Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores

The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model. We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field. EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side-chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores, but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores. These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.

Genetically correlated leaf tensile and morphological traits are driven by growing season length in a widespread perennial grass

PREMISE

Leaf tensile resistance, a leaf's ability to withstand pulling forces, is an important determinant of plant ecological strategies. One potential driver of leaf tensile resistance is growing season length. When growing seasons are long, strong leaves, which often require more time and resources to construct than weak leaves, may be more advantageous than when growing seasons are short. Growing season length and other ecological conditions may also impact the morphological traits that underlie leaf tensile resistance.

METHODS

To understand variation in leaf tensile resistance, we measured size-dependent leaf strength and size-independent leaf toughness in diverse genotypes of the widespread perennial grass Panicum virgatum (switchgrass) in a common garden. We then used quantitative genetic approaches to estimate the heritability of leaf tensile resistance and whether there were genetic correlations between leaf tensile resistance and other morphological traits.

RESULTS

Leaf tensile resistance was positively associated with aboveground biomass (a proxy for fitness). Moreover, both measures of leaf tensile resistance exhibited high heritability and were positively genetically correlated with leaf lamina thickness and leaf mass per area (LMA). Leaf tensile resistance also increased with the growing season length in the habitat of origin, and this effect was mediated by both LMA and leaf thickness.

CONCLUSIONS

Differences in growing season length may promote selection for different leaf lifespans and may explain existing variation in leaf tensile resistance in P. virgatum. In addition, the high heritability of leaf tensile resistance suggests that P. virgatum will be able to respond to climate change as growing seasons lengthen.

Crop Diversity in Agroecosystems for Pest Management and Food Production

During the past 30 years, there has been a growing belief in and promotion of agroecosystem diversity for pest management and future food production as an agroecological or nature-based approach. Monoculture agriculture, which produces most of our food, is considered to be highly vulnerable to pests in contrast to plant species-diverse agroecosystems which may possess a greater abundance of natural enemies, keeping pest populations under control. In this paper, we question the role of crop diversity for pest management and explore the relationship between crop and associated diversity and pests through the following processes: environmental stresses that favor monodominance; evolutionary adaptations that resist insect herbivores (genetic resistance response); mechanisms of spatial escape from insect herbivores (escape response); and the role of crop-associated biodiversity. We present strong evidence that not only questions the high vulnerability of monocultures to pest damage but also supports why monocultures continue to produce most of the world's food. Reference is made to the importance of targeted plant breeding and the role of trans-continental crop introduction supported by efficient quarantine for pest management. We conclude that-with the exception of irrigated rice-much more research is needed to better understand the role of crop diversity in agroecosystems for pest management and food production.

Vertical stratification and defensive traits of caterpillars against parasitoids in a lowland tropical forest in Cameroon

Insect herbivores and their parasitoids play a crucial role in terrestrial trophic interactions in tropical forests. These interactions occur across the entire vertical gradient of the forest. This study compares how caterpillar communities, and their parasitism rates, vary across vertical strata and between caterpillar defensive strategies in a semi deciduous tropical forest in Nditam, Cameroon. Within a 0.1 ha plot, all trees with a diameter at breast height (DBH) ≥ 5 cm were felled and systematically searched for caterpillars. We divided the entire vertical gradient of the forest into eight, five-metre strata. All caterpillars were assigned to a stratum based on their collection height, reared, identified, and classified into one of three defensive traits: aposematic, cryptic and shelter-building. Caterpillar species richness and diversity showed a midstory peak, whereas density followed the opposite pattern, decreasing in the midstory and then increasing towards the highest strata. This trend was driven by some highly dense shelter-building caterpillars in the upper canopy. Specialisation indices indicated decreasing levels of caterpillar generality with increasing height, a midstory peak in vulnerability, and increasing connectance towards the upper canopy, although the latter was likely driven by decreasing network size. Both aposematic and shelter-building caterpillars had significantly higher parasitism rates than cryptic caterpillars. Our results highlight nuanced changes in caterpillar communities across forest strata and provide evidence that defences strategies are important indicators of parasitism rates in caterpillars and that both aposematic and shelter-building caterpillars could be considered a "safe haven" for parasitoids.

Functional traits of fossil plants

A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo-ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait-based ecology, to evaluate which well-established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait-based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi-quantitative ranking of 26 paleo-functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo-ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo-demography (life history), and Earth system history can be derived through the application of paleo-functional traits to fossil plants.

Search for Snail Repellents: Antimollusc Activities from Stemona parviflora and Six Other Chinese Stemona Species

Snails are important agricultural pests difficult to control, but data regarding molluscicidal assays are scant. Stemona alkaloids are typical secondary metabolites for the taxa and have been broadly investigated for their pharmacological and toxicological effects. This makes it possible for us to further develop the toxicities of these compounds to snails. In this work, we tested the antifeedant properties of leaves from seven Chinese Stemona species against the land snail species Bradybaena ravida in choice and non-choice feeding assays. The tested leaves Stemona parviflora exhibited the most deterrent effects, and a further phytochemical investigation of aerial parts led to the identification of 16 alkaloids. Among them, three novel alkaloids could be identified. The alkaloidal fraction and single alkaloids were further assayed against this snail species, and the results suggest a cocktail effect because the impact of the alkaloidal fraction was higher than the effects caused by single alkaloids. The study can promote the search process of natural antimollusc products from plants to control snails.

Plant physical defenses contribute to a latitudinal gradient in resistance to insect herbivory within a widespread perennial grass

PREMISE

Herbivore pressure can vary across the range of a species, resulting in different defensive strategies. If herbivory is greater at lower latitudes, plants may be better defended there, potentially driving a latitudinal gradient in defense. However, relationships that manifest across the entire range of a species may be confounded by differences within genetic subpopulations, which may obscure the drivers of these latitudinal gradients.

METHODS

We grew plants of the widespread perennial grass Panicum virgatum in a common garden that included genotypes from three genetic subpopulations spanning an 18.5° latitudinal gradient. We then assessed defensive strategies of these plants by measuring two physical resistance traits-leaf mass per area (LMA) and leaf ash, a proxy for silica-and multiple measures of herbivory by caterpillars of the generalist herbivore fall armyworm (Spodoptera frugiperda).

RESULTS

Across all genetic subpopulations, low-latitude plants experienced less herbivory than high-latitude plants. Within genetic subpopulations, however, this relationship was inconsistent-the most widely distributed and phenotypically variable subpopulation (Atlantic) exhibited more consistent latitudinal trends than either of the other two subpopulations. The two physical resistance traits, LMA and leaf ash, were both highly heritable and positively associated with resistance to different measures of herbivory across all subpopulations, indicating their importance in defense against herbivores. Again, however, these relationships were inconsistent within subpopulations.

CONCLUSIONS

Defensive gradients that occur across the entire species range may not arise within localized subpopulations. Thus, identifying the drivers of latitudinal gradients in herbivory defense may depend on adequately sampling the diversity within a species.

The associational effects of host plant and mistletoe functional traits on leaf herbivory in mistletoe

Associational effects are a phenomenon in which herbivore damage on co-occurring plant species is influenced by neighboring plants. Mistletoes are a group of shrubs that obtain nutrients from host plants through haustoria. Despite the potential for mistletoe herbivory to be affected by associational effects with their hosts, the effects of host and mistletoe functional traits on mistletoe herbivory have been largely overlooked. This study aimed to evaluate the associational effects of host plants and the direct effects of mistletoe functional traits on mistletoe herbivory. To achieve this, we measured leaf herbivory and leaf traits of three mistletoe species (Dendrophthoe pentandra, Scurrula chingii var. yunnanensis, and Helixanthera parasitica) and their associated 11 host species during both dry and wet seasons. Our results showed that leaf herbivory of D. pentandra and S. chingii var. yunnanensis differed significantly on their respective host species, but H. parasitica did not. The relationships between mistletoe and the paired host herbivory differed between seasons, with a stronger positive relationship observed during the dry season. Furthermore, significant relationships were observed between paired leaf carbon, leaf nitrogen, and condensed tannin in mistletoes and their host plants, indicating that host plants can affect mistletoes' leaf functional traits. A group of mistletoe leaf traits provided significant predictions for leaf herbivory: leaves with higher leaf thickness and leaf total nitrogen showed higher herbivory. Overall, our study reveals that mistletoe leaf herbivory is directly affected by its leaf traits and indirectly affected by host associational effects, primarily through changes in mistletoes' leaf traits.

Leaf traits linked to structure and palatability drive plant-insect interactions within three forested ecosystems

PREMISE

Plant traits and insect herbivory have been highly studied within the modern record but only to a limited extent within the paleontological. Preservation influences what can be measured within the fossil record, but modern methods are also not compatible with paleobotanical methods. To remedy this knowledge gap, a comparable framework was created here using modern and paleobotanical methods, allowing for future comparisons within the fossil record.

METHODS

Insect feeding damage on selected tree species at Harvard Forest, the Smithsonian Environmental Research Center, and La Selva were characterized using the damage type system prevalent within paleobotanical studies and compared with leaf traits. Linear models and random forest analyses tested the influence of leaf traits on total, specialized, gall, and mine frequency and diversity.

RESULTS

Structural traits like leaf dry mass per area and palatability traits, including lignin and phosphorus concentrations, are important variables affecting gall and mine damage. The significance and strength of trait-herbivory relationships varied across forest types, which is likely driven by differences in local insect populations.

CONCLUSIONS

This work addresses the persistent gap between modern and paleoecological studies focusing on the influence of leaf traits on insect herbivory. This is important as modern climate change alters our understanding of plant-insect interactions, providing a need for contextualizing these relationships within evolutionary time. The fossil record provides information on terrestrial response to past climatic events and, thus, should be implemented when considering how to preserve biodiversity under current and future global change.

Cardiac glycosides protect wormseed wallflower (Erysimum cheiranthoides) against some, but not all, glucosinolate-adapted herbivores

The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well-defended plant lineage impacts interactions with diverse herbivores. Erysimum cheiranthoides (Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model.We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes in E. cheiranthoides and characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. Using E. cheiranthoides cardiac glycoside-deficient lines, we conducted insect experiments in both the laboratory and field.EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side chain cleavage, and EcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126 knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate-tolerant specialist herbivores but did not protect against all crucifer-feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores.These results begin elucidation of the E. cheiranthoides cardiac glycoside biosynthetic pathway and demonstrate in vivo that cardiac glycoside production allows Erysimum to escape from some, but not all, specialist herbivores.

Diet Mixing and Supplementation Present an Opportunity to Increase the Use of Encroaching Woody Plants by Goats

Along with the woody plant expansion that is predicted to continue at the expense of the grassy layer is the increasing societal demand for animal protein and livestock products. Unless concerted efforts by land users, ecologists, and animal scientists are made to increase the utilization of trees and shrubs as forage, it will be impossible to meet future demand for meat and meat products. We conducted two short-term pen experiments to determine the effects of (1) supplementation with polyethylene glycol (PEG-a polymer purported to bind and neutralize the negative effects of tannins), a high-protein source (soybean meal), and a high-energy source (yellow maize grain) and (2) diet mixing (single-species vs. multispecies diets) on the intake of condensed tannin-rich woody plants (i.e., Searsia lancea, S. pyroides, and Euclea crispa) by goats. While all three forage species were used in the diet mixing experiment (Exp. 2), only E. crispa was used in the supplementation experiment (Exp. 1). Supplementing goats with energy- and protein-rich sources significantly increased the intake of E. crispa (p < 0.05), 713.4 g ± 13.5 and 760 g ± 28.9, respectively, whereas those on the control diet maintained their intake at 540.32 g ± 11.2. Although PEG tended to increase the consumption of E. crispa by goats, the observed increase was not significant (p > 0.05) from that observed in other treatments. In the diet mixing experiment, goats offered a combination of all three forage species attained substantially higher dry matter intakes compared to the goats offered these species individually (p < 0.05). While longer-term field experiments are needed in the African savannas, we postulate from the current results that management strategies that provide animals with (1) a variety of species in the diet vs. monocultures and (2) a combination of nutrient-rich and tannin-rich species may improve the ability of goats to consume chemically defended woody plants.

Linking Anthropogenic Landscape Perturbation to Herbivory and Pathogen Leaf Damage in Tropical Tree Communities

Anthropogenic disturbance of tropical humid forests leads to habitat loss, biodiversity decline, landscape fragmentation, altered nutrient cycling and carbon sequestration, soil erosion, pest/pathogen outbreaks, among others. Nevertheless, the impact of these alterations in multitrophic interactions, including host-pathogen and vector-pathogen dynamics, is still not well understood in wild plants. This study aimed to provide insights into the main drivers for the incidence of herbivory and plant pathogen damage, specifically, into how vegetation traits at the local and landscape scale modulate such interactions. For this purpose, in the tropical forest of Calakmul (Campeche, Mexico), we characterised the foliar damage caused by herbivores and pathogens in woody vegetation of 13 sampling sites representing a gradient of forest disturbance and fragmentation in an anthropogenic landscape from well preserved to highly disturbed and fragmented areas. We also evaluated how the incidence of such damage was modulated by the vegetation and landscape attributes. We found that the incidence of damage caused by larger, mobile, generalist herbivores, was more sensitive to changes in landscape configuration, while the incidence of damage caused by small and specialised herbivores with low dispersal capacity was more influenced by vegetation and landscape composition. In relation to pathogen symptoms, the herbivore-induced foliar damage seems to be the main factor related to their incidence, indicating the enormous importance of herbivorous insects in the modulation of disease dynamics across tropical vegetation, as they could be acting as vectors and/or facilitating the entry of pathogens by breaking the foliar tissue and the plant defensive barriers. The incidence of pathogen damage also responded to vegetation structure and landscape configuration; the incidence of anthracnose, black spot, and chlorosis, for example, were favoured in sites surrounded by smaller patches and a higher edge density, as well as those with a greater aggregation of semi-evergreen forest patches. Fungal pathogens were shown to be an important cause of foliar damage for many woody species. Our results indicate that an increasing transformation and fragmentation of the tropical forest of southern Mexico could reduce the degree of specialisation in plant-herbivore interactions and enhance the proliferation of generalist herbivores (chewers and scrapers) and of mobile leaf suckers, and consequently, the proliferation of some symptoms associated with fungal pathogens such as fungus black spots and anthracnose. The symptoms associated with viral and bacterial diseases and to nutrient deficiency, such as chlorosis, could also increase in the vegetation in fragmented landscapes with important consequences in the health and productivity of wild and cultivated plant species. This is a pioneering study evaluating the effect of disturbances on multitrophic interactions, offering key insights on the main drivers of the changes in herbivory interactions and incidence of plant pathogens in tropical forests.

Leaf defenses of subtropical deciduous and evergreen trees to varying intensities of herbivory

Generally, deciduous and evergreen trees coexist in subtropical forests, and both types of leaves are attacked by numerous insect herbivores. However, trees respond and defend themselves from herbivores in different ways, and these responses may vary between evergreen and deciduous species. We examined both the percentage of leaf area removed by herbivores as well as the percentage of leaves attacked by herbivores to evaluate leaf herbivore damage across 14 subtropical deciduous and evergreen tree species, and quantified plant defenses to varying intensities of herbivory. We found that there was no significant difference in mean percentage of leaf area removed between deciduous and evergreen species, yet a higher mean percentage of deciduous leaves were damaged compared to evergreen leaves (73.7% versus 60.2%). Although percent leaf area removed was mainly influenced by hemicellulose concentrations, there was some evidence that the ratio of non-structural carbohydrates:lignin and the concentration of tannins contribute to herbivory. We also highlight that leaf defenses to varying intensities of herbivory varied greatly among subtropical plant species and there was a stronger response for deciduous trees to leaf herbivore (e.g., increased nitrogen or lignin) attack than that of evergreen trees. This work elucidates how leaves respond to varying intensities of herbivory, and explores some of the underlying relationships between leaf traits and herbivore attack in subtropical forests.

Effects of leaf traits of tropical trees on the abundance and body mass of herbivorous arthropod communities

In tropical forests, herbivorous arthropods remove between 7% up to 48% of leaf area, which has forced plants to evolve defense strategies. These strategies influence the palatability of leaves. Palatability, which reflects a syndrome of leaf traits, in turn influences both the abundance and the mean body mass not only of particular arthropod taxa but also of the total communities. In this study, we tested two hypotheses: (H1) The abundance of two important chewer guilds ('leaf chewers' and 'rostrum chewers'), dominant components of arthropod communities, is positively related to the palatability of host trees. (H2) Lower palatability leads to an increased mean body mass of chewers (Jarman-Bell principle). Arthropods were collected by fogging the canopies of 90 tropical trees representing 31 species in three plots at 1000 m and three at 2000 m a.s.l. Palatability was assessed by measuring several 'leaf traits' of each host tree and by conducting a feeding trial with the generalist herbivore Gryllus assimilis (Orthoptera, Gryllidae). Leaf traits provided partial support for H1, as abundance of leaf chewers but not of rostrum chewers was positively affected by the experimentally estimated palatability. There was no support for H2 as neither leaf traits nor experimentally estimated palatability affected the mean body mass of leaf chewers. The mean body mass of rostrum chewers was positively related to palatability. Thus, leaf traits and experimentally estimated palatability influenced the abundance and mean body mass of chewing arthropods on the community level. However, the data were not consistent with the Jarman-Bell principle. Overall, our results suggest that the palatability of leaves is not among the dominant factors influencing abundance and mean body mass of the community of chewing arthropod herbivores. If other factors, such as the microclimate, predation or further (a-)biotic interactions are more important has to be analyzed in refined studies.

Size-focused conservation may fail to protect the world's oldest trees

Old trees are irreplaceable natural resources that provide multifaceted benefits to humans. Current conservation strategies focus primarily on large-sized trees that were often considered old. However, some studies have demonstrated that small trees can be more than thousands of years old, suggesting that conventional size-focused perceptions may hamper the efficiency of current conservation strategies for old trees. Here, we compiled paired age and diameter data using tree-ring records sampled from 121,918 trees from 269 species around the world to detect whether tree size is a strong predictor of age for old trees and whether the spatial distribution of small old trees differs from that of large old trees. We found that tree size was a weak predictor of age for old trees, and diameter explained only 10% of the total age variance of old trees. Unlike large-sized trees that are mainly in warm, wet environments and protected, small old trees are predominantly in cold, dry environments and mostly unprotected, indicating that size-focused conservation failed to protect some of the oldest trees. To conserve old trees, comprehensive old-tree recognition systems are needed that consider not only tree size but also age and external characteristics. Protected areas designed for small old trees are urgently needed.

Phytochemical diversity impacts herbivory in a tropical rainforest tree community

Metabolomics provides an unprecedented window into diverse plant secondary metabolites that represent a potentially critical niche dimension in tropical forests underlying species coexistence. Here, we used untargeted metabolomics to evaluate chemical composition of 358 tree species and its relationship with phylogeny and variation in light environment, soil nutrients, and insect herbivore leaf damage in a tropical rainforest plot. We report no phylogenetic signal in most compound classes, indicating rapid diversification in tree metabolomes. We found that locally co-occurring species were more chemically dissimilar than random and that local chemical dispersion and metabolite diversity were associated with lower herbivory, especially that of specialist insect herbivores. Our results highlight the role of secondary metabolites in mediating plant-herbivore interactions and their potential to facilitate niche differentiation in a manner that contributes to species coexistence. Furthermore, our findings suggest that specialist herbivore pressure is an important mechanism promoting phytochemical diversity in tropical forests.

Silicon-based anti-herbivore defense in tropical tree seedlings

Silicon-based defenses deter insect herbivores in many cultivated and wild grass species. Furthermore, in some of these species, silicon (Si) uptake and defense can be induced by herbivory. Tropical trees also take up Si and leaf Si concentrations vary greatly across and within species. As herbivory is a major driver of seedling mortality and niche differentiation of tropical tree species, understanding anti-herbivore defenses is pivotal. Yet, whether silicon is a constitutive and inducible herbivory defense in tropical forest tree species remains unknown. We grew seedlings of eight tropical tree species in a full factorial experiment, including two levels of plant-available soil Si concentrations (-Si/+Si) and a simulated herbivory treatment (-H/+H). The simulated herbivory treatment was a combination of clipping and application of methyl jasmonate. We then carried out multiple-choice feeding trials, separately for each tree species, in which leaves of each treatment combination were offered to a generalist caterpillar (Spodoptera frugiperda). Leaf damage was assessed. Three species showed a significant decrease in leaf damage under high compared to low Si conditions (by up to 72%), consistent with our expectation of Si-based defenses acting in tropical tree species. In one species, leaf damage was increased by increasing soil Si and in four species, no effect of soil Si on leaf damage was observed. Opposite to our expectation of Si uptake and defense being inducible by herbivory damage, simulated herbivory increased leaf damage in two species. Furthermore, simulated herbivory reduced Si concentrations in one species. Our results showed that tropical tree seedlings can be better defended when growing in Si-rich compared to Si-poor soils, and that the effects of Si on plant defense vary strongly across species. Furthermore, Si-based defenses may not be inducible in tropical tree species. Overall, constitutive Si-based defense should be considered part of the vast array of anti-herbivore defenses of tropical tree species. Our finding that Si-based defenses are highly species-specific combined with the fact that herbivory is a major driver of mortality in tropical tree seedling, suggests that variation in soil Si concentrations may have pervasive consequences for regeneration and performance across tropical tree species.

A latitudinal gradient in Darwin's naturalization conundrum at the global scale for flowering plants

Darwin's naturalization conundrum describes two seemingly contradictory hypotheses regarding whether alien species closely or distantly related to native species should be more likely to naturalize in regional floras. Both expectations have accumulated empirical support, and whether such apparent inconsistency can be reconciled at the global scale is unclear. Here, using 219,520 native and 9,531 naturalized alien plant species across 487 globally distributed regions, we found a latitudinal gradient in Darwin's naturalization conundrum. Naturalized alien plant species are more closely related to native species at higher latitudes than they are at lower latitudes, indicating a greater influence of preadaptation in harsher climates. Human landscape modification resulted in even steeper latitudinal clines by selecting aliens distantly related to natives in warmer and drier regions. Our results demonstrate that joint consideration of climatic and anthropogenic conditions is critical to reconciling Darwin's naturalization conundrum.

Overlaps and trade-offs in the diversity and inducibility of volatile chemical profiles among diverse sympatric neotropical canopy trees

A central goal in ecology is to understand the mechanisms by which biological diversity is maintained. The diversity of plant chemical defences and the strategies by which they are deployed in nature may influence biological diversity. Trees in neotropical forests are subject to relatively high herbivore pressure. Such consistent pressure is thought to select for constitutive, non-flexible defence-related phytochemistry with limited capacity for inducible phytochemical responses. However, this has not been explored for volatile organic compounds (VOCs) that have a relatively low ratio of production costs to ecological benefits. To test this, I sampled VOCs emitted from canopy leaves of 10 phylogenetically diverse tree species (3 Magnoliids and 7 Rosids) in the Peruvian Amazon before and after induction with the phytohormone methyl jasmonate (MeJA). There was no phylogenetic signal in induction or magnitude of MeJA-induced VOC emissions from intact leaves: all trees induced VOC profiles dominated by β-ocimene, linalool, and α-farnesene of varying ratios. Moreover, overall inducibility of VOCs from intact leaves was unrelated to phytochemical diversity or richness. In contrast, experimentally wounded leaves showed considerable phylogeny-based and MeJA-independent variation the richness and diversity of constitutive wound-emitted VOCs. Moreover, VOC inducibility from wounded leaves correlated negatively with phytochemical richness and diversity, potentially indicating a tradeoff in constitutive and inducible defence strategies for non-volatile specialised metabolites but not for inducible VOCs. Importantly, there was no correlation between any chemical profile and either natural herbivory or leaf toughness. The coexistence of multiple phytochemical strategies in a hyper-diverse forest has broad implications for competitive and multitrophic interactions, and the evolutionary forces that maintain the exceptional plant biodiversity in neotropical forests.

Are rare plant species less resistant than common ones to herbivores? A multi-plant species study using above- and below-ground generalist herbivores

Rare plant species are suggested to be less resistant to herbivores than common species. Their lower apparency and the fact that they often live in isolated populations, resulting in fewer herbivore encounters, might have led to the evolution of reduced defences. Moreover, their frequent lower levels of genetic diversity compared with common species could negatively affect their resistance against enemies. However, the hypothesis that plant resistance depends on plant regional and local rarity, independently of habitat and competitive and growth strategy, lacks evidence. To test this hypothesis, we assessed the performance and preference of one belowground and three aboveground generalist invertebrate herbivores from different taxonomic groups as indicators of plant resistance. Herbivores were fed a total of 62 regionally and locally rare and common plant species from Switzerland. We accounted for differences in a plant's growth and competitive strategy and habitat resource availability. We found that regionally and locally rare and common plant species did not generally differ in their resistance to most generalist herbivores. However, one herbivore species even performed better and preferred locally and regionally common plant species over rarer ones, indicating that common species are not more resistant, but tend to be less resistant. We also found that all herbivore species consistently performed better on competitive and large plant species, although different herbivore species generally preferred and performed better on different plant species. The latter indicates that the use of generalist herbivores as indicators of plant-resistance levels can be misleading. Synthesis: Our results show that rare plant species are not inherently less resistant than common ones to herbivores. Instead, our results suggest that the ability of plants to allocate resources away from defence towards enhancing their competitive ability might have allowed plants to tolerate herbivory, and to become locally and regionally common.

Distribution of Dalbulus maidis (DeLong) (Hemiptera: Cicadellidae) and incidence of maize rayado fino virus and Candidatus Phytoplasma asteris in corn succession planting systems

BACKGROUND

Corn is one of the main crops grown globally to produce food for human consumption and animal feed, including raw materials for bioenergy. Effective pest management is critical for the economic viability of corn production. The leafhopper Dalbulus maidis and the diseases transmitted by it have become relevant to corn production. Our study aimed to determine environmental parameters that affect D. maidis populations and the impacts of pathogen dispersion on corn productivity under different rotation systems and sowing seasons.

RESULTS

The population density of leafhoppers found in the studied crops was low but capable of establishing the diseases and spreading them widely in the crops. The leafhopper's highest occurrence was in the corn vegetative development stage, and its population peaks were earlier in the corn off-season. The incidence of maize rayado fino virus and maize bushy stunt phytoplasma were higher in corn off-season than in the growing season. The incidence of diseases was higher in the final stages of the cultivation cycle. Yield losses were significantly higher for maize bushy stunt phytoplasma and not significant for maize rayado fino virus.

CONCLUSION

Our study observed that corn's physiological stage was the main factor influencing D. maidis dynamics. The occurrence of D. maidis at low densities was sufficient to ensure the efficient transmission and dissemination of maize rayado fino virus and maize bushy stunt phytoplasma, which had a higher incidence in the reproductive stage and the corn sowed off-season. © 2023 Society of Chemical Industry.

Trees harbouring ants are better defended than con-generic and sympatric ant-free trees

Plant strategies against herbivores are classically divided into chemical, physical, biotic defences. However, little is known about the relative importance of each type of plant defence, especially in the same species. Using the myrmecophyte Triplaris americana (both with and without ants), and the congeneric non-myrmecophyte T. gardneriana, we tested whether ant defence is more effective than other defences of naturally ant-free myrmecophytes and the non-myrmecophyte congeneric species, all spatially co-occurring. In addition, we investigated how plant traits vary among plant groups, and how these traits modulate herbivory. We sampled data on leaf area loss and plant traits from these tree groups in the Brazilian Pantanal floodplain, and found that herbivory is sixfold lower in plants with ants than in ant-free plants, supporting a major role of biotic defences against herbivory. Whereas ant-free plants had more physical defences (sclerophylly and trichomes), they had little effect on herbivory-only sclerophylly modulated herbivory, but with opposite effects depending on ants' presence and species identity. Despite little variation in the chemicals among plant groups, tannin concentrations and δ¹³C signatures negatively affected herbivory in T. americana plants with ants and in T. gardneriana, respectively. We showed that ant defence in myrmecophytic systems is the most effective against herbivory, as the studied plants could not fully compensate the lack of this biotic defence. We highlight the importance of positive insect-plant interactions in limiting herbivory, and therefore potentially plant fitness.

Landscape-level variability and insect herbivore outbreak captured within modern forests provides a framework for interpreting the fossil record

Temporal patterns of plant-insect interactions are readily observed within fossil datasets but spatial variability is harder to disentangle without comparable modern methods due to limitations in preservation. This is problematic as spatial variability influences community structure and interactions. To address this we replicated paleobotanical methods within three modern forests, creating an analogous dataset that rigorously tested inter- and intra-forest plant-insect variability. Random mixed effects models, non-metric multidimensional scaling (NMDS) ordinations, and bipartite network- and node-level metrics were used. Total damage frequency and diversity did not differ across forests but differences in functional feeding groups (FFGs) were observed across forests, correlating with plant diversity, evenness, and latitude. Overall, we found higher generalized herbivory within the temperate forests than the wet-tropical, a finding also supported by co-occurrence and network analyses at multiple spatial scales. Intra-forest analyses captured consistent damage type communities, supporting paleobotanical efforts. Bipartite networks captured the feeding outbreak of Lymantria dispar caterpillars; an exciting result as insect outbreaks have long been unidentifiable within fossil datasets. These results support paleobotanical assumptions about fossil insect herbivore communities, provide a comparative framework between paleobotanical and modern communities, and suggest a new analytical framework for targeting modern and fossil outbreaks of insect feeding.

Extremely high relative growth rate makes the cabbage white, Pieris rapae, a global pest with highly abundant and migratory nature

The small cabbage white butterfly, Pieris rapae, is an extraordinarily abundant migratory pest of cabbage that causes severe damage worldwide without known reasons. I here show that the average relative growth rate (RGR: the ratio of the daily increase of biomass to total biomass) of herbivore (Gh; an indicator of the growth speed of herbivore) of P. rapae on cabbage during the larval period is larger by far than those of all other insect-plant pairs tested. It exceeds 1.15 (/day),-meaning that the biomass more than doubles each day-compared to 0.1-0.7 for most insect-plant pairs, including that of Pieris melete, a sibling of P. rapae which never becomes a pest of cabbage. My data further showed the RGR in the larval stage (larval Gh), positively correlates with abundance and/or migratoriness of insect herbivores. These results together with my mathematical food web model suggest that the extraordinarily high larval Gh of P. rapae is the primary reason for its ubiquitously severe pest status accompanied with its abundance and migratoriness, and that the RGR of herbivores, Gh, characterizing the plant-herbivore interface at the bottom of the food webs is an important factor affecting whole ecosystems, including animal abundance, fauna size, plant damage levels, competitiveness among herbivorous species, determination of hostplant, invasiveness, and the evolution of animal traits involved in the so-called r/K strategy, such as migratoriness. Knowledge about Gh will be crucial to controlling pests and improving the negative effects of human activity on ecosystems including faunal decline (or defaunation).

Effects of plant diversity on species-specific herbivory: patterns and mechanisms

Invertebrate herbivory can shape plant communities when impacting growth and fitness of some plant species more than other species. Previous studies showed that herbivory varies among plant species and that species-specific herbivory is affected by the diversity of the surrounding plant community. However, mechanisms underlying this variation are still poorly understood. In this study, we investigate how plant traits and plant apparency explain differences in herbivory among plant species and we explore the effect of plant community diversity on these species-specific relationships. We found that species differed in the herbivory they experienced. Forbs were three times more damaged by herbivores than grasses. Variability within grasses was caused by differences in leaf dry matter content (LDMC). Furthermore, higher plant diversity increased herbivory on 15 plant species and decreased herbivory on nine species. Variation within forb and grass species in their response to changing plant diversity was best explained by species' physical resistance (LDMC, forbs) and biomass (grasses). Overall, our results show that herbivory and diversity effects on herbivory differ among species, and that, depending on the plant functional group, either species-specific traits or apparency are driving those differences. Thus, herbivores might selectively consume palatable forbs or abundant grasses with contrasting consequences for plant community composition in grasslands dominated by either forbs or grasses.

Interpreting random forest analysis of ecological models to move from prediction to explanation

As modeling tools and approaches become more advanced, ecological models are becoming more complex. Traditional sensitivity analyses can struggle to identify the nonlinearities and interactions emergent from such complexity, especially across broad swaths of parameter space. This limits understanding of the ecological mechanisms underlying model behavior. Machine learning approaches are a potential answer to this issue, given their predictive ability when applied to complex large datasets. While perceptions that machine learning is a "black box" linger, we seek to illuminate its interpretive potential in ecological modeling. To do so, we detail our process of applying random forests to complex model dynamics to produce both high predictive accuracy and elucidate the ecological mechanisms driving our predictions. Specifically, we employ an empirically rooted ontogenetically stage-structured consumer-resource simulation model. Using simulation parameters as feature inputs and simulation output as dependent variables in our random forests, we extended feature analyses into a simple graphical analysis from which we reduced model behavior to three core ecological mechanisms. These ecological mechanisms reveal the complex interactions between internal plant demography and trophic allocation driving community dynamics while preserving the predictive accuracy achieved by our random forests.

Bottom-up and top-down pressures mediate competition between two generalist insects

The effects of competition can have far-reaching consequences for individuals, populations, and communities and therefore we should strive toward a deeper understanding of competitive interactions. In some cases, dietary generalists may be predicted to experience weak competition effects because of their ability to use a wide range of host plants. However, competition between insects frequently occurs indirectly, which can hinder insects' abilities to avoid competitive interactions. Therefore, competition may be as strong among dietary generalists as among dietary specialists. Yet competition between insects that are dietary generalists is infrequently studied. We tested for evidence of competitive interactions between two common, temporally separated, generalist insects: the western tent caterpillar (Malacosoma californicum), which feeds early in the season, and the fall webworm (Hyphantria cunea), which feeds later in the season. Both species frequently use a common host plant species (chokecherry) as a preferred host at our field sites. We tested the relative strength of bottom-up effects resulting from competitive interactions between these two generalists with laboratory-rearing trials at the relevant time of year for each insect. We recorded three common fitness measures (development time, pupal mass, and survival) for caterpillars reared on chokecherry with no damage from either of our focal species, with tent caterpillar damage, and with fall webworm damage. To test the strength of top-down pressures on fall webworm larval fitness and any potential interactions with bottom-up effects, we reared larvae in the field either exposed to or protected from predators on host plants that either did or did not have tent caterpillars feeding on them earlier in the season. We found evidence of bottom-up fitness effects on tent caterpillars and top-down and bottom-up fitness effects on fall webworms confirming that tent caterpillars and fall webworms compete indirectly. Tent caterpillars had lower pupal mass when reared on leaves from shrubs damaged by fall webworms. Fall webworms had lower pupal mass and longer development time when reared on leaves from shrubs damaged by tent caterpillars. In field trials, fall webworms reared on shrubs damaged by tent caterpillars had a lower survival and pupal mass. We show evidence of indirect competition in temporally separated generalists through leaf quality (bottom-up effects) and natural enemies (top-down effects).

Ecological patterns and processes in the vertical dimension of terrestrial ecosystems

Climatic gradients such as latitude and elevation are considered primary drivers of global biogeography. Yet, alongside these macro-gradients, the vertical space and structure generated by terrestrial plants form comparable climatic gradients but at a fraction of the distance. These vertical gradients provide a spectrum of ecological space for species to occur and coexist, increasing biodiversity. Furthermore, vertical gradients can serve as pathways for evolutionary adaptation of species traits, leading to a range of ecological specialisations. In this review, we explore the ecological evidence supporting the proposition that the vertical gradient serves as an engine driving the ecology and evolution of species and shaping larger biogeographical patterns in space and time akin to elevation and latitude. Focusing on vertebrate and invertebrate taxa, we synthesised how ecological patterns within the vertical dimension shape species composition, distribution and biotic interactions. We identify three key ecological mechanisms associated with species traits that facilitate persistence within the vertical environment and draw on empirical examples from the literature to explore these processes. Looking forward, we propose that the vertical dimension provides an excellent study template to explore timely ecological and evolutionary questions. We encourage future research to also consider how the vertical dimension will influence the resilience and response of animal taxa to global change.

The global biomass and number of terrestrial arthropods

Insects and other arthropods are central to terrestrial ecosystems. However, data are lacking regarding their global population abundance. We synthesized thousands of evaluations from around 500 sites worldwide, estimating the absolute biomass and abundance of terrestrial arthropods across different taxa and habitats. We found that there are ≈1 × 1019 (twofold uncertainty range) soil arthropods on Earth, ≈95% of which are soil mites and springtails. The soil contains ≈200 (twofold uncertainty range) million metric tons (Mt) of dry biomass. Termites contribute ≈40% of the soil biomass, much more than ants at ≈10%. Our estimate for the global biomass of above-ground arthropods is more uncertain, highlighting a knowledge gap that future research should aim to close. We estimate the combined dry biomass of all terrestrial arthropods at ≈300 Mt (uncertainty range, 100 to 500), similar to the mass of humanity and its livestock. These estimates enhance the quantitative understanding of arthropods in terrestrial ecosystems and provide an initial holistic benchmark on their decline.

Season and herbivore defence trait mediate tri-trophic interactions in tropical rainforest

Bottom-up effects from host plants and top-down effects from predators on herbivore abundance and distribution vary with physical environment, plant chemistry, predator and herbivore trait and diversity. Tri-trophic interactions in tropical ecosystems may follow different patterns from temperate ecosystems due to differences in above abiotic and biotic conditions. We sampled leaf-chewing larvae of Lepidoptera (caterpillars) from a dominant host tree species in a seasonal rainforest in Southwest China. We reared out parasitoids and grouped herbivores based on their diet preferences, feeding habits and defence mechanisms. We compared caterpillar abundance with leaf numbers ('bottom-up' effects) and parasitoid abundance ('top-down' effects) between different seasons and herbivore traits. We found bottom-up effects were stronger than top-down effects. Both bottom-up and top-down effects were stronger in the dry season than in the wet season, which were driven by polyphagous rare species and host plant phenology. Contrary to our predictions, herbivore traits did not influence differences in the bottom-up or top-down effects except for stronger top-down effects for shelter-builders. Our study shows season is the main predictor of the bottom-up and top-down effects in the tropics and highlights the complexity of these interactions.

Does Ungulate Herbivory Translate into Diversity of Woody Plants? A Long-Term Study in a Montane Forest Ecosystem in Austria

Different species-specific traits of woody plant species, feeding preferences of herbivores together with resulting effects on plant competition are expected to translate into different plant community structures and expressions of biodiversity. We studied the diversity of woody plant species (trees and shrubs) and structural diversity of forest trees, using a 30-year and an 18-year dataset of ungulate exclosure-control plot pairs in a mixed alpine forest community in Austria. We surveyed the tallest individuals per tree species and plot and analyzed the collective of top-height individuals per plot pair. Incidence data for exclosure and control plots were aggregated. Comparing species diversity and diversity of height classes on the plots throughout time, we calculated diversity profiles based on Hill numbers. Diversity of top height individuals and structural diversity, expressed by height classes, were two diversity aspects that differed between exclosures and control plots. Other diversity estimates of woody plant species showed huge variation without significant differences between plots. Height growth was significantly suppressed by ungulate herbivory. Effects of ungulate herbivores in forest ecosystems are highly complex and context-dependent and thus not reducible to simple top-down forces. Long-term surveys provide data that reflect “ultimate” effects of herbivory interacting with other drivers of community dynamics.

Wet and dry extremes reduce arthropod biomass independently of leaf phenology in the wet tropics

Warming temperatures are increasing rainfall extremes, yet arthropod responses to climatic fluctuations remain poorly understood. Here, we used spatiotemporal variation in tropical montane climate as a natural experiment to compare the importance of biotic versus abiotic drivers in regulating arthropod biomass. We combined intensive field data on arthropods, leaf phenology and in situ weather across a 1700-3100 m elevation and rainfall gradient, along with desiccation-resistance experiments and multi-decadal modelling. We found limited support for biotic drivers with weak increases in some herbivorous taxa on shrubs with new leaves, but no landscape-scale effects of leaf phenology, which tracked light and cloud cover. Instead, rainfall explained extensive interannual variability with maximum biomass at intermediate rainfall (130 mm month^-1 ) as both 3 months of high and low rainfall reduced arthropods by half. Based on 50 years of regional rainfall, our dynamic arthropod model predicted shifts in the timing of biomass maxima within cloud forests before plant communities transition to seasonally deciduous dry forests (mean annual rainfall 1000-2500 mm vs. <800 mm). Rainfall magnitude was the primary driver, but during high solar insolation, the 'drying power of air' (VPD_max ) reduced biomass within days contributing to drought related to the El Niño-Southern Oscillation (ENSO). Highlighting risks from drought, experiments demonstrated community-wide susceptibility to desiccation except for some caterpillars in which melanin-based coloration appeared to reduce the effects of evaporative drying. Overall, we provide multiple lines of evidence that several months of heavy rain or drought reduce arthropod biomass independently of deep-rooted plants with the potential to destabilize insectivore food webs.

Phytophagous insects and natural enemies on Sapindus saponaria L. (Sapindales: Sapindaceae) plants fertilized with or without dehydrated sewage sludge

Management programs and efficient techniques are necessary to recover degraded ecosystems. The sewage sludge is rich in nitrogen (N) and with the potential to fertilize Sapindus saponaria L. (Sapinales: Sapindaceae), used in the recovery of degraded areas; this can affect the insect fauna. The study's objective was to evaluate, for 24 months, the abundance of chewing insects, dipterans, pollinators, and predators on S. saponaria plants fertilized with or without dehydrated sewage sludge in a degraded area. The experimental design was completely randomized (with the same characteristics) with two treatments (with or without dehydrated sewage sludge) and 24 replicates, each with one plant. The abundance of Anastrepha sp. (Tephritidae), Cerotoma sp. (Chrysomelidae), Curculionidae, Musca domestica L. (Muscidae), Mantis religiosa L. (Mantodea: Mantidae), Oxyopidae, Salticidae, Tettigoniidae (Orthoptera), and Teudis sp. (Anyphaenidae) was higher on fertilized plants. The abundances of Teudis sp. and Tmarus sp. (Thomisidae) and M. religiosa and Teudis sp. were positively correlated with chewing insects and Diptera, respectively. The population increase of insects and spiders on S. saponaria plants fertilized with dehydrated sewage sludge (bigger crowns) has shown to be suitable for recovering degraded areas with a higher number of niches and better food quality, improving the ecological indices of the area.

An integrated leaf trait analysis of two Paleogene leaf floras

Objectives

This study presents the Integrated Leaf Trait Analysis (ILTA), a workflow for the combined application of methodologies in leaf trait and insect herbivory analyses on fossil dicot leaf assemblages. The objectives were (1) to record the leaf morphological variability, (2) to describe the herbivory pattern on fossil leaves, (3) to explore relations between leaf morphological trait combination types (TCTs), quantitative leaf traits, and other plant characteristics (e.g., phenology), and (4) to explore relations of leaf traits and insect herbivory.

Material and Methods

The leaves of the early Oligocene floras Seifhennersdorf (Saxony, Germany) and Suletice-Berand (Ústí nad Labem Region, Czech Republic) were analyzed. The TCT approach was used to record the leaf morphological patterns. Metrics based on damage types on leaves were used to describe the kind and extent of insect herbivory. The leaf assemblages were characterized quantitatively (e.g., leaf area and leaf mass per area (LM_A)) based on subsamples of 400 leaves per site. Multivariate analyses were performed to explore trait variations.

Results

In Seifhennersdorf, toothed leaves of TCT F from deciduous fossil-species are most frequent. The flora of Suletice-Berand is dominated by evergreen fossil-species, which is reflected by the occurrence of toothed and untoothed leaves with closed secondary venation types (TCTs A or E). Significant differences are observed for mean leaf area and LM_A, with larger leaves tending to lower LM_A in Seifhennersdorf and smaller leaves tending to higher LM_A in Suletice-Berand. The frequency and richness of damage types are significantly higher in Suletice-Berand than in Seifhennersdorf. In Seifhennersdorf, the evidence of damage types is highest on deciduous fossil-species, whereas it is highest on evergreen fossil-species in Suletice-Berand. Overall, insect herbivory tends to be more frequently to occur on toothed leaves (TCTs E, F, and P) that are of low LM_A. The frequency, richness, and occurrence of damage types vary among fossil-species with similar phenology and TCT. In general, they are highest on leaves of abundant fossil-species.

Discussion

TCTs reflect the diversity and abundance of leaf architectural types of fossil floras. Differences in TCT proportions and quantitative leaf traits may be consistent with local variations in the proportion of broad-leaved deciduous and evergreen elements in the ecotonal vegetation of the early Oligocene. A correlation between leaf size, LM_A, and fossil-species indicates that trait variations are partly dependent on the taxonomic composition. Leaf morphology or TCTs itself cannot explain the difference in insect herbivory on leaves. It is a more complex relationship where leaf morphology, LM_A, phenology, and taxonomic affiliation are crucial.

Herbivory in Myrtillocactus geometrizans (Cactaceae): Do Parasitoids Provide Indirect Defense or a Direct Advantage?

Plants respond to herbivory in diverse, complex ways, ranging from avoidance or tolerance to indirect defense mechanisms such as attracting natural enemies of herbivores, i.e., parasitoids or predators, to strengthen their defense. Defense provided by parasitoids to cultivated plants is well documented and is used in biological control programs. However, its effectiveness on wild plants under natural conditions has been little studied. Such is the case of the cactus Myrtilllocactus geometrizans (known in Mexico as garambullo), which is consumed by the soft-scale insect Toumeyella martinezae (herbivore) which, in turn, is host to the parasitoid wasp Mexidalgus toumeyellus, and mutualist with the ant Liometopum apiculatum, that tenders and protects it. This study explores the role of the parasitoid as an indirect defense, by examining its effect on both the herbivore and the plant, and how this interaction is affected by the presence of the mutualistic ant. We found that scales adversely affect the cactus' growth, flower, and fruit production, as well as its progeny's performance, as seedlings from scale-infested garambullo plants were shorter, and it also favors the presence of fungus (sooty mold). The parasitoid responded positively to herbivore abundance, but the presence of ants reduced the intensity of parasitism. Our results show that parasitoids can function as an indirect defense, but their effectiveness is reduced by the presence of the herbivore's mutualistic ant.

Widespread herbivory cost in tropical nitrogen-fixing tree species

Recent observations suggest that the large carbon sink in mature and recovering forests may be strongly limited by nitrogen^1-3. Nitrogen-fixing trees (fixers) in symbiosis with bacteria provide the main natural source of new nitrogen to tropical forests^3,4. However, abundances of fixers are tightly constrained^5-7, highlighting the fundamental unanswered question of what limits new nitrogen entering tropical ecosystems. Here we examine whether herbivory by animals is responsible for limiting symbiotic nitrogen fixation in tropical forests. We evaluate whether nitrogen-fixing trees experience more herbivory than other trees, whether herbivory carries a substantial carbon cost, and whether high herbivory is a result of herbivores targeting the nitrogen-rich leaves of fixers^8,9. We analysed 1,626 leaves from 350 seedlings of 43 tropical tree species in Panama and found that: (1) although herbivory reduces the growth and survival of all seedlings, nitrogen-fixing trees undergo 26% more herbivory than non-fixers; (2) fixers have 34% higher carbon opportunity costs owing to herbivory than non-fixers, exceeding the metabolic cost of fixing nitrogen; and (3) the high herbivory of fixers is not driven by high leaf nitrogen. Our findings reveal that herbivory may be sufficient to limit tropical symbiotic nitrogen fixation and could constrain its role in alleviating nitrogen limitation on the tropical carbon sink.

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.

Insect herbivory within modern forests is greater than fossil localities

Fossilized leaves provide the longest running record of hyperdiverse plant-insect herbivore associations. Reconstructions of these relationships over deep time indicate strong links between environmental conditions, herbivore diversity, and feeding damage on leaves. However, herbivory has not been compared between the past and the modern era, which is characterized by intense anthropogenic environmental change. Here, we present estimates for damage frequencies and diversities on fossil leaves from the Late Cretaceous (66.8 Ma) through the Pleistocene (2.06 Ma) and compare these estimates with Recent (post-1955) leaves collected via paleobotanical methods from modern ecosystems: Harvard Forest, United States; the Smithsonian Environmental Research Center, United States; and La Selva, Costa Rica. Total damage frequency, measured as the percentage of leaves with any herbivore damage, within modern ecosystems is greater than any fossil locality within this record. This pattern is driven by increased frequencies across nearly all functional feeding groups within the Recent. Diversities of total, specialized, and mining damage types are elevated within the Recent compared with fossil floras. Our results demonstrate that plants in the modern era are experiencing unprecedented levels of insect damage, despite widespread insect declines. Human influence, such as the rate of global climate warming, influencing insect feeding and timing of life cycle processes along with urbanization and the introduction of invasive plant and insect species may drive elevated herbivory. This research suggests that the strength of human influence on plant-insect interactions is not controlled by climate change alone but rather, the way in which humans interact with terrestrial landscape.

The phenomenon of red and yellow autumn leaves: Hypotheses, agreements and disagreements

Yellow and red autumn leaves are typical of many temperate/boreal woody plants. Since the 19^th century, it has been either considered the non-functional outcome of chlorophyll degradation that unmasks the pre-existing yellow and red pigments or that the de novo synthesis of red anthocyanins in autumn leaves indicated that it should have a physiological function, although it was commonly ignored. Defending free amino acids and various other resources released especially following the breakdown of the photosynthetic system, and mobilizing them for storage in other organs before leaf fall, is the cornerstone of both the physiological and anti-herbivory hypotheses about the functions of yellow and red autumn leaf colouration. The complicated phenomenon of conspicuous autumn leaf colouration has received significant attention since the year 2000, especially because ecologists started paying attention to its anti-herbivory potential. The obvious imperfection of the hypotheses put forth in several papers stimulated many other scientists. Hot debates among physiologists, among ecologists, and between physiologists and ecologists have been common since the year 2000, first because the various functions of yellow and red autumn leaf colouration are non-exclusive, and second because many scientists were trained to focus on a single subject. Here, I will review the debates, especially between the photoprotective and the anti-herbivory hypotheses, and describe both the progress in their understanding and the required progress.

Tree mycorrhizal type mediates conspecific negative density dependence effects on seedling herbivory, growth, and survival

Tree mycorrhizal type plays an important role in promoting plant species diversity and coexistence, via its mediating role in conspecific negative density dependence (CNDD), i.e., the process by which an individual's performance is impaired by the density of conspecific plants. Previous findings suggest that ectomycorrhizal (EM) tree species are generally less susceptible to CNDD than arbuscular mycorrhizal (AM) tree species, due to the chemical and physical protection that EM fungi provide their host with. We examined how CNDD effects on leaf herbivory, seedling growth, and survival differ between AM and EM seedlings of ten tree species collected over 3 years in an old-growth temperate forest in northeastern China. We found that AM and EM seedlings differed in how conspecific density affected their leaf herbivory, seedling growth, and survival. Specifically, AM seedlings leaf herbivory rates significantly increased with increasing conspecific seedling and adult density, and their growth and survival rates decreased with increasing conspecific adult density, these patterns were, however, absent in EM seedlings. Our work suggests that AM seedlings have a performance disadvantage relative to EM seedlings related to the negative effects from conspecific neighbors. We highlight the importance of integrating information on seedling leaf herbivory, seedling growth, to provide further understanding on potential mechanisms driving differences in CNDD between AM and EM tree seedlings.

Traits along the leaf economics spectrum are associated with communities of foliar endophytic symbionts

Leaf traits of plants worldwide are classified according to the Leaf Economics Spectrum (LES), which links leaf functional traits to evolutionary life history strategies. As a continuum ranging from thicker, tough leaves that are low in nitrogen (N) to thinner, softer, leaves that are high in N, the LES brings together physical, chemical, and ecological traits. Fungal endophytes are common foliar symbionts that occur in healthy, living leaves, especially in tropical forests. Their community composition often differs among co-occurring host species in ways that cannot be explained by environmental conditions or host phylogenetic relationships. Here, we tested the over-arching hypothesis that LES traits act as habitat filters that shape communities of endophytes both in terms of composition, and in terms of selecting for endophytes with particular suites of functional traits. We used culture-based and culture-free surveys to characterize foliar endophytes in mature leaves of 30 phylogenetically diverse plant species with divergent LES traits in lowland Panama, and then measured functional traits of dominant endophyte taxa in vitro. Endophytes were less abundant and less diverse in thick, tough, leaves compared to thin, softer, leaves in the same forest, even in closely related plants. Endophyte communities differed according to leaf traits, including leaf punch strength and carbon and nitrogen content. The most common endophyte taxa in leaves at different ends of the LES differ in their cellulase, protease, chitinase, and antipathogen activity. Our results extend the LES framework for the first time to diverse and ecologically important endophytes, opening new hypotheses regarding the degree to which foliar symbionts respond to, and extend, the functional traits of leaves they inhabit.

Size matters for linking traits to ecosystem multifunctionality

A priority research field addresses how to optimize diverse ecosystem services to people, including biodiversity support, regulatory, utilitarian and cultural services. This field may benefit from linking ecosystem services to the sizes of different body parts of organisms, with functional traits as the go-between. Using woody ecosystems to explore such linkages, we hypothesize that across stem diameter classes from trunk via branches to twigs, key wood and bark functional traits (especially those defining size-shape and resource economics spectra) vary both within individual trees and shrubs and across woody species, thereby together boosting ecosystem multifunctionality. While we focus on woody plants aboveground, we discuss promising extensions to belowground organs of trees and shrubs and analogs with other organisms, for example, vertebrate animals.

Do patterns of insect mortality in temperate and tropical zones have broader implications for insect ecology and pest management?

Background

Understanding how biotic and abiotic factors affect insect mortality is crucial for both fundamental knowledge of population ecology and for successful pest management. However, because these factors are difficult to quantify and interpret, patterns and dynamics of insect mortality remain unclear, especially comparative mortality across climate zones. Life table analysis provides robust information for quantifying population mortality and population parameters.

Methods

In this study, we estimated cause-of-death probabilities and irreplaceable mortality (the portion of mortality that cannot be replaced by another cause or combination of causes) using a Multiple Decrement Life Table (MDLT) analysis of 268 insect life tables from 107 peer-reviewed journal articles. In particular, we analyzed insect mortality between temperate and tropical climate zones.

Results

Surprisingly, our results suggest that non-natural enemy factors (abiotic) were the major source of insect mortality in both temperate and tropical zones. In addition, we observed that irreplaceable mortality from predators in tropical zones was 3.7-fold greater than in temperate zones. In contrast, irreplaceable mortality from parasitoids and pathogens was low and not different between temperate and tropical zones. Surprisingly, we did not observe differences in natural enemy and non-natural enemy factors based on whether the insect species was native or non-native. We suggest that characterizing predation should be a high priority in tropical conditions. Furthermore, because mortality from parasitoids was low in both tropical and temperate zones, this mortality needs to be better understood, especially as it relates to biological control and integrated pest management.

Tomato Defense against Whiteflies under Drought Stress: Non-Additive Effects and Cultivar-Specific Responses

Two of the main causes of losses in tomato production are the greenhouse whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), and drought, which is becoming a central problem in agriculture due to global climate change. The separate effects of whitefly infestation and drought have been amply studied in many crop systems. However, less is known about their combined effects. To evaluate whether drought stress (DS) affects plant defense against whiteflies, we assessed the joint effects of whitefly infestation and DS on plant vegetative and reproductive performance in four tomato cultivars, and assessed the effects of DS on plant resistance and tolerance (compensatory ability) to whiteflies in a greenhouse experiment. Generally, we found negative effects of DS and whiteflies on plant performance, but the combined effects of DS and herbivory were not worse than those of either stress alone. In fact, plant performance under the combined effect of both stresses was usually similar to that in the presence of whiteflies without DS. Plants growing under DS had greater trichome density. However, plant resistance—as measured by whitefly population growth—decreased under DS in two cultivars and was unaffected in the other two. Compensatory ability decreased under DS in all but one cultivar. These cultivar-specific responses suggest genetic variation in resistance and tolerance to whiteflies and could be associated with differences in drought tolerance among cultivars. Our findings underscore the difficulty in predicting the combined effects of DS and herbivory and point to the need for a better understanding of the mechanisms underlying plant responses to both stresses at the molecular, cellular, and organismal levels.

Patterns of Plumericin Concentration in Leaves of Himatanthus tarapotensis (Apocynaceae) and Its Interactions with Herbivory in the Peruvian Amazon

We explored the concentration patterns of the bioactive metabolite plumericin produced by Himatanthus tarapotensis (Apocynaceae) under different edaphic conditions and variations in rainfall intensity, as well as its potential role in the chemical defense against insect herbivores. Values of plumericin concentration from leaves were obtained by High-Performance Liquid Chromatography, and evaluated as a function of differences in soil types, variation of precipitation, and variation of the abundance of insect herbivores, using first a Repeated Measures Correlation (rmcorr) and then a Generalized Linear Mixed Model (GLMM) analysis. Plumericin concentration is highly variable among plants, but with a significantly higher concentration in plants growing on clay soil compared to that of the white-sand soil habitat (p < 0.001). Plumericin concentration is not affected by precipitation. The caterpillar of Isognathus leachii (Lepidoptera: Sphingidae) is the most conspicuous herbivore of H. tarapotensis, and its presence is continuous but not related to plumericin concentration, probably because of its capacity to elude the chemical defense of this plant. Nevertheless, our multivariate model revealed that plumericin concentration is related to the abundance of Hymenoptera (Formicidae), and this relationship is significantly influenced by the soil parameters of carbon percentage, clay percentage, and phosphorous percentage (p < 0.001). Plumericin is a mediating agent in the interaction between H. tarapotensis and its natural environment. Variation in plumericin concentration would be induced by the abundance of Hymenoptera (Formicidae), probably as a chemical response against these insects, and by differences in soil nutrient availability.