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Leaf traits often vary with plant neighbourhood composition, which in turn may mediate plant susceptibility to herbivory. However, it is unknown whether there are any common patterns of change in leaf trait expression in response to neighbourhood diversity, and whether these responses confer increased resistance or susceptibility to herbivores.We used meta‐analysis to combine data from 43 studies that examined the influence of neighbourhood diversity on eight physical and chemical leaf traits that could affect herbivory. All leaf traits apart from leaf thickness were highly plastic and exhibited significant differences between plant monocultures and species mixtures, but the direction of effect was variable. Leaf toughness was the only trait that displayed a significant decrease with plant diversity, whereas specific leaf area (SLA) and leaf nitrogen were both marginally increased in species mixtures.The magnitude and direction of leaf trait responses to neighbourhood diversity were independent of plant density and phylogenetic diversity, but changes in SLA correlated positively with plant species richness. SLA was also significantly increased in experimental studies, but not in observational studies, while neighbourhoods containing nitrogen‐fixers were associated with increased leaf nitrogen and reduced phenolics. When studies on the over‐represented species Betula pendula were removed from the analysis, the effect of neighbourhood diversity on leaf toughness became nonsignificant, but phenolics were significantly reduced in diverse neighbourhoods composed of mature trees, and marginally reduced in species mixtures across all studies.Increases in plant neighbourhood diversity are often associated with reductions of herbivory, although in some cases, the reverse occurs, and plants growing in species mixtures are found to suffer greater herbivory than those in monocultures. This study offers a potential explanation for the latter phenomenon, as our results show that leaf trait expression is highly plastic in response to neighbourhood diversity, and in certain cases could lead to increased leaf quality, which in turn could promote greater rates of herbivory. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Ongoing climate change may impact alpine plant populations via both direct effects of increased temperature and climate‐driven changes in interactions between plants and other organisms, such as insect herbivores. Rates of herbivory in high‐elevation environments are predicted to increase with warmer temperatures, which may also lead to changes in morphological and physiological traits that influence plant resistance. Yet, we currently know little about how temperature‐mediated changes in traits will impact alpine plant vulnerability to herbivores, as well as the extent to which populations from high‐elevation environments might need to rapidly adapt to increasing herbivore pressure with rising temperatures.We assessed the effect of experimental warming on the relative vulnerability of populations of the alpine plant Arabis alpina from different elevations to a specialist herbivore. Herbivore performance was measured on plants from nine populations grown in climate chambers at two temperatures, representing low (warm) and high (cold) elevations. We also measured changes in putative drivers of performance: plant phenological, chemical and defence traits. Assuming populations would be adapted to local climates and levels of herbivory, we predicted that low‐elevation populations would be more resistant to herbivores under warmer temperatures than high‐elevation populations.We found reduced performance of a specialist herbivore on A. alpina grown under warm rather than cold conditions, though this effect varied with elevation. Larvae grew faster on high‐elevation populations than low‐elevation populations when grown under warm temperatures, whereas similar growth rates were observed for plants grown under colder temperatures, consistent with plant adaptation to the lower existing herbivore pressure in cold, high‐elevation environments. Regression analyses suggested that polar metabolite variation explained more variance in larval performance than changes in defensive glucosinolates or morphological traits.Our results suggest that although physiological responses to warming may increase the resistance of cold‐adapted plants to herbivory, populations from different elevations may differ in their interactions with herbivores under climate warming. Without genetic adaptation, existing physiological responses of high‐elevation populations to warmer temperatures may leave these populations vulnerable to the increases in herbivore pressure predicted under climate change. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

When feeding on a plant, herbivorous insects alter the quality of the plant as a food source. This affects other organisms interacting with the same plant. These so‐called 'plant‐mediated interactions' can be altered by parasitoids that attack the herbivores. So far, this research area has mainly focussed on interactions at the leaf level, and very little is known about plant‐mediated interactions via seeds. It is still poorly understood if seeds that survive insect damage have fewer resources to allocate to plant growth and defence against leaf herbivores, and whether parasitoids that kill seed‐feeding insects mitigate such negative effects.Using seeds of wild lima bean plants Phaseolus lunatus, we studied the effect of the intensity of infestation by seed beetles Zabrotes subfasciatus and their parasitoids Stenocorse bruchivora on the following parameters under lab conditions: seed mass and germination, plant growth and defensive compounds (cyanogenic glycosides and flavonoids) and performance of a leaf herbivore species Spodoptera latifascia. In addition, we performed a field experiment using seeds with or without insect damage to investigate the consequences on plant performance and fitness in the wild.Seed beetle infestation had an overall negative impact on seed germination. Lab experiments revealed that damaged seeds produced plants with slower growth and reduced concentration of defensive compounds, which increased the performance of the leaf herbivores. Effects of seed‐feeding on seed germination and plant growth were attenuated by parasitism, resulting in a net increase of the number of viable offspring. In the field, we observed that seed damage impaired germination, delayed flowering time and increased leaf herbivory.Our results show that plant‐mediated interactions between insect herbivores are not limited to leaf herbivores, but extend to seed herbivores. In our study system, parasitoids had no apparent effect on these interactions, despite their strong beneficial effects on germination and plant performance. These findings confirm the long‐lasting consequences of indirect plant‐mediated interactions in a community‐wide ecological context. Furthermore, they contribute to a better understanding of the important but understudied effects of parasitoids on plant fitness. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

In many animal species, individuals engage in fights with conspecifics over access to limited resources (e.g. mates, food, or shelter). Most theory about these intraspecific fights assumes that damage has an important role in determining the contest winner. Thus, defensive structures that reduce the amount of damage an individual accrues during intraspecific competition should provide a fighting advantage.Examples of such damage‐reducing structures include the dermal shields of goats, the dorsal osteoderms of crocodiles, and the armoured telsons of mantis shrimps. Although numerous studies have identified these defensive structures, no study has investigated whether they influence the outcomes of intraspecific fights.Here we investigated whether inhibiting damage by enhancing an individual's armour influenced fighting behaviour and success in the giant mesquite bug, Thasus neocalifornicus (Insecta: Hemiptera: Coreidae).We found that experimentally manipulated individuals (i.e. those provided with additional armour) were 1.6 times more likely to win a fight when compared to the control. These results demonstrate that damage, and damage‐reducing structures, can influence fighting success.The implications of these results are twofold. First, our results experimentally support a fundamental assumption of most theoretical fighting models: that damage is a fighting cost that can influence contest outcomes. Second, these results highlight the importance of an individual's defensive capacity, and why defence should not be ignored. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Vascular plants and lichens often produce a diversity of carbon‐based secondary compounds (CBSCs) to protect them against biotic and abiotic stresses. These compounds play important but often compound‐specific roles in community and ecosystem processes by affecting herbivore and decomposer activity. However, our understanding of what drives community‐level CBSCs among ecosystems or across environmental gradients is limited.We measured concentrations and compositions of CBSCs for all dominant vascular plant and lichen species present across a 500‐m alpine elevational gradient. These measurements were combined with data on species composition and abundance to obtain whole‐community measures of plant and lichen CBSCs across the gradient.At the whole community level, plant CBSCs had the lowest concentrations while lichen CBSCs had the highest concentrations at the highest elevations. Further, plant CBCSs shifted from those associated with herbivore defence towards those protecting against light and oxidative stress as elevation increased, while lichen CBSCs showed the opposite pattern.Synthesis. Our findings that individual compounds show contrasting responses to the same environmental gradient highlight the importance of studying qualitative as well as quantitative changes in CBSCs. Further, the divergent responses between vascular plants and lichens reveal that in systems where both groups are abundant, they need to be considered simultaneously to better understand how future environmental changes may impact on ecosystem‐level processes. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Ecological research has increasingly highlighted the importance of intraspecific variation in shaping the structure and function of communities and ecosystems. Indeed, the effects of intraspecific variation can match or exceed those of interspecific variation. Previous reviews of intraspecific variation in plant traits across heterogeneous environments have focused primarily on mean phenotypic effects. We propose that a richer and fuller understanding of the ecological causes and consequences of intraspecific variation would be provided by partitioning trait variance into its subcomponents (genetic, environment, genotype by environment interaction).We used a meta‐analysis of 352 sets of genetic, environment and genotype by environment (G×E) variation estimates from 72 studies of Salicaceae to compare these sources of variation across plant traits (growth, foliar nitrogen, defence compounds), insect herbivore performance metrics (e.g., survival, growth, fecundity) and environmental conditions (e.g., soil nutrients, water, defoliation).Our findings revealed that variation in levels of defence compounds (both condensed tannins and salicinoids) and insect herbivore performance were primarily genetically determined, while variation in plant growth and foliar nitrogen was more environmentally determined.Plasticity in plant growth, foliar nitrogen levels and insect herbivore performance varied substantially across different sites (year × location), and nutrient, water and carbon dioxide environments. Plasticity was lowest for chemical defence traits and all traits in contrasting ozone and defoliation environments.Our quantitative review also revealed several gaps in the literature, including a need for surveying more mature plants, a wider variety of insect herbivore species (e.g., leaf‐galling insects, specialist insects) and underrepresented environmental treatments (e.g., competition, defoliation, disease, light and water availability).Findings from this analysis highlight the importance of, and patterns within, intraspecific variation with respect to shaping the evolvability and plasticity of traits and governing the interactions of plants and insects. A plain language summary is available for this article. Plain Language Summary [ABSTRACT FROM AUTHOR]

Abstract: Changes in resource availability, functional demands, hormonal regulation and developmental constraints can promote differences in the expression of leaf traits during plant development and foster changes in the targets of natural selection. As a consequence, the pattern and magnitude of covariation among traits, and therefore their phenotypic integration and modularity are equally expected to change throughout ontogeny. However, these changes have not been described yet. We measured leaf economic, defensive and morphological traits in plants of Turnera velutina and estimated the magnitude and pattern of foliar integration and modularity for juvenile and reproductive individuals. In addition, we assessed the relationship between plant biomass and foliar integration within and among ontogenetic stages. Both the pattern and magnitude of foliar integration changed across plant ontogeny. Foliar integration was lower in juvenile than in reproductive plants, and the pattern of phenotypic integration and modularity was different between ontogenetic stages, whereas leaves from juvenile plants showed two functional modules related to plant defence and leaf economy, traits from reproductive plants had greater interconnectivity and hence lower modularity. The relationship between plant biomass and foliar integration was negative within each ontogenetic stage but positive between ontogenetic stages, suggesting that processes intrinsic to plant development influenced the magnitude of foliar integration to a greater extent than plant size. Our findings indicate that plants can change the patterns of covariation among leaf traits during their development. However, a lower foliar integration in juvenile plants could allow for greater lability to explore a multi‐trait phenotypic space, canalisation of leaf attributes along ontogeny should promote greater phenotypic integration, constraining the number of multi‐trait combinations that plants can express. Hence, we suggest that ontogenetic changes in foliar integration allow plants to deal with changing selective dynamics and physiological priorities along their development. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Herbivores often defend themselves from predation by transmitting toxic plant-produced chemicals to their enemies. Polyphagous herbivores sometimes exhibit differential toxicity when found on various host plant species, which is generally assumed to reflect variation in plant chemistry., Here, however, we provide evidence that host-associated herbivore lineages can intrinsically differ in their toxic properties. Lineages of Aphis craccivora originating from black locust ( Robinia pseudoacacia) are unsuitable food for the ladybeetle Harmonia axyridis, resulting in death of both larvae and adults, whereas aphid lineages originating from alfalfa ( Medicago sativa) support larval development and adult reproduction. We show that locust-origin aphids remain toxic and alfalfa-origin aphids remain non-toxic when reared on any of three legume plants (fava, alfalfa or locust)., Furthermore, toxicity is not a function of the facultative bacterial symbiont Arsenophonus, which is naturally present in locust-origin aphid lineages and facilitates aphid use of locust. Experimentally cured locust-origin lineages remain toxic, and an experimentally transinfected alfalfa-origin lineage remains non-toxic to H. axyridis., Instead, Arsenophonus plays an indirect role in the distribution of toxic aphid lineages by facilitating aphid use of locust. It is the parthenogenetic coinheritance of Arsenophonus and the toxic trait that observationally correlates locust-feeding with toxicity in A. craccivora, rather than host plant chemistry per se., Our results clearly demonstrate that aphid lineages intrinsically vary in their toxic properties in a way that neither plant chemistry nor bacterial symbionts can explain. A more inclusive paradigm is needed for understanding variation in herbivore defence against predators. [ABSTRACT FROM AUTHOR]

Bark is crucial to trees because it protects their stems against fire and other hazards and because of its importance for assimilate transport, water relationships and repair. We evaluate size-dependent changes in bark thickness for 50 woody species from a moist forest and 50 species from a dry forest in Bolivia and relate bark thickness to their other bark characteristics, species life-history strategies and wood properties., For 71% of the evaluated species, the allometric coefficient describing the relationship between bark thickness and stem diameter was significantly <1 (average 0·74; range 0·38-1·20), indicating that species attain an absolute increase in bark thickness with increasing stem diameter but invest relatively less in bark thickness at larger diameters., We hypothesized that in response to more frequent fires, dry-forest species should have thicker barked trees. Contrary to this prediction, dry- and moist-forest tree species were similar in allometric bark coefficients and bark thickness., In both forest types, about 50% of the species never developed bark thick enough to avoid fire damage to their vascular cambia. Recent increases in fire frequency and extent may therefore have potentially large effects on the composition of these forests., Within each forest, coexisting species displayed a diversity of bark investment strategies, and bark thickness of trees 40 cm stem diameter varied up to 15-fold across species (ranging from 1·7 to 25·7 mm)., In both forests, sapling bark thickness was positively related to adult stature (maximum height) of the species, possibly because trees of long-lived species are more likely to experience fire during their life span, whereas for species that are characteristically small or short-lived, it may not pay off to invest heavily in bark and they may follow a resprouter strategy instead. Sapling bark thickness was not related to species' shade tolerance., Bark and wood traits were closely associated, showing a trade-off between species with tough tissues (high densities of bark and wood) on the one hand vs. species with watery tissues (high water contents of bark and wood) and thick bark on the other hand., Species with different bark investment strategies coexist in both the moist and the dry tropical forest studied. Bark and wood fulfil many functions, and the observed trade-offs may reflect different plant strategies to deal with fire, avoidance and repair of stem damage, avoidance and resistance of drought stress, and mechanical stability. [ABSTRACT FROM AUTHOR]

Understanding the factors that limit species distributions along environmental gradients is a central question of ecology. Here, we evaluate the hypothesis that the traits that result in performance trade-offs between habitats contribute to the turnover of woody species along a rainfall gradient in the Isthmus of Panama., We studied 24 plant species with contrasting distributions along this rainfall gradient. We measured 18 morphological and physiological traits, and three performance variables in seedlings planted in common garden experiments in two contrasting sites across the Isthmus., We found evidence for a trade-off suggesting that better survival during the dry season corresponded to a lower growth rate in the forest understorey. This trade-off correlated well with the distribution of the species along the rainfall gradient and was explained mostly by variation in photosynthetic capacity., While not all species fit into this trade-off, most dry-distribution species, which we had previously reported to have higher drought survival, were associated with higher stem hydraulic conductance and higher capacity for CO2 assimilation. Our interpretation is that this combination of traits may be associated mostly with desiccation avoidance (deep roots) or desiccation delay (deciduousness) rather than desiccation tolerance. Despite their higher photosynthetic capacity, these species had lower growth in the low-light understorey, probably because of higher maintenance costs (dark respiration rates)., Wet-distribution species, on the other hand, had lower photosynthetic capacity and higher leaf area ratio. This strategy is typical of shade-tolerant species and may explain their higher growth rates in the low-light understorey., In conclusion, our results suggest that habitat associations along the rainfall gradient in the Isthmus of Panama may result in part from a trade-off between traits that are favourable to species that avoid or delay desiccation but that otherwise limit shade tolerance. This trade-off may limit the capacity of some dry-distribution species to colonize wet forests. [ABSTRACT FROM AUTHOR]

1. Plants use a range of sophisticated strategies to protect themselves against herbivores and pathogens, such as the production of jasmonates, a group of plant hormones that prime the plant’s defense system upon attack. However, defense-related mechanisms, such as the jasmonate response, play a more diverse role than previously appreciated. Jasmonates also regulate key mutualist relationships, leading to a suit of potential conflicting selection pressures as a single response is employed to mediate multiple species interactions. 2. Here, we experimentally manipulate the host’s jasmonate response and document the impact on two key plant mutualisms: (i) changes to arbuscular mycorrhizal symbionts belowground (ii) modifications to floral traits affecting pollinator mutualists aboveground. By exogenously applying a range of methyl jasmonate solutions to cucumber plant roots grown with and without mycorrhizal fungi, we are able to examine the potential costs of the jasmonate response to both above and belowground mutualists. 3. We demonstrate that the negative effect of jasmonates on floral traits depends on whether the plant is mycorrhizal or nonmycorrhizal. Mycorrhization had a positive effect on floral traits, but benefits were lost with jasmonate application. While low levels of jasmonate decreased floral traits, these same jasmonate levels increased colonization by the mycorrhizal symbiont three-fold, but only under high phosphorus conditions. 4. Our results highlight potential conflicts for the host in the regulation of their mutualists under different conditions and suggest that the overall impact of the jasmonate response depends on the plant mycorrhizal status and its nutrient context. 5. These findings suggest that increasing the jasmonate response may lead to differential costs and benefits for plants and their mutualists, and highlight potential conflict in planta, with mycorrhizal symbionts benefiting from intermediate levels of jasmonates while certain floral traits can be depressed at this same level. [ABSTRACT FROM AUTHOR]

1. The faecal shield of cassidine larvae is generally thought to have evolved as a defence against predators, but field evidence for this is only anecdotal. We investigated the effectiveness of the shield as defence against natural enemies and as protection from ultraviolet (UV-B) radiation in the shield beetleCassida rubiginosaMüller (Coleoptera, Chrysomelidae). We also investigated if the construction and bearing of the shield is associated with fitness costs for the larvae in the absence of natural enemies.2. Using continuous video surveillance of individual prey, we determined the predator complex of the larvae in the field. Against the main predator, the paper waspPolistes dominulusChrist (Hymenoptera, Vespidae), the faecal shield was not effective at all. Only few other generalist predators attacked the larvae and thus represented no major mortality factor. By contrast, the faecal shield was highly effective against parasitoids, an unexpected trait described here for the first time.3. In dual-choice bioassays we found no attractive effect of the faecal shield on the endoparasitoidFoersterella reptans(Hymenoptera, Tetracampidae), but a removed faecal shield facilitated oviposition.4. There was no protective effect of the faecal shield against UV-B radiation in a laboratory experiment. However, irradiated larvae suffered from a higher mortality rate and reached a lower pupal weight, independent of whether or not they carried a shield.5. We found no measurable fitness costs for the larvae associated with bearing the faecal shield. Advantages (protection from desiccation, wind) seem to outweigh the costs, even in the absence of enemies.6. Although many studies on the defensive function of Cassidine shields have been published, this is to our knowledge the first to consider the actual predation pressure in natural situations. This step is rarely undertaken in studies on proposed defensive traits. However, verification that a proposed selection pressure is in fact present in the field is crucial for demonstrating the value and function of any trait.Functional Ecology(2005)19, 263–272doi: 10.1111/j.1365-2435.2005.00954.x [ABSTRACT FROM AUTHOR]