Does plant trait diversity reduce the ability of herbivores to defend against predators? The plant variability-gut acclimation hypothesis

A domestic plant differs from its wild relative along multiple axes of within-plant trait variability and diversity

For 10,000 years humans have altered plant traits through domestication and ongoing crop improvement, shaping plant form and function in agroecosystems. To date, studies have focused on how these processes shape whole-plant or average traits; however, plants also have characteristic levels of trait variability among their repeated parts, which can be heritable and mediate critical ecological interactions. Here, we examine an underappreciated scale of trait variation-among leaves, within plants-that may have changed through the process of domestication and improvement. Variability at this scale may itself be a target of selection, or be shaped as a by-product of the domestication process. We explore how levels of among-leaf trait variability differ between cultivars and wild relatives of alfalfa (Medicago sativa), a key forage crop with a 7,000-year domestication history. We grew individual plants from 30 wild populations and 30 cultivars, and quantified variability in a broad suite of physical, nutritive, and chemical leaf traits, including measures of chemical dissimilarity (beta diversity) among leaves within each plant. We find that trait variability has changed over the course of domestication, with effects often larger than changes in trait means. Domestic alfalfa had elevated among-leaf variability in SLA, trichomes, and C:N; increased diversity in defensive compounds; and reduced variability in phytochemical composition. We also elucidate fundamental relationships between trait means and variability, and between overall production of secondary metabolites and patterns of chemical diversity. We conclude that within-plant variability is an overlooked dimension of trait diversity in a globally critical agricultural crop. Trait variability is actually higher in cultivated plants compared to wild progenitors for multiple nutritive, physical, and chemical traits, highlighting a scale of variation that may mitigate loss of trait diversity at other scales in alfalfa agroecosystems, and in other crops with similar histories of domestication and improvement.

Chemical diversity rather than cultivar diversity predicts natural enemy control of herbivore pests

Cultivar mixtures have been studied for decades as a means for pest suppression. The literature, however, shows a large variability in outcomes, suggesting that we are unable to create mixtures that consistently suppress insect pests and attract natural enemies. A key gap in our understanding of how cultivar mixtures influence pest control is that few studies have examined the plant traits or mechanisms by which cultivar diversity affects pests and their interactions with natural enemies. The diversity of plant chemistry in a cultivar mixture is one trait dimension that is likely influential for insect ecology because chemical traits alter how predators and herbivores forage and interact. To understand how plant chemical diversity influences herbivores and their interactions with predators, we fully crossed predator presence or absence with monocultures, bicultures, and tricultures of three chemotypes of tomato that differed in odor diversity (terpenes) or surface chemistry (acyl sugars) in a caged field experiment. We found that the direct effects of plant chemotype diversity on herbivore performance were strongest in bicultures and depended on herbivore sex, and these effects typically acted through growth rather than survival. The effects of chemotype diversity on top-down pest suppression by natural enemies differed between classes of chemical diversity. Odor diversity (terpenes) interfered with the ability of predators to hunt effectively, whereas diversity in surface chemistry (acyl sugars) did not. Our results suggest that phytochemical diversity can contribute to pest suppression in agroecosystems, but that implementing it will require engineering cultivar mixtures using trait-based approaches that account for the biology of the pests and natural enemies in the system.

Landscape-dependent effects of varietal mixtures on insect pest control and implications for farmer profits

Intraspecific plant diversity can significantly impact insect herbivore populations in natural systems. Yet, its role as an insect pest control strategy in agriculture has received less attention, and little is known about which crop traits are important to herbivores in different landscape contexts. Moreover, empirical economic analyses on the cost-effectiveness of varietal mixtures are lacking. We used varietal mixtures of Brassica oleracea crops on working farms to examine how two metrics of intraspecific crop diversity, varietal richness and number of plant colors (color richness), affect crop damage and the incidence and abundance of two insect pest species: Pieris rapae and Phyllotreta spp. We evaluated the context-dependency of varietal mixtures by sampling early- and late-season plantings of B. oleracea crops in farms across a gradient of landscape composition. We developed crop budgets and used a net present value analysis to assess the impact of varietal mixtures on input and labor costs, crop revenues, and profit. We found context-dependent effects of varietal mixtures on both pests. In early-season plantings, color richness did not affect Phyllotreta spp. populations. However, increasing varietal richness reduced Phyllotreta spp. incidence in simple landscapes dominated by cropland, but this trend was reversed in complex landscapes dominated by natural habitats. In late-season plantings, color richness reduced the incidence and abundance of P. rapae larvae, but only in complex landscapes where their populations were highest. Varietal richness had the same effect on P. rapae larvae as color richness. Unexpectedly, we consistently found lower pest pressure and reduced crop damage in simple landscapes. Although varietal mixtures did not affect crop damage, increasing color richness corresponded with increased profits, due to increased revenue and a marginal reduction in labor and input costs. We demonstrate varietal mixtures can significantly impact pest populations, and this effect can be mediated by intraspecific variation in crop color. However, the strength and direction of these effects vary by season, landscape composition, and pest species. The association between varietal color richness and profitability indicates farmers could design mixtures to enhance economic returns. We recommend additional research on the benefits of intraspecific trait variation for farmers.

Disentangling dimensions of phytochemical diversity: alpha and beta have contrasting effects on an insect herbivore

Phytochemical diversity is comprised of two main dimensions-the average (alpha) within-plant neighbors or the difference (beta) in the composition of chemicals between plant neighbors. Research, however, has primarily examined the consequences of phytochemical diversity on herbivore performance through a single dimension, even though diversity is multidimensional. Furthermore, the ecological role of phytochemical diversity is not well understood because each of these dimensions exhibits unique biological effects on herbivore performance. Therefore, it has been difficult to tease apart the relative importance of alpha and beta chemical diversities on plant-herbivore interactions. We experimentally manipulated alpha and beta diversities along a chemical gradient to disentangle the relative effects of these dimensions on the performance of a mobile generalist herbivore, Trichoplusia ni (Hübner), using 16 genotypes from the Solanum pennellii introgression lines. First, we found contrasting effects of alpha and beta diversities on herbivore performance. Second, when comparing diversity across and within chemical classes, herbivore performance was reduced when plant neighbors had greater diversity within chemical classes that are biologically inhibiting at higher quantities (i.e., quantitative defenses such as phenolics and acyl sugars). However, herbivore performance was enhanced when plant neighbors had higher levels of chemical classes that are biologically toxic (i.e., qualitative defenses such as alkaloids). Finally, herbivores performed better on plant dicultures compared to monocultures, and performance was positively associated with plant dicultures only when there were high levels of average alpha diversity within plant neighbors. Our results suggest T. ni generalist caterpillars do better when plant neighbors are chemically different because differences provide options for them to choose or to switch between plants to balance chemical uptake. Overall, herbivores interact with a large diversity of plant chemicals at multiple scales, and our results indicate that not all chemical diversity is equal: specific dimensions of phytochemical diversity have unique effects on the dynamics of herbivore performance.

Influence of dietary aconitine and nicotine on the gut microbiota of two lepidopteran herbivores

The gut microbiota plays an important role in pheromone production, pesticide degradation, vitamin synthesis, and pathogen prevention in the host animal. Therefore, similar to gut morphology and digestive enzyme activity, the gut microbiota may also get altered under plant defensive compound-induced stress. To test this hypothesis, Dendrolimus superans larvae were fed either aconitine- or nicotine-treated fresh leaves of Larix gmelinii, and Lymantria dispar larvae were fed either aconitine- or nicotine-treated fresh leaves of Salix matsudana. Subsequently, the larvae were sampled 72hr after diet administration and DNA extracted from larval enteric canals were employed for gut microbial 16S ribosomal RNA gene sequencing (338 F and 806 R primers). The sequence analysis revealed that dietary nicotine and aconitine influenced the dominant bacteria in the larval gut and determined their abundance. Moreover, the effect of either aconitine or nicotine on D. superans and L. dispar larvae had a greater dependence on insect species than on secondary plant metabolites. These findings further our understanding of the interaction between herbivores and host plants and the coevolution of plants and insects.

The many dimensions of phytochemical diversity: linking theory to practice

Research on the ecological and evolutionary roles of phytochemicals has recently progressed from studying single compounds to examining chemical diversity itself. A key conceptual advance enabling this progression is the use of species diversity metrics for quantifying phytochemical diversity. In this perspective, we extend the theory developed for species diversity to further our understanding of what exactly phytochemical diversity is and how its many dimensions impact ecological and evolutionary processes. First, we discuss the major dimensions of phytochemical diversity - richness, evenness, functional diversity, and alpha, gamma and beta diversity. We describe their potential independent roles in biotic interactions and the practical challenges associated with their analysis. Second, we re-analyse the published and unpublished datasets to reveal that the phytochemical diversity experienced by an organism (or observed by a researcher) depends strongly on the scale of the interaction and the total amount of phytochemicals involved. We argue that we must account for these frames of reference to meaningfully understand diversity. Moving from a general notion of phytochemical diversity as a single measure to a precise definition of its multidimensional and multiscale nature yields overlooked testable predictions that will facilitate novel insights about the evolutionary ecology of plant biotic interactions.

Feeding in the frequency domain: coarser-grained environments increase consumer sensitivity to resource variability, covariance and phase

Theory predicts that resource variability hinders consumer performance. How this effect depends on the temporal structure of resource fluctuations encountered by individuals remains poorly understood. Combining modelling and growth experiments with Daphnia magna, we decompose the complexity of resource fluctuations and test the effect of resource variance, supply peak timing (i.e. phase) and co-limiting resource covariance along a gradient from high to low frequencies reflecting fine- to coarse-grained environments. Our results show that resource storage can buffer growth at high frequencies, but yields a sensitivity of growth to resource peak timing at lower ones. When two resources covary, negative covariance causes stronger growth depression at low frequencies. However, negative covariance might be beneficial at intermediate frequencies, an effect that can be explained by digestive acclimation. Our study provides a mechanistic basis for understanding how alterations of the environmental grain size affect consumers experiencing variable nutritional quality in nature.

Ontogenetic strategies in insect herbivores and their impact on tri-trophic interactions

Insect herbivores express tremendous ontogenetic variation in traits related to growth and maturation, but also as an evolutionary consequence of ecological interactions with plants and predators. These selective pressures can either reinforce or restrict expression of particular ontogenetic strategies, allowing herbivores to simultaneously cope with plant resistance and risk of predation through ontogenetic change. For example, whereas an increase in defense-sabotaging behavior, aposematism and sequestration along herbivore ontogeny seems to be reinforced by both bottom-up and top-down forces, some ontogenetic trends in anti-predator behavior can be limited by plant resistance. Communication among plants, herbivores and their natural enemies is also influenced by insect ontogenies. The study of ontogenetic strategies of herbivores requires the assessment of the genetic variation, heritability and adaptive value across herbivore development, considering the variation in plant quality and predation risk.

Plant Secondary Metabolite Diversity and Species Interactions

Ever since the first plant secondary metabolites (PSMs) were isolated and identified, questions about their ecological functions and diversity have been raised. Recent advances in analytical chemistry and complex data computation, as well as progress in chemical ecology from mechanistic to functional and evolutionary questions, open a new box of hypotheses. Addressing these hypotheses includes the measurement of complex traits, such as chemodiversity, in a context-dependent manner and allows for a deeper understanding of the multifunctionality and functional redundancy of PSMs. Here we review a hypothesis framework that addresses PSM diversity on multiple ecological levels (α, β, and γ chemodiversity), its variation in space and time, and the potential agents of natural selection. We use the concept of chemical information transfer as mediator of antagonistic and mutualistic interaction to interpret functional and microevolutionary studies and create a hypothesis framework for understanding chemodiversity as a factor driving ecological processes.

Plant genotypic diversity interacts with predation risk to influence an insect herbivore across its ontogeny

A growing number of studies have manipulated intraspecific plant diversity and found dramatic changes in the densities of associated insect herbivores and their predators. While these studies have been essential for quantifying the net ecological consequences of intraspecific plant diversity, they have been less effective at uncovering the ways in which plant diversity alters trophic interactions within arthropod communities. We manipulated intraspecific plant diversity and predation risk in the field in a factorial design to reveal how a mixture of plant genotypes changes the response of an herbivorous beetle (Leptinotarsa decemlineata) to a common stink bug predator (Podisus maculiventris). We repeated the manipulations twice across the ontogeny of the beetle to examine how the effects of diversity on the predator-prey interaction differ between larval and adult stages. We found that intraspecific plant diversity, mixtures of susceptible and resistant varieties of potato (Solanum tuberosum), reduced larval survival by 20% and adult oviposition by 34%, which surprisingly put survival and oviposition lower in the mixed-genotype plots than in the resistant monocultures. Moreover, we found that predation risk reduced larval survival 25% and 11% in resistant and susceptible monocultures, respectively, but had no effect in the mixture. This result indicated that our genotypic mixing treatment interacted nonadditively with predation risk such that plant diversity altered the predator-prey interaction by changing the responses of the beetles to their stink bug predators. In addition, even though predation risk reduced larval survival, it increased adult overwintering survival by 9%, independently of plant treatment, suggesting that these interactions change through ontogeny. A key implication of our study is that plant diversity influences arthropod communities not only by changing resource quality, as past studies have suggested, but also by changing interactions between species within the arthropod community.