Richness-productivity Relationship Research Articles

Integrative framework of multiple processes to explain plant productivity–richness relationships

Plant diversity and productivity, two crucial properties that sustain ecosystem structures, functions, and services, are intrinsically linked to numerous ecological fields, making productivity–richness relationships (PRR) a central ecological concern. Despite extensive research from the Darwinian era to the 21st century, the various shapes of PRR and their underlying theories have sparked ongoing debates. While several processes, theories, and integrative models have been proposed to explain PRR, a comprehensive understanding of the types of PRR, the effects of these processes on plant productivity and richness, and the relationships between PRR shapes remains elusive. This paper proposes a new integrative framework that focuses on these aspects, aiming to elucidate the diverse shapes of PRR and their interconnections. We review recent integrative methods that explain the roles of processes and the varying shapes in PRR to support this new framework. The paper traces the distinct phases in PRR research, including the discovery of PRR shapes, tests of influencing processes, and integrative research. We discuss the application of the Structural Equation Model (SEM), Statistical Dynamical Model (SDM), and Differential Dynamical Model (DDM) in integrative research. This integrative framework can guide theoretical and applied ecologists in identifying, deriving, explaining, and predicting the interconnected but distinct shapes of PRR. The humped, asymptotic, positive, negative, and irregular shapes of PRR are interconnected, with one shape potentially transforming into another. The balance between the positive and negative effects of different processes determines the different shapes of PRR, ultimately leading to a globally positive effect of plant diversity on plant productivity and other ecosystem functions.

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.

Evenness mediates the global relationship between forest productivity and richness

Abstract1. Biodiversity is an important component of natural ecosystems, with higher species richness often correlating with an increase in ecosystem productivity. Yet, this relationship varies substantially across environments, typically becoming less pronounced at high levels of species richness. However, species richness alone cannot reflect all important properties of a community, including community evenness, which may mediate the relationship between biodiversity and productivity. If the evenness of a community correlates negatively with richness across forests globally, then a greater number of species may not always increase overall diversity and productivity of the system. Theoretical work and local empirical studies have shown that the effect of evenness on ecosystem functioning may be especially strong at high richness levels, yet the consistency of this remains untested at a global scale.2. Here, we used a dataset of forests from across the globe, which includes composition, biomass accumulation and net primary productivity, to explore whether productivity correlates with community evenness and richness in a way that evenness appears to buffer the effect of richness. Specifically, we evaluated whether low levels of evenness in speciose communities correlate with the attenuation of the richness–productivity relationship.3. We found that tree species richness and evenness are negatively correlated across forests globally, with highly speciose forests typically comprising a few dominant and many rare species. Furthermore, we found that the correlation between diversity and productivity changes with evenness: at low richness, uneven communities are more productive, while at high richness, even communities are more productive.4. Synthesis. Collectively, these results demonstrate that evenness is an integral component of the relationship between biodiversity and productivity, and that the attenuating effect of richness on forest productivity might be partly explained by low evenness in speciose communities. Productivity generally increases with species richness, until reduced evenness limits the overall increases in community diversity. Our research suggests that evenness is a fundamental component of biodiversity–ecosystem function relationships, and is of critical importance for guiding conservation and sustainable ecosystem management decisions.

Environment and time drive the links between the species richness and ecosystem multifunctionality from multitrophic freshwater mesocosms

Understanding the link between biodiversity and ecosystem functioning is imperative for ecosystem-based management. The role of multitrophic diversity in sustaining multifunctionality remains unclear, especially in highly diverse aquatic ecosystems. We performed a species-addition experiment consisting of one, two, three, and five species in simulated multitrophic ecosystems to evaluate biodiversity and ecosystem functioning relationships within and across trophic levels. Our results showed that there are positive species richness–productivity relationships within and across trophic levels. We found significant negative correlations between species richness and the cumulative variation of total phosphorus, and between species richness and ecosystem multifunctionality across trophic levels. Also, we found that the relationships between ecosystem multifunctionality and species richness within and across the trophic levels are mediated by a combination of environmental factors, including water temperature, dissolved oxygen, pH, irradiance, and time, rather than by species richness. Our results imply that species richness–ecosystem functioning relationships vary for different ecological functions; the individual ecosystem functions selected and the way multifunctionality calculated are critical when examining links between biodiversity and ecosystem multifunctionality. Our study highlights that multitrophic richness, such as for consumers, is crucial for driving ecosystem multifunctionality. Furthermore, our study implies that management practices for restoring the diversity of aquatic macrophytes in wetlands should consider not only macrophyte richness but also different functional groups and life-forms.

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.

More individuals or specialized niches? Distinguishing support for hypotheses explaining positive species–energy relationships

AbstractAimPositive productivity–richness relationships have been observed across taxa and ecosystems. We assessed support for two hypotheses explaining these relationships, the More Individuals Hypothesis (MIH) and the Niche Specialization Hypothesis (NSH) in two complementary datasets of avian communities in North America, the Breeding Bird Survey (BBS), which has been widely used to study productivity–richness relationships, and the Breeding Bird Census (BBC), which has been used rarely yet offers accurate population density estimates and more homogeneous landscapes, eliminating confounding variables that may make it more difficult to test predictions of the NSH and MIH in the BBS alone.LocationCoterminous United States, 1988–2009.TaxonBirds.MethodsWe first evaluated the strength of productivity–richness relationships in the BBS and BBC, and tested predictions from the MIH and NSH in the two datasets by examining patterns in community abundance, productivity niche breadth and number of foraging guilds along a gradient of normalized difference vegetation index (a measure of vegetation productivity). We also implemented a null model to examine the contribution of sampling effects due to increasing species richness in explaining patterns in number of foraging guilds in both datasets.ResultsWe found that the BBS had a much stronger productivity–richness relationship than the BBC, potentially as a result of increased landscape diversity with increasing productivity at BBS survey sites. Although the MIH and NSH may not be mutually exclusive, we found weaker support for the MIH in the BBC, and stronger support for the NSH in the BBC and BBS.Main ConclusionsResearchers should consider the role of confounding variables such as landscape diversity and focus on developing direct measurements of food resources at macroecological scales to determine the relative importance of mechanisms driving productivity–richness relationships.

Effects of enemy exclusion on biodiversity-productivity relationships in a subtropical forest experiment.

Interspecific niche complementarity is a key mechanism posited to explain positive species richness–productivity relationships in plant communities. However, the exact nature of the niche dimensions that plant species partition remains poorly known.Species may partition abiotic resources that limit their growth, but species may also be specialized with respect to their set of biotic interactions with other trophic levels, in particular with enemies including pathogens and consumers. The lower host densities present in more species‐diverse plant communities may therefore result in smaller populations of specialized enemies, and in a smaller associated negative feedback these enemies exert on plant productivity.To test whether such host density‐dependent effects of enemies drive diversity–productivity relationships in young forest stands, we experimentally manipulated leaf fungal pathogens and insect herbivores in a large subtropical forest biodiversity–ecosystem functioning experiment in China (BEF‐China).We found that fungicide spraying of tree canopies removed the positive tree‐species richness–productivity relationship present in untreated control plots. The tree species that contributed the most to this effect were the ones with the highest fungicide‐induced growth increase in monoculture. Insecticide application did not cause comparable effects. Synthesis. Our findings suggest that tree species diversity may not only promote productivity by interspecific resource‐niche partitioning but also by trophic niche partitioning. Most likely, partitioning occurred with respect to enemies such as pathogenic fungi. Alternatively, similar effects on tree growth would have occurred if fungicide had eliminated positive effects of a higher diversity of beneficial fungi (e.g. mycorrhizal symbionts) that may have occurred in mixed tree species communities.

Climatic resources mediate the shape and strength of grassland productivity-richness relationships from local to regional scales

The plant productivity and species richness (P-SR) relationship has been a central topic in ecology for decades. It is increasingly recognized that climate resources (CR, temperature and precipitation) might affect the P-SR relationship as (1) the response of plant productivity and species richness to CR are different; (2) plant-plant interactions change along the CR gradient. However, little is known about how these effects mediate the shape and strength of the P-SR relationship from local-site to regional scales. Here, we proposed a cross-scale framework to disentangle the climatic control over the P-SR relationship in grasslands using the HerbDivNet dataset, which covers 28 natural grasslands in 19 countries and 6 continents with wide precipitation and temperature gradients. Our results showed that, at the local-site scale, a humped-back P-SR relationship is common at sites under poor and rich CR conditions, whereas both the shape and strength of the P-SR relationship strongly vary among sites under moderate CR condition. However, at the regional scale, the humped-back P-SR relationship is only observed under poor CR condition. We suggested that the asynchronous responses of productivity and species richness to increasing CR is an abiotic driver of the inconsistent P-SR relationships along the climatic gradient in addition to the changing plant-plant interactions suggested by the stress-gradient hypothesis. These findings help to reconcile the long-standing debate over the P-SR relationship and highlight the importance of CR conditions in developing or comparing grassland ecosystem theories and management at small scales.

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.

Reprint of: Functional-structural plant models to boost understanding of complementarity in light capture and use in mixed-species forests

Positive relationships between tree species richness and productivity have been observed in both natural and experimental forests. Complementarity in resource capture and use has been put forward as one important mechanism for the positive richness-productivity relationship. However, inference that complementarity drives this relationship is often based on statistical modelling or the use of functional diversity indices and these methods do not consider resource capture or use among individuals from various species within a community. Here I introduce functional-structural plant models as tool to study species complementarity in light capture and use in mixed-species forests. These models consider the interplay between structure, physiology and the environment in 3D and scale from organ-specific characteristics to tree and community performance. Functional-structural plant models that represent mixed-species forests have therefore the potential to disentangle the effects of structural and functional differences among species on community light capture and use using the 3D setting of the forest. Knowing how community light capture and use in mixed-species forests depends on structural and functional differences among species will shed light on the potential of species complementarity in light capture and use to drive species richness-productivity relationships.

Drivers of understorey biomass: tree species identity is more important than richness in a young forest

Abstract Aims Positive biodiversity–ecosystem functioning relationships have been widely reported, predominately from grassland ecosystems. However, this does not necessarily have to apply accordingly in more complex situations such as in forests across different vertical strata. For instance, overstorey tree species richness has been shown to be associated with a lower understorey productivity. Whether or not tree species richness effects add to understorey productivity by increasing (i.e. due to habitat heterogeneity) or reducing resource availability (i.e. through increasing competition) and whether understorey productivity is indeed being governed more strongly by tree species identity are likely to change over time. Moreover, studies also suggested that richness–productivity relationships change with the environmental context. Using an experimental forest plantation with manipulated tree species richness, this study examined these temporal and environmental dynamics across strata. Methods In the context of the Biodiversity-Ecosystem Functioning project in subtropical China (BEF-China), we made use of understorey biomass samples repeatedly collected over a time period of 3 years along a tree species richness gradient. The effects of tree species richness, tree species identities and time were studied across different environmental treatments for their impact on understorey biomass. Important Findings While we found significant and consistent tree layer identity effects on understorey biomass, no such effect was encountered for tree species richness. Our results also indicate that among structural layers in forests, there might not be a single, generalizable overstorey species richness–understorey productivity relationship, and that the extent as to which overstorey-related environmental factors such as light transmittance contribute to understorey productivity change with time. Overall, we demonstrate that temporal dynamics should be considered when studying relationship among structural layers in forests.

Functional-structural plant models to boost understanding of complementarity in light capture and use in mixed-species forests

Positive relationships between tree species richness and productivity have been observed in both natural and experimental forests. Complementarity in resource capture and use has been put forward as one important mechanism for the positive richness-productivity relationship. However, inference that complementarity drives this relationship is often based on statistical modelling or the use of functional diversity indices and these methods do not consider resource capture or use among individuals from various species within a community. Here I introduce functional-structural plant models as tool to study species complementarity in light capture and use in mixed-species forests. These models consider the interplay between structure, physiology and the environment in 3D and scale from organ-specific characteristics to tree and community performance. Functional-structural plant models that represent mixed-species forests have therefore the potential to disentangle the effects of structural and functional differences among species on community light capture and use using the 3D setting of the forest. Knowing how community light capture and use in mixed-species forests depends on structural and functional differences among species will shed light on the potential of species complementarity in light capture and use to drive species richness-productivity relationships.

Soil fungal guilds as important drivers of the plant richness-productivity relationship.

This article is a Commentary on https://doi.org/10.1111/nph.16390.

Fertility-related interplay between fungal guilds underlies plant richness-productivity relationships in natural grasslands.

The plant richness-productivity relationship is a central subject in ecology, yet the mechanisms behind this pattern remain debated. Soil fungi are closely associated with the dynamics of plant communities, however empirical evidence on how fungal communities integrate into the richness-productivity relationships of natural environments is lacking. We used Illumina high-throughput sequencing to identify rhizosphere fungal communities across a natural plant richness gradient at two sites with different fertility conditions, and related the subsequent information to plant richness and productivity to elucidate the role of fungal guilds in integrating the linkages of both plant components. Saprotrophs, mycorrhizal fungi and potential plant pathogens interacted differently between the sites, with saprotrophic and mycorrhizal fungal abundances being positively correlated at the high-nutrient site and abundances of mycorrhizal fungi and potential plant pathogens being negatively correlated at the low-nutrient site. The synergistic associations between these fungal guilds with plant richness and productivity operated in concert to promote positive richness-productivity relationships. Our findings provide empirical evidence for the importance of soil fungal guilds in integrating the linkages of plant richness and productivity, and suggest that future work incorporating soil fungal communities into richness-productivity relationships would advance our mechanistic understanding of their linkages.

Biomass allocation and productivity-richness relationship across four grassland types at the Qinghai Plateau.

Aboveground biomass (AGB) and belowground biomass (BGB) allocation and productivity–richness relationship are controversial. Here, we assessed AGB and BGB allocation and the productivity–richness relationship at community level across four grassland types based on the biomass data collected from 80 sites across the Qinghai Plateau during 2011–2012. The reduced major axis regression and general linear models were used and showed that (a) the median values of AGB were significantly higher in alpine meadow than in other three grassland types; the ratio of root to shoot (R/S) was significantly higher in desert grassland (36.06) than intemperate grassland (16.60), alpine meadow (13.35), and meadow steppe (19.46). The temperate grassland had deeper root distribution than the other three grasslands, with about 91.45% roots distributed in the top 30 cm soil layer. (b) The slopes between log AGB and log BGB in the temperate grassland and meadow steppe were 1.09 and 1, respectively, whereas that in the desert grassland was 1.12, which was significantly different from the isometric allocation relationship. A competitive relationship between AGB and BGB was observed in the alpine meadow with a slope of −1.83, indicating a trade‐off between AGB and BGB in the alpine meadow. (c) A positive productivity–richness relationship existed across the four grassland types, suggesting that the positive productivity–richness relationship might not be affected by the environmental factors at the plant location. Our results provide a new insight for biomass allocation and biodiversity–ecosystem functioning research.

Spatial scaling of species richness–productivity relationships for local communities: analytical results from a neutral model

The relationship between species richness and productivity changes with spatial scale, but the way in which it changes and the underlying mechanisms remain unclear. We address this critical knowledge gap using a new mechanistic model of the spatial scaling of species richness–productivity (SP) relationships for a local community. Our model is neutral and hence assumes that species dynamics are driven by dispersal limitation and demographic stochasticity. We showed analytically that SP relationships predicted by our model are typically unimodal. Consistent with previous simulation-based studies, the positive phase of our unimodal SP relationship was driven by a sampling effect (“more-individuals effect”) whereas the negative phase was driven by relatively more propagules being of local as opposed to external origin (“dilution effect”). Our main novel finding related to the spatial scaling of the unimodal SP relationship: the peak shifted to the left with increasing spatial scale, such that the decreasing phase covered a greater range of productivity. This was driven by an increase in the strength of the dilution effect relative to the more-individuals effect, reflecting higher area/perimeter ratios at larger spatial scales. Our theoretical predictions are qualitatively consistent with the spatial scaling of SP relationships documented for trees in localized forest communities across the world.

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.

Niche packing and expansion account for species richness–productivity relationships in global bird assemblages

AbstractAimNiche theory proposes that increases in species richness along an environmental gradient are associated with a packing of species inside the niche space or an expansion of the niche space. We test whether and under what conditions an increase in bird species richness along a gradient of resource availability is associated with an expansion or packing of the niche as measured based on traits related to resource use.LocationGlobal.Time periodCurrent.Major taxa studiedBirds.MethodsWe measured birds' realized niche space as the standardized departure between observed total trait range and its null expectation (functional richness: SES.FRic) in 12,188 cells worldwide. We first correlated both species richness and this measurement along the global net primary productivity (NPP) gradient using linear regressions. Second, we investigated the non‐stationarity of the species richness–NPP relationship with Lee's bivariate correlation, a measure of the spatial association of two variables. We then assessed the number of cells exhibiting a significant positive species richness–NPP association and a significant negative or positive SES.FRic. Third, we assessed whether species of species‐rich assemblages occur within or outside the niche space of species‐poor assemblages.ResultsAt a global scale, we found that species richness and SES.FRic increased with NPP. We also showed that cells with a significant positive association between species richness and NPP exhibited niche packing (1,699 assemblages out of 12,188) more than niche expansion (five assemblages). Niche packing was associated with complex biomes such as tropical rain forests. Finally, by showing that species in species‐rich assemblages predominantly occur within the niche space of species‐poor assemblages, we showed that the increase in SES.FRic with NPP contributed little to the increase in species richness.Main conclusionAlthough niche volume increases with species richness along an NPP gradient, we confirmed that niche packing is the pattern most associated with the species richness–NPP relationship at a global scale.

Dynamics and drivers of phytoplankton richness and composition along productivity gradient

The shape of the productivity–richness relationship (PRR) for phytoplankton has been suggested to be unimodal, that is, the richness peaks at intermediate productivity levels. However, the mechanistic understanding for this pattern is still widely debated. In this study, we observed a unimodal PRR within 71 lakes along the Yangtze River encompassing an altitude range of 0–2700m, and an over 2200km distance from the upper reaches to the lower reaches. At low productivity, the competition for resources and regulatory processes jointly affected phytoplankton richness and composition, and their explanatory power depend on the gradient scale of driving factors. The variation of temperature attributing to altitudinal difference explained the majority of the variations of phytoplankton. If the altitude variation in temperature was eliminated, the explanatory power of temperature decreased from 31.7 to 7.6, and the independent effect of each resource and regulatory variable were limited and not decisive. At high productivity, the negative feedback of increased productivity (light limitation) affected the phytoplankton species richness and composition. The light-sensitive species disappeared, low-light-adapted species was retained and the phytoplankton composition gradually became similar with an increase in productivity. The findings contribute to an increased understanding of the mechanisms resulting in a hump-shaped PRR for phytoplankton.