturbance. Absence of grazing in the eutrophic communities would lead to an investment in the soil of the unconsumed organic matter.     

A functional trait-based approach to understand community assembly and diversity–productivity relationships over 7 years in experimental grasslands

Several multi-year biodiversity experiments have shown positive species richness–productivity relationships which strengthen over time, but the mechanisms which control productivity are not well understood. We used experimental grasslands (Jena Experiment) with mixtures containing different numbers of species (4, 8, 16 and 60) and plant functional groups (1–4; grasses, legumes, small herbs, tall herbs) to explore patterns of variation in functional trait composition as well as climatic variables as predictors for community biomass production across several years (from 2003 to 2009). Over this time span, high community mean trait values shifted from the dominance of trait values associated with fast growth to trait values suggesting a conservation of growth-related resources and successful reproduction. Increasing between-community convergence in means of several productivity-related traits indicated that environmental filtering and exclusion of competitively weaker species played a role during community assembly. A general trend for increasing functional trait diversity within and convergence among communities suggested niche differentiation through limiting similarity in the longer term and that similar mechanisms operated in communities sown with different diversity. Community biomass production was primarily explained by a few key mean traits (tall growth, large seed mass and leaf nitrogen concentration) and to a smaller extent by functional diversity in nitrogen acquisition strategies, functional richness in multiple traits and functional evenness in light-acquisition traits. Increasing species richness, presence of an exceptionally productive legume species (Onobrychis viciifolia) and climatic variables explained an additional proportion of variation in community biomass. In general, community biomass production decreased through time, but communities with higher functional richness in multiple traits had high productivities over several years. Our results suggest that assembly processes within communities with an artificially maintained species composition maximize functional diversity through niche differentiation and exclusion of weaker competitors, thereby maintaining their potential for high productivity.