Effects of plant intraspecific diversity across three trophic levels: Underlying mechanisms and plant traits

Effects of phytochemical diversity on multitrophic interactions

The ecological effects of plant diversity have been well studied, but the extent to which they are driven by variation in specialized metabolites is not well understood. Here, we provide theoretical background on phytochemical diversity effects on herbivory and its expanded consequences for higher trophic levels. We then review empirical evidence for effects on predation and parasitism by focusing on a handful of studies that have undertaken manipulative approaches and link back their results to theory on mechanisms. We close by summarizing key aspects for future research, building on knowledge gained thus far.

Host-plant sex and phenology of Buddleja cordata Kunth interact to influence arthropod communities

Intraspecific variation in plants is expected to have profound impacts on the arthropod communities associated with them. Because sexual dimorphism in plants is expected to provide consistent variation among individuals of the same species, researchers have often studied the effect it has on associated arthropods. Nevertheless, most studies have focused on the effect of sexual dimorphism in a single or a few herbivores, thus overlooking the potential effects on the whole arthropod community. Our main objective was to evaluate effects of Buddleja cordata's plant-sex on its associated arthropod community. We surveyed 13 pairs of male and female plants every 2 months during a year (June 2010 to April 2011). Every sampling date, we measured plant traits (water content and leaf thickness), herbivory, and the arthropod community. We did not find differences in herbivory between plant sex or through time. However, we found differences in water content through time, with leaf water-content matching the environmental seasonality. For arthropod richness, we found 68 morphospecies associated with female and 72 with male plants, from which 53 were shared by both sexes. We did not observe differences in morphospecies richness; however, we found sex-associated differences in the diversity of all species and differences on the diversity of the most abundant species with an interesting temporal component. During peak flowering season, male plants showed higher values on both parameters, but during the peak fructification season female plants showed the higher values on both diversity parameters. Our research exemplifies the interaction between plant-phenology and plant-sex as drivers of arthropod communities' diversity, even when plant sexual-dimorphism is inconspicuous, and highlighting the importance of accounting for seasonal variation. We stress the need of conducting more studies that test this time-dependent framework in other dioecious systems, as it has the potential to reconcile previous contrasting observations reported in the literature.

Early damage enhances compensatory responses to herbivory in wild lima bean

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

Growing up aspen: ontogeny and trade-offs shape growth, defence and reproduction in a foundation species

BACKGROUND AND AIMS

Intraspecific variation in foundation species of forest ecosystems can shape community and ecosystem properties, particularly when that variation has a genetic basis. Traits mediating interactions with other species are predicted by simple allocation models to follow ontogenetic patterns that are rarely studied in trees. The aim of this research was to identify the roles of genotype, ontogeny and genotypic trade-offs shaping growth, defence and reproduction in aspen.

METHODS

We established a common garden replicating >500 aspen genets in Wisconsin, USA. Trees were measured through the juvenile period into the onset of reproduction, for growth, defence chemistry (phenolic glycosides and condensed tannins), nitrogen, extrafloral nectaries, leaf morphology (specific leaf area), flower production and foliar herbivory and disease. We also assayed the TOZ19 sex marker and heterozygosity at ten microsatellite loci.

KEY RESULTS

We found high levels of genotypic variation for all traits, and high heritabilities for both the traits and their ontogenetic trajectories. Ontogeny strongly shaped intraspecific variation, and trade-offs among growth, defence and reproduction supported some predictions while contradicting others. Both direct resistance (chemical defence) and indirect defence (extrafloral nectaries) declined during the juvenile stage, prior to the onset of reproduction. Reproduction was higher in trees that were larger, male and had higher individual heterozygosity. Growth was diminished by genotypic allocation to both direct and indirect defence as well as to reproduction, but we found no evidence of trade-offs between defence and reproduction.

CONCLUSIONS

Key traits affecting the ecological communities of aspen have high levels of genotypic variation and heritability, strong patterns of ontogeny and clear trade-offs among growth, defence and reproduction. The architecture of aspen's community genetics - its ontogeny, trade-offs and especially its great variability - is shaped by both its broad range and the diverse community of associates, and in turn further fosters that diversity.

Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth

Research on plant-herbivore interactions has long recognized that plant genetic variation plays a central role in driving insect abundance and herbivory, as well as in determining plant defense. However, how plant genes influence herbivore feeding performances, and which plant defensive traits mediate these effects, remain poorly understood. Here we investigated the feeding performances of two insect leaf chewers with contrasting diet breadth (the generalist Lymantria dispar L. and the specialist Thaumetopoea processionea L.) on different genotypes of pedunculate oak (Quercus robur L.) and tested the role of leaf phenolics. We used leaves from four clones of 30 Q. robur full-sibs grown in a common garden to estimate the performance of both herbivores in laboratory feeding trials and to quantify the concentration of constitutive chemical defences (phenolic compounds). We found that tree genetics influenced leaf consumption by T. processionea but not by L. dispar. However genetic variation among trees did not explain growth rate variation in either herbivore nor in leaf phenolics. Interestingly, all phenolic compounds displayed a positive relationship with L. dispar growth rate, and leaf consumption by both herbivores displayed a positive relationship with the concentrations of condensed tannins, suggesting that highly defended leaves could induce a compensatory feeding response. While genetic variation in oaks did not explain herbivore growth rate, we found positive genetic correlations between the two herbivores for leaf consumption and digestion. Overall, we found that oak genotype and phenolic compounds partly and independently contribute to variability in herbivore performance. We challenged the current view of plant-insect interaction and provided little support to the idea that the effect of plant genotype on associated organisms is driven by plant defences. Together, our results point to the existence of genetically determined resistance traits in oaks whose effects differ between herbivores and motivate further research on mechanisms governing oak-herbivore interactions.

A random survival forest illustrates the importance of natural enemies compared to host plant quality on leaf beetle survival rates

Background

Wetlands are habitats where variation in soil moisture content and associated environmental conditions can strongly affect the survival of herbivorous insects by changing host plant quality and natural enemy densities. In this study, we combined natural enemy exclusion experiments with random survival forest analyses to study the importance of local variation in host plant quality and predation by natural enemies on the egg and larval survival of the leaf beetle Galerucella sagittariae along a soil moisture gradient.

Results

Our results showed that the exclusion of natural enemies substantially increased the survival probability of G. sagittariae eggs and larvae. Interestingly, the egg survival probability decreased with soil moisture content, while the larval survival probability instead increased with soil moisture content. For both the egg and larval survival, we found that host plant height, the number of eggs or larvae, and vegetation height explained more of the variation than the soil moisture gradient by itself. Moreover, host plant quality related variables, such as leaf nitrogen, carbon and phosphorus content did not influence the survival of G. sagittariae eggs and larvae.

Conclusion

Our results suggest that the soil moisture content is not an overarching factor that determines the interplay between factors related to host plant quality and factors relating to natural enemies on the survival of G. sagittariae in different microhabitats. Moreover, the natural enemy exclusion experiments and the random survival forest analysis suggest that natural enemies have a stronger indirect impact on the survival of G. sagittariae offspring than host plant quality.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0187-7) contains supplementary material, which is available to authorized users.

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.

A Conceptual Framework for Integrated Pest Management

The concept of integrated pest management (IPM) has been accepted and incorporated in public policies and regulations in the European Union and elsewhere, but a holistic science of IPM has not yet been developed. Hence, current IPM programs may often be considerably less efficient than the sum of separately applied individual crop protection actions. Thus, there is a clear need to formulate general principles for synergistically combining traditional and novel IPM actions to improve efforts to optimize plant protection solutions. This paper addresses this need by presenting a conceptual framework for a modern science of IPM. The framework may assist attempts to realize the full potential of IPM and reduce risks of deficiencies in the implementation of new policies and regulations.

Bottom-up and top-down effects of tree species diversity on leaf insect herbivory

The diversity of plant neighbors commonly results in direct, bottom-up effects on herbivore ability to locate their host, and in indirect effects on herbivores involving changes in plant traits and a top-down control by their enemies. Yet, the relative contribution of bottom-up and top-down forces remains poorly understood. We also lack knowledge on the effect of abiotic constraints such as summer drought on the strength and direction of these effects. We measured leaf damage on pedunculate oak (Quercus robur), alone or associated with birch, pine or both in a long-term tree diversity experiment (ORPHEE), where half of the plots were irrigated while the other half remained without irrigation and received only rainfall. We tested three mechanisms likely to explain the effects of oak neighbors on herbivory: (1) Direct bottom-up effects of heterospecific neighbors on oak accessibility to herbivores, (2) indirect bottom-up effects of neighbors on the expression of leaf traits, and (3) top-down control of herbivores by predators. Insect herbivory increased during the growth season but was independent of neighbor identity and irrigation. Specific leaf area, leaf toughness, and thickness varied with neighbor identity while leaf dry matter content or C:N ratio did not. When summarized in a principal component analysis (PCA), neighbor identity explained 87% of variability in leaf traits. PCA axes partially predicted herbivory. Despite greater rates of attack on dummy caterpillars in irrigated plots, avian predation, and insect herbivory remained unrelated. Our study suggests that neighbor identity can indirectly influence insect herbivory in mixed forests by modifying leaf traits. However, we found only partial evidence for these trait-mediated effects and suggest that more attention should be paid to some unmeasured plant traits such as secondary metabolites, including volatile organic compounds, to better anticipate the effects of climate change on plant-insect interactions in the future.