The fate of carbon in a mature forest under carbon dioxide enrichment.

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1-5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration6. Although evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth3-5, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands7-10, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO2 unclear4,5,7-11. Here using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO2 exposure. We show that, although the eCO2 treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for half of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO2 fertilization as a driver of increased carbon sinks in global forests.

Plant Silicon Defences Suppress Herbivore Performance, but Mode of Feeding Is Key.

The performance of herbivorous animals depends on the nutritional and defensive traits of the plants they consume. The uptake and deposition of biogenic silicon in plant tissues is arguably the most basic and ubiquitous anti-herbivore defence used by plants, especially grasses. We conducted meta-analyses of 150 studies reporting how vertebrate and invertebrate herbivores performed when feeding on silicon-rich plants relative to those feeding on low-silicon plants. Silicon levels were 52% higher and 32% more variable in silicon-rich plants compared to plants with low silicon, which resulted in an overall 33% decline in herbivore performance. Fluid-feeding herbivore performance was less adversely impacted (-14%) than tissue-chewing herbivores, including mammals (-45%), chewing arthropods (-33%) and plant-boring arthropods (-39%). Fluid-feeding arthropods with a wide diet breadth or those feeding on perennial plant species were mostly unaffected by silicon defences. Unlike many other plant defences, where diet specialisation often helps herbivores overcome their effects, silicon negatively impacts chewing herbivores regardless of diet breadth. We conclude that silicon defences primarily target chewing herbivores and impact vertebrate and invertebrate herbivores to a similar degree.

Elevated atmospheric CO2 changes defence allocation in wheat but herbivore resistance persists.

Predicting how plants allocate to different anti-herbivore defences in response to elevated carbon dioxide (CO2) concentrations is important for understanding future patterns of crop susceptibility to herbivory. Theories of defence allocation, especially in the context of environmental change, largely overlook the role of silicon (Si), despite it being the major anti-herbivore defence in the Poaceae. We demonstrated that elevated levels of atmospheric CO2 (e[CO2]) promoted plant growth by 33% and caused wheat (Triticum aestivum) to switch from Si (-19%) to phenolic (+44%) defences. Despite the lower levels of Si under e[CO2], resistance to the global pest Helicoverpa armigera persisted; relative growth rates (RGRs) were reduced by at least 33% on Si-supplied plants, irrespective of CO2 levels. RGR was negatively correlated with leaf Si concentrations. Mandible wear was c. 30% higher when feeding on Si-supplemented plants compared to those feeding on plants with no Si supply. We conclude that higher carbon availability under e[CO2] reduces silicification and causes wheat to increase concentrations of phenolics. However, Si supply, at all levels, suppressed the growth of H. armigera under both CO2 regimes, suggesting that shifts in defence allocation under future climate change may not compromise herbivore resistance in wheat.

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 eCO2, 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.

Silicon deposition on guard cells increases stomatal sensitivity as mediated by K+ efflux and consequently reduces stomatal conductance.

Silicon (Si) has been widely reported to improve plant resistance to water stress via various mechanisms including cuticular Si deposition to reduce leaf transpiration. However, there is limited understanding of the effects of Si on stomatal physiology, including the underlying mechanisms and implications for resistance to water stress. We grew tall fescue (Festuca arundinacea Schreb. cv. Fortuna) hydroponically, with or without Si, and treated half of the plants with 20% polyethylene glycol to impose physiological drought (osmotic stress). Scanning electron microscopy in conjunction with X-ray mapping found that Si was deposited on stomatal guard cells and as a sub-cuticular layer in Si-treated plants. Plants grown in Si had a 28% reduction in stomatal conductance and a 23% reduction in cuticular conductance. When abscisic acid was applied exogenously to epidermal leaf peels to promote stomatal closure, Si plants had 19% lower stomatal aperture compared to control plants (i.e. increased stomatal sensitivity) and an increased efflux of guard cell K+ ions. However, the changes in stomatal physiology with Si were not substantial enough to improve water stress resistance, as shown by a lack of significant effect of Si on water potential, growth, photosynthesis and water-use efficiency. Our findings suggest a novel underlying mechanism for reduced stomatal conductance with Si application; specifically, that Si deposition on stomatal guard cells promotes greater stomatal sensitivity as mediated by guard cell K+ efflux.

Plant silicon application alters leaf alkaloid concentrations and impacts parasitoids more adversely than their aphid hosts.

Grasses accumulate large amounts of silicon (Si) which acts as a highly effective physical defence against insect herbivory, however recent evidence shows that Si supplementation also modifies plant secondary metabolite concetrations. Changes in plant secondary metabolites concentrations can have cascading effects on higher trophic levels, such as parasitoids, as they are dependent on the host herbivore for growth and development. However, relatively little is known about how Si application affects higher trophic levels. We examined the effects of Si addition on alkaloid content in leaves of Phalaris aquatica (Poaceae) and the effect on interactions between an aphid (Rhopalosiphum padi) and its parasitoid (Aphidius colemani). Si supplementation had no effect on aphid abundance or parasitism rate. Adult aphids, aphid mummies (parasitised aphids) and the emergent parasitoids were, however, significantly smaller on Si+ plants. Parasitoid traits (size and emergence) were correlated with aphid mummy size. Si addition reduced parasitoid emergence rate and size due to reduced host mummy size, in addition, significantly fewer females emerged from mummies on Si+ plants. Aphid infestation significantly altered alkaloids concentrations, reducing gramine by 80% while increasing tryptamine by 91% in Si- plants. Si addition reduced aphid-induced tryptamine concentrations by 64% and increased 5-MeO-tryptamine by over 800% in control and 142% in aphid infested plants. Our results show that while Si addition has modest impacts on the herbivore, it significantly alters secondary metabolites and has stronger effects on the higher trophic level through changes in the quality of the parasitised host.

Contrasting effects of Miocene and Anthropocene levels of atmospheric CO2 on silicon accumulation in a model grass.

Grasses are hyper-accumulators of silicon (Si), which they acquire from the soil and deposit in tissues to resist environmental stresses. Given the high metabolic costs of herbivore defensive chemicals and structural constituents (e.g. cellulose), grasses may substitute Si for these components when carbon is limited. Indeed, high Si uptake grasses evolved in the Miocene when atmospheric CO2 concentration was much lower than present levels. It is, however, unknown how pre-industrial CO2 concentrations affect Si accumulation in grasses. Using Brachypodium distachyon, we hydroponically manipulated Si-supply (0.0, 0.5, 1, 1.5, 2 mM) and grew plants under Miocene (200 ppm) and Anthropocene levels of CO2 comprising ambient (410 ppm) and elevated (640 ppm) CO2 concentrations. We showed that regardless of Si treatments, the Miocene CO2 levels increased foliar Si concentrations by 47% and 56% relative to plants grown under ambient and elevated CO2, respectively. This is owing to higher accumulation overall, but also the reallocation of Si from the roots into the shoots. Our results suggest that grasses may accumulate high Si concentrations in foliage when carbon is less available (i.e. pre-industrial CO2 levels) but this is likely to decline under future climate change scenarios, potentially leaving grasses more susceptible to environmental stresses.

Silicon is an inducible and effective herbivore defence against Helicoverpa punctigera (Lepidoptera: Noctuidae) in soybean.

The role of silicon (Si) in alleviating the effects of biotic and abiotic stresses, including defence against insect herbivores, in plants is widely reported. Si defence against insect herbivores is overwhelmingly studied in grasses (especially the cereals), many of which are hyper-accumulators of Si. Despite being neglected, legumes such as soybean (Glycine max) have the capacity to control Si accumulation and benefit from increased Si supply. We tested how Si supplementation via potassium, sodium or calcium silicate affected a soybean pest, the native budworm Helicoverpa punctigera Wallengren (Lepidoptera: Noctuidae). Herbivory reduced leaf biomass similarly in Si-supplemented (+Si) and non-supplemented (-Si) plants (c. 29 and 23%, respectively) relative to herbivore-free plants. Both Si supplementation and herbivory increased leaf Si concentrations. In relative terms, herbivores induced Si uptake by c. 19% in both +Si and -Si plants. All Si treatments reduced H. punctigera relative growth rates (RGR) to a similar extent for potassium (-41%), sodium (-49%) and calcium (-48%) silicate. Moreover, there was a strong negative correlation between Si accumulation in leaves and herbivore RGR. To our knowledge, this is only the second report of Si-based herbivore defence in soybean; the rapid increase in leaf Si following herbivory being indicative of an induced defence. Taken together with the other benefits of Si supplementation of legumes, Si could prove an effective herbivore defence in legumes as well as grasses.

When resistance is futile, tolerate instead: silicon promotes plant compensatory growth when attacked by above- and belowground herbivores.

Plants have evolved numerous herbivore defences that are resistance- or tolerance-based. Resistance involves physical and chemical traits that deter and/or harm herbivores whereas tolerance minimizes fitness costs of herbivory, often via compensatory growth. The Poaceae frequently accumulate large amounts of silicon (Si), which can be used for herbivore resistance, including biomechanical and (indirectly) biochemical defences. To date, it is unclear whether Si improves tolerance of herbivory. Here we report how Si enabled a cereal (Triticum aestivum) to tolerate damage inflicted by above- and belowground herbivores. Leaf herbivory increased Si concentrations in the leaves by greater than 50% relative to herbivore-free plants, indicating it was an inducible defensive response. In plants without Si supplementation, leaf herbivory reduced shoot biomass by 52% and root herbivory reduced root biomass by 68%. Si supplementation, however, facilitated compensatory growth such that shoot losses were more than compensated for (+14% greater than herbivore-free plants) and root losses were minimized to -16%. Si supplementation did not improve plant resistance since Si did not enhance biomechanical resistance (i.e. force of fracture) or reduce leaf consumption and herbivore relative growth rates. We propose that Si-based defence operates in wheat via tolerance either in addition or as an alternative to resistance-based defence.

Elevated carbon dioxide and warming impact silicon and phenolic-based defences differently in native and exotic grasses.

Global climate change may increase invasions of exotic plant species by directly promoting the success of invasive/exotic species or by reducing the competitive abilities of native species. Changes in plant chemistry, leading to altered susceptibility to stress, could mediate these effects. Grasses are hyper-accumulators of silicon, which play a crucial function in the alleviation of diverse biotic and abiotic stresses. It is unknown how predicted increases in atmospheric carbon dioxide (CO2 ) and air temperature affect silicon accumulation in grasses, especially in relation to primary and secondary metabolites. We tested how elevated CO2 (eCO2 ) (+240 ppm) and temperature (eT) (+4°C) affected chemical composition (silicon, phenolics, carbon and nitrogen) and plant growth in eight grass species, either native or exotic to Australia. eCO2 increased phenolic concentrations by 11%, but caused silicon accumulation to decline by 12%. Moreover, declines in silicon occurred mainly in native species (-19%), but remained largely unchanged in exotic species. Conversely, eT increased silicon accumulation in native species (+19%) but decreased silicon accumulation in exotic species (-10%). Silicon and phenolic concentrations were negatively correlated with each other, potentially reflecting a defensive trade-off. Moreover, both defences were negatively correlated with plant mass, compatible with a growth-defence trade-off. Grasses responded in a species-specific manner, suggesting that the relative susceptibility of different species may differ under future climates compared to current species rankings of resource quality. For example, the native Microlaena stipoides was less well defended under eCO2 in terms of both phenolics and silicon, and thus could suffer greater vulnerability to herbivores. To our knowledge, this is the first demonstration of the impacts of eCO2 and eT on silicon accumulation in grasses. We speculate that the greater plasticity in silicon uptake shown by Australian native grasses may be partly a consequence of evolving in a low nutrient and seasonally arid environment.

Elevated atmospheric carbon dioxide concentrations promote ant tending of aphids.

Animal mutualisms, which involve beneficial interactions between individuals of different species, are common in nature. Insect-insect mutualism, for example, is widely regarded as a keystone ecological interaction. Some mutualisms are anticipated to be modified by climate change, but the focus has largely been on plant-microbe and plant-animal mutualisms rather than those between animals. Ant-aphid mutualisms, whereby ants tend aphids to harvest their honeydew excretions and, in return, provide protection for the aphids, are widespread. The mutualism is heavily influenced by the quality and quantity of honeydew produced by aphids, which is directly affected by host plant quality. As predicted increases in concentrations of atmospheric carbon dioxide (eCO2 ) are widely reported to affect plant nutritional chemistry, this may also alter honeydew quality and hence the nature of ant-aphid mutualisms. Using glasshouse chambers and field-based open-top chambers, we determined the effect of eCO2 on the growth and nutritional quality (foliar amino acids) of lucerne (Medicago sativa). We determined how cowpea aphid (Aphis craccivora) populations and honeydew production were impacted when feeding on such plants and how this affected the tending behaviour of ants (Iridomyrmex sp.). eCO2 stimulated plant growth but decreased concentrations of foliar amino acids by 29% and 14% on aphid-infested plants and aphid-free plants, respectively. Despite the deterioration in host plant quality under eCO2 , aphids maintained performance and populations were unchanged by eCO2 . Aphids induced higher concentrations of amino acids (glutamine, asparagine, glutamic acid and aspartic acid) important for endosymbiont-mediated synthesis of essential amino acids. Aphids feeding under eCO2 also produced over three times more honeydew than aphids feeding under ambient CO2 , suggesting they were imbibing more phloem sap at eCO2 . The frequency of ant tending of aphids more than doubled in response to eCO2 . To our knowledge, this is the first study to demonstrate the effects of atmospheric change on an ant-aphid mutualism. In particular, these results highlight how impending changes to concentrations of atmospheric CO2 may alter mutualistic behaviour between animals. These could include positive impacts, as reported here, shifts from mutualism to antagonism, partner switches and mutualism abandonment.

Drought negates growth stimulation due to root herbivory in pasture grasses.

Predicted increases in extreme weather are likely to alter the interactions between organisms within ecosystems. Whilst many studies have investigated the impacts of climate change on aboveground plant-insect interactions, those belowground remain relatively unexplored. Root herbivores can be the dominant taxa in grasslands, potentially altering plant community dynamics. To better predict the impact of climate change on grasslands, we subjected four Australian pasture grasses (Cynodon dactylon, Paspalum dilatatum, Microlaena stipoides and Lolium perenne) to contrasting rainfall regimes [a press drought (i.e. sustained, moderate water stress), a pulse drought (water stress followed by periodic, infrequent deluge event) and a well-watered control], with and without root herbivores; a manual root cutting treatment was also included for comparison. Plant growth, rooting strategy, phenology and biochemistry were measured to evaluate above and belowground treatment responses. Watering treatments had a larger effect on plant productivity than root damage treatments: press drought and pulse drought treatments reduced biomass by 58% and 47%, respectively. Root herbivore damage effects were species dependent and were not always equivalent to root cutting. The combination of pulse drought and root herbivory resulted in increased root:shoot ratios for both P. dilatatum and L. perenne, as well as decreased biomass and delayed flowering time for P. dilatatum. Plant biomass responses to root damage were greatest under well-watered conditions; however, root damage also delayed or prevented investment in reproduction in at least one species. Our findings highlight the important role of soil-dwelling invertebrates for forecasting growth responses of grassland communities to future rainfall regime changes.

Silicon uptake by a pasture grass experiencing simulated grazing is greatest under elevated precipitation.

BACKGROUND: Grasses are hyper-accumulators of silicon (Si) and often up-regulate Si following herbivory. Positive correlations exist between Si and plant water content, yet the extent to which Si uptake responses can be mediated by changes in soil water availability has rarely been studied and never, to our knowledge, under field conditions. We used field-based rain-exclusion shelters to investigate how simulated grazing (shoot clipping) and altered rainfall patterns (drought and elevated precipitation, representing 50% and 150% of ambient precipitation levels, respectively) affected initial patterns of root- and shoot-Si uptake in a native Australian grass (Microlaena stipoides) in Si-supplemented and untreated soils. RESULTS: Si supplementation increased soil water retention under ambient and elevated precipitation but not under drought, although this had little effect on Si uptake and growth (tiller numbers or root biomass) of M. stipoides. Changes in rainfall patterns and clipping had strong individual effects on plant growth and Si uptake and storage, whereby clipping increased Si uptake by M. stipoides under all rainfall treatments but to the greatest extent under elevated precipitation. Moreover, above-ground-below-ground Si distribution only changed following elevated precipitation by decreasing the ratio of root:shoot Si concentrations. CONCLUSIONS: Results highlight the importance of soil water availability for Si uptake and suggest a role for both active and passive Si transport mechanisms. Such manipulative field studies may provide a more realistic insight into how grasses initially respond to herbivory in terms of Si-based defence under different environmental conditions.

Unravelling mummies: cryptic diversity, host specificity, trophic and coevolutionary interactions in psyllid - parasitoid food webs.

BACKGROUND: Parasitoids are hyperdiverse and can contain morphologically and functionally cryptic species, making them challenging to study. Parasitoid speciation can arise from specialisation on niches or diverging hosts. However, which process dominates is unclear because cospeciation across multiple parasitoid and host species has rarely been tested. Host specificity and trophic interactions of the parasitoids of psyllids (Hemiptera) remain mostly unknown, but these factors are fundamentally important for understanding of species diversity, and have important applied implications for biological control. RESULTS: We sampled diverse parasitoid communities from eight Eucalyptus-feeding psyllid species in the genera Cardiaspina and Spondyliaspis, and characterised their phylogenetic and trophic relationships using a novel approach that forensically linked emerging parasitoids with the presence of their DNA in post-emergence insect mummies. We also tested whether parasitoids have cospeciated with their psyllid hosts. The parasitoid communities included three Psyllaephagus morphospecies (two primary and, unexpectedly, one heteronomous hyperparasitoid that uses different host species for male and female development), and the hyperparasitoid, Coccidoctonus psyllae. However, the number of genetically delimited Psyllaephagus species was three times higher than the number of recognisable morphospecies, while the hyperparasitoid formed a single generalist species. In spite of this, cophylogenetic analysis revealed unprecedented codivergence of this hyperparasitoid with its primary parasitoid host, suggesting that this single hyperparasitoid species is possibly diverging into host-specific species. Overall, parasitoid and hyperparasitoid diversification was characterised by functional conservation of morphospecies, high host specificity and some host switching between sympatric psyllid hosts. CONCLUSIONS: We conclude that host specialisation, host codivergence and host switching are important factors driving the species diversity of endoparasitoid communities of specialist host herbivores. Specialisation in parasitoids can also result in heteronomous life histories that may be more common than appreciated. A host generalist strategy may be rare in endoparasitoids of specialist herbivores despite the high conservation of morphology and trophic roles, and endoparasitoid species richness is likely to be much higher than previously estimated. This also implies that the success of biological control requires detailed investigation to enable accurate identification of parasitoid-host interactions before candidate parasitoid species are selected as biological control agents for target pests.

Impacts of silicon-based grass defences across trophic levels under both current and future atmospheric CO2 scenarios.

Silicon (Si) has important functional roles in plants, including resistance against herbivores. Environmental change, such as increasing atmospheric concentrations of CO2, may alter allocation to Si defences in grasses, potentially changing the feeding behaviour and performance of herbivores, which may in turn impact on higher trophic groups. Using Si-treated and untreated grasses (Phalaris aquatica) maintained under ambient (400 ppm) and elevated (640 and 800 ppm) CO2 concentrations, we show that Si reduced feeding by crickets (Acheta domesticus), resulting in smaller body mass. This, in turn, reduced predatory behaviour by praying mantids (Tenodera sinensis), which consequently performed worse. Despite elevated CO2 decreasing Si concentrations in P. aquatica, this reduction was not large enough to affect the feeding behaviour of crickets or their predator. Our results suggest that Si-based defences in plants have adverse impacts on both primary and secondary trophic taxa, and these are not likely to decline under future climate change scenarios.

Codivergence of the primary bacterial endosymbiont of psyllids versus host switches and replacement of their secondary bacterial endosymbionts.

Coevolution between insects and bacterial endosymbionts contributes to the success of many insect lineages. For the first time, we tested for phylogenetic codivergence across multiple taxonomic scales, from within genera to superfamily between 36 psyllid species of seven recognised families (Hemiptera: Psylloidea), their exclusive primary endosymbiont Carsonella and more diverse secondary endosymbionts (S-endosymbionts). Within Aphalaridae, we found that Carsonella and S-endosymbionts were fixed in one Glycaspis and 12 Cardiaspina populations. The dominant S-endosymbiont was Arsenophonus, while Sodalis was detected in one Cardiaspina species. We demonstrated vertical transmission for Carsonella and Arsenophonus in three Cardiaspina species. We found strong support for strict cospeciation and validated the informative content of Carsonella as extended host genome for inference of psyllid relationships. However, S-endosymbiont and host phylogenies were incongruent, and displayed signs of host switching and endosymbiont replacement. The high incidence of Arsenophonus in psyllids and other plant sap-feeding Hemiptera may be due to repeated host switching within this group. In two psyllid lineages, Arsenophonus and Sodalis genes exhibited accelerated evolutionary rates and AT-biases characteristic of long-term host associations. Together with strict vertical transmission and 100% prevalence within host populations, our results suggest an obligate, and not facultative, symbiosis between psyllids and some S-endosymbionts.

Roots under attack: contrasting plant responses to below- and aboveground insect herbivory.

The distinctive ecology of root herbivores, the complexity and diversity of root-microbe interactions, and the physical nature of the soil matrix mean that plant responses to root herbivory extrapolate poorly from our understanding of responses to aboveground herbivores. For example, root attack induces different changes in phytohormones to those in damaged leaves, including a lower but more potent burst of jasmonates in several plant species. Root secondary metabolite responses also differ markedly, although patterns between roots and shoots are harder to discern. Root defences must therefore be investigated in their own ecophysiological and evolutionary context, specifically one which incorporates root microbial symbionts and antagonists, if we are to better understand the battle between plants and their hidden herbivores.

Antagonistic interactions between an invasive alien and a native coccinellid species may promote coexistence.

Despite the capacity of invasive alien species to alter ecosystems, the mechanisms underlying their impact remain only partly understood. Invasive alien predators, for example, can significantly disrupt recipient communities by consuming prey species or acting as an intraguild predator (IGP). Behavioural interactions are key components of interspecific competition between predators, yet these are often overlooked invasion processes. Here, we show how behavioural, non-lethal IGP interactions might facilitate the establishment success of an invading alien species. We experimentally assessed changes in feeding behaviour (prey preference and consumption rate) of native UK coccinellid species (Adalia bipunctata and Coccinella septempunctata), whose populations are, respectively, declining and stable, when exposed to the invasive intraguild predator, Harmonia axyridis. Using a population dynamics model parameterized with these experimental data, we predicted how intraguild predation, accommodating interspecific behavioural interactions, might impact the abundance of the native and invasive alien species over time. When competing for the same aphid resource, the feeding rate of A. bipunctata significantly increased compared to the feeding in isolation, while the feeding rate of H. axyridis significantly decreased. This suggests that despite significant declines in the UK, A. bipunctata is a superior competitor to the intraguild predator H. axyridis. In contrast, the behaviour of non-declining C. septempunctata was unaltered by the presence of H. axyridis. Our experimental data show the differential behavioural plasticity of competing native and invasive alien predators, but do not explain A. bipunctata declines observed in the UK. Using behavioural plasticity as a parameter in a population dynamic model for A. bipunctata and H. axyridis, coexistence is predicted between the native and invasive alien following an initial period of decline in the native species. We demonstrate how empirical and theoretical techniques can be combined to understand better the processes and consequences of alien species invasions for native biodiversity.

Trade-Offs between Silicon and Phenolic Defenses may Explain Enhanced Performance of Root Herbivores on Phenolic-Rich Plants.

Phenolic compounds play a role in plant defense against herbivores. For some herbivorous insects, particularly root herbivores, host plants with high phenolic concentrations promote insect performance and tissue consumption. This positive relationship between some insects and phenolics, however, could reflect a negative correlation with other plant defenses acting against insects. Silicon is an important element for plant growth and defense, particularly in grasses, as many grass species take up large amounts of silicon. Negative impact of a high silicon diet on insect herbivore performance has been reported aboveground, but is unreported for belowground herbivores. It has been hypothesized that some silicon accumulating plants exhibit a trade-off between carbon-based defense compounds, such as phenolics, and silicon-based defenses. Here, we investigated the impact of silicon concentrations and total phenolic concentrations in sugarcane roots on the performance of the root-feeding greyback canegrub (Dermolepida albohirtum). Canegrub performance was positively correlated with root phenolics, but negatively correlated with root silicon. We found a negative relationship in the roots between total phenolics and silicon concentrations. This suggests the positive impact of phenolic compounds on some insects may be the effect of lower concentrations of silicon compounds in plant tissue. This is the first demonstration of plant silicon negatively affecting a belowground herbivore.