Species Richness-productivity Relationship Research Articles

Water depth modulates the species richness–biomass relationship in submerged macrophytes

The effect of biodiversity on ecosystem productivity has been a controversial issue in ecological research. The species richness–productivity relationship is highly variable in natural ecosystems, with a positive relationship being one of the most commonly observed relationships. Previous regional studies from terrestrial ecosystems have demonstrated that environmental gradients can regulate the species richness–productivity relationship. However, how this relationship varies in freshwater ecosystems across spatial environment gradients remains unclear. In this study, we propose that the species richness–productivity relationship can be modulated by the water depth. Here, we surveyed the submerged macrophyte community structure by establishing 24 transects and 642 quadrats in Erhai Lake, Yunnan Plateau, China. Our findings highlight that the species richness–productivity relationship gradually changed from slightly positive to strongly positive as the environment became more light-limited with the increasing water depth, supporting the stress-gradient hypothesis. The results from this study provide new insights into the biodiversity–ecosystem functioning relationships and in managing lake macrophyte communities and productivity.

Species Richness Promotes Productivity through Tree Crown Spatial Complementarity in a Species-Rich Natural Forest

Ecological theory suggests that species with complementary architectural and physiological traits can optimize crown packing to improve resource efficiency and promote ecosystem productivity in forest communities. However, empirical evidence of this prediction is rare in species-rich natural forests, as little is known about how crown spatial complementarity regulates community species richness-productivity relationship (SRPR). In this study, we measured tree architectural traits (stem diameter, height, crown depth and width) for 11,337 trees, and quantified species richness, functional diversity, crown spatial complementarity, soil fertility and forest productivity for 44 quadrats (20 m × 20 m per quadrat) in the Badagongshan 25 ha forest plot, central China. We tested bivariate correlations between species richness, crown complementarity, functional diversity and forest productivity. We employed linear mixed effects models to predict crown complementarity and examined its relationship with functional diversity. Finally, we applied structural equation modeling to quantify the mediation effects of crown complementarity on SRPRs. Species richness promoted crown complementarity and forest productivity. Crown complementarity varied across quadrats, with increases driven primarily by changes in tree height. Crown complementarity was positively related to functional diversity and forest productivity. Species richness increased with soil total phosphorus, while functional diversity decreased with soil bulk density. Forest productivity increased with soil organic carbon and total nitrogen, but decreased with bulk density. Crown complementarity partially mediated the positive effect of species richness on forest productivity, and the mediation effect was mainly through functional diversity. Our results suggest that the crown complementarity index accurately reflects the niche complementarity through light utilization and carbon reallocation. Our study emphasizes that species richness can promote crown complementarity, leading to greater forest productivity, which provides greater insight into the mechanical understanding of the SRPRs.

The species richness–productivity relationship varies among regions and productivity estimates, but not with spatial resolution

The relationship between species richness and productivity (SRPR) has been a long‐studied and hotly debated topic in ecology. Different studies have reported different results with variable shapes (i.e. unimodal, linear) and directions (i.e. positive, negative) of SRPRs depending on spatial grain (i.e. size of sampling unit for species richness), productivity estimates and study extent. In this study, we quantified the effect of multiple estimates of productivity (aboveground, belowground and total biomass, and various measures of soil fertility) on species richness across three spatial grains (0.04, 1 and 25 m2) across temperate grasslands from two regions in central Europe. We analyzed SRPR in each of the two regional datasets separately, as well as the two datasets pooled together. Our results have revealed that differences caused by spatial grain were unexpectedly small, and the direction of the SRPR was consistent within each productivity estimate, but differed between regions. Productivity estimates (across all spatial scales) had different, sometimes contrasting effects on SRPR (together with predictive power) within a region, and this pattern was more pronounced when compared between regions. The combination of different datasets led to very different results than when these were analyzed separately. We did not find any evidence for a unimodal response. This study points to the necessity of careful assessing when combining datasets from different regions, even if the plant communities belong to the same vegetation type. The dataset combination may blur the role of different drivers, which likely determine the shape and strength of SRPR. We suggest that data and study comparability may be enhanced by consistently using the same productivity estimates, which would allow for more robust interpretation of possible ecological drivers underlying the SRPR.

Correction: Process strengths determine the forms of the relationship between plant species richness and primary productivity.

[This corrects the article DOI: 10.1371/journal.pone.0185884.].

The type of nutrient limitation affects the plant species richness–productivity relationship: Evidence from dry grasslands across Eurasia

Abstract The species richness–productivity relationship is one of the most debated patterns in ecology. Species coexistence theory suggests that it could be tightly linked to the type of nutrient limitation (no limitation, single‐nutrient limitation, colimitation by several nutrients). Yet, the effects of nutrient limitation on the species richness–productivity relationship have been rarely studied at the regional and continental scales. Combining the predictions of the humped‐back model and the niche dimension hypothesis, we hypothesized that an increase in plant species richness with the number of different limiting nutrients is detectable only at higher productivity levels, at which competition for nutrients is more intense. Therefore, we expected the shape of the diversity–productivity relationship to differ between sites colimited by nitrogen (N) and phosphorus (P), sites limited by a single nutrient (either N or P), and sites not limited by any of these nutrients. To test this hypothesis, we used species richness data collected in 10 m × 10 m plots at 694 temperate dry grassland sites across eight regions in northern Eurasia. Productivity ranged from 10 to ~500 g/m2 of above‐ground standing biomass. The type of nutrient limitation was identified by critical nutrient ratios alone and their combination with critical nutrient concentrations measured in the plant tissue. Relationships were analysed using generalized linear and mixed‐effect models. In line with our expectations, species richness of Eurasian temperate dry grasslands increased more steeply and peaked higher under higher productivity levels at N&P‐colimited sites. When nutrient limitation was assessed by both ratios and concentrations, species richness at N&P‐colimited sites continued to increase monotonically until the maximum productivity sampled in this study. In contrast, at sites with a single‐nutrient limitation or no limitation, the peak in species richness was lower and occurred at a lower productivity of about 300–400 g/m2. Synthesis. We provide the first evidence that the species richness–productivity relationship may depend on the type of nutrient limitation as predicted by the species coexistence theory. To generalize these findings, the role of nutrient limitation needs to be tested in other ecosystems, including more productive plant communities.

Process strengths determine the forms of the relationship between plant species richness and primary productivity.

The current rates of biodiversity loss have exceeded the rates observed during the earth’s major extinction events, which spurs the studies of the ecological relationships between biodiversity and ecosystem functions, stability, and services to determine the consequences of biodiversity loss. Plant species richness-productivity relationship (SRPR) is crucial to the understanding of these relationships in plants. Most ecologists have reached a widespread consensus that the loss of plant diversity undoubtedly impairs ecosystem functions, and have proposed many processes to explain the SRPR. However, none of the available studies has satisfactorily described the forms and mechanisms clarifying the SRPR. Observed results of the SRPR forms are inconsistent, and studies have long debated the ecological processes explaining the SRPR. Here, I have developed a simple model that combines the positive and/or negative effects of sixteen ecological processes on the SRPR and models that describe the dynamics of complementary-selection effect, density effect, and the interspecific competitive stress influenced by other ecological processes. I can regulate the strengths of the effects of these ecological processes to derive the asymptotic, positive, humped, negative, and irregular forms of the SRPR, and verify these forms using the observed data. The results demonstrated that the different strengths of the ecological processes determine the forms of the SRPR. The forms of the SRPR can change with variations in the strengths of the ecological processes. The dynamic characteristics of the complementary-selection effect, density effect, and the interspecific competitive stress on the SRPR are diverse, and are dependent on the strengths and variation of the ecological processes. This report explains the diverse forms of the SRPR, clarifies the integrative effects of the different ecological processes on the SRPR, and deepens our understanding of the interactions that occur among these ecological processes.

Is there any support for the humped-back model in some steppe and semi steppe regions of Iran?

One of the famous theories in ecology is the humped-back model (HBM) which explains that habitats with intermediate productivity and disturbance support the most species. Most studies on HBM patterns have been conducted for herbaceous species. Knowledge about this theory in arid and semi-arid communities is still weak, especially with severe environments and disturbances containing woody plants. In this study, an analysis is presented on species richness–productivity relationship along a gradient of disturbance and stress in some steppe and semi steppe rangelands in Iran. A nonlinear curve-fitting approach was used that fitted a three-parameter Gaussian curve between species richness and productivity across 180 plots (1 m2). Mitchell-Olds and Shaw 's test (MOS) was used to assess the significance of the humped-back curve. Data were collected from six rangelands, including all life forms of species (grasses, forbs and small shrubs) with litter. Fitted Gaussian curve showed a positive trend between productivity and species richness across a decreasing gradient of disturbance in which species richness was increased. Maximum values for species richness correspond to values of plant productivity in undisturbed sites with a moderate stress. The MOS test showed a significant positive trend in the curve which means that the observed shape in the species richness–productivity relationship covers the mechanisms located in low to medium productivity in the left-hand side of the humped-back model. In other words, this research did not detect a full unimodal curve. This is because of less productivity in the arid and semi-arid ecosystems with abiotic stress. However, there was a partial support for the humped-back model in this study. Hence, more studies to test the existence of a humped-back model in such areas, would be required to reveal all aspects of this relationship.

Seasonal Variation in the NDVI–Species Richness Relationship in a Prairie Grassland Experiment (Cedar Creek)

Species richness generally promotes ecosystem productivity, although the shape of the relationship varies and remains the subject of debate. One reason for this uncertainty lies in the multitude of methodological approaches to sampling biodiversity and productivity, some of which can be subjective. Remote sensing offers new, objective ways of assessing productivity and biodiversity. In this study, we tested the species richness–productivity relationship using a common remote sensing index, the Normalized Difference Vegetation Index (NDVI), as a measure of productivity in experimental prairie grassland plots (Cedar Creek). Our study spanned a growing season (May to October, 2014) to evaluate dynamic changes in the NDVI–species richness relationship through time and in relation to environmental variables and phenology. We show that NDVI, which is strongly associated with vegetation percent cover and biomass, is related to biodiversity for this prairie site, but it is also strongly influenced by other factors, including canopy growth stage, short-term water stress and shifting flowering patterns. Remarkably, the NDVI-biodiversity correlation peaked at mid-season, a period of warm, dry conditions and anthesis, when NDVI reached a local minimum. These findings confirm a positive, but dynamic, productivity–diversity relationship and highlight the benefit of optical remote sensing as an objective and non-invasive tool for assessing diversity–productivity relationships.

Density and assemblage influence the nature of the species richness–productivity relationship in Australian dry sclerophyll forest species

AbstractThe relationship between species richness and productivity is important from both a basic, theoretical perspective and also because it has important ramifications for applied ecology including ecosystem restoration and the design of carbon offset plantings. While a more species‐rich community is often believed to be more productive than a species‐poor community, findings from observational and experimental studies differ and our understanding of the relationship comes largely from grasslands. Consequently, we aimed to determine for the first time the nature of the species richness–productivity relationship in a southern‐hemisphere dry sclerophyll ecosystem. We investigated the impact of species richness on productivity, plant density and mean plant biomass at three sowing densities in three species assemblages. Eucalyptus globulus, Acacia mearnsii and Allocasuarina verticillata were each grown as monocultures and included in every subsequent level of species richness, forming three distinct species assemblages. Communities were grown in a glasshouse pot experiment for four months, then harvested and above‐ground biomass measured. We found no general species richness–productivity relationship in the communities studied. There were no overall increases in productivity as species richness increased and in fact in most cases the productivity of communities with 4 and 8 species was lower than monocultures of the dominants. Importantly, density influenced the way richness affected productivity and this effect was dependent upon assemblage, indicating that species identity is a key determinant of productivity. These results demonstrate important ecological principles in a previously untested system. A key outcome of this experiment is that density alters the relationship between species richness and initial productivity in assemblages of Australian dry sclerophyll species.

Species richness-productivity relationships of tropical terrestrial ferns at regional and local scales

Summary The species richness–productivity relationship (SRPR), by which the species richness of habitats or ecosystems is related to the productivity of the ecosystem or the taxon, has been documented both on regional and local scales, but its generality, biological meaning and underlying mechanisms remain debated. We evaluated the SRPR and three mechanistic hypotheses using terrestrial ferns in 18 study plots along an elevational gradient (500–4000 m) in Ecuador. We measured annual increases in above-ground biomass of 6175 fern individuals from 91 species over 2 years, and estimated plot-level tree productivity from increases in above-ground woody biomass of 560 trees. Analyses were conducted by (i) comparing plots along the elevational gradient (regional scale) and (ii) comparing plots within each elevational belt (local scale). Fern diversity was related to the productivity of the fern assemblages, but not to above-ground productivity of the trees. At the regional scale, we found a positive relationship of fern species richness to fern productivity that appeared to be determined by an increase in the number of fern individuals and niche availability. In contrast, at the local scale, this relationship was negative and likely driven by interspecific competition. Synthesis. Plot diversity of ferns appears to be limited by the number of available niches and competition to occupy these niches. At the local scale, this is reflected in a negative SRPR probably driven by competition, whereas with increasing scale, the positive influence of productivity emerges. This represents the first evidence that productivity and competition affect the diversity of tropical herb assemblages at the plot scale.

Impact of management on biodiversity-biomass relations in Estonian flooded meadows

A negative species richness-productivity relationship is often described in grasslands at smaller scales. We aimed to study the effect of management on this relationship. In particular, we addressed the relative importance of biomass cutting, hay removal and nutrient impoverishment on species richness and growth form structure. We conducted fieldwork in flooded meadows in Alam-Pedja Nature Reserve, central Estonia. We sampled vegetation in managed and abandoned stands of two types of alluvial meadows, sedge and tall forb meadow. Aboveground biomass and litter were harvested, weighed and analysed for major plant nutrients by near infrared reflectance spectroscopy. Three groups of general additive models were developed and compared, addressing the impact of (i) productivity, (ii) nutrients and (iii) management regime on species richness. Management—mowing and hay removal—reduced the amount of litter but not aboveground biomass. Management led to a decrease in nitrogen in the biomass and enhanced species richness, particularly in the tall forb meadow. The strongest determinant of species richness was the amount of litter, exhibiting a hump-shaped relationship with richness. The effect of nitrogen supply was significant, but explained less variation. Management increased the proportion of sedges in the sedge meadow and of small herbs in the tall forb meadow. We conclude that litter removal is the most important management means to support biodiversity. On highly productive sites, reducing nutrients via hay removal is of secondary importance within a timeframe of 10 years.

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

AbstractThis 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.

Relationship between species richness and productivity in plants: the role of sampling effect, heterogeneity and species pool

The relationship between environmental productivity and the number of species [species richness–productivity relationship (SRPR)] has been thoroughly studied, but the mechanisms responsible for its form are still largely unknown, possibly because the majority of studies have focused on evaluating the sole effect of a single hypothesis. We tested whether variation in species richness along a productivity gradient is due to variation in (i) the number of individuals, (ii) the number of species in the species pool or (iii) habitat heterogeneity. We measured species richness (S), individual abundance (N) and productivity (P) estimated as standing biomass in different herbaceous communities in the Czech Republic at two spatial scales. Species pool (S pool) was obtained from a database concerning individual habitats, and habitat heterogeneity (H) was measured using the community dissimilarity index. The SRPR was scale-dependent: at the smaller spatial scale of individual plots, there was a significant curvilinearly negative relationship between S and P, whereas at the larger site scale it turned into a non-significant relationship. Species richness was significantly affected by a combined effect of N and S pool at the plot scale and by a combined effect of S pool and H at the site scale. None of these variables was sufficient to explain the SRPR by itself. Synthesis. Our findings indicate that there is no universal form of the species–productivity relationship, and the SRPR is driven by multiple scale-dependent mechanisms. It is important to consider the joint effect of different factors in explaining species richness patterns rather than to focus on the sole effect of productivity.

Estimation of herbaceous biomass from species composition and cover

AbstractQuestionsBiomass is an important ecological property, but its measurement is destructive and time‐consuming and therefore generally missing for historical vegetation plots. Here we propose and test indirect estimation of herbaceous biomass using models based on easily obtainable variables, namely plant height and cover. We compare these models with Ellenberg indicator values for nutrients (EIVs Nutrients), which are sometimes used as an alternative measure of productivity.LocationCzech Republic, western Slovakia.MethodsAbove‐ground biomass (dry weight; g·m−2) was regressed against the following explanatory variables: (1) Cover E1, total percentage cover of the herb layer visually estimated in the field; (2) Biomass estimate‐raw, ‐adjusted and ‐median, calculated from plant covers and heights (according to a local flora); and (3) mean EIVs Nutrients calculated per plot. For the analyses, we used four data sets containing a total of 469 plots from different vegetation types: ‘Wet meadows’, ‘Dry grasslands’, ‘Fen–dry grassland transects’ and ‘Forest herb layer’. To test the applicability of different biomass estimates we chose an example of a species richness–productivity relationship in the ‘Wet meadows’ data set and describe differences in resulting patterns.ResultsBoth cover of herb layer and calculated ‘biomass volumes’ were more accurate in predicting biomass dry weight than EIVs Nutrients. The best results were obtained from the Biomass estimate‐median model that combines median stand height and total cover of the herb layer. Cover E1 showed relatively tight correlations with biomass, particularly in sparse vegetation, but was a rather poor predictor when cover values were high. This was especially noticeable in application of the Cover E1 model in analysis of the species richness–productivity relationship.ConclusionsIn contrast to biomass, cover of the herb layer has a fixed upper limit (100%), which may lead to misinterpretations in dense, structurally diverse vegetation. Most promising is the Biomass estimate‐median method, which can be applied both to already sampled plots by calculating median height from average species heights according to local floras and to newly sampled plots using the median of plant heights measured in the field. Therefore, we propose it as a rapid, non‐destructive alternative to biomass harvest.

The species richness–productivity relationship in the herb layer of European deciduous forests

ABSTRACTAim In contrast to non‐forest vegetation, the species richness–productivity (SR‐P) relationship in forests still remains insufficiently explored. Several studies have focused on the diversity of the tree layer, but the species richness of temperate deciduous forests is mainly determined by their species‐rich herb layer. The factors controlling herb‐layer productivity may differ from those affecting tree layers or open herbaceous vegetation, and thus the SR‐P relationship and its underlying processes may differ. However, the few relevant studies have reported controversial results. Here we explore the SR‐P relationship in the forest herb layer across different areas from oceanic to continental Europe, and put the effect of habitat productivity on species richness into context with other key factors, namely soil pH and light availability.Location North‐western Germany, Czech Republic, Slovakia and southern Urals (Russia).Methods We measured herb‐layer species richness and biomass, soil pH and tree‐layer cover in 156 vegetation plots of 100 m2 in deciduous forests. We analysed the SR‐P relationship and the relative importance of environmental variables using regression models for particular areas and separate forest types.Results We found a consistent monotonic increase in the herb‐layer species richness with productivity across all study areas and all forest types. Soil pH and light availability also affected species richness, but their relative importance differed among areas.Main conclusions We suggest that the monotonically increasing SR‐P relationship in the forest herb layer results from the fact that herb‐layer productivity is limited by canopy shading; competition within the herb layer is therefore not strong enough to exclude many species. This differs fundamentally from open herbaceous vegetation, which is not subject to such productivity limits and consequently exhibits a unimodal SR‐P relationship. We present a conceptual model that might explain the differences in the SR‐P relationship between the forest herb layer and open herbaceous vegetation.

Meta‐analyses and mega‐mistakes: calling time on meta‐analysis of the species richness–productivity relationship

The form of the species richness-productivity relationship (SRPR) is both theoretically important and contentious. In an effort to distill general patterns, ecologists have undertaken meta-analyses, within which each SRPR data set is first classified into one of five alternative forms: positive, humped (unimodal), negative, U-shaped (unimodal), and no relationship. Herein, I first provide a critique of this approach, based on 68 plant data sets/ studies used in three meta-analyses published in Ecology. The meta-analyses are shown to have resulted in highly divergent outcomes, inconsistent and often highly inappropriate classification of data sets, and the introduction and multiplication of errors from one meta-analysis to the next. I therefore call on the ecological community at large to adopt a far more rigorous and critical attitude to the use of meta-analysis. Second, I develop the argument that the literature on the SRPR continues to be bedeviled by a common failing to appreciate the fundamental importance of the scale of analysis, beginning with the confusion evident between concepts of grain, focus, and extent. I postulate that variation in the form of the SRPR at fine scales of analysis owes much to artifacts of the sampling regime adopted. An improved understanding may emerge from combining sampling theory with an understanding of the factors controlling the form of species abundance distributions and species accumulation curves.

Mega mistakes in meta‐analyses: devil in the detail

Science is a work in progress, with each new study attempting to add to, or improve upon, those that have come before. In this way we have moved initially from a characterization of the species richness-productivity relationship (SRPR) as being ubiquitously unimodel ' (Rosenzweig and Abramsky 1993, Huston and deAn gelis 1994) to a transitional view of the relationship as one in which unimodel relationships were seen not to be dominant but to instead depend on the geographic scale of study (Mittelbach et al. 2001). More recently there has been a re-characterization in which the dominant form of the relationship has been found to be positive at both fine and course grains and at all but very local geographic scales (Gillman and Wright 2006). However, Robert Whittaker's (Whittaker 2010) , analysis gives the impression that on this issue we have recently descended into chaos. He first suggests that a set of prescribed criteria should be followed, but then concludes that we should entirely abandon meta analyses in favor of narrative review or more directed primary data collection. We have some sympathy with Whittaker's arguments but we take issue with the analysis he has undertaken and the conclusions he makes. We revisit his analysis and in so doing conclude that he has overstated the problem and that the way forward is not to abandon meta-analyses, but to ensure that greater caution is exercised when undertaking, reviewing and citing them. The habitual problem with any meta-analysis is, we believe, not primarily with the statistical analysis, but with the widespread indiscrim inate use of studies that are fed into the analysis, that are entirely inappropriate to the question being asked. Statistical meta-analysis can resolve some of these issues. However, in some cases the statistical analysis that has been performed, rather than overcoming problems, adds to the deception with a veneer of re spectability. Unfortunately, poorly derived meta-anal yses continue to be cited without question.

In the dragon's den: a response to the meta‐analysis forum contributions

Secondary analysis of previously published data has a long tradition in ecological science and is widely and successfully practiced as a means of efficiently address ing new questions and hypotheses. Meta-analysis is, in essence, the class of such analyses in which the findings of multiple primary studies are subject to further statistical analysis of emergent outcomes, and is a more recent practice within ecology. I recognize that this is a loose definition of (Ellison 2010, Gur evitch and Mengersen 2010) but continue to refer to the studies I critique using this common broader usage. Owing to the apparent power of such synthetic analyses, papers can be highly influential (Mittel bach 2010). This forum, together with other recent critical assessments (e.g., Englund et al. 1999, Gates 2002), demonstrates that there are good reasons to call for great care, improved rigor and transparency in the use of meta-analysis tools in ecology. However, in the article that initiated this forum exchange (Whittaker 2010), all the specific criticisms I made were restricted to recent meta-analyses of just one problem, which concerns the form of the species richness-productivity relationship (SRPR) in plants. In this brief response to the seven other contributions, I retain this focus while aiming to resolve several misconstructions of points made in my paper, and to comment on a few key points of disagreement regarding analyses of the SRPR. Use of proxies.?First, there have been relatively few studies that have specifically set out to gather data to determine the form of the SRPR and so, in order to increase the power of analysis and refine the questions asked, those undertaking meta-analyses have sought other data sets that were initially gathered for different purposes. There are many published papers providing diversity data, but few that provide direct measurements of productivity, which is a difficult property to estimate accurately. Hence the reliance in Mittelbach et al. (2001), P?rtel et al. (2007), and Laanisto et al. (2008)

Why does the unimodal species richness–productivity relationship not apply to woody species: a lack of clonality or a legacy of tropical evolutionary history?

ABSTRACTAim To study how differences in species richness patterns of woody and herbaceous plants may be influenced by ecological and evolutionary factors. Unimodal species richness–productivity relationships (SRPRs) have been of interest to ecologists since they were first described three decades ago for British herbaceous vegetation by J. P. Grime. The decrease in richness at high productivity may be due to competitive exclusion of subordinate species, or diverse factors related to evolution and dispersal. Unimodal SRPRs are most often reported for plants, but there are exceptions. For example, unimodal SRPRs are common in the temperate zone but not in the tropics. Similarly, woody species and forest communities in the Northern Hemisphere do not tend to show unimodal SRPRs.Location Global.Methods We used data from the literature to test whether a unimodal SRPR applies to woody species and forest communities on a global scale. We explored whether the shape of SRPRs may be related to the lack of clonality in woody species (which may prevent their being competitively superior), or the legacy of evolutionary history (most temperate woody species originate from tropical lineages, and due to niche conservatism they may still demonstrate ‘tropical patterns’). We used case studies that reported the names of the dominant or most abundant species for productive sites.Results Woody species were indeed less clonal than herbaceous species. Both clonality and the temperate evolutionary background of dominating species were associated with unimodality in SRPRs, with woodiness modifying the clonality effect.Main conclusions The unimodal SRPR has been common in the ecological literature because most such studies originate from temperate herbaceous communities with many clonal species. Consequently, both evolutionary and ecological factors may influence species richness patterns.

Detecting the role of individual species for overyielding in experimental grassland communities composed of potentially dominant species

Several studies have shown that the contribution of individual species to the positive relationship between species richness and community biomass production cannot be easily predicted from species monocultures. Here, we used a biodiversity experiment with a pool of nine potentially dominant grassland species to relate the species richness-productivity relationship to responses in density, size and aboveground allocation patterns of individual species. Aboveground community biomass increased strongly with the transition from monocultures to two-species mixtures but only slightly with the transition from two- to nine-species mixtures. Tripartite partitioning showed that the strong increase shown by the former was due to trait-independent complementarity effects, while the slight increase shown by the latter was due to dominance effects. Trait-dependent complementarity effects depended on species composition. Relative yield total (RYT) was greater than 1 (RYT>1) in mixtures but did not increase with species richness, which is consistent with the constant complementarity effect. The relative yield (RY) of only one species, Arrhenatherum elatius, continually increased with species richness, while those of the other species studied decreased with species richness or varied among different species compositions within richness levels. High observed/expected RYs (RYo/RYe>1) of individual species were mainly due to increased module densities, whereas low observed/expected RYs (RYo/RYe<1) were due to more pronounced decreases in module density (species with stoloniferous or creeping growth) or module size (species with clearly-defined plant individuals). The trade-off between module density and size, typical for plant populations under the law of constant final yield, was compensated among species. The positive trait-independent complementarity effect could be explained by an increase in community module density, which reached a maximum at low species richness. In contrast, the increasing dominance effect was attributable to the species-specific ability, in particular that of A. elatius, to increase module size, while intrinsic growth limitations led to a suppression of the remaining species in many mixtures.