Experimental warming transforms multiple predator effects in a grassland food web

This experimental study tests new theory for multiple predator effects on communities by using warming to alter predator habitat use and hence direct and indirect interactions in a grassland food web containing two dominant spider predator species, a dominant grasshopper herbivore and grass and herb plants. Experimental warming further offers insight into how climate change might alter direct and indirect effects. Under ambient environmental conditions, spiders used habitat in spatially complementary locations. Consistent with predictions, the multiple predator effect on grasshoppers and on plants was the average of the individual predator effects. Warming strengthened the single predator effects. It also caused the spider species to overlap lower in the vegetation canopy. Consistent with predictions, the system was transformed into an intraguild predation system with the consequent extinction of one spider species. The results portend climate caused loss of predator diversity with important consequences for food web structure and function.     

Predator–prey interactions in a grassland food chain vary with temperature and food quality

Because species interactions are often context‐dependent, abiotic factors such as temperature and biotic factors such as food quality may alter species interactions with potential consequences to ecosystem structure and function. For example, altered predator–prey interactions may influence the dynamics of trophic cascades, affecting net primary production. In a three‐year field experiment, we manipulated a plant–grasshopper–spider food chain in mesic tallgrass prairie to investigate the effects of temperature and food quality on grasshopper performance, and to understand the direct and indirect tritrophic interactions that contribute to trophic cascades. Because spiders are active at cooler temperatures than grasshoppers in our system, we hypothesized that predator effects would be strongest in cooled treatments, and weakest in warmed treatments. Grasshopper spider interactions were highly context‐dependent and varied significantly with food quality, temperature treatment and year. Spiders most often reduced grasshopper survival in the cooled and ambient temperature treatments, but had little to no effect on grasshopper survival in the warmed treatments, as hypothesized. In some years, plants compensated for grasshopper herbivory and trophic cascades were not observed despite significant effects of predators on grasshopper survival. However, in the year they were observed, trophic cascades only occurred in cooled treatments where predator effects on grasshoppers were strongest. Predicting ecosystem responses to climate change will require an understanding of how temperature influences species interactions. Our results demonstrate that changes in daily temperature regimes can alter predator–prey interactions among arthropods with consequences for ecosystem processes such as primary production and the relative importance of top–down and bottom–up processes.     

Effects of experimental watering but not warming on herbivory vary across a gradient of precipitation

Climate change can affect biotic interactions, and the impacts of climate on biotic interactions may vary across climate gradients. Climate affects biotic interactions through multiple drivers, although few studies have investigated multiple climate drivers in experiments. We examined the effects of experimental watering, warming, and predator access on leaf water content and herbivory rates of woolly bear caterpillars (Arctia virginalis) on a native perennial plant, pacific silverweed (Argentina anserina ssp. pacifica), at two sites across a gradient of precipitation in coastal California. Based on theory, we predicted that watering should increase herbivory at the drier end of the gradient, predation should decrease herbivory, and watering and warming should have positive interacting effects on herbivory. Consistent with our predictions, we found that watering only increased herbivory under drier conditions. However, watering increased leaf water content at both wetter and drier sites. Warming increased herbivory irrespective of local climate and did not interact with watering. Predation did not affect herbivory rates. Given predictions that the study locales will become warmer and drier with climate change, our results suggest that the effects of future warming and drying on herbivory may counteract each other in drier regions of the range of Argentina anserina. Our findings suggest a useful role for range‐limit theory and the stress‐gradient hypothesis in predicting climate change effects on herbivory across stress gradients. Specifically, if climate change decreases stress, herbivory may increase, and vice versa for increasing stress. In addition, our work supports previous suggestions that multiple climate drivers are likely to have dampening effects on biotic interactions due to effects in different directions, though this is context‐dependent.     

Temperature dependency of predation: Increased killing rates and prey mass consumption by predators with warming

Temperature dependency of consumer–resource interactions is fundamentally important for understanding and predicting the responses of food webs to climate change. Previous studies have shown temperature‐driven shifts in herbivore consumption rates and resource preference, but these effects remain poorly understood for predatory arthropods. Here, we investigate how predator killing rates, prey mass consumption, and macronutrient intake respond to increased temperatures using a laboratory and a field reciprocal transplant experiment. Ectothermic predators, wolf spiders (Pardosa sp.), in the lab experiment, were exposed to increased temperatures and different prey macronutrient content (high lipid/low protein and low lipid/high protein) to assess changes in their killing rates and nutritional demands. Additionally, we investigate prey mass and lipid consumption by spiders under contrasting temperatures, along an elevation gradient. We used a field reciprocal transplant experiment between low (420 masl; 26°C) and high (2,100 masl; 15°C) elevations in the Ecuadorian Andes, using wild populations of two common orb‐weaver spider species (Leucauge sp. and Cyclosa sp.) present along the elevation gradient. We found that killing rates of wolf spiders increased with warmer temperatures but were not significantly affected by prey macronutrient content, although spiders consumed significantly more lipids from lipid‐rich prey. The field reciprocal transplant experiment showed no consistent predator responses to changes in temperature along the elevational gradient. Transplanting Cyclosa sp. spiders to low‐ or high‐elevation sites did not affect their prey mass or lipid consumption rate, whereas Leucauge sp. individuals increased prey mass consumption when transplanted from the high to the low warm elevation. Our findings show that increases in temperature intensify predator killing rates, prey consumption, and lipid intake, but the responses to temperature vary between species, which may be a result of species‐specific differences in their hunting behavior and sensitivity to temperature.     

Forecasting the effects of global change scenarios on bioaccumulation patterns in great lakes species

Climate change will have substantial impacts on biodiversity, particularly for aquatic species. Warming temperatures and changing weather patterns will also remobilize and modify chemical partitioning. Holding millions of cubic yards of sediments contaminated with persistent legacy chemicals such as polychlorinated biphenyls (PCBs) and dioxins, the Laurentian Great Lakes are a laboratory for observing interactions between biological and chemical responses to climate change. They provide a wide range of habitat to a variety of species, from littoral forage fish to deep‐water predators. In this paper, we couple bioenergetic and bioaccumulation models to investigate the biological and chemical effects of climate change in the Great Lakes. We consider three species: round goby, a warm‐water invasive forage fish; mottled sculpin, a cool‐water native forage fish; and lake trout, a cold‐water native predator. Using our coupled models, we calculate the accumulation of a representative persistent chemical, PCB‐77, under four climate scenarios for Lake Erie and Lake Superior. Predator–prey (lake trout–round goby) interactions and food availability (high–low) are incorporated into our simulations. For cool‐ to cold‐water species (sculpin, lake trout) we find that warm temperatures limit growth. For warm‐water species (round goby) cold temperatures limit growth. The impact of climate warming on growth depends on the winter lows as well as the summer highs of the scenario, in combination with the species' critical upper and lower thermal limits. We find conditions for high growth and consumption rates generally lead to high bioaccumulation. However, this can be confounded by predator–prey dynamics, as mismatches in the temperature preferences of predator and prey can lead to mismatches in relative growth and uptake rates. As predator–prey dynamics are expected to undergo substantial shifts with changing climate, these relative thermal sensitivities will be key in determining the implications of climate change for bioaccumulation, particularly in top predator species.     

Responses of foliage‐living spider assemblage composition and traits to a climatic gradient in Themeda grasslands

For speciose, but poorly known groups, such as terrestrial arthropods, functional traits present a potential avenue to assist in predicting responses to environmental change. Species turnover is common along environmental gradients, but it is unclear how this is reflected in species traits. Community‐level change in arthropod traits, other than body size, has rarely been explored across spatial scales comparable to those examined here. We hypothesized that the composition and morphological traits of spider assemblages would differ across a gradient of climate and habitat structure. We examined foliage‐living spider assemblages associated with Themeda triandra grasslands along a 900 km climatic gradient in south‐eastern Australia. We used sweep‐netting to collect T. triandra‐associated spiders and counted juveniles and identified adults. We also measured morphological traits of adult spiders and noted their hunting mode. Associations with measures of habitat structure were less consistent than relationships with climate. Both juvenile and adult spiders were more abundant in warmer sites, although species richness was not affected by temperature. We found distinct turnover in species composition along the climatic gradient, with hunting spiders, particularly crab spiders (Thomisidae), making up a greater proportion of assemblages in warmer climates. A range of traits of spiders correlated with the climatic gradient. For example, larger spider species and species that were active hunters were more common in warmer climates. Changes in morphological traits across species, rather than within species drove the morphology‐climate relationship. Strong climate‐trait correlations suggest that it may be possible to predict changes in functional traits of assemblages in response to anthropogenic disturbances such as climate change.     

Asynchronous range shifts drive alpine plant–pollinator interactions and reduce plant fitness

Climate change is driving species' range shifts, which are in turn disrupting species interactions due to species‐specific differences in their abilities to migrate in response to climate. We evaluated the consequences of asynchronous range shifts in an alpine plant–pollinator community by transplanting replicated alpine meadow turfs downslope along an elevational gradient thereby introducing them to warmer climates and novel plant and pollinator communities. We asked how these novel plant–pollinator interactions affect plant reproduction. We found that pollinator communities differed substantially across the elevation/temperature gradient, suggesting that these plants will likely interact with different pollinator communities with warming climate. Contrary to the expectation that floral visitation would increase monotonically with warmer temperatures at lower elevations, visitation rate to the transplanted communities peaked under intermediate warming at midelevation sites. In contrast, visitation rate generally increased with temperature for the local, lower elevation plant communities surrounding the experimental alpine turfs. For two of three focal plant species in the transplanted high‐elevation community, reproduction declined at warmer sites. For these species, reproduction appears to be dependent on pollinator identity such that reduced reproduction may be attributable to decreased visitation from key pollinator species, such as bumble bees, at warmer sites. Reproduction in the third focal species appears to be primarily driven by overall pollinator visitation rate, regardless of pollinator identity. Taken together, the results suggest climate warming can indirectly affect plant reproduction via changes in plant–pollinator interactions. More broadly, the experiment provides a case study for predicting the outcome of novel species interactions formed under changing climates.     

Experimental warming causes large and rapid species loss, dampened by simulated grazing, on the Tibetan Plateau

We investigated the independent and combined effects of experimental warming and grazing on plant species diversity on the north‐eastern Tibetan Plateau, a region highly vulnerable to ongoing climate and land use changes. Experimental warming caused a 26–36% decrease in species richness, a response that was generally dampened by experimental grazing. Higher species losses occurred at the drier sites where N was less available. Moreover, we observed an indirect effect of climate change on species richness as mediated by plant–plant interactions. Heat stress and warming‐induced litter accumulation are potential explanations for the species’ responses to experimental warming. This is the first reported experimental evidence that climate warming could cause dramatic declines in plant species diversity in high elevation ecosystems over short time frames and supports model predictions of species losses with anthropogenic climate change.     

Climate warming increases biological control agent impact on a non‐target species

Climate change may shift interactions of invasive plants, herbivorous insects and native plants, potentially affecting biological control efficacy and non‐target effects on native species. Here, we show how climate warming affects impacts of a multivoltine introduced biocontrol beetle on the non‐target native plant Alternanthera sessilis in China. In field surveys across a latitudinal gradient covering their full distributions, we found beetle damage on A. sessilis increased with rising temperature and plant life history changed from perennial to annual. Experiments showed that elevated temperature changed plant life history and increased insect overwintering, damage and impacts on seedling recruitment. These results suggest that warming can shift phenologies, increase non‐target effect magnitude and increase non‐target effect occurrence by beetle range expansion to additional areas where A. sessilis occurs. This study highlights the importance of understanding how climate change affects species interactions for future biological control of invasive species and conservation of native species.     

Predator diversity and abundance provide little support for the enemies hypothesis in forests of high tree diversity

Predatory arthropods can exert strong top-down control on ecosystem functions. However, despite extensive theory and experimental manipulations of predator diversity, our knowledge about relationships between plant and predator diversity--and thus information on the relevance of experimental findings--for species-rich, natural ecosystems is limited. We studied activity abundance and species richness of epigeic spiders in a highly diverse forest ecosystem in subtropical China across 27 forest stands which formed a gradient in tree diversity of 25-69 species per plot. The enemies hypothesis predicts higher predator abundance and diversity, and concomitantly more effective top-down control of food webs, with increasing plant diversity. However, in our study, activity abundance and observed species richness of spiders decreased with increasing tree species richness. There was only a weak, non-significant relationship with tree richness when spider richness was rarefied, i.e. corrected for different total abundances of spiders. Only foraging guild richness (i.e. the diversity of hunting modes) of spiders was positively related to tree species richness. Plant species richness in the herb layer had no significant effects on spiders. Our results thus provide little support for the enemies hypothesis--derived from studies in less diverse ecosystems--of a positive relationship between predator and plant diversity. Our findings for an important group of generalist predators question whether stronger top-down control of food webs can be expected in the more plant diverse stands of our forest ecosystem. Biotic interactions could play important roles in mediating the observed relationships between spider and plant diversity, but further testing is required for a more detailed mechanistic understanding. Our findings have implications for evaluating the way in which theoretical predictions and experimental findings of functional predator effects apply to species-rich forest ecosystems, in which trophic interactions are often considered to be of crucial importance for the maintenance of high plant diversity.     

Direct and indirect effects of predation and predation risk in old-field interaction webs

ABSTRACT Indirect effects emerge when a change in the abundance of one species indirectly affects another by changing the abundances of intermediate species-called density-mediated indirect effects-or they arise when one species modifies how two other species interact-called trait-mediated indirect effects. I report on field experiments that evaluated how grass and herb biomass in old-field interaction webs was influenced indirectly by a spider carnivore through its interactions with a generalist and a grass-specialist grasshopper species. I manipulated interaction pathways between the spider and the plants using different combinations of the grasshopper species. I changed the modality of predator-prey interactions to isolate density-mediated from trait-mediated effects using natural spiders (predation spiders) or spiders that were prevented from subduing prey by mouthpart manipulation (risk spiders). I found that indirect effects were stronger in speciose, reticulate food webs than in linear food chains owing to a trait-mediated effect, a diet shift by herbivores in response to predation risk. Spiders alone did not have significant effects on grasshopper densities in the field experiments, removing any possibility of density-mediated indirect effects. The study illustrates that ecologists should not underestimate the importance of behavioral ecology in determining community-level interactions.     

Size‐based ecological interactions drive food web responses to climate warming

Predicting climate change impacts on animal communities requires knowledge of how physiological effects are mediated by ecological interactions. Food‐dependent growth and within‐species size variation depend on temperature and affect community dynamics through feedbacks between individual performance and population size structure. Still, we know little about how warming affects these feedbacks. Using a dynamic stage‐structured biomass model with food‐, size‐ and temperature‐dependent life history processes, we analyse how temperature affects coexistence, stability and size structure in a tri‐trophic food chain, and find that warming effects on community stability depend on ecological interactions. Predator biomass densities generally decline with warming – gradually or through collapses – depending on which consumer life stage predators feed on. Collapses occur when warming induces alternative stable states via Allee effects. This suggests that predator persistence in warmer climates may be lower than previously acknowledged and that effects of warming on food web stability largely depend on species interactions.     

Temperature alters food web body-size structure

The increased temperature associated with climate change may have important effects on body size and predator–prey interactions. The consequences of these effects for food web structure are unclear because the relationships between temperature and aspects of food web structure such as predator–prey body-size relationships are unknown. Here, we use the largest reported dataset for marine predator–prey interactions to assess how temperature affects predator–prey body-size relationships among different habitats ranging from the tropics to the poles. We found that prey size selection depends on predator body size, temperature and the interaction between the two. Our results indicate that (i) predator–prey body-size ratios decrease with predator size at below-average temperatures and increase with predator size at above-average temperatures, and (ii) that the effect of temperature on predator–prey body-size structure will be stronger at small and large body sizes and relatively weak at intermediate sizes. This systematic interaction may help to simplify forecasting the potentially complex consequences of warming on interaction strengths and food web stability.     

Climate change enhances the negative effects of predation risk on an intermediate consumer

Predators are a major source of stress in natural systems because their prey must balance the benefits of feeding with the risk of being eaten. Although this ‘fear’ of being eaten often drives the organization and dynamics of many natural systems, we know little about how such risk effects will be altered by climate change. Here, we examined the interactive consequences of predator avoidance and projected climate warming in a three‐level rocky intertidal food chain. We found that both predation risk and increased air and sea temperatures suppressed the foraging of prey in the middle trophic level, suggesting that warming may further enhance the top‐down control of predators on communities. Prey growth efficiency, which measures the efficiency of energy transfer between trophic levels, became negative when prey were subjected to predation risk and warming. Thus, the combined effects of these stressors may represent an important tipping point for individual fitness and the efficiency of energy transfer in natural food chains. In contrast, we detected no adverse effects of warming on the top predator and the basal resources. Hence, the consequences of projected warming may be particularly challenging for intermediate consumers residing in food chains where risk dominates predator‐prey interactions.     

Thermal acclimation modulates the impacts of temperature and enrichment on trophic interaction strengths and population dynamics

Global change affects individual phenotypes and biotic interactions, which can have cascading effects up to the ecosystem level. However, the role of environmentally induced phenotypic plasticity in species interactions is poorly understood, leaving a substantial gap in our knowledge of the impacts of global change on ecosystems. Using a cladoceran–dragonfly system, we experimentally investigated the effects of thermal acclimation, acute temperature change and enrichment on predator functional response and metabolic rate. Using our experimental data, we next parameterized a population dynamics model to determine the consequences of these effects on trophic interaction strength and food‐chain stability. We found that (1) predation and metabolic rates of the dragonfly larvae increase with acute warming, (2) warm‐acclimated larvae have a higher maximum predation rate than cold‐acclimated ones, and (3) long‐term interaction strength increases with enrichment but decreases with both acclimation and acute temperatures. Overall, our experimental results show that thermal acclimation can buffer negative impacts of environmental change on predators and increase food‐web stability and persistence. We conclude that the effect of acclimation and, more generally, phenotypic plasticity on trophic interactions should not be overlooked if we aim to understand the effects of climate change and enrichment on species interaction strength and food‐web stability.     

Experimental climate warming alters aspen and birch phytochemistry and performance traits for an outbreak insect herbivore

Climate change and insect outbreaks are key factors contributing to regional and global patterns of increased tree mortality. While links between these environmental stressors have been established, our understanding of the mechanisms by which elevated temperature may affect tree–insect interactions is limited. Using a forest warming mesocosm, we investigated the influence of elevated temperature on phytochemistry, tree resistance traits, and insect performance. Specifically, we examined warming effects on forest tent caterpillar (Malacosoma disstria) and host trees aspen (Populus tremuloides) and birch (Betula papyrifera). Trees were grown under one of three temperature treatments (ambient, +1.7 °C, +3.4 °C) in a multiyear open‐air warming experiment. In the third and fourth years of warming (2011, 2012), we assessed foliar nutrients and defense chemistry. Elevated temperatures altered foliar nitrogen, carbohydrates, lignin, and condensed tannins, with differences in responses between species and years. In 2012, we performed bioassays using a common environment approach to evaluate plant‐mediated indirect warming effects on larval performance. Warming resulted in decreased food conversion efficiency and increased consumption, ultimately with minimal effect on larval development and biomass. These changes suggest that insects exhibited compensatory feeding due to reduced host quality. Within the context of observed phytochemical variation, primary metabolites were stronger predictors of insect performance than secondary metabolites. Between‐year differences in phytochemical shifts corresponded with substantially different weather conditions during these two years. By sampling across years within an ecologically realistic and environmentally open setting, our study demonstrates that plant and insect responses to warming can be temporally variable and context dependent. Results indicate that elevated temperatures can alter phytochemistry, tree resistance traits, and herbivore feeding, but that annual weather variability may modulate warming effects leading to uncertain consequences for plant–insect interactions with projected climate change.     

Relative impacts of disturbance and temperature: persistent changes in microenvironment and vegetation in retrogressive thaw slumps

In the Low Arctic, a warming climate is increasing rates of permafrost degradation and altering vegetation. Disturbance associated with warming permafrost can change microclimate and expose areas of ion-rich mineral substrate for colonization by plants. Consequently, the response of vegetation to warming air temperatures may differ significantly from disturbed to undisturbed tundra. Across a latitudinal air temperature gradient, we tested the hypothesis that the microenvironment in thaw slumps would be warmer and more nutrient rich than undisturbed tundra, resulting in altered plant community composition and increased green alder ( Alnus viridis subsp. fruticosa ) growth and reproduction. Our results show increased nutrient availability, soil pH, snow pack, ground temperatures, and active layer thickness in disturbed terrain and suggest that these variables are important drivers of plant community structure. We also found increased productivity, catkin production, and seed viability of green alder at disturbed sites. Altered community composition and enhancement of alder growth and reproduction show that disturbances exert a strong influence on deciduous shrubs that make slumps potential seed sources for undisturbed tundra. Overall, these results indicate that accelerated disturbance regimes have the potential to magnify the effects of warming temperature on vegetation. Consequently, understanding the relative effects of temperature and disturbance on Arctic plant communities is critical to predicting feedbacks between northern ecosystems and global climate change.     

气候变化对作物矿质元素利用率影响研究进展

作物矿质元素利用率对气候变化的响应是目前全球变化研究中既重要、又复杂,且认知最少的科学领域。这个科学问题的研究关系到解密或预测陆地植物及农作物矿质胁迫对全球气候变化响应的机理,为将来农业投入提供理论依据,是应对气候变化的当务之急。目前只有少数研究,通过模拟试验,探索性地开展了CO_2浓度或温度升高的环境条件下,矿质元素在土壤-植物系统迁移、分布和储存特征的研究。从相关的文献报道来看,CO_2浓度升高环境条件下,小麦和水稻作物籽粒中大量和痕量元素的富集水平一般呈下降趋势。但温度升高情况下,作物各器官对对矿质元素的吸收情况则更为复杂。正由于气候因素与植物矿质元素利用率之间关系的复杂性,在气候变化背景下,解密作物矿质胁迫对全球气候变化响应的科学问题,尚需改进试验方法、手段,从土壤性质、作物生态生理,以及农业生态系统中矿质元素在土壤-作物系统中迁移转化的过程,全面考察作物矿质元素利用率对气候变化的响应机理。     

A common thermal niche among geographically diverse populations of the widely distributed tree species Eucalyptus tereticornis: No evidence for adaptation to climate‐of‐origin

Impacts of climate warming depend on the degree to which plants are constrained by adaptation to their climate‐of‐origin or exhibit broad climatic suitability. We grew cool‐origin, central and warm‐origin provenances of Eucalyptus tereticornis in an array of common temperature environments from 18 to 35.5°C to determine if this widely distributed tree species consists of geographically contrasting provenances with differentiated and narrow thermal niches, or if provenances share a common thermal niche. The temperature responses of photosynthesis, respiration, and growth were equivalent across the three provenances, reflecting a common thermal niche despite a 2,200 km geographic distance and 13°C difference in mean annual temperature at seed origin. The temperature dependence of growth was primarily mediated by changes in leaf area per unit plant mass, photosynthesis, and whole‐plant respiration. Thermal acclimation of leaf, stem, and root respiration moderated the increase in respiration with temperature, but acclimation was constrained at high temperatures. We conclude that this species consists of provenances that are not differentiated in their thermal responses, thus rejecting our hypothesis of adaptation to climate‐of‐origin and suggesting a shared thermal niche. In addition, growth declines with warming above the temperature optima were driven by reductions in whole‐plant leaf area and increased respiratory carbon losses. The impacts of climate warming will nonetheless vary across the geographic range of this and other such species, depending primarily on each provenance's climate position on the temperature response curves for photosynthesis, respiration, and growth.