The Interaction between Competition and Predation: A Meta-analysis of Field Experiments

Habitat engineering by an apex predator generates spatial trophic dynamics across a temporal environmental stress gradient

Ecosystem engineering is a facilitative interaction that generates bottom-up extrinsic variability that may increase species coexistence, particularly along a stress/disturbance gradient. American alligators (Alligator mississippiensis) create and maintain 'alligator ponds' that serve as dry-season refuges for other animals. During seasonal water recession, these ponds present an opportunity to examine predictions of the stress-gradient (SGH) and intermediate disturbance hypotheses (IDH). To test the assumption that engineering would facilitate species coexistence in ponds along a stress gradient (seasonal drying), we modelled fish catch-per-unit-effort (CPUE) in ponds and marshes using a long-term dataset (1997-2022). Stomach contents (n = 1677 from 46 species) and stable isotopes of carbon and nitrogen (n = 3978 representing 91 taxa) from 2018 to 2019 were used to evaluate effects of engineering on trophic dynamics. We quantified diets, trophic niche areas, trophic positions and basal-resource use among habitats and between seasons. As environmental stress increases, we used seasonal changes in trophic niche areas as a proxy for competition to examine SGH and IDH. Across long-term data, fish CPUE increased by a factor of 12 in alligator ponds as the marsh dried. This validates the assumption that ponds are an important dry-season refuge. We found that 73% of diet shifts occurred during the dry season but that diets differed among habitats in only 11% of comparisons. From wet season to dry season, both stomach contents and stable isotopes revealed changes in niche areas. Direction of change depended on trophic guild but was opposite between stable-isotope and stomach-content niches, except for detritivores. Stomach-content niches generally increased suggesting decreased competition in the dry season consistent with existing theory, but stable-isotope niches yielded the opposite. This may result from a temporal mismatch with stomach contents reflecting diets over hours, while stable isotopes integrate diet over weeks. Consumptive effects may have a stronger effect than competition on niche areas over longer time intervals. Overall, our results demonstrated that alligators ameliorated dry-season stress by engineering deep-water habitats and altering food-web dynamics. We propose that ecosystem engineers facilitate coexistence at intermediate values of stress/disturbance consistent with predictions of both the SGH and IDH.

Sky High or Grounded: Nest Site Selection of Herons and Egrets in a Mixed-Species Colony

Resource partitioning is crucial for the coexistence of colonial herons, as it allows multiple species to share the same habitat while minimising competition. This study took advantage of a natural experiment in 2006 and 2007 when Black-crowned Night Herons were prevented from breeding at Lake Fetzara in the first year due to the presence of a feral cat. This event provided valuable insight into the spatial and temporal dynamics of nest site selection among coexisting heron species, which consisted of Cattle Egrets (Ardea ibis), Little Egrets (Egretta garzetta) and Squacco Herons (Ardeola ralloides). After the cat was removed, egg-laying began in the core areas of the colony and gradually spread to the periphery. Species that initiated nesting early selected mid-elevation sites near the tree trunk, which likely offered protection from both ground and aerial predators, while also providing some shielding from solar radiation and strong winds. These early selected sites featured larger branches, which conferred greater nest stability. Vertical stratification was evident among the heron species; however, contrary to long-standing assumptions, it was not directly related to body size. Both vertical and horizontal stratification were observed, with nests progressively moving higher and further from the tree trunk as the breeding season advanced. The following year, Black-crowned Night Herons displaced other species to lower heights and positions further from the trunk, highlighting the significant influence of interspecific interactions on nest site selection. This study underscores the complex interplay between nest site selection, biotic interactions and abiotic factors in heron colonies, emphasising the importance of resource partitioning in maintaining species coexistence in densely populated breeding sites.

Incorporating stressor interactions into spatially explicit cumulative impact assessments

Human-induced stressors are impacting the oceans and reducing the biodiversity of marine ecosystems. The many stressors affecting marine environments do not act in isolation. However, their cumulative impact is difficult to predict. Most of the available methods for quantifying cumulative impacts on marine ecosystems sum the impact of individual stressors to estimate cumulative impact. We demonstrate how experimental evidence from interacting stressors can be accounted for in cumulative impact assessments. We adapted a widely used additive model to incorporate nonadditive stressor interactions into a marine spatially explicit cumulative impact assessment for seagrasses. We combined experimental data on the impact of multiple stressors with spatial data on stressor intensity to test whether stressor interactions impact seagrasses in a case study region in South Australia. We also assessed how uncertainty about cumulative impacts changes when uncertainty in stressor interactions is included in the impact mapping. The results from an additive spatial cumulative impact assessment model were compared with results from the model incorporating interactions. Cumulative effects from the interaction model were more variable than those produced by the additive model. Five of the 15 stressor interactions that we tested produced impacts that significantly deviated from those predicted by an additive model. Areas of our study region that showed the largest discrepancies between the additive and interactive outputs were also associated with higher uncertainty. Our study demonstrates that the inclusion of stressor interactions changes the pattern and intensity of modeled spatial cumulative impact. Additive models have the potential to misrepresent cumulative impact intensity and do not provide the opportunity for targeted mitigation measures when managing the interactive effects of stressors. Appropriate inclusion of interacting stressor data may have implications for the identification of key stressors and the subsequent spatial planning and management of marine ecosystems and biodiversity.

Additive Effects of Multiple Global Change Factors on Plant Invasions Are Common

Quantifying how co-acting global change factors (GCFs) influence plant invasion is crucial for predicting future invasion dynamics. We did a meta-analysis to assess pairwise effects of five GCFs (elevated CO2, drought, eutrophication, increased rainfall and warming) on native and alien plants. We found that alien plants, compared to native plants, suffered less or benefited more for four of the eight pairwise GCF combinations, and that all GCFs acted additively. Subgroup analysis showed that the relative benefits of alien over native plants were particularly apparent when they grew in competition with one another, and that the results were largely the same when the aliens were restricted to naturalised or invasive species. Our meta-analysis provides evidence that additive effects of multiple global change factors on plant invasions are common, and thus that with the ongoing global environmental changes, the risk of plant invasion continues to increase.

Intra-specific diversity and adaptation modify regime shifts dynamics under environmental change

Environmental changes are a growing concern, as they exert pressures on ecosystems. In some cases, such changes lead to shifts in ecosystem structure. However, species can adapt to changes through evolution, and it is unclear how evolution interacts with regime shifts, which restricts ecosystem management strategies. Here, we used a model of prey population with evolution and intra-specific trait diversity, and simulated regime shifts through changes in predation pressure. We then explored interactions between evolution, diversity, and shifts in population density. Evolution induced delayed or early regime shifts, and altered the recovery of populations. Such changes depended on the relative speed of evolution and change of predation pressure, as well as on the initial state of the population. Evolution also influenced population resilience, which was important when considering strong environmental variability. For instance, storms can spontaneously increase mortality and induce shifts. Furthermore, environmental variability induced even higher mortality if the phenotypic diversity of populations is large. Some phenotypes were more vulnerable to environmental changes, and such increases in mortality favor shifts to decreases in density. Thus, population management needs to consider diversity, evolution, and environmental change altogether to better anticipate regime shifts on eco-evolutionary time scales. Here, evolution and diversity showed complex interactions with population shift dynamics. Investigating the influence of higher diversity levels, such as diversity at a community level, should be another step towards anticipating changes in ecosystems and communities.

The impact of global change factors on the functional genes of soil nitrogen and methane cycles in grassland ecosystems: a meta-analysis

Soil functional genes in grasslands are crucial for processes like nitrogen fixation, nitrification, denitrification, methane production, and oxidation, integral to nitrogen and methane cycles. However, the impact of global changes on these genes is not well understood. We reviewed 84 studies to examine the effects of nitrogen addition (N), warming (W), increased precipitation (PPT +), decreased precipitation (PPT-), and elevated CO2 (eCO2) on these functional genes. For nitrogen cycling, global changes mainly boost genes involved in nitrification but reduce those in denitrification, with nirK being the most sensitive. Most nitrogen fixation-related genes did not show a significant response. Among single factors, N and PPT + have the most significant effects. The impact of global changes on nitrogen cycling genes is largely additive, and their interaction with N is particularly influential. For methane cycling, global changes notably affect mcrA, while only PPT + significantly reduces pmoA. The magnitude and duration of global change treatments are more critical than the treatment form for nitrogen cycling genes. For methane cycling, the form and intensity of nitrogen addition, along with treatment duration, affect pmoA abundance. We also identified a competitive relationship between methane oxidation and nitrification and a complex coupling with denitrification. This study provides new insights into microbial responses in nitrogen and methane cycling under global changes, with significant implications for experimental design and management strategies in grassland ecosystems.

Trophic niche segregation in a guild of top predators within the Mediterranean Basin

Niche theory predicts that closely related and ecologically similar species with overlapping distribution ranges can coexist through resource partitioning that limits interspecific competition. However, studies examining the mechanisms promoting coexistence of top predators at a large geographical scale are still scant. Here, we describe the foraging ecology of 3 sympatric owl species (Northern long-eared owl [Asio otus], Tawny owl [Strix aluco], Eurasian eagle owl [Bubo bubo]) in the Mediterranean Basin. We review 160 studies reporting diet information (212,236 vertebrate preys) and investigate among-species differences in diet metrics (diversity, evenness, prey size, and proportion of mammals) and their variation along geographical and environmental gradients. Moreover, we test whether diet metrics differ in presence or absence of the other predators. All the 3 species mainly rely on small mammals, but they significantly differ in diet metrics. The smallest predator (i.e., long-eared owl) shows a higher level of specialism on small mammals (highest proportion but lowest diversity of mammals in the diet) compared to the larger ones. In addition, mean prey size significantly increases with predator body size (long-eared owl < tawny owl < eagle owl). Finally, interspecific competition results in an increase of diet diversity and evenness in the long-eared owl, and species' diet also varies in response to environmental factors. The 3 species thus segregate along several dietary niche axes over a large spatial scale and according to both morphological characteristics (i.e., body size) and environmental variables. Such dietary niche segregation may adaptively buffer interspecific competition costs, ultimately allowing coexistence.

Interactive Effects of Climate Change and Pathogens on Plant Performance: A Global Meta-Analysis

Plant health is increasingly threatened by abiotic and biotic stressors linked to anthropogenic global change. These stressors are frequently studied in isolation. However, they might have non-additive (antagonistic or synergistic) interactive effects that affect plant communities in unexpected ways. We conducted a global meta-analysis to summarize existing evidence on the joint effects of climate change (drought and warming) and biotic attack (pathogens) on plant performance. We also investigated the effect of drought and warming on pathogen performance, as this information is crucial for a mechanistic interpretation of potential indirect effects of climate change on plant performance mediated by pathogens. The final databases included 1230 pairwise cases extracted from 117 recently published scientific articles (from 2006) on a global scale. We found that the combined negative effects of drought and pathogens on plant growth were lower than expected based on their main effects, supporting the existence of antagonistic interactions. Thus, the larger the magnitude of the drought, the lower the pathogen capacity to limit plant growth. On the other hand, the combination of warming and pathogens caused larger plant damage than expected, supporting the existence of synergistic interactions. Our results on the effects of drought and warming on pathogens revealed a limitation of their growth rates and abundance in vitro but an improvement under natural conditions, where multiple factors operate across the microbiome. Further research on the impact of climate change on traits explicitly defining the infective ability of pathogens would enhance the assessment of its indirect effects on plants. The evaluated plant and pathogen responses were conditioned by the intensity of drought or warming and by moderator categorical variables defining the pathosystems. Overall, our findings reveal the need to incorporate the joint effect of climatic and biotic components of global change into predictive models of plant performance to identify non-additive interactions.

Interactions between ocean acidification and multiple environmental drivers on the biochemical traits of marine primary producers: A meta-analysis

Ocean acidification (OA) interacts with multiple environmental drivers, such as temperature, nutrients, and ultraviolet radiation (UVR), posing a threat to marine primary producers. In this study, we conducted a quantitative meta-analysis of 1001 experimental assessments from 68 studies to examine the combined effects of OA and multiple environmental drivers (e.g., light, nutrient) on the biochemical compositions of marine primary producers. The results revealed significant positive effects of each environmental driver and their interactions with OA according to Hedge's d analysis. The results revealed significant positive effects of multiple environmental drivers and their interactions with OA. Additive effects dominated (71%), with smaller proportions of antagonistic (20%) and synergistic interactions (9%). The antagonistic interactions, although fewer, had a substantial impact, causing OA and other environmental drivers to interact antagonistically. Significant differences were observed among taxonomic groups: haptophytes and rhodophytes were negatively affected, while bacillariophytes were positively affected by OA. Our findings also indicated that the interactions between OA and multiple environmental drivers varied depending on specific type of the environmental driver, suggesting a modulating role of OA on the biochemical compositions of marine primary producers in response to global change. In summary, our study elucidates the complex interactions between OA and multiple environmental drivers on marine primary producers, highlighting the varied impacts on biochemical compositions and elemental stoichiometry.

The Effect of Temperature Variability on Biological Responses of Ectothermic Animals-A Meta-Analysis

Climate change is altering temperature means and variation, and both need to be considered in predictions underpinning conservation. However, there is no consensus in the literature regarding the effects of temperature fluctuations on biological functions. Fluctuations may affect biological responses because of inequalities from non-linear responses, endocrine regulation or exposure to damaging temperatures. Here we establish the current state of knowledge of how temperature fluctuations impact biological responses within individuals and populations compared to constant temperatures with the same mean. We conducted a meta-analysis of 143 studies on ectothermic animals (1492 effect sizes, 118 species). In this study, 89% of effect sizes were derived from diel cycles, but there were no significant differences between diel cycles and shorter (<8 h) or longer (>48 h) cycles in their effect on biological responses. We show that temperature fluctuations have little effect overall on trait mean and variance. Nonetheless, temperature fluctuations can be stressful: fluctuations increased 'gene expression' in aquatic animals, which was driven mainly by increased hsp70. Fluctuating temperatures also decreased longevity, and increased amplitudes had negative effects on population responses in aquatic organisms. We conclude that mean temperatures and extreme events such as heat waves are important to consider, but regular (particularly diel) temperature fluctuations are less so.

Grazing herbivores reduce herbaceous biomass and fire activity across African savannas

Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one-zebra increase in herbivore biomass density (~100 kg/km2 of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass.

Micro- and nanoplastics interact with conventional pollutants on microalgae: Synthesis through meta-analysis

Micro- and nanoplastics (MNPs) have been found to occur intensively in aquatic environments, along with other conventional pollutants (Po) such as heavy metals, pesticides, pharmaceuticals, etc. However, our understanding of how MNPs and Po interact on aquatic primary producers is fragmented. We performed a quantitative meta-analysis based on 933 published experimental assessments from 44 studies to examine the coupled effects of MNPs and Po on microalgae. Although the results based on interaction type frequency (the proportion of each interaction type in all results) revealed dominantly additive interactions (56%) for overall physiological performance, an overall antagonistic effect was observed based on the mean interaction effect sizes. A higher proportion of antagonistic interaction type frequency was found in marine species compared to fresh species. The antagonistic effects were particularly significant for growth, oxidative responses, and photosynthesis, which could be attributed to the adsorption effect of MNPs on Po and thus the decreasing concentrations of pollutants in the medium. Larger-sized, negatively charged or uncharged and aged MNPs had higher proportions of antagonistic effects compared to smaller-sized, positively charged and virgin MNPs, due to their stronger adsorption capacity to Po. This study provides a comprehensive insight into the interactive effects of MNPs and Po on microalgae.

Does periphyton turn less palatable under grazing pressure?

Periphyton acts as an important primary producer in stream food webs with bottom-up grazing pressure and is also subject to effects of top-down grazing pressure. However, the underlying mechanisms of these interactions remain unclear. In this study we conducted a mesocosm experiment to explore the periphyton response to grazing pressure by the freshwater snail Bellamya aeruginosa in relation to food quality indicated by polyunsaturated fatty acid (PUFA) biomarkers, including eicosapentaenoic acid (20:5n3) and the 22C fatty acid docosahexaenoic acid (22:6n3), which are essential for cell growth and reproduction and cannot be synthesized by most consumers of periphyton. Results indicated that periphyton grazing pressure led to a decrease in Bacillariophyta, which contain high-quality PUFAs such as eicsapentaenoic acid and docosahexaenoic acid, and an increase in Cyanophyta and Chlorophyta, which are rich in 18C PUFAs such as linoleic acid (18:2n6) and alpha-linolenic acid (18:3n3). We observed upregulation of genes that participate in lipid metabolism promoting unsaturated fatty acid biosynthesis, alpha-linolenic acid metabolism, and glycerophospholipid metabolism, which are related to the carbohydrate and energy metabolism maintaining the energy stability of periphyton. These results demonstrate that the food quality of periphyton decreased under grazing pressure and also elucidate the compositional, chemical, and molecular perspectives of the interactive bottom-up and top-down effects on structuring stream food webs.

Size- and surface charge-dependent hormetic effects of microplastics on bacterial resistance and their interactive effects with quinolone antibiotic

The facilitation of microplastics (MPs) on bacterial resistance has attracted wide concern, due to the widespread presence of MPs in environmental media and their ubiquitous contact with bacteria strains. Furthermore, MPs possibly co-exist with antibiotics to trigger combined stress on bacterial survival. Therefore, it is significant to reveal the dose-responses of MPs and MP-antibiotic mixtures on bacterial endogenous and exogenous resistance. In this study, 0.1 and 5 μm polystyrenes with no surface functionalization (PS-NF, no charge), surface functionalized with amino groups (PS-NH2, positive charge) and carboxyl groups (PS-COOH, negative charge) were selected as the test MPs, and norfloxacin (NOR) was set as the representative of antibiotics. It was found that six types of PS all inhibited the growth of Escherichia coli (E. coli) but induced hormetic dose-responses on the mutation frequency (MF) and conjugative transfer frequency (CTF) of RP4 plasmid in E. coli. Moreover, these hormetic effects exhibited size- and surface charge-dependent features, where 0.1 μm PS-NH2 (100 mg/L) triggered the maximum stimulatory rates on MF (363.63 %) and CTF (74.80 %). The hormetic phenomena of MF and CTF were also observed in the treatments of PS-NOR mixtures, which varied with the particle size and surface charge of PS. In addition, the interactive effects between PS and NOR indicated that the co-existence of PS and NOR might trigger greater resistance risk than the single pollutants. Mechanistic exploration demonstrated that the increase of cellular reactive oxygen species and the variation of cell membrane permeability participated in the hormetic effects of PS and PS-NOR mixtures on bacterial resistance. This study provides new insights into the individual effects of MPs and the combined effects of MP-antibiotic mixtures on bacterial resistance, which will promote the development of environmental risk assessment of MPs from the perspective of bacterial resistance.

Using meta-analysis to explore the roles of global upwelling exposure and experimental design in bivalve responses to low pH

Low pH conditions, associated with ocean acidification, represent threats to many commercially and ecologically important organisms, including bivalves. However, there are knowledge gaps regarding factors explaining observed differences in biological responses to low pH in laboratory experiments. Specific sources of local adaptation such as upwelling exposure and the role of experimental design, such as carbonate chemistry parameter changes, should be considered. Linking upwelling exposure, as an individual oceanographic phenomenon, to responses measured in laboratory experiments may further our understanding of local adaptation to global change. Here, meta-analysis is used to test the hypotheses that upwelling exposure and experimental design affect outcomes of individual, laboratory-based studies that assess bivalve metabolic (clearance and respiration rate) responses to low pH. Results show that while bivalves generally decrease metabolic activity in response to low pH, upwelling exposure and experimental design can significantly impact outcomes. Bivalves from downwelling or weak upwelling areas decrease metabolic activity in response to low pH, but bivalves from strong upwelling areas increase or do not change metabolic activity in response to low pH. Furthermore, experimental temperature, exposure time and magnitude of the change in carbonate chemistry parameters all significantly affect outcomes. These results suggest that bivalves from strong upwelling areas may be less sensitive to low pH. This furthers our understanding of local adaptation to global change by demonstrating that upwelling alone can explain up to 49 % of the variability associated with bivalve metabolic responses to low pH. Furthermore, when interpreting outcomes of individual, laboratory experiments, scientists should be aware that higher temperatures, shorter exposure times and larger changes in carbonate chemistry parameters may increase the chance of suppressed metabolic activity.

Predation and competition drive trait diversity across space and time

Competition should play a key role in shaping community assembly and thereby local and regional biodiversity patterns. However, identifying its relative importance and effects in natural communities is challenging because theory suggests that competition can lead to different and even opposing patterns depending on the underlying mechanisms. Here, we have taken a different approach: rather than attempting to indirectly infer competition from diversity patterns, we compared trait diversity patterns in odonate (dragonfly and damselfly) communities across different spatial and temporal scales along a natural competition-predation gradient. At the local scale (within a community), we found that trait diversity increased with the size of top predators (from invertebrates to fish). This relationship is consistent with differences in taxonomic diversity, suggesting that competition reduces local trait diversity through competitive exclusion. Spatial (across communities) and temporal (within communities over time) trait variation peaked in communities with intermediate predators indicating that both high levels of competition or predation select for trait convergence of communities. This indicates that competition acts as a deterministic force that reduces trait diversity at the local, regional, and temporal scales, which contrasts with patterns at the taxonomic level. Overall, results from this natural experiment reveal how competition and predation interact to shape biodiversity patterns in natural communities across spatial and temporal scales and provide new insights into the underlying mechanisms.

Meta-analysis of variation in sport and exercise science: Examples of application within resistance training research

Meta-analysis has become commonplace within sport and exercise science for synthesising and summarising empirical studies. However, most research in the field focuses upon mean effects, particularly the effects of interventions to improve outcomes such as fitness or performance. It is thought that individual responses to interventions vary considerably. Hence, interest has increased in exploring precision or personalised exercise approaches. Not only is the mean often affected by interventions, but variation may also be impacted. Exploration of variation in studies such as randomised controlled trials (RCTs) can yield insight into interindividual heterogeneity in response to interventions and help determine generalisability of effects. Yet, larger samples sizes than those used for typical mean effects are required when probing variation. Thus, in a field with small samples such as sport and exercise science, exploration of variation through a meta-analytic framework is appealing. Despite the value of embracing and exploring variation alongside mean effects in sport and exercise science, it is rarely applied to research synthesis through meta-analysis. We introduce and evaluate different effect size calculations along with models for meta-analysis of variation using relatable examples from resistance training RCTs.

Quantitative evidence synthesis: a practical guide on meta-analysis, meta-regression, and publication bias tests for environmental sciences

Meta-analysis is a quantitative way of synthesizing results from multiple studies to obtain reliable evidence of an intervention or phenomenon. Indeed, an increasing number of meta-analyses are conducted in environmental sciences, and resulting meta-analytic evidence is often used in environmental policies and decision-making. We conducted a survey of recent meta-analyses in environmental sciences and found poor standards of current meta-analytic practice and reporting. For example, only ~ 40% of the 73 reviewed meta-analyses reported heterogeneity (variation among effect sizes beyond sampling error), and publication bias was assessed in fewer than half. Furthermore, although almost all the meta-analyses had multiple effect sizes originating from the same studies, non-independence among effect sizes was considered in only half of the meta-analyses. To improve the implementation of meta-analysis in environmental sciences, we here outline practical guidance for conducting a meta-analysis in environmental sciences. We describe the key concepts of effect size and meta-analysis and detail procedures for fitting multilevel meta-analysis and meta-regression models and performing associated publication bias tests. We demonstrate a clear need for environmental scientists to embrace multilevel meta-analytic models, which explicitly model dependence among effect sizes, rather than the commonly used random-effects models. Further, we discuss how reporting and visual presentations of meta-analytic results can be much improved by following reporting guidelines such as PRISMA-EcoEvo (Preferred Reporting Items for Systematic Reviews and Meta-Analyses for Ecology and Evolutionary Biology). This paper, along with the accompanying online tutorial, serves as a practical guide on conducting a complete set of meta-analytic procedures (i.e., meta-analysis, heterogeneity quantification, meta-regression, publication bias tests and sensitivity analysis) and also as a gateway to more advanced, yet appropriate, methods.

Nitrification, denitrification, and related functional genes under elevated CO2 : A meta-analysis in terrestrial ecosystems

Global change may have profound effects on soil nitrogen (N) cycling that can induce positive feedback to climate change through increased nitrous oxide (N₂ O) emissions mediated by nitrification and denitrification. We conducted a meta-analysis of the effects of elevated CO₂ on nitrification and denitrification based on 879 observations from 58 publications and 46 independent elevated CO₂ experiments in terrestrial ecosystems. We investigated the effects of elevated CO₂ alone or combined with elevated temperature, increased precipitation, drought, and N addition. We assessed the response to elevated CO₂ of gross and potential nitrification, potential denitrification, and abundances of related functional genes (archaeal amoA, bacterial amoA, nirK, nirS, and nosZ). Elevated CO₂ increased potential nitrification (+28%) and the abundance of bacterial amoA functional gene (+62%) in cropland ecosystems. Elevated CO₂ increased potential denitrification when combined with N addition and higher precipitation (+116%). Elevated CO₂ also increased the abundance of nirK (+25%) and nirS (+27%) functional genes in terrestrial ecosystems and of nosZ (+32%) functional gene in cropland ecosystems. The increase in the abundance of nosZ under elevated CO₂ was larger at elevated temperature and high N (+62%). Four out of 14 two-way interactions tested between elevated CO₂ and elevated temperature, elevated CO₂ and increased precipitation, and elevated CO₂ and N addition were marginally significant and mostly synergistic. The effects of elevated CO₂ on potential nitrification and abundances of bacterial amoA and nirS functional genes increased with mean annual temperature and mean annual precipitation. Our meta-analysis thus suggests that warming and increased precipitation in large areas of the world could reinforce positive responses of nitrification and denitrification to elevated CO₂ and urges the need for more investigations in the tropical zone and on interactive effects among multiple global change factors, as we may largely underestimate the effects of global change on soil N₂ O emissions.

Warmer water, high light intensity, lithium and microplastics: Dangerous environmental combinations to zooplankton and Global Health?

Nowadays there is a high concern about the combined effects of global warming and emerging environmental contaminants with significant increasing trends of use, such as lithium (Li) and microplastics (MPs), both on wildlife and human health. Therefore, the effects of high light intensity (26,000 lx) or warmer water temperature (25 °C) on the long-term toxicity of Li and mixtures of Li and MPs (Li-MPs mixtures) were investigated using model populations of the freshwater zooplankton species Daphnia magna. Three 21-day bioassays were done in the laboratory at the following water temperatures and light intensities: (i) 20 °C/10830 lx; (ii) 20 °C/26000 lx (high light intensity); (iii) 25 °C/10830 lx (warmer temperature). Based on the 21-day EC50s on reproduction, high light intensity increased the reproductive toxicity of Li and Li-MPs mixtures by ~1.3 fold; warmer temperature increased the toxicity of Li by ~1.2 fold, and the toxicity of Li-MPs mixtures by ~1.4 fold based on the concentration of Li, and by ~2 fold based on the concentrations of MPs. At high light intensity, Li (0.04 mg/L) and Li-MPs mixtures (0.04 Li + 0.09 MPs mg/L) reduced the population fitness by 32 % and 41 %, respectively. Warmer temperature, Li (0.05 mg/L) and Li-MPs mixtures (0.05 Li + 0.09 MPs mg/L) reduced it by 63 % and 71 %, respectively. At warmer temperature or high light intensity, higher concentrations of Li and Li-MPs mixtures lead to population extinction. Based on the population growth rate and using data of bioassays with MPs alone done simultaneously, Li and MPs interactions were antagonistic or synergistic depending on the scenario. High light intensity and chemical stress generally acted synergistically. Warmer temperature and chemical stress always acted synergistically. These findings highlight the threats of long-term exposure to Li and Li-MPs mixtures to freshwater zooplankton and Global Health in a warmer world.

Season and prey identity mediate the effect of predators on parasites in rodents: a test of the healthy herds hypothesis

The healthy herds hypothesis (HHH) suggests that predators decrease parasitism in their prey. Repeated tests of this hypothesis across a range of taxa and ecosystems have revealed significant variation in the effect of predators on parasites in prey. Differences in the response to predators (1) between prey taxa, (2) between seasons, and (3) before and after catastrophic disturbance are common in natural systems, but typically ignored in empirical tests of the HHH. We used a predator exclusion experiment to measure the effect of these heterogeneities on the tri-trophic interaction among predators, parasites and prey. We experimentally excluded mammalian predators from the habitats of hispid cotton rats (Sigmodon hispidus) and cotton mice (Peromyscus gossypinus) and measured the effect of exclusion on gastrointestinal parasites in these rodents. Our experiment spanned multiple seasons and before and after a prescribed burn. We found that the exclusion of the same predators had opposite effects on the parasites of small mammal prey species. Additionally, we found that the effect of mammal exclusion on parasitism differed before versus after fire disturbance. Finally, we saw that the effect of predator exclusion was highly dependent on prey capture season. Significant effects of exclusion emerged primarily in the fall and winter months. The presence of so many different effects in one relatively simple system suggests that predator effects on parasites in prey are highly context dependent.

Resetting our expectations for parasites and their effects on species interactions: a meta-analysis

Despite the ubiquitous nature of parasitism, how parasitism alters the outcome of host-species interactions such as competition, mutualism and predation remains unknown. Using a phylogenetically informed meta-analysis of 154 studies, we examined how the mean and variance in the outcomes of species interactions differed between parasitized and non-parasitized hosts. Overall, parasitism did not significantly affect the mean or variance of host-species interaction outcomes, nor did the shared evolutionary histories of hosts and parasites have an effect. Instead, there was considerable variation in outcomes, ranging from strongly detrimental to strongly beneficial for infected hosts. Trophically-transmitted parasites increased the negative effects of predation, parasites increased and decreased the negative effects of interspecific competition for parasitized and non-parasitized heterospecifics, respectively, and parasites had particularly strong negative effects on host species interactions in freshwater and marine habitats, yet were beneficial in terrestrial environments. Our results illuminate the diverse ways in which parasites modify critical linkages in ecological networks, implying that whether the cumulative effects of parasitism are considered detrimental depends not only on the interactions between hosts and their parasites but also on the many other interactions that hosts experience.

Non-Additive Effects of Environmental Factors on Growth and Physiology of Invasive Solidago canadensis and a Co-Occurring Native Species (Artemisia argyi)

Changes in environmental factors, such as temperature and UV, have significant impacts on the growth and development of both native and invasive plant species. However, few studies examine the combined effects of warming and enhanced UV on plant growth and performance in invasive species. Here, we investigated single and combined effects of warming and UV radiation on growth, leaf functional and photosynthesis traits, and nutrient content (i.e., total organic carbon, nitrogen and phosphorous) of invasive Solidago canadensis and its co-occurring native species, Artemisia argyi, when grown in culture racks in the greenhouse. The species were grown in monoculture and together in a mixed community, with and without warming, and with and without increased UV in a full factorial design. We found that growth in S. canadensis and A. argyi were inhibited and more affected by warming than UV-B radiation. Additionally, there were both antagonistic and synergistic interactions between warming and UV-B on growth and performance in both species. Overall, our results suggested that S. canadensis was more tolerant to elevated temperatures and high UV radiation compared to the native species. Therefore, substantial increases in temperature and UV-B may favour invasive S. canadensis over native A. argyi. Research focusing on the effects of a wider range of temperatures and UV levels is required to improve our understanding of the responses of these two species to greater environmental variability and the impacts of climate change.

Multiple drivers and lineage-specific insect extinctions during the Permo-Triassic

The Permo-Triassic interval encompasses three extinction events including the most dramatic biological crisis of the Phanerozoic, the latest Permian mass extinction. However, their drivers and outcomes are poorly quantified and understood for terrestrial invertebrates, which we assess here for insects. We find a pattern with three extinctions: the Roadian/Wordian (≈266.9 Ma; extinction of 64.5% insect genera), the Permian/Triassic (≈252 Ma; extinction of 82.6% insect genera), and the Ladinian/Carnian boundaries (≈237 Ma; extinction of 74.8% insect genera). We also unveil a heterogeneous effect of these extinction events across the major insect clades. Because extinction events have impacted Permo-Triassic ecosystems, we investigate the influence of abiotic and biotic factors on insect diversification dynamics and find that changes in floral assemblages are likely the strongest drivers of insects' responses throughout the Permo-Triassic. We also assess the effect of diversity dependence between three insect guilds; an effect ubiquitously found in current ecosystems. We find that herbivores held a central position in the Permo-Triassic interaction network. Our study reveals high levels of insect extinction that profoundly shaped the evolutionary history of the most diverse non-microbial lineage.

Influence of multiple global change drivers on plant invasion: Additive effects are uncommon

Invasive plants threaten biodiversity and cause huge economic losses. It is thought that global change factors (GCFs) associated with climate change (including shifts in temperature, precipitation, nitrogen, and atmospheric CO₂) will amplify their impacts. However, only few studies assessed mixed factors on plant invasion. We collated the literature on plant responses to GCFs to explore independent, combined, and interactive effects on performance and competitiveness of native and invasive plants. From 176 plant species, our results showed that: (1) when native and invasive plants are affected by both independent and multiple GCFs, there is an overall positive effect on plant performance, but a negative effect on plant competitiveness; (2) under increased precipitation or in combination with temperature, most invasive plants gain advantages over natives; and (3) interactions between GCFs on plant performance and competitiveness were mostly synergistic or antagonistic. Our results indicate that native and invasive plants may be affected by independent or combined GCFs, and invasive plants likely gain advantages over native plants. The interactive effects of factors on plants were non-additive, but the advantages of invasive plants may not increase indefinitely. Our findings show that inferring the impacts of climate change on plant invasion from factors individually could be misleading. More mixed factor studies are needed to predict plant invasions under global change.

Predator dispersal influences predator distribution but not prey diversity in pitcher plant microbial metacommunities

The spatial distribution of predators can affect both the distribution and diversity of their prey. Therefore, differences in predator dispersal ability that affect their spatial distribution, could also affect prey communities. Here, we use the microbial communities within pitcher plant leaves as a model system to test the relationship between predator (protozoa) dispersal ability and distribution, and its consequences for prey (bacteria) diversity and composition. We hypothesized that limited predator dispersal results in clustered distributions and heterogeneous patches for prey species, whereas wide predator dispersal and distribution could homogenize prey metacommunities. We analyzed the distribution of two prominent bacterivore protozoans from a 2-year survey of an intact field of Sarracenia purpurea pitcher plants, and found a clustered distribution of Tetrahymena and homogeneous distribution of Poterioochromonas. We manipulated the sources of protozoan colonists and recorded protozoan recruitment and bacterial diversity in target leaves in a field experiment. We found the large ciliate, Tetrahymena, was dispersal limited and occupied few leaves, whereas the small flagellate Poterioochromonas was widely dispersed. However, the bacterial communities these protozoans feed on was unaffected by clustering of Tetrahymena, but likely influenced by Poterioochromonas and other bacterivores dispersing in the field. We propose that bacterial communities in this system are structured by a combination of well dispersed bacterivores, bacterial dispersal, and bottom-up mechanisms. Clustered predators could become strong drivers of prey communities if they were specialists or keystone predators, or if they exerted a dominant influence on other predators in top-down controlled systems. Linking dispersal ability within trophic levels and its consequences for trophic dynamics can lead to a more robust perspective on trophic metacommunities.

Long-term effects of lithium and lithium-microplastic mixtures on the model species Daphnia magna: Toxicological interactions and implications to 'One Health'

Environmental contamination with lithium (Li) and microplastics (MP) has been steadily increasing and this trend is expected to continue in the future. Many freshwater ecosystems, which are crucial to reach the United Nations Sustainable Development Goals, are particularly vulnerable to Li and MP contamination, and other pressures. The long-term effects of Li, either alone or combined with MP (Li-MP mixtures), were investigated using the freshwater zooplankton micro-crustacean Daphnia magna as model species. In the laboratory, D. magna females were exposed for 21 days to water concentrations of Li (0.02, 0.04, 0.08 mg/L) or Li-MP mixtures (0.02 Li + 0.04 MP, 0.04 Li + 0.09 MP mg/L, 0.08 Li + 0.19 MP mg/L). In the range of concentrations tested, Li and Li-MP mixtures caused parental mortality, and decreased the somatic growth (up to 20% and 40% reduction, respectively) and the reproductive success (up to 93% and 90% reduction, respectively). The 21-day EC50s of Li and Li-MP mixtures on D. magna reproduction were 0.039 mg/L and 0.039 Li + 0.086 MP mg/L, respectively. Under exposure to the highest concentration of Li (0.08 mg/L) and Li-MP mixtures (0.08 Li + 0.19 MP mg/L), the mean of D. magna population growth rate was reduced by 67% and 58%, respectively. Based on the population growth rate and using data from a bioassay testing the same concentrations of MP alone and carried simultaneously, the toxicological interaction between Li and MP was antagonism under exposure to the lowest and the highest concentrations of Li-MP mixtures, and synergism under exposure to the medium concentration of Li-MP mixtures. These findings highlight the need of further investigating the combined effects of contaminants, and the threat of long-term environmental contamination with Li and MP to freshwater zooplankton, biodiversity, ecosystem services and 'One Health'.

Are experiment sample sizes adequate to detect biologically important interactions between multiple stressors?

As most ecosystems are being challenged by multiple, co-occurring stressors, an important challenge is to understand and predict how stressors interact to affect biological responses. A popular approach is to design factorial experiments that measure biological responses to pairs of stressors and compare the observed response to a null model expectation. Unfortunately, we believe experiment sample sizes are inadequate to detect most non-null stressor interaction responses, greatly hindering progress. Using both real and simulated data, we show sample sizes typical of many experiments (<6) can (i) only detect very large deviations from the additive null model, implying many important non-null stressor-pair interactions are being missed, and (ii) potentially lead to mostly statistical outliers being reported. Computer code that simulates data under either additive or multiplicative null models is provided to estimate statistical power for user-defined responses and sample sizes, and we recommend this is used to aid experimental design and interpretation of results. We suspect that most experiments may require 20 or more replicates per treatment to have adequate power to detect nonadditive. However, estimates of power need to be made while considering the smallest interaction of interest, i.e., the lower limit for a biologically important interaction, which is likely to be system-specific, meaning a general guide is unavailable. We discuss ways in which the smallest interaction of interest can be chosen, and how sample sizes can be increased. Our main analyses relate to the additive null model, but we show similar problems occur for the multiplicative null model, and we encourage similar investigations into the statistical power of other null models and inference methods. Without knowledge of the detection abilities of the statistical tools at hand or the definition of the smallest meaningful interaction, we will undoubtedly continue to miss important ecosystem stressor interactions.

Are experiment sample sizes adequate to detect biologically important interactions between multiple stressors?

As most ecosystems are being challenged by multiple, co-occurring stressors, an important challenge is to understand and predict how stressors interact to affect biological responses. A popular approach is to design factorial experiments that measure biological responses to pairs of stressors and compare the observed response to a null model expectation. Unfortunately, we believe experiment sample sizes are inadequate to detect most non-null stressor interaction responses, greatly hindering progress. Using both real and simulated data, we show sample sizes typical of many experiments (<6) can (i) only detect very large deviations from the additive null model, implying many important non-null stressor-pair interactions are being missed, and (ii) potentially lead to mostly statistical outliers being reported. Computer code that simulates data under either additive or multiplicative null models is provided to estimate statistical power for user-defined responses and sample sizes, and we recommend this is used to aid experimental design and interpretation of results. We suspect that most experiments may require 20 or more replicates per treatment to have adequate power to detect nonadditive. However, estimates of power need to be made while considering the smallest interaction of interest, i.e., the lower limit for a biologically important interaction, which is likely to be system-specific, meaning a general guide is unavailable. We discuss ways in which the smallest interaction of interest can be chosen, and how sample sizes can be increased. Our main analyses relate to the additive null model, but we show similar problems occur for the multiplicative null model, and we encourage similar investigations into the statistical power of other null models and inference methods. Without knowledge of the detection abilities of the statistical tools at hand or the definition of the smallest meaningful interaction, we will undoubtedly continue to miss important ecosystem stressor interactions.

Grazing and global change factors differentially affect biodiversity-ecosystem functioning relationships in grassland ecosystems

Grazing and global change (e.g., warming, nitrogen deposition, and altered precipitation) both contribute to biodiversity loss and alter ecosystem structure and functioning. However, how grazing and global change interactively influence plant diversity and ecosystem productivity, and their relationship remains unclear at the global scale. Here, we synthesized 73 field studies to quantify the individual and/or interactive effects of grazing and global change factors on biodiversity-productivity relationship in grasslands. Our results showed that grazing significantly reduced plant richness by 3.7% and aboveground net primary productivity (ANPP) by 29.1%, but increased belowground net primary productivity (BNPP) by 9.3%. Global change factors, however, decreased richness by 8.0% but increased ANPP and BNPP by 13.4% and 14.9%, respectively. Interestingly, the strength of the change in biodiversity in response to grazing was positively correlated with the strength of the change in BNPP. Yet, global change flipped these relationships from positive to negative even when combined with grazing. These results indicate that the impacts of global change factors are more dominant than grazing on the belowground biodiversity-productivity relationship, which is contrary to the pattern of aboveground one. Therefore, incorporating global change factors with herbivore grazing into Earth system models is necessary to accurately predict climate-grassland carbon cycle feedbacks in the Anthropocene.

Spatial and temporal variations in interspecific interaction: impact of a recreational landscape

Anthropogenic activities, such as outdoor recreation, have the potential to change complex interactions between wildlife and livestock, with further consequences for the management of both animals, the environment, and disease transmission. We present the interaction amongst wildlife, livestock, and outdoor recreationists as a three-way interaction. Little is known about how recreational activities alter the interaction between herbivores in areas extensively used for recreational purposes. We investigate how hiking activity affects spatio-temporal co-occurrence between domestic sheep (Ovis aries) and red deer (Cervus elaphus). We used camera traps to capture the spatio-temporal distribution of red deer and sheep and used the distance from the hiking path as a proxy of hiking activity. We used generalized linear models to investigate the spatial distribution of sheep and deer. We analysed the activity patterns of sheep and deer and then calculated their coefficients of temporal overlap for each camera trap location. We compared these coefficients in relation to the distance from the hiking path. Finally, we used a generalized linear mixed-model to investigate which factors influence the spatio-temporal succession between deer and sheep. We do not find that sheep and red deer spatially avoid each other. The coefficient of temporal overlap varied with distance from the hiking trail, with stronger temporal co-occurrence at greater distances from the hiking trail. Red deer were more likely to be detected further from the path during the day, which increased the temporal overlap with sheep in these areas. This suggests that hiking pressure influences spatio-temporal interactions between sheep and deer, leading to greater temporal overlap in areas further from the hiking path due to red deer spatial avoidance of hikers. This impact of recreationists on the wildlife and livestock interaction can have consequences for the animals’ welfare, the vegetation they graze, their management, and disease transmission.

Should I stay or should I go: escape behaviour of Russell’s vipers, Daboia russelii (Shaw & Nodder, 1797) in India’s agricultural landscapes

Predation exerts a strong selective force on prey, and hence prey species have evolved a multitude of ways to escape predation. One strategy by which many mobile species escape predation is by fleeing when approached by predators. However, fleeing too early can have fitness costs. Thus, optimal escape theory suggests that escape behaviour in prey depends on the risk of being eaten and the fleeing costs. Several studies on mammals, birds and lizards lend support to this hypothesis. However, few studies have explored escape behaviour in snakes. Here, using radio telemetry to track snakes in the field, we study the escape behaviour in Russell’s vipers, a highly venomous and cryptic snake, responsible for the highest number of snakebite deaths in India. We show that escape response, i.e., the decision to stay or flee, was influenced by intrinsic factors such as the snake’s behaviour and body temperature. We also show that the flight initiation distance, the distance at which the snake flees, was mostly determined by habitat selection, i.e., the visibility of the snake and the distance to the nearest cover. Overall, we show that different factors could determine the decision to flee and when to flee. We also highlight how understanding escape response in such highly venomous, medically important yet secretive snakes could potentially help reduce human-snake encounters and mitigate the snakebite crisis.

Additive impacts of ocean acidification and ambient ultraviolet radiation threaten calcifying marine primary producers

Ocean acidification (OA) represents a threat to marine organisms and ecosystems. However, OA rarely exists in isolation but occurs concomitantly with other stressors such as ultraviolet radiation (UVR), whose effects have been neglected in oceanographical observations. Here, we perform a quantitative meta-analysis based on 373 published experimental assessments from 26 studies to examine the combined effects of OA and UVR on marine primary producers. The results reveal predominantly additive stressor interactions (69-84% depending on the UV waveband), with synergistic and antagonistic interactions being rare but significantly different between micro- and macro-algae. In microalgae, variations in interaction type frequencies are related to cell volume, with antagonistic interactions accounting for a higher proportion in larger sized species. Despite additive interactions being most frequent, the small proportion of antagonistic interactions appears to have a stronger power, leading to neutral effects of OA in combination with UVR. High levels of UVR at near in situ conditions in combination with OA showed additive inhibition of calcification, but not when UVR was low. The results also reveal that the magnitude of responses is strongly dependent on experimental duration, with the negative effects of OA on calcification and pigmentation being buffered and amplified by increasing durations, respectively. Tropical primary producers were more vulnerable to OA or UVR alone compared to conspecifics from other climatic regions. Our analysis highlights that further multi-stressor long-term adaptation experiments with marine organisms of different cell volumes (especially microalgae) from different climatic regions are needed to fully disclose future impacts of OA and UVR.

High morphological disparity in a bizarre Paleocene fauna of predatory freshwater reptiles

BACKGROUND

The consequences of the K-Pg mass extinction are reflected across present biodiversity, but many faunas that appeared immediately after the extinction event were very different from current ones. Choristodera is a clade of reptiles of uncertain phylogenetic placement that have an extremely poor fossil record throughout their 150-million-year history. Yet, choristoderes survived the K-Pg event and persisted until the Miocene.

RESULTS

I describe the skulls and skeletons of two new choristoderes from a single Paleocene ecosystem in western North America that reveal the hidden Cenozoic diversity of this reptile clade. Despite their similar size, the new species deviate dramatically in morphology. Kosmodraco magnicornis gen. et sp. nov. possesses an extremely short snout and extensive cranial ornamentation. The sacrum of K. magnicornis bears enlarged muscle attachment sites and other modifications reminiscent of some giant crocodylians. In contrast, Champsosaurus norelli sp. nov. is a longirostrine species with an uninflated and ventrally divergent postorbital skull. Together with a North American choristodere previously classified in the European genus Simoedosaurus, K. magnicornis substantiates a new clade of giant, short-snouted taxa endemic to the Americas. C. norelli is found to be an early-diverging member of the genus Champsosaurus from the Cretaceous-Paleogene of the northern hemisphere. This suggests the presence of several ghost lineages of champsosaurid that crossed the K-Pg boundary.

CONCLUSIONS

The new taxa greatly increase Cenozoic choristodere richness and strengthen the evidence for the existence of distinctive freshwater faunas in Paleogene Eurasia and North America, where this clade diversified to exploit newly available macropredatory niches in the aftermath of the asteroid impact. The new choristoderes also reveal the distinct ecological context in which extant freshwater predators of the Americas like alligatoroids and gars have their origins: Paleocene fluviolacustrine ecosystems in North America displayed high large predator diversity and morphological disparity relative to modern ones.

Projecting coral responses to intensifying marine heatwaves under ocean acidification

Over this century, coral reefs will run the gauntlet of climate change, as marine heatwaves (MHWs) become more intense and frequent, and ocean acidification (OA) progresses. However, we still lack a quantitative assessment of how, and to what degree, OA will moderate the responses of corals to MHWs as they intensify throughout this century. Here, we first projected future MHW intensities for tropical regions under three future greenhouse gas emissions scenario (representative concentration pathways, RCP2.6, RCP4.5 and RCP8.5) for the near-term (2021-2040), mid-century (2041-2060) and late-century (2081-2100). We then combined these MHW intensity projections with a global data set of 1,788 experiments to assess coral attribute performance and survival under the three emissions scenarios for the near-term, mid-century and late-century in the presence and absence of OA. Although warming and OA had predominately additive impacts on the coral responses, the contribution of OA in affecting most coral attributes was minor relative to the dominant role of intensifying MHWs. However, the addition of OA led to greater decreases in photosynthesis and survival under intermediate and unrestricted emissions scenario for the mid- and late-century than if intensifying MHWs were considered as the only driver. These results show that role of OA in modulating coral responses to intensifying MHWs depended on the focal coral attribute and extremity of the scenario examined. Specifically, intensifying MHWs and OA will cause increasing instances of coral bleaching and substantial declines in coral productivity, calcification and survival within the next two decades under the low and intermediate emissions scenario. These projections suggest that corals must rapidly adapt or acclimatize to projected ocean conditions to persist, which is far more likely under a low emissions scenario and with increasing efforts to manage reefs to enhance resilience.

Rapid evolution generates synergism between multiple stressors: Linking theory and an evolution experiment

Global change encompasses many co-occurring anthropogenic stressors. Understanding the interactions between these multiple stressors, whether they be additive, antagonistic or synergistic, is critical for ecosystem managers when prioritizing which stressors to mitigate in the face of global change. While such interactions between stressors appear prevalent, it remains unclear if and how these interactions change over time, as the majority of multiple-stressor studies rarely span multiple generations of study organisms. Although meta-analyses have reported some intriguing temporal trends in stressor interactions, for example that synergism may take time to emerge, the mechanistic basis for such observations is unknown. In this study, by analysing data from an evolution experiment with the rotifer Brachionus calyciflorus (~35 generations and 31,320 observations), we show that adaptation to multiple stressors shifts stressor interactions towards synergism. We show that trade-offs, where populations cannot optimally perform multiple tasks (i.e. adapting to multiple stressors), generate this bias towards synergism. We also show that removal of stressors from evolved populations does not necessarily increase fitness and that there is variation in the evolutionary trajectories of populations that experienced the same stressor regimes. Our results highlight outstanding questions at the interface between evolution and global change biology, and illustrate the importance of considering rapid adaptation when managing or restoring ecosystems subjected to multiple stressors under global change.

The Relative Benefits of Environmental Enrichment on Learning and Memory are Greater When Stressed: A Meta-analysis of Interactions in Rodents

Environmental enrichment ("EE") is expected to alleviate the negative effects of stress on cognitive performance. However, there are complexities associated with interpreting interactions that obscure determining the benefit EE may play in mitigating the negative effects of stress. To clarify these complexities, we conducted a systematic review with meta-analysis on the main and interactive effects of EE and stress on learning and memory in rodents. We show that EE and stress interact 'synergistically' where EE provides a greater relative benefit to stressed individuals compared to those reared in conventional housing. Importantly, EE can fully-compensate for the negative effects of stress where stressed individuals with EE performed equally to enriched individuals without a stress manipulation. Additionally, we show the importance of other mediating factors, including the order of treatment exposure, duration and type of stress, type of EE, and type of cognitive assays used. This study not only quantifies the interactions between EE and stress, but also provides a clear example for how to conduct and interpret meta-analysis of interactions.

Effects of phenotypic variation on consumer coexistence and prey community structure

A popular idea in ecology is that trait variation among individuals from the same species may promote the coexistence of competing species. However, theoretical and empirical tests of this idea have yielded inconsistent findings. We manipulated intraspecific trait diversity in a ciliate competing with a nematode for bacterial prey in experimental microcosms. We found that intraspecific trait variation inverted the original competitive hierarchy to favour the consumer with variable traits, ultimately resulting in competitive exclusion. This competitive outcome was driven by foraging traits (size, speed and directionality) that increased the ciliate's fitness ratio and niche overlap with the nematode. The interplay between consumer trait variation and competition resulted in non-additive cascading effects-mediated through prey defence traits-on prey community assembly. Our results suggest that predicting consumer competitive population dynamics and the assembly of prey communities will require understanding the complexities of trait variation within consumer species.

The Opposite of Biotic Resistance: Herbivory and Competition Suppress Regeneration of Native but Not Introduced Mangroves in Southern China

Mangrove forests are increasingly threatened by plant invasions worldwide, but some mangrove species are invasive and threaten salt marsh and native mangrove ecosystems. The southern coast of China is invaded by the cordgrass Spartina alterniflora, and the mangrove Sonneratia apetala, providing a model system for studying the processes and mechanisms through which non-native species establish and spread. We used a transplant experiment to test the overarching hypothesis that native herbivores and plants provided biotic resistance against invasion by S. apetala, and that the importance of these factors would vary geographically. Survival of transplanted mangrove seedlings was lower in Zhangjiang Estuary (23°55′ N) than in Leizhou Bay (20°56′ N), and varied with species and habitats. S. apetala had higher survival and growth rates than native mangroves at both sites, and S. apetala grew taller than the S. alterniflora canopy at Leizhou Bay in only two growing seasons. In contrast, native mangroves grew poorly in S. alterniflora. Grazing by rodents and insects suppressed the growth and survival of Kandelia obovata and Avicennia marina in Zhangjiang Estuary and Leizhou Bay, but had little effect on S. apetala. Competition with vegetation (S. alterniflora and native mangroves) exacerbated the reduced survival of native mangroves, and these effects varied across study sites. Low survival of non-native S. apetala in vegetated habitats at Zhangjiang Estuary was likely due to a synergistic effect of low winter temperatures and low light intensity. Escape from herbivory (the opposite to biotic resistance) and fast growth may drive the quick expansion of non-native S. apetala in China. Rapid encroachment of S. apetala may transform the native mangrove forests and monospecific intertidal Spartina grasslands into non-native mangrove forests in the southern coast of China.

Surrounded by challenges: The simulated presence of competitors and predators modulates perianal secretion marking behaviour in the European mink (Mustela lutreola)

Animals face a variety of daily challenges to their reproduction and survival that can detect in time through to the environment cues. By using an individual focal sampling, we evaluated the variations in the time devoted to the perianal secretion marking behaviour (PSMB) of European mink when they were exposed to the simulated presence of conspecifics and two potential predators. Model results indicated that males and adult individuals dedicated more time to PSMB than females and subadults. The presence of conspecifics increased PSMB time only in adult and males, probably as an intrasexual territorial competence response. The heightened decrease in PSMB time in presence of a dog suggests an innate response due to the detection of volatile substances from faeces of carnivorous. In addition, simulated conspecific presence increased PSMB in absence of odours (control) and with owl faeces. However, when facing dog faeces, the simulated conspecific presence had no effects on PSMB. Thus, minks seem to prioritize the imminent risk of predation to avoid being detected. The stimulation of PSMB in captivity by simulated cues from conspecifics and potential predators could be useful to facilitate the reintroduction of individuals into nature, as well as their adaptation and survival.

Long-term adverse effects of microplastics on Daphnia magna reproduction and population growth rate at increased water temperature and light intensity: Combined effects of stressors and interactions

In many ecosystems, the zooplankton community has been pressured simultaneously by microplastic pollution and alterations resulting from global climate changes. The potential influence of light intensity rise (from 10,830 lx to 26,000 lx) and water temperature rise (from 20 °C to 25 °C) on the long term-toxicity of microplastics (MPs) to Daphnia magna were investigated. Three 21-day laboratory bioassays with model MPs (1-5 μm diameter) were carried out at (i) 20 °C/10830 lx, (ii) 20 °C/26000 lx, and (iii) 25 °C/10830 lx. In each bioassay, one control (no MPs) and three MP concentrations (0.04, 0.09, 0.19 mg/L) were tested. In all the bioassays, MPs caused parental and juvenile mortality, and reduced the somatic growth, reproduction and population growth rate. The MP EC₅₀s on living offspring (95% confidence interval within brackets) were 0.146 mg/L (0.142-0.151 mg/L) at 20 °C/10830 lx, 0.102 mg/L (0.099-0.105 mg/L) at 20 °C/26000 lx, and 0.101 mg/L (0.098-0.104 mg/L) at 25 °C/10830 lx. Relatively to the respective control group, 0.19 mg/L of MPs decreased the mean of the population growth rate by 27% at 20 °C/10830 lx, 38% at 20 °C/26000 lx and 59% at 25 °C/10830 lx. Based on the population growth rate and in relation to 20 °C/10830 lx (control, no MPs), the interaction between increased light intensity (26,000 lx) and MPs was synergism (at all the MP concentrations tested). The interaction between water temperature rise (25 °C) and MPs was antagonism at 0.04 mg/L of MPs and synergism at 0.09 and 0.19 mg/L of MPs. In the present scenario of climate changes and global MP pollution such findings raise high concern because zooplankton communities are crucial for aquatic biodiversity conservation, ecosystem functioning and services provided to humans. Further studies on the combined effects of MPs, other common pollutants, and alterations due to climate changes are needed.

The role of competition and herbivory in biotic resistance against invaders: a synergistic effect

Invasive species pose a major threat to global diversity, and once they are well established their eradication typically becomes unfeasible. However, certain natural mechanisms can increase the resistance of native communities to invaders and can be used to guide effective management policies. Both competition and herbivory have been identified as potential biotic resistance mechanisms that can limit plant invasiveness, but it is still under debate to what extent they might be effective against well-established invaders. Surprisingly, whereas biotic mechanisms are known to interact strongly, most studies to date have examined single biotic mechanisms separately, which likely influences our understanding of the strength and effectiveness of biotic resistance against invaders. Here we use long-term field data, benthic assemblage sampling, and exclusion experiments to assess the effect of native assemblage complexity and herbivory on the invasion dynamics of a successful invasive species, the alga Caulerpa cylindracea. A higher complexity of the native algal assemblage limited C. cylindracea invasion, probably through competition by canopy-forming and erect algae. Additionally, high herbivory pressure by the fish Sarpa salpa reduced C. cylindracea abundance by more than four times. However, long-term data of the invasion reflects that biotic resistance strength can vary across the invasion process and it is only where high assemblage complexity is concomitant with high herbivory pressure, that the most significant limitation is observed (synergistic effect). Overall, the findings reported in this study highlight that neglecting the interactions between biotic mechanisms during invasive processes and restricting the studied time scales may lead to underestimations of the true capacity of native assemblages to develop resistance to invaders.

Classifying ecosystem stressor interactions: Theory highlights the data limitations of the additive null model and the difficulty in revealing ecological surprises

Understanding how multiple co-occurring environmental stressors combine to affect biodiversity and ecosystem services is an on-going grand challenge for ecology. Currently, progress has been made through accumulating large numbers of smaller-scale empirical studies that are then investigated by meta-analyses to detect general patterns. There is particular interest in detecting, understanding and predicting 'ecological surprises' where stressors interact in a non-additive (e.g. antagonistic or synergistic) manner, but so far few general results have emerged. However, the ability of the statistical tools to recover non-additive interactions in the face of data uncertainty is unstudied, so crucially, we do not know how well the empirical results reflect the true stressor interactions. Here, we investigate the performance of the commonly implemented additive null model. A meta-analysis of a large (545 interactions) empirical dataset for the effects of pairs of stressors on freshwater communities reveals additive interactions dominate individual studies, whereas pooling the data leads to an antagonistic summary interaction class. However, analyses of simulated data from food chain models, where the underlying interactions are known, suggest both sets of results may be due to observation error within the data. Specifically, we show that the additive null model is highly sensitive to observation error, with non-additive interactions being reliably detected at only unrealistically low levels of data uncertainty. Similarly, plausible levels of observation error lead to meta-analyses reporting antagonistic summary interaction classifications even when synergies co-dominate. Therefore, while our empirical results broadly agree with those of previous freshwater meta-analyses, we conclude these patterns may be driven by statistical sampling rather than any ecological mechanisms. Further investigation of candidate null models used to define stressor-pair interactions is essential, and once any artefacts are accounted for, the so-called 'ecological surprises' may be more frequent than was previously assumed.

Environmental drivers of Cheirogaleidae population density: Remarkable resilience of Madagascar's smallest lemurs to habitat degradation

AIM

Global animal populations are in decline due to destruction and degradation of their natural habitat. Understanding the factors that determine the distribution and density of threatened animal populations is therefore now a crucial component of their study and conservation. The Cheirogaleidae are a diverse family of small-bodied, nocturnal lemurs that are widespread throughout the forests of Madagascar. However, many cheirogaleid lemurs are now highly threatened with extinction and the environmental factors that determine their distribution and population density are still little known. Here, I investigated the environmental drivers of Cheirogaleidae population density at genus level.

LOCATION

Various forest sites across Madagascar.

METHODS

I investigated how six environmental variables affect Cheirogaleidae population density at the genus level via random-effect meta-analyses. I then used a generalized linear mixed-effects model to identify the primary predictors of Cheirogaleidae population density. Finally, I investigated how the population density of this family of lemurs varies between protected and unprotected areas of Madagascar via a GLM analysis.

RESULTS

My results indicate that the relationships between the tested environmental factors and population density are genus-specific among the Cheirogaleidae. Rather remarkably, the density of Microcebus appears to have a profoundly positive relationship with anthropogenic disturbance and a negative relationship with forest cover, a finding that is also reflected by larger population densities within unprotected areas in comparison with localities within Madagascar's protected area network.

MAIN CONCLUSIONS

The results of this study are highly encouraging for the conservation of the Cheirogaleidae and highlight the remarkable resilience of these lemurs to habitat degradation and anthropogenic activity. However, this study also outlines the dearth of knowledge that we have for many species, and why these data are urgently needed to understand the biogeography and ecology of threatened animal populations and implement successful conservation.

Natural enemies have inconsistent impacts on the coexistence of competing species

The role of natural enemies in promoting coexistence of competing species has generated substantial debate. Modern coexistence theory provides a detailed framework to investigate this topic, but there have been remarkably few empirical applications to the impact of natural enemies. We tested experimentally the capacity for a generalist enemy to promote coexistence of competing insect species, and the extent to which any impact can be predicted by trade-offs between reproductive rate and susceptibility to natural enemies. We used experimental mesocosms to conduct a fully factorial pairwise competition experiment for six rainforest Drosophila species, with and without a generalist pupal parasitoid. We then parameterised models of competition and examined the coexistence of each pair of Drosophila species within the framework of modern coexistence theory. We found idiosyncratic impacts of parasitism on pairwise coexistence, mediated through changes in fitness differences, not niche differences. There was no evidence of an overall reproductive rate-susceptibility trade-off. Pairwise reproductive rate-susceptibility relationships were not useful shortcuts for predicting the impact of parasitism on coexistence. Our results exemplify the value of modern coexistence theory in multi-trophic contexts and the importance of contextualising the impact of generalist natural enemies to determine their impact. In the set of species investigated, competition was affected by the higher trophic level, but the overall impact on coexistence cannot be easily predicted just from knowledge of relative susceptibility. Methodologically, our Bayesian approach highlights issues with the separability of model parameters within modern coexistence theory and shows how using the full posterior parameter distribution improves inferences. This method should be widely applicable for understanding species coexistence in a range of systems.

Experimental warming influences species abundances in a Drosophila host community through direct effects on species performance rather than altered competition and parasitism

Global warming is expected to have direct effects on species through their sensitivity to temperature, and also via their biotic interactions, with cascading indirect effects on species, communities, and entire ecosystems. To predict the community-level consequences of global climate change we need to understand the relative roles of both the direct and indirect effects of warming. We used a laboratory experiment to investigate how warming affects a tropical community of three species of Drosophila hosts interacting with two species of parasitoids over a single generation. Our experimental design allowed us to distinguish between the direct effects of temperature on host species performance, and indirect effects through altered biotic interactions (competition among hosts and parasitism by parasitoid wasps). Although experimental warming significantly decreased parasitism for all host-parasitoid pairs, the effects of parasitism and competition on host abundances and host frequencies did not vary across temperatures. Instead, effects on host relative abundances were species-specific, with one host species dominating the community at warmer temperatures, irrespective of parasitism and competition treatments. Our results show that temperature shaped a Drosophila host community directly through differences in species’ thermal performance, and not via its influences on biotic interactions.