Predator diversity and the functioning of ecosystems: the role of intraguild predation in dampening trophic cascades

Single trophic‐level studies of the relationship between biodiversity and ecosystem functioning highlight the importance of mechanisms such as resource partitioning, facilitation, and sampling effect. In a multi‐trophic context, trophic interactions such as intraguild predation may also be an important mediator of this relationship. Using a salt‐marsh food web, we investigated the interactive effects of predator species richness (one to three species) and trophic composition (strict predators, intraguild predators, or a mixture of the two) on ecosystem functions such as prey suppression and primary production via trophic cascades. We found that the trophic composition of the predator assemblage determined the impact of increasing predator species richness on the occurrence of trophic cascades. In addition, increasing the proportion of intraguild predator species present diminished herbivore suppression and reduced primary productivity. Therefore, trophic composition of the predator assemblage can play an important role in determining the nature of the relationship between predator diversity and ecosystem function.     

Using root traits to understand temporal changes in biodiversity effects in grassland mixtures

Biodiversity–ecosystem functioning (BEF) studies typically show that species richness enhances community biomass, but the underlying mechanisms remain debated. Here, we combine metrics from BEF research that distinguish the contribution of dominant species (selection effects, SE) from those due to positive interactions such as resource partitioning (complementarity effects, CE) with a functional trait approach in an attempt to reveal the functional characteristics of species that drive community biomass in species mixtures. In a biodiversity experiment with 16 plant species in monocultures, 4‐species and 16‐species mixtures, we used aboveground biomass to determine the relative contributions of CE and SE to biomass production in mixtures in the second, dry year of the experiment. We also measured root traits (specific root length, root length density, root tissue density and the deep root fraction) of each species in monocultures and linked the calculated community weighted mean (CWM) trait values and trait diversity of mixtures to CE and SE. In the second year of the experiment, community biomass, CE and SE increased compared to the first year. The contribution of SE to this positive effect was greater than that of CE. The increased contribution of SE was associated with root traits: SE increased most in communities with high abundance of species with deep, thick and dense roots. In contrast, changes in CE were not related to trait diversity or CWM trait values. Together, these results suggest that increased positive effects of species richness on community biomass in a dry year were mainly driven by increased dominance of deep‐rooting species, supporting the insurance hypothesis of biodiversity. Positive CE indicates that other positive interactions did occur, but we could not find evidence that belowground resource partitioning or facilitation via root trait diversity was important for community productivity in our biodiversity experiment.     

Determining the mechanism by which fish diversity influences production

Understanding the ability of biodiversity to govern ecosystem function is essential with current pressures on natural communities from species invasions and extirpations. Changes in fish communities can be a major determinant of food web dynamics, and even small shifts in species composition or richness can translate into large effects on ecosystems. In addition, there is a large information gap in extrapolating results of small-scale biodiversity–ecosystem function experiments to natural systems with realistic environmental complexity. Thus, we tested the key mechanisms (resource complementarity and selection effect) for biodiversity to influence fish production in mesocosms and ponds. Fish diversity treatments were created by replicating species richness and species composition within each richness level. In mesocosms, increasing richness had a positive effect on fish biomass with an overyielding pattern indicating species mixtures were more productive than any individual species. Additive partitioning confirmed a positive net effect of biodiversity driven by a complementarity effect. Productivity was less affected by species diversity when species were more similar. Thus, the primary mechanism driving fish production in the mesocosms was resource complementarity. In the ponds, the mechanism driving fish production changed through time. The key mechanism was initially resource complementarity until production was influenced by the selection effect. Varying strength of intraspecific interactions resulting from differences in resource levels and heterogeneity likely caused differences in mechanisms between the mesocosm and pond experiments, as well as changes through time in the ponds. Understanding the mechanisms by which fish diversity governs ecosystem function and how environmental complexity and resource levels alter these relationships can be used to improve predictions for natural systems.     

Steeply Increasing Growth Differential Between Mixture and Monocultures of Tropical Trees

Studies of biodiversity and terrestrial ecosystem functioning have concentrated almost exclusively on temperate grasslands. To broaden the reach of biodiversity‐functioning research, five fast growing species, comprising three eudicot trees and two congeneric palms (none symbiotic with nitrogen‐fixing microorganisms), were grown for 13 yr in a replacement‐series mixture and monocultures on a fertile soil in a high‐rainfall area of lowland Costa Rica. The mixture accrued more biomass and had greater net productivity than the average, but not the most productive, monoculture. Relative Land Output (a measure of comparative yield) increased steeply. The combined evidence points to an increase in resource partitioning or facilitation among species over time. Spatial partitioning of aboveground space (for light capture) and soil (possibly for retrieval of deep nitrogen), and facilitation of phosphorus availability by one species, are mechanisms that may account for the inferred complementarity. Extending the generalized findings on biodiversity–productivity relationships from well‐studied grasslands to tropical forests is warranted. Mixtures of fast growing trees can out‐perform the average of their component monocultures, whether the metric is biomass accrual or productivity. The modular growth of long‐lived structure enables arborescent species to retain crown space previously captured and may lead to increased spatial partitioning and facilitation of resources over time.     

Focusing on individual plants to understand community scale biodiversity effects: the case of root distribution in grasslands

Spatial resource partitioning between species via differences in rooting depth is one of the main explanations for the positive biodiversity–productivity relationship. However, evidence for the importance of this mechanism is limited. This may be due to the community scale at which these interactions are often investigated. Community measures represent net outcomes of species interactions and may obscure the mechanisms underlying belowground interactions. Here, we assess the performance of ~1700 individual plants and their heterospecific neighbours over three growing seasons in experimental grassland plots containing one, four or sixteen different plant species and tested whether their performance in mixtures compared to monocultures was related to their own rooting depth versus the rooting depth of their heterospecific neighbours. Overall, individuals of deep-rooting species performed better in mixtures and this effect significantly increased when surrounded by more shallow-rooting species. This effect was not apparent for the shallow rooting species. Together, including both deep and shallow rooting species increased mixture performance. Our results show that taking the perspective of the individual rather than the community can elucidate the interactions between species that contribute to positive biodiversity effects, emphasizing the need for studies at different scales to disentangle the myriad interactions that take place in diverse communities.     

A synthesis of subdisciplines: predator–prey interactions, and biodiversity and ecosystem functioning

The last 15 years has seen parallel surges of interest in two research areas that have rarely intersected: biodiversity and ecosystem functioning (BEF), and multispecies predator–prey interactions (PPI). Research addressing role of biodiversity in ecosystem functioning has focused primarily on single trophic‐level systems, emphasizing additive effects of diversity that manifest through resource partitioning and the sampling effect. Conversely, research addressing predator–prey interactions has focused on two trophic‐level systems, emphasizing indirect and non‐additive interactions among species. Here, we use a suite of consumer‐resource models to organize and synthesize the ways in which consumer species diversity affects the densities of both resources and consumer species. Specifically, we consider sampling effects, resource partitioning, indirect effects caused by intraguild interactions and non‐additive effects. We show that the relationship between consumer diversity and the density of resources and consumer species are broadly similar for systems with one vs. two trophic levels, and that indirect and non‐additive interactions generally do little more than modify the impacts of diversity established by the sampling effect and resource partitioning. The broad similarities between systems with one vs. two trophic levels argue for greater communication between researchers studying BEF, and researchers studying multispecies PPI.     

Water source niche overlap increases with site moisture availability in woody perennials

Classical niche partitioning theory posits increased competition for and partitioning of the most limiting resource among coexisting species. Coexisting plant species may vary in rooting depth, reflecting niche partitioning in water source use. Our goal was to assess the soil water partitioning of woody plant communities across northern Arizona along an elevational moisture gradient using stem and soil water isotopes from two sampling periods to estimate the use of different water sources. We hypothesized that niche overlap of water sources would be higher and monsoon precipitation uptake would be lower at sites with higher moisture availability. Pairwise niche overlap of coexisting species was calculated using mixing model estimates of proportional water use for three sources. Across the moisture gradient, niche overlap increased with site moisture index (precipitation/potential evapotranspiration) across seasons, and site moisture index explained 37% of the variation in niche overlap of intermediate and deeper sources of water. Desert trees utilized more winter source water than desert shrubs, suggesting the partitioning of water sources between functional groups. However, seasonal differences in surface water use were primarily found at intermediate levels of site moisture availability. Our findings support classical niche partitioning theory in that plants exhibit higher overlap of water sources when water is not a limiting resource.     

Biodiversity, productivity and the temporal stability of productivity: patterns and processes

Theory predicts that the temporal stability of productivity, measured as the ratio of the mean to the standard deviation of community biomass, increases with species richness and evenness. We used experimental species mixtures of grassland plants to test this hypothesis and identified the mechanisms involved. Additionally, we tested whether biodiversity, productivity and temporal stability were similarly influenced by particular types of species interactions. We found that productivity was less variable among years in plots planted with more species. Temporal stability did not depend on whether the species were planted equally abundant (high evenness) or not (realistically low evenness). Greater richness increased temporal stability by increasing overyielding, asynchrony of species fluctuations and statistical averaging. Species interactions that favoured unproductive species increased both biodiversity and temporal stability. Species interactions that resulted in niche partitioning or facilitation increased both productivity and temporal stability. Thus, species interactions can promote biodiversity and ecosystem services.     

Facilitation versus competition in grazing herbivore assemblages

The importance of facilitation versus competition in structuring herbivore species assemblages is a critical issue in theoretical ecology as well as for practical wildlife management. This paper examines the evidence for facilitation and clarifies our understanding in relation to the mechanisms and the spatial and temporal scales where they occur. Evidence for facilitation through stimulation of grass regrowth during the growing season appears stronger than that for increased resource access through removal of obstructing grass structures during the dormant season. Although facilitation may benefit the nutritional gains obtained by certain species in the short term, these benefits do not appear to be translated into the expected population consequences. We suggest this could be due to seasonal tradeoffs between facilitation and competition, as well as to restrictions on the spatial extent of trophic overlap.     

Resource partitioning along multiple niche dimensions in differently sized African savanna grazers

Resource partitioning among mammalian savanna herbivores is thought to be predominantly driven by differences in body size. In general, large herbivore species utilize abundant low quality forage while small herbivores focus on scarcer high quality food items. However, in a natural system other factors such as digestive strategy, season and the presence of megaherbivores (body size > 1000 kg) are likely to complicate allometric predictions. Non‐ruminants are probably better able to cope with abundant low quality food than ruminants of the same size causing a non‐ruminant to act ‘larger’ than allometrically predicted. Also, the effect of alternating seasons with high and low food availability on diet choice and hence the competitive interactions between co‐occurring herbivores is still poorly understood. Lastly, how megaherbivores deviate from allometric predictions (based on smaller species) is still not well quantified. In this study we examine resource partitioning among three ruminant and three non‐ruminant grazers: impala, wildebeest, buffalo, warthog, zebra and white rhinoceros (megaherbivore) in the savanna of Hluhluwe iMfolozi Park, South Africa. We analysed habitat and diet overlap, specifically grass species (something not commonly investigated) and grass height eaten, in both the wet and the dry seasons. We found that habitat utilization differences among the species were generally small and did not vary between seasons. Diets within feeding patches overlapped during the wet season but highly diverged during the dry season. Body mass differences among species explained their dry season resource partitioning for all species except for comparisons with the megaherbivore (white rhino), while differences in digestive strategy were not related to niche overlap in either season. We conclude that savanna herbivores in this system coexist mostly through body size‐driven resource partitioning in the dry‐season, with the exception of the white rhino (megaherbivore).     

Genotypic richness and dissimilarity opposingly affect ecosystem functioning

Biodiversity is an essential determinant of ecosystem functioning. Numerous studies described positive effects of diversity on the functioning of communities arising from complementary resource use and facilitation. However, high biodiversity may also increase competitive interactions, fostering antagonism and negatively affecting community performance. Using experimental bacterial communities we differentiated diversity effects based on genotypic richness and dissimilarity. We show that these diversity characteristics have opposite effects on ecosystem functioning. Genotypic dissimilarity governed complementary resource use, improving ecosystem functioning in complex resource environments. Contrastingly, genotypic richness drove allelopathic interactions, mostly reducing ecosystem functioning. The net biodiversity effect on community performance resulted from the interplay between the genetic structure of the community and resource complexity. These results demonstrate that increasing richness, without concomitantly increasing dissimilarity, can decrease ecosystem functioning in simple environments due to antagonistic interactions, an effect insufficiently considered so far in mechanistic models of the biodiversity-ecosystem functioning relationship.     

The interplay between above‐ and below‐ground plant–plant interactions along an environmental gradient: insights from two‐layer zone‐of‐influence models

The changes in plant–plant interactions along environmental gradients have been a focus of recent ecological research. It has been suggested that both above‐ and below‐ground competition and their interplay vary along gradients, but few studies have investigated this idea, and in most cases, the role of facilitation has not been considered, despite its importance in high stress environments. Here we used two‐layer ‘zone‐of‐influence’ models to simulate the effects of facilitation, size‐asymmetry of competition, abiotic stress, resource availability and the balance of root–shoot growth on shoot and root interactions and their interplay along an environmental gradient. In the absence of facilitation, shoot and total competition became weaker, while root competition and the interplay between shoot and root competition were unchanged under increasing stress when root competition was completely symmetric. In contrast, shoot, root, total interactions and the interplay between shoot and root interactions were all negative, and they increased with increasing stress when root competition was size‐symmetric. When facilitation was included in the models, net effects of shoot, root, total interactions and the interplay of root–shoot interactions were very different from those without facilitation, and many were positive under highly stressful conditions. The type of stress (non‐resource or resource) did not significantly influence the simulation results. Our study provides an alternative interpretation of the interplay between above‐ and below‐ground plant–plant interactions across an environmental gradient.     

Prey diversity as a driver of resource partitioning between river‐dwelling fish species

Although food resource partitioning among sympatric species has often been explored in riverine systems, the potential influence of prey diversity on resource partitioning is little known. Using empirical data, we modeled food resource partitioning (assessed as dietary overlap) of coexisting juvenile Atlantic salmon (Salmo salar) and alpine bullhead (Cottus poecilopus). Explanatory variables incorporated into the model were fish abundance, benthic prey diversity and abundance, and several dietary metrics to give a total of seventeen potential explanatory variables. First, a forward stepwise procedure based on the Akaike information criterion was used to select explanatory variables with significant effects on food resource partitioning. Then, linear mixed‐effect models were constructed using the selected explanatory variables and with sampling site as a random factor. Food resource partitioning between salmon and bullhead increased significantly with increasing prey diversity, and the variation in food resource partitioning was best described by the model that included prey diversity as the only explanatory variable. This study provides empirical support for the notion that prey diversity is a key driver of resource partitioning among competing species.     

Resource manipulation effects on net primary production,biomass allocation and rain-use efficiency of two semiarid grassland sites in Inner Mongolia,China

Productivity of semiarid grasslands is affected by soil water and nutrient availability, with water controlling net primary production under dry conditions and soil nutrients constraining biomass production under wet conditions. In order to investigate limitations on plants by the response of root–shoot biomass allocation to water and nitrogen (N) availability, a field experiment, on restoration plots with rainfed, unfertilized control plots, fertilized plots receiving N (25 kg urea-N ha⁻¹) and water (irrigation simulating a wet season), was conducted at two sites with different grazing histories: moderate (MG) and heavy (HG) grazing. Irrigation and N addition had no effect on belowground biomass. Irrigation increased aboveground (ANPP) and belowground net primary production (BNPP) and rain-use efficiency based on ANPP (RUE_ANPP), whereas N addition on rainfed plots had no effect on any of the measured parameters. N fertilizer application on irrigated plots increased ANPP and RUE_ANPP and reduced the root fraction (RF: root dry matter/total dry matter), resulting in smaller N effects on total net primary production (NPP) and rain-use efficiency based on NPP. This suggests that BNPP should be included in evaluating ecosystem responses to resource availability from the whole-plant perspective. N effects on all measured parameters were similar on both sites. However, site HG responded to irrigation with higher ANPP and a lower RF when compared to site MG, indicating that species composition had a pronounced effect on carbon allocation pattern due to below- and aboveground niche complementarity.     

Small scale genotypic richness stabilizes plot biomass and increases phenotypic variance in the invasive grass Phalaris arundinacea

Aims We aim to understand how small-scale genotypic richness and genotypic interactions influence the biomass and potential invasiveness of the invasive grass, Phalaris arundinacea under two different disturbance treatments: intact plots and disturbed plots, where all the native vegetation has been removed. Specifically, we address the following questions (i) Does genotypic richness increase biomass production? (ii) Do genotypic interactions promote or reduce biomass production? (iii) Does the effect of genotypic richness and genotypic interactions differ in different disturbance treatments? Finally (iv) Is phenotypic variation greater as genotypic richness increases?Methods We conducted a 2-year common garden experiment in which we manipulated genotype richness using eight genotypes planted under both intact and disturbed conditions in a wetland in Burlington, Vermont (44°27′23″N, 73°11′29″W). The experiment consisted of a randomized complete block design of three blocks, each containing 20 plots (0.5 m 2) per disturbed treatment. We calculated total plot biomass and partitioned the net biodiversity effect into three components: dominance effect, trait-dependent complementarity and trait-independent complementarity. We calculated the phenotypic variance for each different genotype richness treatment under the two disturbance treatments.Important findings Our results indicate that local genotypic richness does not increase total biomass production of the invasive grass P. arundinacea in either intact or disturbed treatments. However, genotypic interactions underlying the responses showed very different patterns in response to increasing genotypic richness. In the intact treatment, genotypic interactions resulted in the observed biomass being greater than the predicted biomass from monoculture plots (e.g., overyielding) and this was driven by facilitation. However, facilitation was reduced as genotypic richness increased. In the disturbed treatment, genotypic interactions resulted in underyielding with observed biomass being slightly less than expected from the performance of genotypes in monocultures; however, underyielding was reduced as genotypic richness increased. Thus, in both treatments, higher genotypic richness resulted in plot biomass nearing the additive biomass from individual monocultures. In general, higher genotypic richness buffered populations against interactions that would have reduced biomass and potentially spread. Phenotypic variance also had contrasting patterns in intact and disturbed treatments. In the intact treatment, phenotypic variance was low across all genotypic richness levels, while in the disturbed treatment, phenotypic variance estimates increased as genotypic richness increased. Thus, under the disturbed treatment, plots with higher genotypic richness had a greater potential response to selection. Therefore, limiting the introduction of new genotypes, even if existing genotypes of the invasive species are already present, should be considered a desirable management strategy to limit the invasive behavior of alien species.     

An expanded modern coexistence theory for empirical applications

Understanding long‐term coexistence of numerous competing species is a longstanding challenge in ecology. Progress requires determining which processes and species differences are most important for coexistence when multiple processes operate and species differ in many ways. Modern coexistence theory (MCT), formalised by Chesson, holds out the promise of doing that, but empirical applications remain scarce. We argue that MCT's mathematical complexity and subtlety have obscured the simplicity and power of its underlying ideas and hindered applications. We present a general computational approach that extends our previous solution for the storage effect to all of standard MCT's spatial and temporal coexistence mechanisms, and also process‐defined mechanisms amenable to direct study such as resource partitioning, indirect competition, and life history trade‐offs. The main components are a method to partition population growth rates into contributions from different mechanisms and their interactions, and numerical calculations in which some mechanisms are removed and others retained. We illustrate how our approach handles features that have not been analysed in the standard framework through several case studies: competing diatom species under fluctuating temperature, plant–soil feedbacks in grasslands, facilitation in a beach grass community, and niche differences with independent effects on recruitment, survival and growth in sagebrush steppe.     

Photosynthesis,Transpiration, and Water Use Efficiency of Vegetative and Reproductive Shoots of Grassland Species from North-Eastern China

The differences in net photosynthetic rate (P _N), transpiration rate (E), and water use efficiency (WUE) between the vegetative and reproductive shoots of three native grass species from the grassland of northeastern China [grey-green and yellow green populations of Leymus chinensis (Trin.) Tzvel., Puccinellia tenuiflora (Griseb) Scrib & Merr, Puccinellia chinampoensis Ohwi] were compared. The two type shoots experienced similar habitats, but differed in leaf life-span and leaf area. The leaf P _N and WUE for the vegetative shoots were significantly higher than those for the reproductive shoots in the grasses, while their E were remarked lower in the dry season. Relative lower leaf P _N and WUE for the reproductive shoots of grassland grasses may explain the facts of lower seed production and the subordinate role of seed in the grassland renewal in north-eastern China.     

Why does grassland productivity increase with species richness? Disentangling species richness and composition with tests for overyielding and superyielding in biodiversity experiments.

The causal relationship between the biodiversity of natural and modified environments and their net primary production has been a topic of significant scientific controversy and scrutiny. Early theoretical and empirical results indicated that production was sometimes significantly correlated with species richness when species richness was directly manipulated in experimental systems. Possible mechanisms for this phenomenon include statistical sampling effects, complementary resource use and mutualistic interactions. However, the interpretation of experimental results has sometimes confounded species richness with species composition, and disentangling the effects of species diversity from species identity has proved a formidable challenge. Here, I present a statistical method that is based on simple probability models and does not rely on the species composition of individual plots to distinguish among three phenomena that occur in biodiversity-production experiments: underyielding, overyielding and (a new concept) superyielding. In some cases, distinguishing these phenomena will provide evidence for underlying mechanisms. As a proof-of-concept, I first applied this technique to a simulated dataset, indicating the strengths of the method with both clear and ambiguous cases. I then analysed data from the BIODEPTH experimental biodiversity manipulations. No evidence of either overyielding or superyielding was detected in the BIODEPTH experiment.     

Adaptive foraging allows the maintenance of biodiversity of pollination networks

Pollination systems are recognized as critical for the maintenance of biodiversity in terrestrial ecosystems. Therefore, the understanding of mechanisms that promote the integrity of those mutualistic assemblages is an important issue for the conservation of biodiversity and ecosystem function. In this study we present a new population dynamics model for plant–pollinator interactions that is based on the consumer–resource approach and incorporates a few essential features of pollination ecology. The model was used to project the temporal dynamics of three empirical pollination network, in order to analyze how adaptive foraging of pollinators (AF) shapes the outcome of community dynamics in terms of biodiversity and network robustness to species loss. We found that the incorporation of AF into the dynamics of the pollination networks increased the persistence and diversity of its constituent species, and reduced secondary extinctions of both plants and animals. These findings were best explained by the following underlying processes: 1) AF increased the amount of floral resources extracted by specialist pollinators, and 2) AF raised the visitation rates received by specialist plants. We propose that the main mechanism by which AF enhanced those processes is (trophic) niche partitioning among animals, which in turn generates (pollen vector) niche partitioning among plants. Our results suggest that pollination networks can maintain their stability and diversity by the adaptive foraging of generalist pollinators.