Bottom-up and top-down forces structuring consumer communities in an experimental grassland

Correlated biodiversity change between plant and insect assemblages resurveyed after 80 years across a dynamic habitat mosaic

Historical data on co-occurring taxa are extremely rare. As such, the extent to which distinct co-occurring taxa experience similar long-term patterns in species richness and compositional change (e.g., when exposed to a changing environment) is not clear. Using data from a diverse ecological community surveyed in the 1930s and resurveyed in the 2010s, we investigated whether local plant and insect assemblages displayed cross-taxon congruence-that is, spatiotemporal correlation in species richness and compositional change-across six co-occurring taxa: vascular plants, non-vascular plants, grasshoppers and crickets (Orthoptera), ants (Hymenoptera: Formicinae), hoverflies (Diptera: Syrphidae), and dragonflies and damselflies (Odonata). All taxa exhibited high levels of turnover across the ca. 80-year time period. Despite minimal observed changes at the level of the whole study system, species richness displayed widespread cross-taxon congruence (i.e., correlated temporal change) across local assemblages within the study system. Hierarchical logistic regression models suggest a role for shared responses to environmental change underlying cross-taxon correlations and highlight stronger correlations between vascular plants and their direct consumers, suggesting a possible role for biotic interactions between these groups. These results provide an illustration of cross-taxon congruence in biodiversity change using data unique in its combination of temporal and taxonomic scope, and highlight the potential for cascading and comparable effects of environmental change (abiotic and biotic) on co-occurring plant and insect communities. However, analyses of historical resurveys based on currently available data come with inherent uncertainties. As such, this study highlights a need for well-designed experiments, and monitoring programs incorporating co-occurring taxa, to determine the underlying mechanisms and prevalence of congruent biodiversity change as anthropogenic environmental change accelerates apace.

Habitat attributes mediate herbivory and influence community development in algal metacommunities

Understanding the drivers and impacts of spatiotemporal variation in species abundance on community trajectories is key to understanding the factors contributing to ecosystem resilience. Temporal variation in species trajectories across patches can provide compensation for species loss and can influence successional patterns. However, little is known about the underlying mechanisms that lead to patterns of species or spatial compensation and how those patterns may be mediated by consumer-resource relationships. Here we describe an experiment testing whether habitat attributes (e.g., structural complexity and spatial heterogeneity) mediate the effects of herbivory on tropical marine macroalgal communities by reducing accessibility and detectability, respectively, leading to variable trajectories among algal species at community (within patch) and metacommunity (i.e., among patch) scales. Reduced accessibility (greater habitat complexity) decreased the effects of herbivory (i.e., depressed consumption rate, increased algal species richness), and both accessibility and detectability (spatial heterogeneity) influenced algal community structure. Moreover, decreased accessibility at the community scale and a mosaic of accessibility at the metacommunity scale led to variation in community assembly. We suggest that habitat attributes can be important influencers of consumer-resource interactions on coral reefs, which in turn can increase species diversity, promote species succession, and enhance stability in algal metacommunities.

Vegetation shapes aboveground invertebrate communities more than soil properties and pollution: a preliminary investigation on a metal-contaminated site

Pollution with trace metals (TM) has been shown to affect diversity and/or composition of plant and animal communities. While ecotoxicological studies have estimated the impact of TM contamination on plant and animal communities separately, ecological studies have widely demonstrated that vegetation is an important factor shaping invertebrate communities. It is supposed that changes in invertebrate communities under TM contamination would be explained by both direct impact of TM on invertebrate organisms and indirect effects due to changes in plant communities. However, no study has clearly investigated which would more importantly shape invertebrate communities under TM contamination. Here, we hypothesized that invertebrate communities under TM contamination would be affected more importantly by plant communities which constitute their habitat and/or food than by direct impact of TM. Our analysis showed that diversity and community identity of flying invertebrates were explained only by plant diversity which was not affected by TM contamination. Diversity of ground-dwelling (GD) invertebrates in spring was explained more importantly by plant diversity (27% of variation) than by soil characteristics including TM concentrations (8%), whereas their community identity was evenly explained by plant diversity and soil characteristics (2-7%). In autumn, diversity of GD invertebrates was only explained by plant diversity (12%), and their identity was only explained by soil characteristics (8%). We conclude that vegetation shapes invertebrate communities more importantly than direct effects of TM on invertebrates. Vegetation should be taken into account when addressing the impacts of environmental contamination on animal communities.

Trophic differences regulate grassland food webs: herbivores track food quality and predators select for habitat volume

The impacts of altered biogeochemical cycles on ecological systems are likely to vary with trophic level. Predicting how these changes will affect ecological food webs is further complicated by human activities, which are simultaneously altering the availability of macronutrients like nitrogen (N) and phosphorus (P), and micronutrients such as sodium (Na). Here we contrast three hypotheses that predict how increasing nutrient availability will shape grassland food webs. We conducted a distributed factorial fertilization experiment (N and P crossed with NaCl) across four North American grasslands, quantifying the responses of aboveground plant biomass and volume, plant tissue and soil elemental concentrations, as well as the abundance of five arthropod functional groups. Fertilization with N and P increased plant biomass and foliar N and P concentrations in grasses but not forbs. Fertilization with Na had no effect on plant biomass but increased foliar Na concentrations. Consistent with the nutrient limitation hypothesis, we found strong evidence of nutrient limitation for insect herbivores across the four sites with sucking (phloem and xylem feeding) herbivores increasing in abundance with NP fertilization and chewing herbivores increasing in response to both Na and NP fertilization, and a trend for increased response of arthropods to lower plant nutrient availability. We found no evidence for an interaction of NaCl and NP on arthropod abundance as predicted by the serial colimitation hypothesis. Finally, consistent with the ecosystem size hypothesis, predator and parasitoid abundances increased with plant volume, but not fertilization. Our results suggest these functional group-specific responses to changes in plant nutrients and structure are key to predicting the future of grassland food webs in an era with increasing use of N and P fertilizers, and increasing terrestrial inputs of Na from road salt, saline irrigation water, and aerosols due to rising sea levels.

Bottom-up when it is not top-down: Predators and plants control biomass of grassland arthropods

We investigate where bottom-up and top-down control regulates ecological communities as a mechanism linking ecological gradients to the geography of consumer abundance and biomass. We use standardized surveys of 54 North American grasslands to test alternate hypotheses predicting 100-fold shifts in the biomass of four common grassland arthropod taxa-Auchenorrhyncha, sucking herbivores, Acrididae, chewing herbivores, Tettigoniidae, omnivores, and Araneae, predators. Bottom-up models predict that consumer biomass tracks plant quantity (e.g. productivity and standing biomass) and quality (nutrient content) and that ectotherm access to food increases with temperature. Each of the focal trophic groups responded differently to these drivers: the biomass of sucking herbivores and omnivores increased with plant biomass; that of chewing herbivores tracked plant quality; and predator biomass did not depend on plant quality, plant quantity or temperature. The Exploitation Ecosystem Hypothesis is a top-down hypothesis that predicts a shift from resource limitation of herbivores when plant production is low, to predator limitation when plant production is high. In grasslands where spider biomass was low, herbivore biomass increased with plant biomass, whereas bottom-up structuring was not evident when spiders were abundant. Furthermore, neither predator biomass nor trophic position (via stable isotope analysis) increased with plant biomass, suggesting predators themselves are top-down limited. Stable isotope analysis revealed that trophic position of the chewing herbivore and omnivore increased significantly with plant biomass, suggesting these groups increased scavenging and meat consumption in grasslands with higher carbohydrate availability. Taken together, our snapshot sampling documents gradients of food web structure across 54 grasslands, consistent with multiple hypotheses of bottom-up and top-down regulation.

Tree species richness attenuates the positive relationship between mutualistic ant-hemipteran interactions and leaf chewer herbivory

Interactions across trophic levels influence plant diversity effects on ecosystem functions, but the complexity of these interactions remains poorly explored. For example, the interplay between different interactions (e.g. mutualism, predation) might be an important moderator of biodiversity-ecosystem function relationships. We tested for relationships between trophobioses (facultative ant-hemipteran mutualism) and leaf chewer herbivory in a subtropical forest biodiversity experiment. We analysed trophobiosis and herbivory data of more than 10 000 trees along a tree species richness gradient. Against expectations, chewing damage was higher on trees with trophobioses. However, the net positive relationship between trophobioses and overall herbivory depended on tree species richness, being most pronounced at low richness. Our results point to indirect, positive effects of ant-tended sap suckers on leaf chewers, potentially by altering plant defences. Direct antagonistic relationships of trophobiotic ants and leaf-chewing herbivores-frequently reported to drive community-wide effects of trophobioses in other ecosystems-seemed less relevant. However, antagonistic interactions likely contributed to the attenuating effect of tree species richness, because trophobiotic ant and herbivore communities changed from monocultures to species-rich mixtures. Our findings, therefore, suggest that biodiversity loss might lead to complex changes in higher trophic level effects on ecosystem functions, mediated by both trophic and non-trophic interactions.

Increased grassland arthropod production with mammalian herbivory and eutrophication: a test of mediation pathways

Increases in nutrient availability and alterations to mammalian herbivore communities are a hallmark of the Anthropocene, with consequences for the primary producer communities in many ecosystems. While progress has advanced understanding of plant community responses to these perturbations, the consequences for energy flow to higher trophic levels in the form of secondary production are less well understood. We quantified arthropod biomass after manipulating soil nutrient availability and wild mammalian herbivory, using identical methods across 13 temperate grasslands. Of experimental increases in nitrogen, phosphorus, and potassium, only treatments including nitrogen resulted in significantly increased arthropod biomass. Wild mammalian herbivore removal had a marginal, negative effect on arthropod biomass, with no interaction with nutrient availability. Path analysis including all sites implicated nutrient content of the primary producers as a driver of increased arthropod mean size, which we confirmed using 10 sites for which we had foliar nutrient data. Plant biomass and physical structure mediated the increase in arthropod abundance, while the nitrogen treatments accounted for additional variation not explained by our measured plant variables. The mean size of arthropod individuals was 2.5 times more influential on the plot-level total arthropod biomass than was the number of individuals. The eutrophication of grasslands through human activity, especially nitrogen deposition, thus may contribute to higher production of arthropod consumers through increases in nutrient availability across trophic levels.

Factors shaping community assemblages and species co-occurrence of different trophic levels

Species assemblages are the results of various processes, including dispersion and habitat filtering. Disentangling the effects of these different processes is challenging for statistical analysis, especially when biotic interactions should be considered. In this study, we used plants (producers) and leafhoppers (phytophagous) as model organisms, and we investigated the relative importance of abiotic versus biotic factors that shape community assemblages, and we infer on their biotic interactions by applying three-step statistical analysis. We applied a novel statistical analysis, that is, multiblock Redundancy Analysis (mbRA, step 1) and showed that 51.8% and 54.1% of the overall variation in plant and leafhopper assemblages are, respectively, explained by the two multiblock models. The most important blocks of variables to explain the variations in plant and leafhopper assemblages were local topography and biotic factors. Variation partitioning analysis (step 2) showed that pure abiotic filtering and pure biotic processes were relatively less important than their combinations, suggesting that biotic relationships are strongly structured by abiotic conditions. Pairwise co-occurrence analysis (step 3) on generalist leafhoppers and the most common plants identified 40 segregated species pairs (mainly between plant species) and 16 aggregated pairs (mainly between leafhopper species). Pairwise analysis on specialist leafhoppers and potential host plants clearly revealed aggregated patterns. Plant segregation suggests heterogeneous resource availability and competitive interactions, while leafhopper aggregation suggests host feeding differentiation at the local level, different feeding microhabitats on host plants, and similar environmental requirements of the species. Using the novel mbRA, we disentangle for the first time the relative importance of more than five distinct groups of variables shaping local species communities. We highlighted the important role of abiotic processes mediated by bottom-up effects of plants on leafhopper communities. Our results revealed that in-field structure diversification and trophic interactions are the main factors causing the co-occurrence patterns observed.

Belowground top-down and aboveground bottom-up effects structure multitrophic community relationships in a biodiverse forest

Ecosystem functioning and human well-being critically depend on numerous species interactions above- and belowground. However, unraveling the structure of multitrophic interaction webs at the ecosystem level is challenging for biodiverse ecosystems. Attempts to identify major relationships between trophic levels usually rely on simplified proxies, such as species diversity. Here, we propose to consider the full information on species composition across trophic levels, using Procrustes correlation and structural equation models. We show that species composition data of a highly diverse subtropical forest―with 5,716 taxa across 25 trophic groups― reveal strong interrelationships among plants, arthropods, and microorganisms, indicating complex multitrophic interactions. We found substantial support for top-down effects of microorganisms belowground, indicating important feedbacks of microbial symbionts, pathogens, and decomposers on plant communities. In contrast, aboveground pathways were characterized by bottom-up control of plants on arthropods, including many non-trophic links. Additional analyses based on diversity patterns revealed much weaker interrelationships. Our study suggests that multitrophic communities in our forest system are structured via top-down effects of belowground biota on plants, which in turn affect aboveground arthropod communities across trophic levels. Moreover, the study shows that the consequences of species loss will be more complex than indicated by studies based solely on diversity.

Predators regulate prey species sorting and spatial distribution in microbial landscapes

The role of predation in determining the metacommunity assembly model of prey communities is understudied relative to that of interspecific competition among prey. Previous work on metacommunity dynamics of competing species has shown that sorting by habitat patch type and spatial patterning can be affected by disturbances. Microcosms offer a useful model system to test the effect of multi-trophic interactions and disturbance on metacommunity dynamics. Here, we investigated the potential role of predators in enhancing or disrupting sorting and spatial pattern among prey in experimental landscapes. We exposed multi-trophic protist microcosm landscapes with one predator, two competing prey, two patch resource types, and localized dispersal to three disturbance regimes (none, low, and high). Then, we used variation partitioning and spatial clustering analysis to analyse the results. In contrast with previous experiments that did not manipulate predators, we found that patch type did not structure prey communities very well. Instead, we found that it was the distribution of the predator that most strongly predicted the composition of the prey community. The predator impacted species sorting by (1) preferentially consuming one prey, thereby acting as a strong local environmental driver, and by (2) indirectly magnifying the impact of patch food resources on the less preferred prey. The predator also enhanced spatial signal in the prey community because of its limited dispersal. Our results indicate that predators can strongly influence prey species sorting and spatial patterning in metacommunities in ways that would otherwise be attributed to stochastic effects, such as dispersal limitation or demographic drift. Therefore, whenever possible, predators should be explicitly included as separate explanatory factors in variation partitioning analyses.

Transgenic Bt Cotton Does Not Disrupt the Top-Down Forces Regulating the Cotton Aphid in Central China

Top-down force is referred to arthropod pest management delivered by the organisms from higher trophic levels. In the context of prevalent adoption of transgenic Bt crops that produce insecticidal Cry proteins derived from Bacillus thuringiensis (Bt), it still remains elusive whether the top-down forces are affected by the insect-resistant traits that introduced into the Bt crops. We explored how Bt cotton affect the strength of top-down forces via arthropod natural enemies in regulating a non-target pest species, the cotton aphid Aphis gossypii Glover, using a comparative approach (i.e. Bt cotton vs. conventional cotton) under field conditions. To determine top-down forces, we manipulated predation/parasitism exposure of the aphid to their natural enemies using exclusion cages. We found that the aphid population growth was strongly suppressed by the dominant natural enemies including Coccinellids, spiders and Aphidiines parasitoids. Coccinellids, spiders and the assemblage of other arthropod natural enemies (mainly lacewings and Hemipteran bugs) are similarly abundant in both plots, but with the parasitoid mummies less abundant in Bt cotton plots compared to the conventional cotton plots. However, the lower abundance of parasitoids in Bt cotton plots alone did not translate into differential top-down control on A. gossypii populations compared to conventional ones. Overall, the top-down forces were equally strong in both plots. We conclude that transgenic Bt cotton does not disrupt the top-down forces regulating the cotton aphid in central China.

Disentangling effects of abiotic factors and biotic interactions on cross-taxon congruence in species turnover patterns of plants, moths and beetles

High cross-taxon congruence in species diversity patterns is essential for the use of surrogate taxa in biodiversity conservation, but presence and strength of congruence in species turnover patterns, and the relative contributions of abiotic environmental factors and biotic interaction towards this congruence, remain poorly understood. In our study, we used variation partitioning in multiple regressions to quantify cross-taxon congruence in community dissimilarities of vascular plants, geometrid and arciinid moths and carabid beetles, subsequently investigating their respective underpinning by abiotic factors and biotic interactions. Significant cross-taxon congruence observed across all taxon pairs was linked to their similar responses towards elevation change. Changes in the vegetation composition were closely linked to carabid turnover, with vegetation structure and associated microclimatic conditions proposed causes of this link. In contrast, moth assemblages appeared to be dominated by generalist species whose turnover was weakly associated with vegetation changes. Overall, abiotic factors exerted a stronger influence on cross-taxon congruence across our study sites than biotic interactions. The weak congruence in turnover observed particularly between plants and moths highlights the importance of multi-taxon approaches based on groupings of taxa with similar turnovers, rather than the use of single surrogate taxa or environmental proxies, in biodiversity assessments.

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

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

Cascading effects of artificial light at night: resource-mediated control of herbivores in a grassland ecosystem

Artificial light at night has a wide range of biological effects on both plants and animals. Here, we review mechanisms by which artificial light at night may restructure ecological communities by modifying the interactions between species. Such mechanisms may be top-down (predator, parasite or grazer controlled), bottom-up (resource-controlled) or involve non-trophic processes, such as pollination, seed dispersal or competition. We present results from an experiment investigating both top-down and bottom-up effects of artificial light at night on the population density of pea aphids Acyrthosiphon pisum in a diverse artificial grassland community in the presence and absence of predators and under low-level light of different spectral composition. We found no evidence for top-down control of A. pisum in this system, but did find evidence for bottom-up effects mediated through the impact of light on flower head density in a leguminous food plant. These results suggest that physiological effects of light on a plant species within a diverse plant community can have detectable demographic effects on a specialist herbivore.

Spatially Heterogeneous Perturbations Homogenize the Regulation of Insect Herbivores

Anthropogenic influences on resources and consumers can affect food web regulation, with impacts on trophic structure and ecosystem processes. Identifying how these impacts unfold is challenging because alterations to one or both resources and consumers can similarly transform community structure, especially for intermediate consumers. To date, empirical testing of perturbations on trophic regulation has been limited by the difficulty in separating the direct effect of perturbations on species composition and diversity from those unfolding indirectly via altered feeding pathways. Moreover, disentangling the independent and interactive impacts of covarying stressors that characterize human-altered systems has been an ongoing analytical challenge. We used a large-scale metacommunity experiment in grasslands to test how resource inputs, stand perturbation, and spatial factors affect regulation of insect herbivores in tritrophic grassland food webs. Using path-model comparisons, we observed significant simplification of food web regulation on insect herbivores, shifting from mixed predator-resource regulation in unaltered mainland areas to strictly resource-based regulation with landscape perturbation and fragmentation. Most changes were attributed to homogenization of plant community caused by landscape fragmentation and the deterministic influence of eutrophication that reduced among-patch beta diversity. This led to a simplified food web dominated by fewer but more abundant herbivore taxa. Our work implies that anthropogenic perturbation relating to resources and spatial isolation can transform the regulation of food web diversity, structure, and function.

Plant diversity impacts decomposition and herbivory via changes in aboveground arthropods

Loss of plant diversity influences essential ecosystem processes as aboveground productivity, and can have cascading effects on the arthropod communities in adjacent trophic levels. However, few studies have examined how those changes in arthropod communities can have additional impacts on ecosystem processes caused by them (e.g. pollination, bioturbation, predation, decomposition, herbivory). Therefore, including arthropod effects in predictions of the impact of plant diversity loss on such ecosystem processes is an important but little studied piece of information. In a grassland biodiversity experiment, we addressed this gap by assessing aboveground decomposer and herbivore communities and linking their abundance and diversity to rates of decomposition and herbivory. Path analyses showed that increasing plant diversity led to higher abundance and diversity of decomposing arthropods through higher plant biomass. Higher species richness of decomposers, in turn, enhanced decomposition. Similarly, species-rich plant communities hosted a higher abundance and diversity of herbivores through elevated plant biomass and C:N ratio, leading to higher herbivory rates. Integrating trophic interactions into the study of biodiversity effects is required to understand the multiple pathways by which biodiversity affects ecosystem functioning.