Effects of elevated atmospheric CO2 on the nutritional ecology of C3 and C4 grass-feeding caterpillars

Silicon-mediated herbivore defence in a pasture grass under reduced and Anthropocene levels of CO2

The uptake and accumulation of silicon (Si) in grass plants play a crucial role in alleviating both biotic and abiotic stresses. Si supplementation has been reported to increase activity of defence-related antioxidant enzyme, which helps to reduce oxidative stress caused by reactive oxygen species (ROS) following herbivore attack. Atmospheric CO2 levels are known to affect Si accumulation in grasses; reduced CO2 concentrations increase Si accumulation whereas elevated CO2 concentrations often decrease Si accumulation. This can potentially affect antioxidant enzyme activity and subsequently insect herbivory, but this remains untested. We examined the effects of Si supplementation and herbivory by Helicoverpa armigera on antioxidant enzyme (catalase, CAT; superoxide dismutase, SOD; and ascorbate peroxidase, APX) activity in tall fescue grass (Festuca arundinacea) grown under CO2 concentrations of 200, 410, and 640 ppm representing reduced, ambient, and elevated CO2 levels, respectively. We also quantified foliar Si, carbon (C), and nitrogen (N) concentrations and determined how changes in enzymes and elemental chemistry affected H. armigera relative growth rates and plant consumption. Rising CO2 concentrations increased plant mass and foliar C but decreased foliar N and Si. Si supplementation enhanced APX and SOD activity under the ranging CO2 regimes. Si accumulation and antioxidant enzyme activity were at their highest level under reduced CO2 conditions and their lowest level under future levels of CO2. The latter corresponded with increased herbivore growth rates and plant consumption, suggesting that some grasses could become more susceptible to herbivory under projected CO2 conditions.

Climate change alters slug abundance but not herbivory in a temperate grassland

Climate change will significantly impact the world's ecosystems, in part by altering species interactions and ecological processes, such as herbivory and plant community dynamics, which may impact forage quality and ecosystem production. Yet relatively few field experimental manipulations assessing all of these parameters have been performed to date. To help fill this knowledge gap, we evaluated the effects of increased temperature (+3°C day and night, year-round) and precipitation (+30% of mean annual rainfall) on slug herbivory and abundance and plant community dynamics biweekly in a pasture located in central Kentucky, U.S.A. Warming increased slug abundance once during the winter, likely due to improving conditions for foraging, whereas warming reduced slug abundance at times in late spring, mid-summer, and early fall (from 62-95% reduction depending on month). We found that warming and increased precipitation did not significantly modify slug herbivory at our site, despite altering slug abundance and affecting plant community composition and forage quality. Climate change will alter seasonal patterns of slug abundance through both direct effects on slug biology and indirect effects mediated by changes in the plant community, suggesting that pasture management practices may have to adapt.

Human Impacts on Insect Chemical Communication in the Anthropocene

The planet is presently undergoing dramatic changes caused by human activities. We are living in the era of the Anthropocene, where our activities directly affect all living organisms on Earth. Insects constitute a major part of the world’s biodiversity and currently, we see dwindling insect biomass but also outbreaks of certain populations. Most insects rely on chemical communication to locate food, mates, and suitable oviposition sites, but also to avoid enemies and detrimental microbes. Emissions of, e.g., CO2, NOx, and ozone can all affect the chemical communication channel, as can a rising temperature. Here, we present a review of the present state of the art in the context of anthropogenic impact on insect chemical communication. We concentrate on present knowledge regarding fruit flies, mosquitoes, moths, and bark beetles, as well as presenting our views on future developments and needs in this emerging field of research. We include insights from chemical, physiological, ethological, and ecological directions and we briefly present a new international research project, the Max Planck Centre for Next Generation Insect Chemical Ecology (nGICE), launched to further increase our understanding of the impact of human activities on insect olfaction and chemical communication.

Anti-herbivore silicon defences in a model grass are greatest under Miocene levels of atmospheric CO2

Silicon (Si) has an important role in mitigating diverse biotic and abiotic stresses in plants, mainly via the silicification of plant tissues. Environmental changes such as atmospheric CO₂ concentrations may affect grass Si concentrations which, in turn, can alter herbivore performance. We recently demonstrated that pre-industrial atmospheric CO₂ increased Si accumulation in Brachypodium distachyon grass, yet the patterns of Si deposition in leaves and whether this affects insect herbivore performance remains unknown. Moreover, it is unclear whether CO₂ -driven changes in Si accumulation are linked to changes in gas exchange (e.g. transpiration rates). We therefore investigated how pre-industrial (reduced; rCO₂ , 200 ppm), ambient (aCO₂ , 410 ppm) and elevated (eCO₂ , 640 ppm) CO₂ concentrations, in combination with Si-treatment (Si+ or Si-), affected Si accumulation in B. distachyon and its subsequent effect on the performance of the global insect pest, Helicoverpa armigera. rCO₂ increased Si concentrations by 29% and 36% compared to aCO₂ and eCO₂ respectively. These changes were not related to observed changes in gas exchange under different CO₂ regimes, however. The increased Si accumulation under rCO₂ decreased herbivore relative growth rate (RGR) by 120% relative to eCO_2, whereas rCO₂ caused herbivore RGR to decrease by 26% compared to eCO₂ . Si supplementation also increased the density of macrohairs, silica and prickle cells, which was associated with reduced herbivore performance. There was a negative correlation among macrohair density, silica cell density, prickle cell density and herbivore RGR under rCO₂ suggesting that these changes in leaf surface morphology were linked to reduced performance under this CO₂ regime. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO₂ reduces insect herbivore performance. Contrastingly, we found reduced Si accumulation under higher CO₂ , which suggests that some grasses may become more susceptible to insect herbivores under projected climate change scenarios.

Light environments affect herbivory patterns but not reproductive performance of a multivoltine specialist moth, Pareuchaetes pseudoinsulata

Unravelling the responses of insect herbivores to light-environment-mediated variation in the traits of their host plants is central to our understanding of the nutritional ecology of, and factors driving the population dynamics in, these species. This study examined the effect of light environment (shaded vs full-sun habitat) on leaf toughness and leaf nutritional quality in Chromolaena odorata (an invasive species in West Africa) and related these attributes to the abundance, herbivory patterns and reproductive performance of a multivoltine specialist moth, Pareuchaetes pseudoinsulata (a biological control agent). In this system, plants growing in shaded areas in the field experienced more herbivory and had higher herbivore abundance than those growing in full-sun. In the laboratory, P. pseudoinsulata larvae consumed significantly greater amounts of shaded foliage relative to full-sun foliage. However, reproductive performance metrics such as mating success, pre-oviposition period, number of eggs laid, duration of egg laying, egg hatchability, and adult longevity in P. pseudoinsulata did not differ according to foliage types. Reduced leaf toughness, increased water and nitrogen contents in shaded leaves coincided with increased leaf consumption by the larvae of P. pseudoinsulata. In summary, this study showed for the first time that light environments affect herbivory patterns but not reproductive performance of P. pseudoinsulata and hypothesized that high foliar nitrogen and water contents in shaded leaves resulted in feedback and necessity consumption patterns.

Effects of elevated CO2 and endophytic bacterium on photosynthetic characteristics and cadmium accumulation in Sedum alfredii

Elevated CO₂ and use of endophytic microorganisms have been considered as efficient and novel ways to improve phytoextraction efficiency. However, the interactive effects of elevated CO₂ and endophytes on hyperaccumulator is poorly understood. In this study, a hydroponics experiment was conducted to investigate the combined effect of elevated CO₂ (eCO₂) and inoculation with endophyte SaMR12 (ES) on the photosynthetic characteristics and cadmium (Cd) accumulation in hyperaccumulator Sedum alfredii. The results showed that eCO₂ × ES interaction promoted the growth of S. alfredii, shoot and root biomass net increment were increased by 264.7 and 392.3%, respectively, as compared with plants grown in ambient CO₂ (aCO₂). The interaction of eCO₂ and ES significantly (P < 0.05) increased chlorophyll content (53.2%), Pn (111.6%), Pn_max (59.8%), AQY (65.1%), and Lsp (28.8%), but reduced Gs, Tr, Rd, and Lcp. Increased photosynthetic efficiency was associated with higher activities of rubisco, Ca²⁺-ATPase, and Mg²⁺-ATPase, and linked with over-expression of two photosystem related genes (SaPsbS and SaLhcb2). PS II activities were significantly (P < 0.05) enhanced with Fv/Fm and Φ(II) increased by 12.3 and 13.0%, respectively, compared with plants grown in aCO₂. In addition, the net uptake of Cd in the shoot and root tissue of S. alfredii grown in eCO₂ × ES treatment was increased by 260.7 and 434.9%, respectively, due to increased expression of SaHMA2 and SaCAX2 Cd transporter genes. Our results suggest that eCO₂ × ES can promote the growth of S. alfredii due to increased photosynthetic efficiency, and improve Cd accumulation and showed considerable potential of improving the phytoextraction ability of Cd by S. alfredii.

Opposite macroevolutionary responses to environmental changes in grasses and insects during the Neogene grassland expansion

The rise of Neogene C4 grasslands is one of the most drastic changes recently experienced by the biosphere. A central - and widely debated - hypothesis posits that Neogene grasslands acted as a major adaptive zone for herbivore lineages. We test this hypothesis with a novel model system, the Sesamiina stemborer moths and their associated host-grasses. Using a comparative phylogenetic framework integrating paleoenvironmental proxies we recover a negative correlation between the evolutionary trajectories of insects and plants. Our results show that paleoenvironmental changes generated opposing macroevolutionary dynamics in this insect-plant system and call into question the role of grasslands as a universal adaptive cradle. This study illustrates the importance of implementing environmental proxies in diversification analyses to disentangle the relative impacts of biotic and abiotic drivers of macroevolutionary dynamics.

Structural inflexibility of the rhizosphere microbiome in mangrove plant Kandelia obovata under elevated CO2

Rhizosphere microbial communities play an important role in mediating the decomposition of soil organic matter. Increased CO₂ concentration may increase plant growth by stimulating photosynthesis or improving water use efficiency. However, possible eco-physiological influences of this greenhouse gas in mangrove plants are not well understood, especially how rhizosphere microbial communities respond to CO₂ increase. We characterized the effect of elevated CO₂ (eCO₂) on rhizospheric microbial communities associated with the mangrove plant Kandelia candel for 20 weeks, eCO₂ increased plant chlorophyll a levels and root microbial biomass. Operational taxonomic unit analysis revealed no significant effects of eCO₂ on rhizospheric bacterial communities; however, some influence on archaeal community structure was observed, especially on the ammonia-oxidizing archaea. Principal component analysis showed that microbial biomass C, total nitrogen, C/N ratio, nitrate nitrogen, and salinity were the main factors structuring the microbial community. The relative contribution of environmental parameters to variability among samples was 31.0%. In addition, functional analysis by average well color development showed that carbon source utilization under eCO₂ occurred in the order amino acids > carbohydrates > polymers > carboxylic acids > amines > phenolic acids; whereas, sugars, amino acids, and carboxylic acids were the preferred carbon sources in control groups. Differences in utilization ability of carbohydrates and amino acids resulted in changes in carbon metabolism between the two groups. Rhizosphere microbial communities appear to have some buffering ability in response to short-term (20 weeks) CO₂ increase, during which the metabolic efficiency of carbon sources is changed. The results will help better understand the structural inflexibility and functional plasticity of the rhizosphere microbiome in mangrove plants facing a changing environment (such as global climate change).

Stable isotope analysis of diet confirms niche separation of two sympatric species of Namib Desert lizard

We used stable isotopes of carbon and nitrogen to study the trophic niche of two species of insectivorous lizards, the Husab sand lizard Pedioplanis husabensis and Bradfield's Namib day gecko living sympatrically in the Namib Desert. We measured the δ(13) C and δ(15) N ratios in lizard blood tissues with different turnover times (whole blood, red blood cells and plasma) to investigate lizard diet in different seasons. We also measured the δ(13) C and δ(15) N ratios in available arthropod prey and plant tissues on the site, to identify the avenues of nutrient movement between lizards and their prey. Through the use of stable isotope mixing models, we found that the two lizard species relied on a largely non-overlapping but seasonally variable array of arthropods: P. husabensis primarily fed on termites, beetles and wasps, while R. bradfieldi fed mainly on ants, wasps and hemipterans. Nutrients originating from C3 plants were proportionally higher for R. bradfieldi than for P. husabensis during autumn and late autumn/early winter, although not summer. Contrary to the few available data estimating the trophic transfer of nutrients in ectotherms in mixed C3 and C4 /crassulacean acid metabolism (CAM) plant landscapes, we found that our lizard species primarily acquired nutrients that originated from C4 /CAM plants. This work adds an important dimension to the general lack of studies using stable isotope analyses to estimate lizard niche partitioning and resource use.

Preference for C4 shade grasses increases hatchling performance in the butterfly, Bicyclus safitza

The Miocene radiation of C4 grasses under high-temperature and low ambient CO 2 levels occurred alongside the transformation of a largely forested landscape into savanna. This inevitably changed the host plant regime of herbivores, and the simultaneous diversification of many consumer lineages, including Bicyclus butterflies in Africa, suggests that the radiations of grasses and grazers may be evolutionary linked. We examined mechanisms for this plant-herbivore interaction with the grass-feeding Bicyclus safitza in South Africa. In a controlled environment, we tested oviposition preference and hatchling performance on local grasses with C3 or C4 photosynthetic pathways that grow either in open or shaded habitats. We predicted preference for C3 plants due to a hypothesized lower processing cost and higher palatability to herbivores. In contrast, we found that females preferred C4 shade grasses rather than either C4 grasses from open habitats or C3 grasses. The oviposition preference broadly followed hatchling performance, although hatchling survival was equally good on C4 or C3 shade grasses. This finding was explained by leaf toughness; shade grasses were softer than grasses from open habitats. Field monitoring revealed a preference of adults for shaded habitats, and stable isotope analysis of field-sampled individuals confirmed their preference for C4 grasses as host plants. Our findings suggest that plant-herbivore interactions can influence the direction of selection in a grass-feeding butterfly. Based on this work, we postulate future research to test whether these interactions more generally contribute to radiations in herbivorous insects via expansions into new, unexploited ecological niches.

Genome-Wide Transcriptional Profiling and Metabolic Analysis Uncover Multiple Molecular Responses of the Grass Species Lolium perenne Under Low-Intensity Xenobiotic Stress

Lolium perenne, which is a major component of pastures, lawns, and grass strips, can be exposed to xenobiotic stresses due to diffuse and residual contaminations of soil. L. perenne was recently shown to undergo metabolic adjustments in response to sub-toxic levels of xenobiotics. To gain insight in such chemical stress responses, a de novo transcriptome analysis was carried out on leaves from plants subjected at the root level to low levels of xenobiotics, glyphosate, tebuconazole, and a combination of the two, leading to no adverse physiological effect. Chemical treatments influenced significantly the relative proportions of functional categories and of transcripts related to carbohydrate processes, to signaling, to protein-kinase cascades, such as Serine/Threonine-protein kinases, to transcriptional regulations, to responses to abiotic or biotic stimuli and to responses to phytohormones. Transcriptomics-based expressions of genes encoding different types of SNF1 (sucrose non-fermenting 1)-related kinases involved in sugar and stress signaling or encoding key metabolic enzymes were in line with specific qRT-PCR analysis or with the important metabolic and regulatory changes revealed by metabolomic analysis. The effects of pesticide treatments on metabolites and gene expression strongly suggest that pesticides at low levels, as single molecule or as mixture, affect cell signaling and functioning even in the absence of major physiological impact. This global analysis of L. perenne therefore highlighted the interactions between molecular regulation of responses to xenobiotics, and also carbohydrate dynamics, energy dysfunction, phytohormones and calcium signaling.

A field experiment with elevated atmospheric CO2-mediated changes to C4 crop-herbivore interactions

The effects of elevated CO₂ (E-CO₂) on maize and Asian corn borer (ACB), Ostrinia furnacalis, in open-top chambers were studied. The plants were infested with ACB and exposed to ambient and elevated (550 and 750 μl/l) CO₂. E-CO₂ increased the plant height and kernel number per ear. The plants had lower nitrogen contents and higher TNC: N ratios under E-CO₂ than at ambient CO₂. The response of plant height to E-CO₂ was significantly dampened in plants with ACB infestation. However, the weight gain of the survivors declined in plants grown under E-CO₂. Moreover, the plant damage caused by ACB was not different among the treatments. Overwintering larvae developed under E-CO₂ had a lower supercooling point than those developed under ambient CO₂. The results indicated that there was a positive effect of E-CO₂ on the accumulation of maize biomass, i.e., the “air-fertilizer” effect, which led to a nutritional deficiency in the plants. The fitness-related parameters of ACB were adversely affected by the CO₂-mediated decreased in plant nutritional quality, and ACB might alter its food consumption to compensate for these changes. Larval damage to maize under E-CO₂ appears to be offset by this “air-fertilizer” effect, with reductions in larval fitness.

Recent advances in the integrative nutrition of arthropods

In this review we highlight recent advances in four areas in which nutrition shapes the relationships between organisms: between plants and herbivores, between hosts and their microbiota, between individuals within groups and societies, and between species within food webs. We demonstrate that taking an explicitly multidimensional view of nutrition and employing the logic of the geometric framework for nutrition provide novel insights and offer a means of integration across different levels of organization, from individuals to ecosystems.

Coyote (Canis latrans) mammalian prey diet shifts in response to seasonal vegetation change

Drylands typically have strong seasonal variation in rainfall and primary productivity. This study examines the effects of seasonal change in grass-derived resource availability on the base of the food chain of a mammalian predator. Seasonal changes in live grass cover were measured in two vegetation types at the Sevilleta National Wildlife Refuge in central New Mexico, USA. Non-invasive genetic sampling of scat was used to identify individuals in the local coyote (Canis latrans) population. Stable carbon and nitrogen isotope analysis of hair removed from scats of 45 different coyotes was used to assess seasonal variation in the diet of mammalian coyote prey that came from C4 grasses. Live grass cover increased from the spring to the summer and fall; contribution of C4 grasses to the diet of mammalian coyote prey increased from the summer to the fall and was higher in grassland areas. There were significant differences in the seasonal patterns in the prey diet between grassland and shrubland areas.

Elevated atmospheric CO2 triggers compensatory feeding by root herbivores on a C3 but not a C4 grass

Predicted increases in atmospheric carbon dioxide (CO₂) concentrations often reduce nutritional quality for herbivores by increasing the C∶N ratio of plant tissue. This frequently triggers compensatory feeding by aboveground herbivores, whereby they consume more shoot material in an attempt to meet their nutritional needs. Little, however, is known about how root herbivores respond to such changes. Grasslands are particularly vulnerable to root herbivores, which can collectively exceed the mass of mammals grazing aboveground. Here we provide novel evidence for compensatory feeding by a grass root herbivore, Sericesthis nigrolineata, under elevated atmospheric CO₂ (600 µmol mol⁻¹) on a C₃ (Microlaena stipoides) but not a C₄ (Cymbopogon refractus) grass species. At ambient CO₂ (400 µmol mol⁻¹) M. stipoides roots were 44% higher in nitrogen (N) and 7% lower in carbon (C) concentrations than C. refractus, with insects performing better on M. stipoides. Elevated CO₂ decreased N and increased C∶N in M. stipoides roots, but had no impact on C. refractus roots. Root-feeders displayed compensatory feeding on M. stipoides at elevated CO₂, consuming 118% more tissue than at ambient atmospheric CO₂. Despite this, root feeder biomass remained depressed by 24%. These results suggest that compensatory feeding under elevated atmospheric CO₂ may make some grass species particularly vulnerable to attack, potentially leading to future shifts in the community composition of grasslands.

Desert tortoise (Gopherus agassizii) dietary specialization decreases across a precipitation gradient

We studied the plant resource use between and within populations of desert tortoise (Gopherus agassizii) across a precipitation gradient in the Sonoran Desert of Arizona. The carbon and nitrogen stable isotope values in animal tissues are a reflection of the carbon and nitrogen isotope values in diet, and consequently represent a powerful tool to study animal feeding ecology. We measured the δ¹³C and δ¹⁵N values in the growth rings on the shells of tortoises in different populations to characterize dietary specialization and track tortoise use of isotopically distinct C₄/CAM versus C₃ plant resources. Plants using C₃ photosynthesis are generally more nutritious than C₄ plants and these trait differences can have important growth and fitness consequences for consumers. We found that dietary specialization decreases in successively drier and less vegetated sites, and that broader population niche widths are accompanied by an increase in the dietary variability between individuals. Our results highlight how individual consumer plant resource use is bounded under a varying regime of precipitation and plant productivity, lending insight into how intra-individual dietary specialization varies over a spatial scale of environmental variability.

An emerging understanding of mechanisms governing insect herbivory under elevated CO2

By changing the chemical composition of foliage, the increase in atmospheric CO(2) is fundamentally altering insect herbivory. The responses of folivorous insects to these changes is, however, highly variable. In this review we highlight emerging mechanisms by which increasing CO(2) alters the defense chemistry and signaling of plants. The response of allelochemicals affecting insect performance varies under elevated CO(2), and results suggest this is driven by changes in plant hormones. Increasing CO(2) suppresses the production of jasmonates and ethylene and increases the production of salicylic acid, and these differential responses of plant hormones affect specific secondary chemical pathways. In addition to changes in secondary chemistry, elevated CO(2) decreases rates of water loss from leaves, increases temperature and feeding rates, and alters nutritional content. New insights into the mechanistic responses of secondary chemistry to elevated CO(2) increase our ability to predict the ecological and evolutionary responses of plants attacked by insects.

Modification of esterified cell wall phenolics increases vulnerability of tall fescue to herbivory by the fall armyworm

Feruloylation of arabinoxylan in grass cell walls leads to cross-linked xylans. Such cross-linking appears to play a role in plant resistance to pathogens and insect herbivores. In this study, we investigated the effect of ferulate cross-linking on resistance to herbivory by fall armyworm (Spodoptera frugiperda) making use of genetically modified tall fescue [Schedonorus arundinaceus (Festuca arundinacea)] expressing a ferulic acid esterase gene. Mature leaves of these plants have significant reduced levels of cell wall ferulates and diferulates but no change in acid detergent lignin. These reduced levels of esterified cell wall ferulates in transgenic plants had a positive effect on all measures of armyworm larval performance examined. More larvae survived (89 vs. 57 %) and grew faster (pupated 2.1 days sooner) when fed transgenic leaves with reduced levels of cell wall ferulates, than when fed control tall fescue leaves where levels of cell wall ferulates were not altered. Overall, mortality, growth and food utilization were negatively associated with level of esterified cell wall ferulates and diferulates in leaves they were fed. This study is the first to use transgenic plants with modified level of cell wall esterified ferulates to test the role of feruloylation in plant resistance to insects. It is concluded that the accumulation of ferulates and the cross-linking of arabinoxylans via diferulate esters in the leaves of tall fescue underlies the physical barrier to insect herbivory. Reducing ferulate cross-linking in grass cell walls could increase susceptibility of these plants to insect folivores.

Linking precipitation and C3-C4 plant production to resource dynamics in higher-trophic-level consumers

In many ecosystems, seasonal shifts in temperature and precipitation induce pulses of primary productivity that vary in phenology, abundance, and nutritional quality. Variation in these resource pulses could strongly influence community composition and ecosystem function, because these pervasive bottom-up forces play a primary role in determining the biomass, life cycles, and interactions of organisms across trophic levels. The focus of this research is to understand how consumers across trophic levels alter resource use and assimilation over seasonal and interannual timescales in response to climatically driven changes in pulses of primary productivity. We measured the carbon isotope ratios (delta(13)C) of plant, arthropod, and lizard tissues in the northern Chihuahuan Desert to quantify the relative importance of primary production from plants using C3 and C4 photosynthesis for consumers. Summer monsoonal rains on the Sevilleta Long Term Ecological Research (LTER) site in New Mexico support a pulse of C4 plant production that has tissue delta(13)C values distinct from C3 plants. During a year when precipitation patterns were relatively normal, delta(13)C measurements showed that consumers used and assimilated significantly more C4-derived carbon over the course of a summer, tracking the seasonal increase in abundance of C4 plants. In the following spring, after a failure in winter precipitation and the associated failure of spring C3 plant growth, consumers showed elevated assimilation of C4-derived carbon relative to a normal rainfall regime. These findings provide insight into how climate, pulsed resources, and temporal trophic dynamics may interact to shape semiarid grasslands such as the Chihuahuan Desert in the present and future.

Hydrolyzable tannins as "quantitative defenses": limited impact against Lymantria dispar caterpillars on hybrid poplar

The high levels of tannins in many tree leaves are believed to cause decreased insect performance, but few controlled studies have been done. This study tested the hypothesis that higher foliar tannin levels produce higher concentrations of semiquinone radicals (from tannin oxidation) in caterpillar midguts, and that elevated levels of radicals are associated with increased oxidative stress in midgut tissues and decreased larval performance. The tannin-free leaves of hybrid poplar (Populus tremulaxP. alba) were treated with hydrolyzable tannins, producing concentrations of 0%, 7.5% or 15% dry weight, and fed to Lymantria dispar caterpillars. As expected, larvae that ingested control leaves contained no measurable semiquinone radicals in the midgut, those that ingested 7.5% hydrolyzable tannin contained low levels of semiquinone radicals, and those that ingested 15% tannin contained greatly increased levels of semiquinone radicals. Ingested hydrolyzable tannins were also partially hydrolyzed in the midgut. However, increased levels of semiquinone radicals in the midgut were not associated with oxidative stress in midgut tissues. Instead, it appears that tannin consumption was associated with increased metabolic costs, as measured by the decreased efficiency of conversion of digested matter to body mass (ECD). Decreased ECD, in turn, decreased the overall efficiency of conversion of ingested matter to body mass (ECI). Contrary to our hypothesis, L. dispar larvae were able to maintain similar growth rates across all tannin treatment levels, in part, because of compensatory feeding. We conclude that hydrolyzable tannins act as "quantitative defenses" in the sense that high levels appear to be necessary to increase levels of semiquinone radicals in the midguts of caterpillars. However, these putative resistance factors are not sufficient to decrease the performance of tannin-tolerant caterpillars such as L. dispar.

Stable isotopes, ecological integration and environmental change: wolves record atmospheric carbon isotope trend better than tree rings

Large-scale patterns of isotope ratios are detectable in the tissues of organisms, but the variability in these patterns often obscures detection of environmental trends. We show that plants and animals at lower trophic levels are relatively poor indicators of the temporal trend in atmospheric carbon isotope ratios (delta13C) when compared with animals at higher trophic levels. First, we tested how differences in atmospheric delta13C values were transferred across three trophic levels. Second, we compared contemporary delta13C trends (1961-2004) in atmospheric CO2 to delta13C patterns in a tree species (jack pine, Pinus banksiana), large herbivore (moose, Alces alces) and large carnivore (grey wolf, Canis lupus) from North America. Third, we compared palaeontological (approx. 30000 to 12000 14C years before present) atmospheric CO2 trends to delta13C patterns in a tree species (Pinus flexilis, Juniperus sp.), a megaherbivore (bison, Bison antiquus) and a large carnivore (dire wolf, Canis dirus) from the La Brea tar pits (southern California, USA) and Great Basin (western USA). Contrary to previous expectations, we found that the environmental isotope pattern is better represented with increasing trophic level. Our results indicate that museum specimens of large carnivores would best reflect large-scale spatial and temporal patterns of carbon isotopes in the palaeontological record because top predators can act as ecological integrators of environmental change.

Limited impact of elevated levels of polyphenol oxidase on tree-feeding caterpillars: assessing individual plant defenses with transgenic poplar

Polyphenol oxidase (PPO) is commonly believed to function as an effective antiherbivore defense in plants. PPO is induced in plants following herbivory, and insect performance is often negatively correlated with PPO levels. However, induced defenses create numerous changes in plants, and very little work has been done to test the direct effects of PPO on insect herbivores separately from other changes. This study examined the impacts of high levels of PPO on the performance of two species of tree-feeding caterpillars (Lymantria dispar and Orgyia leucostigma) on poplar. Transgenic PPO-overexpressing poplar (Populus tremula x Populus alba) was used as a source of elevated-PPO leaves, thereby controlling for the multiple effects of induction. In addition, the impacts of treating poplar foliage with high levels of purified mushroom PPO were examined on the two caterpillar species. Contrary to expectation, in several cases increased PPO levels had no significant effect on insect consumption or growth rates. Although one of the mechanisms by which PPO is believed to impact herbivores is via increased oxidative stress, the ingestion of large amounts of PPO had little or no effect on semiquinone radical and oxidized protein levels in the gut contents of lymantriid caterpillars. PPO activity in caterpillars is likely limited by the low oxygen and high ascorbate levels commonly found in their gut contents. This study questions whether induced PPO functions as an effective post-ingestive defense against tree-feeding caterpillars, and indicates that controlled, mechanistic studies are needed in other plant-herbivore systems to test for a direct effect of PPO on insect performance.

Performance of a generalist grasshopper on a C3 and a C4 grass: compensation for the effects of elevated CO2 on plant nutritional quality

The increasing CO2 concentration in Earth's atmosphere is expected to cause a greater decline in the nutritional quality of C3 than C4 plants. As a compensatory response, herbivorous insects may increase their feeding disproportionately on C3 plants. These hypotheses were tested by growing the grasses Lolium multiflorum C3) and Bouteloua curtipendula C4) at ambient (370 ppm) and elevated (740 ppm) CO2 levels in open top chambers in the field, and comparing the growth and digestive efficiencies of the generalist grasshopper Melanoplus sanguinipes on each of the four plant x CO2 treatment combinations. As expected, the nutritional quality of the C3 grass declined to a greater extent than did that of the C4 grass at elevated CO2; protein levels declined in the C3 grass, while levels of carbohydrates (sugar, fructan and starch) increased. However, M. sanguinipes did not significantly increase its consumption rate to compensate for the lower nutritional quality of the C3 grass grown under elevated CO2. Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C3 grass under elevated CO2. Consumption rates of the C3 and C4 grasses were also similar, demonstrating a lack of compensatory feeding on the C4 grass. We also examined the relative efficiencies of nutrient utilization from a C3 and C4 grass by M. sanguinipes to test the basis for the C4 plant avoidance hypothesis. Contrary to this hypothesis, neither protein nor sugar was digested with a lower efficiency from the C4 grass than from the C3 grass. A novel finding of this study is that fructan, a potentially large carbohydrate source in C3 grasses, is utilized by grasshoppers. Based on the higher nutrient levels in the C3 grass and the better growth performance of M. sanguinipes on this grass at both CO2 levels, we conclude that C3 grasses are likely to remain better host plants than C4 grasses in future CO2 conditions.