Ecological and evolutionary implications of plant tolerance to herbivory

Functional traits of exotic submerged macrophytes mediate diversity-invasibility relationship in freshwater communities under eutrophication

Plant diversity may respond differently in terms of whether it can drive plant invasions in freshwater ecosystem. Linkages and interactions between diversity and invasibility have not been clearly resolved, and it is unclear how nutrient enrichment (e.g., eutrophication) will affect this relationship. As a key predictor of plant growth, the ability of functional traits to mediate trade-offs in the diversity-invasibility relationship is unknown. Here, we conducted a series of experiments to determine the role of exotic plant functional traits in the diversity-invasibility relationship of submerged macrophyte communities under eutrophication. We selected common native and exotic submerged macrophytes in the subtropics to construct different diverse submerged macrophyte communities to simulate invasion. Meanwhile, to test the adaptability and importance of functional traits, we experimentally verified the differences in functional traits between exotic and native species. Our results showed a positive correlation between native plant diversity and community invasibility. Moreover, the invader's performance was predominantly determined by functional traits of exotic species, such as plant biomass and tissue nutrients, which were significantly altered by species diversity. Furthermore, our results suggested that functional traits contribute significantly more to the invasiveness of exotic submerged macrophytes than the other factors to which they are subjected. Plant functional traits can mediate the diversity-invasibility relationship because of the higher intrinsic dominance of exotic submerged macrophyte species. In summary, our study revealed diversity-invasibility relationship in submerged macrophyte communities and highlighted functional traits as key drivers of invasion of high-risk exotic submerged macrophyte species. Although previous studies have elucidated the importance of functional trait studies for plant invasions, our study provides the only current evidence demonstrating the important role of invaders' functional traits in mediating the diversity-invasibility relationship. This novel perspective offers valuable insights into the management and control of invasive aquatic plants.

Species-specific responses drive browsing impacts on physiological and functional traits in Quercus agrifolia and Umbellularia californica

Herbivory is a fundamental ecological force in the evolution of plant physiological, morphological, and chemical attributes. In this study, we explored how browsing pressure by local deer populations affected leaf form and function in two California native tree species, Quercus agrifolia (coast live oak) and Umbellularia californica (California bay laurel). Specifically, we investigated how leaf and stem vascular attributes differed between browsed and non-browsed zones of each species. Browsing significantly altered traits such as leaf to phloem ratios and leaf area, but we observed few meaningful differences in leaf and stem anatomy between browsed and non-browsed material. We discuss these results in the context of leaf and stem adaptations to herbivory and water use efficiency and explore future research considerations for investigating leaf and stem vascular trait development with herbivore presence.

Plasticity in biomass allocation underlies tolerance to leaf damage in native and non-native populations of Datura stramonium

An introduction to a novel habitat represents a challenge to plants because they likely would face new interactions and possibly different physical context. When plant populations arrive to a new region free from herbivores, we can expect an evolutionary change in their defense level, although this may be contingent on the type of defense, resistance or tolerance, and cost of defense. Here, we addressed questions on the evolution of tolerance to damage in non-native Spanish populations of Datura stramonium by means of two comparative greenhouse experiments. We found differences in seed production, specific leaf area, and biomass allocation to stems and roots between ranges. Compared to the Mexican native populations of this species, non-native populations produced less seeds despite damage and allocate more biomass to roots and less to stems, and had higher specific leaf area values. Plasticity to leaf damage was similar between populations and no difference in tolerance to damage between native and non-native populations was detected. Costs for tolerance were detected in both regions. Two plasticity traits of leaves were associated with tolerance and were similar between regions. These results suggest that tolerance remains beneficial to plants in the non-native region despite it incurs in fitness costs and that damage by herbivores is low in the non-native region. The study of the underlying traits of tolerance can improve our understanding on the evolution of tolerance in novel environments, free from plants' specialist herbivores.

Temperature alters the toxicological impacts of plant terpenoids on the polyphagous model herbivore Vanessa cardui

Terpenes are a major class of secondary metabolites present in all plants, and long hypothesized to have diversified in response to specific plant-herbivore interactions. Herbivory is a major biotic interaction that plays out across broad temporal and spatial scales that vary dramatically in temperature regimes, both due to climatic variation across geographic locations as well as the effect of seasonality. In addition, there is an emerging understanding that global climate change will continue to alter the temperature regimes of nearly every habitat on Earth over the coming centuries. Regardless of source, variation in temperature may influence herbivory, in particular via changes in the efficacy and impacts of plant defensive chemistry. This study aims to characterize temperature-driven variation in toxicological effects across several structural classes of terpenes in the model herbivore Vanessa cardui, the painted lady butterfly. We observed a general increase in monoterpene toxicity to larvae, pupa, and adults at higher temperatures, as well as an increase in development time as terpene concentration increased. Results obtained from this study yield insights into possible drivers of seasonal variation in plant terpene production as well as inform effects of rising global temperatures on plant-insect interactions. In the context of other known effects of climate change on plant-herbivore interactions like carbon fertilization and compensatory feeding, temperature-driven changes in plant chemical defense efficacy may further complicate the prediction of climate change impacts on the fundamental ecological process of herbivory.

Plasticity in Compensatory Growth to Artificial Defoliation and Light Availability in Four Neotropical Understory and Forest Edge Herb Species

Defoliation from falling branches is a major factor in the survival of understory herbs in tropical rainforests. Experimental studies of defoliation under three levels of light environment compared responses to partial and total defoliation in four species of tropical rainforest understory herbs. We predicted that elevated levels of light availability would help compensate for damage to through compensatory growth in both understory and forest edge species and that forest edge species would more effectively compensate under high light conditions than shade-tolerant species from the forest understory All species showed a high tolerance to defoliation under high and intermediate light conditions. Under low-light conditions survival differed dramatically with minimal mortality in forest-edge species compared to high mortality in completely defoliated understory species. Defoliation, and light × defoliation interactions, impacted multiple growth traits in understory species. In contrast, forest-edge species showed no effect of defoliation except on total biomass, and only one light × defoliation interaction was observed. Our results indicate that differences in biomass allocation, leaf ecophysiology, and other growth parameters between forest understory and edge species may be structuring post-damage response in understory and forest edge herbs.

Disentangling the effects of jasmonate and tissue loss on the sex allocation of an annual plant

Selection through pollinators plays a major role in the evolution of reproductive traits. However, herbivory can also induce changes in plant sexual expression and sexual systems, potentially influencing conditions governing transitions between sexual systems. Previous work has shown that herbivory has a strong effect on sex allocation in the wind-pollinated annual plant Mercurialis annua, likely via responses to resource loss. It is also known that many plants respond to herbivory by inducing signaling, and endogenous responses to it, via the plant hormone jasmonate. Here, we attempt to uncouple the effects of herbivory on sex allocation in M. annua through resource limitation (tissue loss) versus plant responses to jasmonate hormone signaling. We used a two-factorial experiment with four treatment combinations: control, herbivory (25% chronic tissue loss), jasmonate, and combined herbivory and jasmonate. We estimated the effects of tissue loss and defense-inducing hormones on reproductive allocation, male reproductive effort, and sex allocation. Tissue loss caused plants to reduce their male reproductive effort, resulting in changes in total sex allocation. However, application of jasmonate after herbivory reversed its effect on male investment. Our results show that herbivory has consequences on plant sex expression and sex allocation, and that defense-related hormones such as jasmonate can buffer the impacts. We discuss the physiological mechanisms that might underpin the effects of herbivory on sex allocation, and their potential implications for the evolution of plant sexual systems.

Priming of Arabidopsis resistance to herbivory by insect egg deposition depends on the plant's developmental stage

While traits of plant resistance to herbivory often change during ontogeny, it is unknown whether the primability of this resistance depends on the plant's developmental stage. Resistance in non-flowering Arabidopsis thaliana against Pieris brassicae larvae is known to be primable by prior egg deposition on leaves. We investigated whether this priming effect is maintained in plants at the flowering stage. Larval performance assays revealed that flowering plants' resistance to herbivory was not primable by egg deposition. Accordingly, transcriptomes of flowering plants showed almost no response to eggs. In contrast, egg deposition on non-flowering plants enhanced the expression of genes induced by subsequent larval feeding. Strikingly, flowering plants showed constitutively high expression levels of these genes. Larvae performed generally worse on flowering than on non-flowering plants, indicating that flowering plants constitutively resist herbivory. Furthermore, we determined the seed weight in regrown plants that had been exposed to eggs and larvae during the non-flowering or flowering stage. Non-flowering plants benefitted from egg priming with a smaller loss in seed yield. The seed yield of flowering plants was unaffected by the treatments, indicating tolerance towards the larvae. Our results show that the primability of anti-herbivore defences in Arabidopsis depends on the plant's developmental stage.

Latitudinal resource gradient shapes multivariate defense strategies in a long‐lived shrub

Plant defense against herbivores is multidimensional, and investment into different defense traits is intertwined due to genetic, physiological, and ecological costs. This relationship is expected to generate a trade-off between direct defense and tolerance that is underlain by resource availability, with increasing resources being associated with increased investment in tolerance and decreased investment in direct resistance. We tested these predictions across populations of the shrub Artemisia californica by growing plants sourced from a latitudinal aridity gradient within common gardens located at the southern (xeric) and northern (mesic) portions of its distribution. We measured plant growth rate, resistance via a damage survey, and tolerance to herbivory by experimentally simulating vertebrate herbivory. Plants from more northern (vs. southern) environments were less resistant (received higher percent damage by vertebrate herbivores) and tended to be more tolerant (marginally significant) with respect to change in biomass measured 12 months after simulated vertebrate herbivory. Also, putative growth and defense traits paralleled patterns of resistance and tolerance, such that leaves from northern populations contained lower concentrations of terpenes and increased N, specific leaf area, and % water. Last, plant growth rate did not demonstrate clear clinal patterns, as northern populations (vs. southern populations) grew more slowly in the southern (xeric) garden, but there was no clinal relationship detected in the northern (mesic) garden. Overall, our findings support the prediction of lower resistance and higher tolerance in plant populations adapted to more resource-rich, mesic environments, but this trade-off was not associated with concomitant trade-offs in growth rate. These findings ultimately suggest that plant adaptation to resource availability and herbivory can shape intraspecific variation in multivariate plant defenses.

Are some species ‘robust’ to exploitation? Explaining persistence in deceptive relationships

Animals and plants trick others in an extraordinary diversity of ways to gain fitness benefits. Mimicry and deception can, for example, lure prey, reduce the costs of parental care or aid in pollination–in ways that impose fitness costs on the exploited party. The evolutionary maintenance of such asymmetric relationships often relies on these costs being mitigated through counter-adaptations, low encounter rates, or indirect fitness benefits. However, these mechanisms do not always explain the evolutionary persistence of some classic deceptive interactions.

Sexually deceptive pollination (in which plants trick male pollinators into mating with their flowers) has evolved multiple times independently, mainly in the southern hemisphere and especially in Australasia and Central and South America. This trickery imposes considerable costs on the males: they miss out on mating opportunities, and in some cases, waste their limited sperm on the flower. These relationships appear stable, yet in some cases there is little evidence suggesting that their persistence relies on counter-adaptations, low encounter rates, or indirect fitness benefits. So, how might these relationships persist?

Here, we introduce and explore an additional hypothesis from systems biology: that some species are robust to exploitation. Robustness arises from a species’ innate traits and means they are robust against costs of exploitation. This allows species to persist where a population without those traits would not, making them ideal candidates for exploitation. We propose that this mechanism may help inform new research approaches and provide insight into how exploited species might persist.

Chymotrypsin is a molecular target of insect resistance of three corn varieties against the Asian corn borer, Ostrinia furnacalis

The Asian corn borer, Ostrinia furnacalis, is a serious insect pest that can infest corn leaves and stems. Due to its internal feeding behavior, its larvae are not exposed to insecticides that are usually sprayed for pest control. To minimize crop damage caused by O. furnacalis, improving insect resistance trait of corn has been considered as an optimal control tactic. This study screened 27 corn varieties for their insect resistance trait and selected three varieties of Ilmichal (IM), P3394, and Kwangpyeongok (KP) that showed insect resistance trait. O. furnacalis larvae did not show any significant difference in preference between these three insect-resistant corn varieties and a control susceptible variety. However, these resistant varieties after ingestion significantly interfered with larval development of O. furnacalis. This suggests that the insect resistance trait is induced by antibiosis, but not by antixenosis. Indeed, larvae fed with these varieties suffered from low chymotrypsin (CHY) activities in the midgut juice. To determine the target CHY inhibited by resistant corn varieties, a total of nine CHY genes (Of-CHY1~Of-CHY9) were predicted from the transcriptome of O. furnacalis. Six genes (Of-CHY1~Of-CHY6) were expressed in all developmental stages and tissues. Especially, Of-CHY3 was highly expressed in the midgut of O. furnacalis larvae. RNA interference (RNAi) using double-stranded RNA (dsRNA) specific to Of-CHY3 (2 μg of dsRNA injected to each L5 larva) resulted in significant reduction of Of-CHY3 expression level at 24 h post-treatment. Feeding L3 larvae with this dsRNA also significantly suppressed the expression level of Of-CHY3 and reduced its enzyme activity at 24 h post-treatment. A recombinant Escherichia coli expressing dsRNA specific to Of-CHY3 was constructed using L4440 vector. Feeding such recombinant bacteria suppressed the expression level of Of-CHY3 and prevented larval development of O. furnacalis. These results suggest that the three resistant varieties can produce a resistance factor(s) to inhibit the CHY activity of O. furnacalis and suppress larval growth. This study suggests that CHY might be an inhibition target in O. furnacalis for breeding insect-resistant corns.

Stomata-mediated interactions between plants, herbivores, and the environment

Stomata play a central role in plant responses to abiotic and biotic stresses. Existing knowledge regarding the roles of stomata in plant stress is centered on abiotic stresses and plant-pathogen interactions, but how stomata influence plant-herbivore interactions remains largely unclear. Here, we summarize the functions of stomata in plant-insect interactions and highlight recent discoveries of how herbivores manipulate plant stomata. Because stomata are linked to interrelated physiological processes in plants, herbivory-induced changes in stomatal dynamics might have cellular, organismic, and/or even community-level impacts. We summarize our current understanding of how stomata mediate plant responses to herbivory and environmental stimuli, propose how herbivores may influence these responses, and identify key knowledge gaps in plant-herbivore interactions.

Early damage enhances compensatory responses to herbivory in wild lima bean

Damage by herbivores can induce various defensive responses. Induced resistance comprises traits that can reduced the damage, while compensatory responses reduce the negative effects of damage on plant fitness. Timing of damage may be essential in determining the patterns of induced defenses. Here, we tested how timing and frequency of leaf damage affect compensatory responses in wild lima bean plants in terms of growth and seed output, as well as their effects on induced resistance to seed beetles. To this end, we applied mechanical damage to plants at different ontogenetical stages, at one time point (juvenile stage only) or two time points (seedling and juvenile stage or juvenile and reproductive stage). We found that plants damaged at the seedling/juvenile stage showed higher compensatory growth, and seed output compared to plants damaged only at the juvenile stage or juvenile/reproductive stage. Seeds from plants damaged at the juvenile and juvenile/reproductive stages had fewer beetles than seeds from undamaged plants, however this was driven by a density dependent effect of seed abundance rather than a direct effect of damage treatments. We did not find differences in parasitism rate by parasitoid wasps on seed beetles among plant treatments. Our results show that damage at the seedling stage triggers compensatory responses which implies that tolerance to herbivory is enhanced or primed by early damage. Herbivory often occurs at several time points throughout plant development and this study illustrates that, for a full understanding of the factors associated with plant induced responses in a dynamic biotic environment, it is important to determine the multitrophic consequences of damage at more than one ontogenetical stage.

Response of Five Miscanthus sinensis Cultivars to Grasshopper Herbivory: Implications for Monitoring of Invasive Grasses in Protected Areas

Introduced grasses can aggressively expand their range and invade native habitats, including protected areas. Miscanthus sinensis is an introduced ornamental grass with 100+ cultivars of various invasive potential. Previous studies have demonstrated that the invasive potential of M. sinensis cultivars may be linked to seed viability, and some of the physiological traits, such as growth rate. Little is known, however, about whether these traits are associated with response of M. sinensis to insect herbivory, and whether plant tolerance and resistance to herbivory vary among its cultivars; which, in turn, can contribute to the invasive potential of some of M. sinensis cultivars. To address this issue, in our study we explored the response of five cultivars of M. sinensis to herbivory by Melanoplus grasshoppers. We demonstrated that plant responses varied among the cultivars during a season; all the cultivars, but “Zebrinus”, demonstrated a significant increase in plant tolerance by the end of the growing season regardless of the amount of sustained leaf damage. Different patterns in plant responses from “solid green” and “striped/spotted” varieties were recorded, with the lowest plant resistance detected for “Autumn Anthem” in the cage experiment. Our results have important applications for monitoring low-risk invaders in protected areas, as well as for biotic resistance of native communities to invasive grasses.

Response of a carnivorous plant to simulated herbivory

Studies addressing the impact of herbivory in carnivorous plants are scarce, despite loss of tissue being expected to be costly, as leaves are involved in both energy and nutrient acquisition. We evaluated the effect of simulated herbivory in a population of the carnivorous plant Pinguicula moranensis. We predicted an overall negative impact of herbivory by reducing growth, flowering probability and survivorship. Specifically, we expected that the increase in the negative effect should be related to the amount of leaf area removed. We performed simulated herbivory in plants growing in situ (2015) and ex situ (greenhouse, 2016) using a paper punch (herbivory ranging from 0 to 50%). In the ex-situ experiment, we also tested the effect of availability of resources by implementing a prey addition trial where half of the plants were supplemented with prey (one fruit fly per week). In both experiments, our hypothesis was not supported, as growth, flowering probability and survivorship were not influenced by herbivory treatments. Plants subjected to prey addition did not show higher performance than plants deprived of prey, contradicting our prediction that availability of resources could ameliorate the effect of herbivory. Overall, the lack of differences in performance between undamaged (control) and damaged plants indicate an unexpected short-term ability of P. moranensis to tolerate herbivory, even at high levels of damage.

Herbivory and jasmonate treatment affect reproductive traits in wild Lima bean, but without transgenerational effects

PREMISE

Plant responses to herbivores and their elicitors include changes in traits associated with phenology, defense, and reproduction. Induced responses by chewing herbivores are known to be hormonally mediated by the jasmonate pathway and can cascade and affect late-season seed predators and pollinators. Moreover, herbivore-induced plant responses can be transmitted to the next generation. Whether herbivore-induced transgenerational effects also apply to phenological traits is less well understood.

METHODS

Here, we explored responses of wild lima bean plants (Phaseolus lunatus) to herbivory and jasmonate treatment and possible transgenerational effects of herbivore-induced early flowering. In a controlled field experiment, we exposed lima bean plants to herbivory by leaf beetles or methyl jasmonate sprays (MJ). We then compared plant development, phenology, reproductive fitness and seed traits among these treatments and undamaged, untreated control plants.

RESULTS

We found that MJ and leaf herbivory induced similar responses, with treated plants growing less, flowering earlier, and producing fewer seeds than undamaged plants. However, seed size, phenolics and cyanogenic glycosides concentrations did not differ among treatments. Seed germination rates and flowering time of the offspring were similar among maternal treatments.

CONCLUSIONS

Overall, the results confirm that responses of lima bean to herbivory by leaf beetles are mediated by jasmonate; however, effects on phenological traits are not transmitted to the next generation. We discuss why transgenerational effects of herbivory might be restricted to traits that directly target herbivores.

Closely related parasitic plants have similar host requirements and related effects on hosts

The performance of root hemiparasites depends strongly on host species identity, but it remains unknown whether there exist general patterns in the quality of species as hosts for hemiparasites and in their sensitivity to parasitism. In a comparative approach, the model root hemiparasites Rhinanthus minor and R. alectorolophus were grown with 25 host species (grasses, forbs, and legumes) at two nutrient levels. Hosts grown without parasites served as a control. Host species identity strongly influenced parasite biomass and other traits, and both parasites grew better with legumes and grasses than with forbs. The biomass of R. alectorolophus was much higher than that of R. minor with all host plants and R. alectorolophus responded much more strongly to higher nutrient availability than R. minor. The performance of the two species of Rhinanthus with individual hosts was strongly correlated, and it was also correlated with that of R. alectorolophus and the related Odontites vulgaris in previous experiments with many of the same hosts, but only weakly with that of the less closely related Melampyrum arvense. The negative effect of R. minor on host biomass was less strong than that of R. alectorolophus, but stronger relative to its own biomass, suggesting that it is more parasitic. The impact of the two parasites on individual hosts did not depend on nutrient level and was correlated. Several legumes and grasses were tolerant of parasitism. While R. minor slightly reduced mean overall productivity, R. alectorolophus increased it with several species, indicating that the loss of host biomass was more than compensated by that of the parasite. The results show that closely related parasites have similar host requirements and correlated negative effects on individual hosts, but that there are also specific interactions between pairs of parasitic plants and their hosts.

Ecological consequences of herbivory by Japanese macaques (Macaca fuscata) on succession patterns of tree assemblages: A case of snowy regions in Japan

Apart from frugivory, we have limited knowledge of the ecological consequences of primate herbivory. We aimed to ascertain the effects of spring folivory and winter bark/bud herbivory by Japanese macaques (Macaca fuscata) on tree species and succession patterns of cool-temperate forests with heavy snow. To evaluate the impact of herbivory on individual trees, we assessed the growth and mortality of trees consumed by simulating herbivory on nine tree species over 4 years. Additionally, we assessed the cumulative impacts of bark/bud herbivory observed at the tree community level by monitoring the patterns of natural herbivory for almost a decade and evaluating the structure of tree assemblages in places with different cumulative impacts of herbivory. The results of simulated herbivory showed that the mortality caused by both spring and winter herbivory was limited (<20%) for almost all tree species monitored; however, the simulated folivory led to delayed tree growth and/or weakening of tree architecture. In contrast, the simulated bark/bud herbivory sometimes resulted in overcompensation of the tree consumed. The multiyear monitoring of natural herbivory demonstrated that, while bark/bud herbivory did not reduced the diversity and biomass of tree assemblages, the cumulative impacts of natural herbivory could have affected the tree succession pattern, resulting in increasing the availability of bark/buds preferably fed by macaques. The key cause for this feedback effect of herbivory on available foods of macaques might be heavy snow conditions, which could physically and physiologically restrain the excessive bark/buds herbivory by macaques.

Compensation of Wild Plants Weakens the Effects of Crop-Wild Gene Flow on Wild Rice Populations

Crop-wild gene flow may alter the fitness of the recipient i.e., crop-wild hybrids, then potentially impact wild populations, especially for the gene flow carrying selective advantageous crop alleles, such as transgenes conferring insect resistance. Given the continuous crop-wild gene flow since crop domestication and the occasionally stressful environments, the extant wild populations of most crops are still "wild." One interpretation for this phenomenon is that wild populations have the mechanism buffered for the effects of crop alleles. However, solid evidence for this has been scarce. We used wild rice (Oryza rufipogon) and transgenic (Bt/CpTI) rice (O. sativa) as a crop-wild gene flow model and established cultivated, wild, and F7 hybrid rice populations under four levels of insect (Chilo suppressalis) pressure. Then, we measured the trait performance of the plants and estimated fitness to test the compensatory response of relatively high fitness compared to the level of insect damage. The performance of all plants varied with the insect pressure level; wild plants had higher insect-tolerance that was expressed as over- or equal-compensatory responses to insect damage, whereas crop and hybrids exhibited under-compensatory responses. The higher compensation resulted in a better performance of wild rice under insect pressure where transgenes conferring insect resistance had a somewhat beneficial effect. Remarkable hybrid vigour and the benefit effect of transgenes increased the fitness of hybrids together, but this joint effect was weakened by the compensation of wild plants. These results suggest that compensation to environmental stress may reduce the potential impacts of crop alleles on wild plants, thereby it is a mechanism maintaining the "wild" characteristics of wild populations under the scenario of continuous crop-wild gene flow.

Historic exposure to herbivores, not constitutive traits, explains plant tolerance to herbivory in the case of two Medicago species (Fabaceae)

Mechanisms that allow plants to survive and reproduce after herbivory are considered to play a key role in plant evolution. In this study, we evaluated how tolerance varies in species with different historic exposure to herbivores considering ontogeny. We exposed the range-restricted species Medicago citrina and its closely related and widespread species M. arborea to one and two herbivory simulations (80 % aerial biomass loss). Physiological and growth parameters related to tolerance capacity were assessed to evaluate constitutive values (without herbivory) and induced tolerance after damage. Constitutive traits were not always related to greater tolerance, and each species compensated for herbivory through different traits. Herbivory damage only led to mortality in M. citrina; adults exhibited root biomass loss and increased oxidative stress after damage, but also compensated aerial biomass. Despite seedlings showed a lower death percentage than adults after herbivory in M. citrina, they showed less capacity to recover control values than adults. Moderate tolerance to M. arborea herbivory and low tolerance to M. citrina is found. Thus, although the constitutive characteristics are maintained in the lineage, the tolerance of plants decreases in M. citrina. That represents how plants respond to the lack of pressure from herbivores in their habitat.

Overgrazing-induced legacy effects may permit Leymus chinensis to cope with herbivory

There is growing evidence that herbivory-induced legacy effects permit plants to cope with herbivory. However, herbivory-induced defense strategies in plants against grazing mammals have received little attention. To further understand the grazing-induced legacy effects on plants, we conducted a greenhouse experiment with Leymus chinensis experiencing different grazing histories. We focused on grazing-induced legacy effects on above-ground spatial avoidance and below-ground biomass allocation. Our results showed that L. chinensis collected from the continuous overgrazing plot (OG) exhibited higher performance under simulated grazing in terms of growth, cloning and colonizing ability than those collected from the 35-year no-grazing plot (NG). The enhanced adaptability of OG was attributed to increased above-ground spatial avoidance, which was mediated by larger leaf angle and shorter height (reduced vertical height and increased leaf angle contributed to the above-ground spatial avoidance at a lower herbivory stubble height, while reduced tiller natural height contributed to above-ground spatial avoidance at a higher herbivory stubble height). Contrary to our prediction, OG pre-allocated less biomass to the rhizome, which does not benefit the herbivory tolerance and avoidance of L. chinensis; however, this also may reflect a tolerance strategy where reduced allocation to rhizomes is associated with increased production of ramets.

The impact of Spodoptera exigua herbivory on Meloidogyne incognita-induced root responses depends on the nematodes' life cycle stages

Induced responses to above-ground and below-ground herbivores may interact via systemic signalling in plants. We investigated whether the impact of above-ground herbivory on root-knot nematode-induced responses depends on the nematode's life cycle stages. Tomato plants were infected with the nematode (Meloidogyne incognita) for 5, 15 or 30 days before receiving Spodoptera exigua caterpillars above-ground. We collected root materials after 24 h of caterpillar feeding. We investigated phytohormones and α-tomatine levels, and the expression of defence and glycoalkaloid metabolism (GAME) marker genes in tomato roots. Nematode infection alone increased the endogenous root levels of jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), α-tomatine and the expression of the GLYCOALKALOID METABOLISM 1 (GAME1) gene mostly at 30 days post-nematode inoculation. Caterpillar feeding alone upregulated Lipoxygenase D and downregulated Basic-β-1-glucanase and GAME1 expression in roots. On nematode-infected plants, caterpillar feeding decreased JA levels, but it increased the expression of Leucine aminopeptidase A. The induction patterns of ABA and SA suggest that caterpillars cause cross-talk between the JA-signalling pathway and the SA and ABA pathways. Our results show that caterpillar feeding attenuated the induction of the JA pathway triggered by nematodes, mostly in the nematodes' reproduction stage. These results generate a better understanding of the molecular and chemical mechanisms underlying frequent nematode-plant-caterpillar interactions in natural and agricultural ecosystems.

Proportional fitness loss and the timing of defensive investment: a cohesive framework across animals and plants

The risk of consumption is a pervasive aspect of ecology and recent work has focused on synthesis of consumer-resource interactions (e.g., enemy-victim ecology). Despite this, theories pertaining to the timing and magnitude of defenses in animals and plants have largely developed independently. However, both animals and plants share the common dilemma of uncertainty of attack, can gather information from the environment to predict future attacks and alter their defensive investment accordingly. Here, we present a novel, unifying framework based on the way an organism's ability to defend itself during an attack can shape their pre-attack investment in defense. This framework provides a useful perspective on the nature of information use and variation in defensive investment across the sequence of attack-related events, both within and among species. It predicts that organisms with greater proportional fitness loss if attacked will gather and respond to risk information earlier in the attack sequence, while those that have lower proportional fitness loss may wait until attack is underway. This framework offers a common platform to compare and discuss consumer effects and provides novel insights into the way risk information can propagate through populations, communities, and ecosystems.

Interspecific variation and elevated CO2 influence the relationship between plant chemical resistance and regrowth tolerance

To understand how comprehensive plant defense phenotypes will respond to global change, we investigated the legacy effects of elevated CO2 on the relationships between chemical resistance (constitutive and induced via mechanical damage) and regrowth tolerance in four milkweed species (Asclepias). We quantified potential resistance and tolerance trade-offs at the physiological level following simulated mowing, which are relevant to milkweed ecology and conservation. We examined the legacy effects of elevated CO2 on four hypothesized trade-offs between the following: (a) plant growth rate and constitutive chemical resistance (foliar cardenolide concentrations), (b) plant growth rate and mechanically induced chemical resistance, (c) constitutive resistance and regrowth tolerance, and (d) regrowth tolerance and mechanically induced resistance. We observed support for one trade-off between plant regrowth tolerance and mechanically induced resistance traits that was, surprisingly, independent of CO2 exposure. Across milkweed species, mechanically induced resistance increased by 28% in those plants previously exposed to elevated CO2. In contrast, constitutive resistance and the diversity of mechanically induced chemical resistance traits declined in response to elevated CO2 in two out of four milkweed species. Finally, previous exposure to elevated CO2 uncoupled the positive relationship between plant growth rate and regrowth tolerance following damage. Our data highlight the complex and dynamic nature of plant defense phenotypes under environmental change and question the generality of physiologically based defense trade-offs.

Disentangling the effects of foliar vs. floral herbivory of leaf-cutting ants on the plant reproductive success of Miconia nervosa (Smith) Triana (Family Melastomataceae)

Flower and leaf herbivory might cause relevant and negative impacts on plant fitness. While flower removal or damage by florivores produces direct negative effects on plant fitness, folivores affect plant fitness by reducing resource allocation to reproduction. In this study, we examine the effects of both flower and leaf herbivory by leaf-cutting ants on the reproductive success of the shrub species Miconia nervosa (Smith) Triana (Family Melastomataceae) in a fragment of Atlantic Forest in Northeast Brazil. We conducted a randomized block-designed field experiment with nine replicates (blocks), in which three plants per block were assigned to one of the three following treatments: undamaged plants (ant exclusion), leaf-damaged plants (ant exclusion from reproductive organs, but not from leaves), and flower + leaf-damaged plants (no exclusion of ants). We then measured flower production, fruit set, and fruit production. Our results showed that flower + leaf-damaged plants reduced flower production nearly twofold in relation to undamaged plants, while flower set in leaf-damaged plants remained constant. The number of flowers that turned into fruits (i.e., fruit set), however, increased by 15% in flower + leaf-damaged plants, while it slightly decreased in leaf-damaged compared to undamaged plants. Contrastingly, fruit production was similar between all treatments. Taken together, our results suggest a prominent role of ant floral herbivory across different stages of the reproductive cycle in M. nervosa, with no consequences on final fruit production. The tolerance of M. nervosa to leaf-cutting ant herbivory might explain its high abundance in human-modified landscapes where leaf-cutting ants are hyper-abundant.

A unified framework for plant life-history strategies shaped by fire and herbivory

Fire and herbivory both remove aboveground biomass. Environmental factors determine the type and intensity of these consumers globally, but the traits of plants can also alter their propensity to burn and the degree to which they are eaten. To understand plant life-history strategies associated with fire and herbivory we need to describe both response and effect functional traits, and how they sort within communities, along resource gradients, and across evolutionary timescales. Fire and herbivore functional traits are generally considered separately, but there are advances made in understanding fire that relate to herbivory, and vice versa. Moreover, fire and herbivory interact: the presence of one consumer affects the type and intensity of the other. Here, we present a unifying conceptual framework to understand plant strategies that enable tolerance and persistence to fire and herbivory. Using grasses as an example, we discuss how flammability and fire tolerance, palatability, and grazing tolerance traits might organize themselves in ecosystems exposed to these consumers, and how these traits might have evolved with reference to other strong selective processes, like aridity. Our framework can be used to predict both the diversity of life-history strategies and plant species diversity under different consumer regimes.

The effect of allometric partitioning on herbivory tolerance in four species in South China

Herbivory tolerance can offset the negative effects of herbivory on plants and plays an important role in both immigration and population establishment. Biomass reallocation is an important potential mechanism of herbivory tolerance. To understand how biomass allocation affects plant herbivory tolerance, it is necessary to distinguish the biomass allocations resulting from environmental gradients or plant growth. There is generally a tight balance between the amounts of biomass invested in different organs, which must be analyzed by means of an allometric model. The allometric exponent is not affected by individual growth and can reflect the changes in biomass allocation patterns of different parts. Therefore, the allometric exponent was chosen to study the relationship between biomass allocation pattern and herbivory tolerance. We selected four species (Wedelia chinensis, Wedelia trilobata, Merremia hederacea, and Mikania micrantha), two of which are invasive species and two of which are accompanying native species, and established three herbivory levels (0%, 25% and 50%) to compare differences in allometry. The biomass allocation in stems was negatively correlated with herbivory tolerance, while that in leaves was positively correlated with herbivory tolerance. Furthermore, the stability of the allometric exponent was related to tolerance, indicating that plants with the ability to maintain their biomass allocation patterns are more tolerant than those without this ability, and the tendency to allocate biomass to leaves rather than to stems or roots helps increase this tolerance. The allometric exponent was used to remove the effects of individual development on allocation pattern, allowing the relationship between biomass allocation and herbivory tolerance to be more accurately explored. This research used an allometric model to fit the nonlinear process of biomass partitioning during the growth and development of plants and provides a new understanding of the relationship between biomass allocation and herbivory tolerance.

Compensation to simulated insect leaf herbivory in wild cotton (Gossypium hirsutum): responses to multiple levels of damage and associated traits

Identifying the mechanisms of compensation to insect herbivory remains a major challenge in plant biology and evolutionary ecology. Most previous studies have addressed plant compensatory responses to one or two levels of insect herbivory, and the underlying traits mediating such responses remain elusive in many cases. We evaluated responses associated with compensation to multiple intensities of leaf damage (0% control, 10%, 25%, 50%, 75% of leaf area removed) by means of mechanical removal of foliar tissue and application of a caterpillar (Spodoptera exigua) oral secretions in 3-month-old wild cotton plants (Gossypium hirsutum). Four weeks post-treatment, we measured plant growth and multiple traits associated with compensation, namely: changes in above- and belowground, biomass and the concentration of nutrients (nitrogen and phosphorus) and non-structural carbon reserves (starch and soluble sugars) in roots, stems and leaves. We found that wild cotton fully compensated in terms of growth and biomass allocation when leaf damage was low (10%), whereas moderate (25%) to high leaf damage in some cases led to under-compensation. Nonetheless, high levels of leaf removal (50% and 75%) in most cases did not cause further reductions in height and allocation to leaf and stem biomass relative to low and moderate damage. There were significant positive effects of leaf damage on P concentration in leaves and stems, but not roots, as well as a negative effect on soluble sugars in roots. These results indicate that wild cotton fully compensated for a low level of leaf damage but under-compensated under moderate to high leaf damage, but can nonetheless sustain growth despite increasing losses to herbivory. Such responses were possibly mediated by a re-allocation of carbohydrate reserves from roots to shoots.

Heritability of growth and leaf loss compensation in a long-lived tropical understorey palm

Introduction

Defoliation and light competition are ubiquitous stressors that can strongly limit plant performance. Tolerance to defoliation is often associated with compensatory growth, which could be positively or negatively related to plant growth. Genetic variation in growth, tolerance and compensation, in turn, plays an important role in the evolutionary adaptation of plants to changing disturbance regimes but this issue has been poorly investigated for long-lived woody species. We quantified genetic variation in plant growth and growth parameters, tolerance to defoliation and compensation for a population of the understorey palm Chamaedorea elegans. In addition, we evaluated genetic correlations between growth and tolerance/compensation.

Methods

We performed a greenhouse experiment with 711 seedlings from 43 families with twelve or more individuals of C. elegans. Seeds were collected in southeast Mexico within a 0.7 ha natural forest area. A two-third defoliation treatment (repeated every two months) was applied to half of the individuals to simulate leaf loss. Compensatory responses in specific leaf area, biomass allocation to leaves and growth per unit leaf area were quantified using iterative growth models.

Results

We found that growth rate was highly heritable and that plants compensated strongly for leaf loss. However, genetic variation in tolerance, compensation, and the individual compensatory responses was low. We found strong correlations between family mean growth rates in control and defoliation treatments. We did not find indications for growth-tolerance/compensation trade-offs: genetic correlation between tolerance/compensation and growth rate were not significant.

Implications

The high genetic variation in growth rate, but low genetic variation in tolerance and compensation observed here suggest high ability to adapt to changes in environment that require different growth rates, but a low potential for evolutionary adaptation to changes in damage or herbivory. The strong correlations between family mean growth rates in control and defoliation treatments suggest that performance differences among families are also maintained under stress of disturbance.

Overcompensation: a 30-year perspective

Biomass removal by herbivores usually incurs a fitness cost for the attacked plants, with the total cost per unit lost tissue depending on the value of the removed tissue (i.e., how costly it is to be replaced by regrowth). Optimal defense theory, first outlined in the 1960s and 1970s, predicted that these fitness costs result in an arms race between plants and herbivores, in which selection favors resistance strategies that either repel herbivores through morphological and chemical resistance traits in order to reduce their consumption, or result in enemy escape through rapid growth or by timing the growth or flowering to the periods when herbivores are absent. Such resistance against herbivores would most likely evolve when herbivores are abundant, cause extensive damage, and consume valuable plant tissues. The purpose of this Special Feature is to celebrate the 30th anniversary of the phenomenon of overcompensation, specifically, where the finding has brought us and where it is leading us 30 yr later. We first provide a short overview of how the phenomenon of overcompensation has led to broader studies on plant tolerance to herbivory, summarize key findings, and then discuss some promising new directions in light of six featured research papers.

Release from natural enemies mitigates inbreeding depression in native and invasive Silene latifolia populations

Inbreeding and enemy infestation are common in plants and can synergistically reduce their performance. This inbreeding ×environment (I × E) interaction may be of particular importance for the success of plant invasions if introduced populations experience a release from attack by natural enemies relative to their native conspecifics. Here, we investigate whether inbreeding affects plant infestation damage, whether inbreeding depression in growth and reproduction is mitigated by enemy release, and whether this effect is more pronounced in invasive than native plant populations. We used the invader Silene latifolia and its natural enemies as a study system. We performed two generations of experimental out- and inbreeding within eight native (European) and eight invasive (North American) populations under controlled conditions using field-collected seeds. Subsequently, we exposed the offspring to an enemy exclusion and inclusion treatment in a common garden in the species' native range to assess the interactive effects of population origin (range), breeding treatment, and enemy treatment on infestation damage, growth, and reproduction. Inbreeding increased flower and leaf infestation damage in plants from both ranges, but had opposing effects on fruit damage in native versus invasive plants. Inbreeding significantly reduced plant fitness; whereby, inbreeding depression in fruit number was higher in enemy inclusions than exclusions. This effect was equally pronounced in populations from both distribution ranges. Moreover, the magnitude of inbreeding depression in fruit number was lower in invasive than native populations. These results support that inbreeding has the potential to reduce plant defenses in S. latifolia, which magnifies inbreeding depression in the presence of enemies. However, future studies are necessary to further explore whether enemy release in the invaded habitat has actually decreased inbreeding depression and thus facilitated the persistence of inbred founder populations and invasion success.

J, Bush SE, Clayton DH. Annual environmental variation influences host tolerance to parasites

When confronted with a parasite or pathogen, hosts can defend themselves by resisting or tolerating the attack. While resistance can be diminished when resources are limited, it is unclear how robust tolerance is to changes in environmental conditions. Here, we investigate the sensitivity of tolerance in a single host population living in a highly variable environment. We manipulated the abundance of an invasive parasitic fly, Philornis downsi, in nests of Galápagos mockingbirds ( Mimus parvulus) over four field seasons and measured host fitness in response to parasitism. Mockingbird tolerance to P. downsi varied significantly among years and decreased when rainfall was limited. Video observations indicate that parental provisioning of nestlings appears key to tolerance: in drought years, mockingbirds likely do not have sufficient resources to compensate for the effects of P. downsi. These results indicate that host tolerance is a labile trait and suggest that environmental variation plays a major role in mediating the consequences of host-parasite interactions.

Unravelling associations between tree-seedling performance, herbivory, competition, and facilitation in high nature value farmlands

Herbivory, plant facilitation, and competition have complex impacts on tree regeneration which are seldom investigated together. Grazing exclosure experiments have allowed quantification of the effects of large herbivores on tree regeneration dynamics but have often ignored the effect of herbivorous insects. We experimentally tested how folivory (percentage of leaf damaged by insects) and microenvironment (tree canopy cover and herbs) jointly alter performance (growth and survival) of seedlings of two dominant Mediterranean oak-species within ungulate exclosures in a 3-year field study. An agroforestry system dominated by cork oak (Quercus suber) and holm oak (Q. rotundifolia) was assessed in south-east Portugal. We aimed also to determine whether the two oak species differed in the interdependences between folivory, microenvironment, covaring factors, and seedling performance. Unexpectedly, under the low-moderate insect defoliation, growth and survival of cork and holm oak seedlings were positively associated with herbivore damage. Herb removal increased oak folivory by 1.4 times. Herb removal was also positively associated with growth, directly and indirectly through its negative effect on oak folivory. Tree canopy favored insect folivory upon cork oak seedlings directly and upon holm oak indirectly via decreasing light availability. Folivory was threefold greater upon cork than upon holm oak-seedlings. Our study shows that tree canopy, herbs, and covarying factors can affect cork and holm oak-seedling performances through complex pathways, which markedly differ for the two species. The combined effect of insect herbivory and positive and negative plant-plant interactions need to be integrated into future tree regeneration efforts toward tackling the regeneration crisis of oak agroforestry systems of the Mediterranean.

Physical, but not chemical, antiherbivore defense expression is related to the clustered spatial distribution of tropical trees in an Amazonian forest

The conspecific negative density dependence hypothesis states that mortality of young trees (seedlings and saplings) is higher near conspecific adults due to mechanisms such as allelopathy, intraspecific competition, and pest facilitation, explaining why in the tropics, most of plant species tend to be rare and live dispersed. However, there are some tree species that defy this expectation and grow in large clusters of conspecific juveniles and adults. We hypothesize that conspecifics living in clusters show higher and/or more variable defensive profiles than conspecifics with dispersed distributions.We evaluated our hypothesis by assessing the expression of physical leaf traits (thickness, and the resistance to punch, tear and shear) and leaf chemical defenses for six clustered and six non-clustered tree species in Yasuní National Park, Ecuadorian Amazon. We ask ourselves whether (a) clustered species have leaves with higher physical resistance to damage and more chemical defenses variability than non-clustered species; (b) saplings of clustered species may show higher physical resistance to damage and higher variation on chemical leaf defenses than their conspecific adults, and (c) saplings of non-clustered species show lower resistance to physical damage and lower variation in chemical defenses compared to conspecific adults.Overall, our study did not support any of our hypotheses. Remarkably, we found that soluble metabolites were significantly species-specific.Our study suggests that plants physical but not chemical leaf antiherbivore defenses may be a crucial strategy for explaining survivorship of clustered species. Trees in Yasuní may also fall along a suite of tolerance/escape/defense strategies based on limitations of each species physiological constraints for survival and establishment. We conclude that other mechanisms, such as those related to indirect defenses, soil nutrient exploitation efficiency, volatile organic compounds, delayed leaf-greening, and seed dispersal mechanisms, shall be evaluated to understand conspecific coexistence in this forest.

Drought weakens the positive effects of defoliation on native rhizomatous grasses but enhances the drought-tolerance traits of native caespitose grasses

The objective of this study was to evaluate the drought tolerance, compensatory growth, and different plant traits between two native perennial caespitose grasses and two native rhizomatous grasses in response to drought and defoliation. A randomized complete block design at the Swift Current Research and Development Centre (SCRDC) of Agriculture and Agri-Food Canada (AAFC) examined the effects of water stress and clipping on the plant biomass, plant morphological traits, and relative leaf chlorophyll content (SPAD value) of four native grasses (caespitose grass: Hesperostipa comata and H. curtiseta; rhizomatous grass: Pascopyrum smithii and Elymus lanceolatus). Drought drastically decreased the shoot and root biomass, plant height, number of tillers and leaf growth of P. smithii and E. lanceolatus, as well as the rhizome biomass and R/S ratio of P. smithii. Defoliation had a positive effect on the shoot biomass of P. smithii and E. lanceolatus under well water treatments (100% and 85% of field capacity). However, the compensatory growth of P. smithii and E. lanceolatus significantly declined with increased water stress. In addition, there are no significant changes in plant biomass, plant height, number of tillers and leaves, and SPAD value of H. comata and H. curtiseta under relative dry condition (70% of field capacity). Consequently, these results demonstrated that the rhizomatous grasses possessed a stronger compensation in response to defoliation under wet conditions, but the positive effects of defoliation can be weakened by drought. The caespitose grasses (Hesperostipa species) exhibited a greater drought tolerance than rhizomatous grasses due to the relatively stable plant traits in response to water stress.

Evolutionary changes in growth, regrowth and carbohydrate storage in an invasive plant

We hypothesized that due to the absence of specialist herbivores in introduced ranges, invasive plants have evolved decreased allocation to carbohydrate storage for regrowth ability and as a consequence allocate more to growth. In this study, we compared plant growth, carbohydrate storage and regrowth ability of invasive and native Jacobaea vulgaris in response to complete shoot defoliation. We used invasive J. vulgaris genotypes from three geographically and climactically distinct regions and compared these with native genotypes from Europe. We found that invasive genotypes initially grew larger while native genotypes regrew larger after defoliation. Before defoliation, the carbohydrate storage in roots of invasive genotypes was 38% lower than native genotypes. Biomass after regrowth increased with root carbohydrate storage while it decreased with structural root mass, showing that it is crucial to study root storage and structural components separately in order to investigate plant regrowth. All studied traits of invasive populations from the three geographically and climatologically distinct regions changed in the same expected direction suggesting that the shifts in herbivore guild were causal to the observed change in growth and regrowth ability rather than environmental factors.

Finish line plant-insect interactions mediated by insect feeding mode and plant interference: a case study of Brassica interactions with diamondback moth and turnip aphid

There are gaps in our understanding of plant responses under different insect phytophagy modes and their subsequent effects on the insect herbivores' performance at late season. Here we compared different types of insect feeding by an aphid, Lipaphis erysimi, and a lepidopteran, Plutella xylostella, and how this affected defensive metabolites in leaves of 2 Brassica species when plants gain maturity. Thiocyanate concentrations after P. xylostella and L. erysimi feeding activities were the same. Total phenolics was higher after the phloem feeder feeding than the folivore activity. The plants compensatory responses (i.e., tolerance) to L. erysimi feeding was significantly higher than the responses to P. xylostella. This study showed that L. erysimi had higher carbon than P. xylostella whereas nitrogen in P. xylostella was 1.42 times that in L. erysimi. Population size of the phloem feeder was not affected by plant species or insect coexistence. However, there was no correlation between plant defensive metabolites and both insects' population size and biomass. This suggests that plant root biomass and tolerance index after different insect herbivory modes are not necessarily unidirectional. Importantly, the interaction between the folivore and the phloem feeder insects is asymmetric and the phloem feeder might be a trickier problem for plants than the folivore. Moreover, as both plants' common and special defenses decreased under interspecific interference, we suggest that specialist insect herbivores can be more challenged in ecosystems in which plants are not involved in interspecific interference.