Negative effects of urbanisation on diurnal and nocturnal pollen-transport networks.

Pollinating insects are declining due to habitat loss and climate change, and cities with limited habitat and floral resources may be particularly vulnerable. The effects of urban landscapes on pollination networks remain poorly understood, and comparative studies of taxa with divergent niches are lacking. Here, for the first time, we simultaneously compare nocturnal moth and diurnal bee pollen-transport networks using DNA metabarcoding and ask how pollination networks are affected by increasing urbanisation. Bees and moths exhibited substantial divergence in the communities of plants they interact with. Increasing urbanisation had comparable negative effects on pollen-transport networks of both taxa, with significant declines in pollen species richness. We show that moths are an important, but overlooked, component of urban pollen-transport networks for wild flowering plants, horticultural crops, and trees. Our findings highlight the need to include both bee and non-bee taxa when assessing the status of critical plant-insect interactions in urbanised landscapes.

Plant defences mediate interactions between herbivory and the direct foliar uptake of atmospheric reactive nitrogen.

Reactive nitrogen from human sources (e.g., nitrogen dioxide, NO2) is taken up by plant roots following deposition to soils, but can also be assimilated by leaves directly from the atmosphere. Leaf uptake should alter plant metabolism and overall nitrogen balance and indirectly influence plant consumers; however, these consequences remain poorly understood. Here we show that direct foliar assimilation of NO2 increases levels of nitrogen-based defensive metabolites in leaves and reduces herbivore consumption and growth. These results suggest that atmospheric reactive nitrogen could have cascading negative effects on communities of herbivorous insects. We further show that herbivory induces a decrease in foliar uptake, indicating that consumers could limit the ability of vegetation to act as a sink for nitrogen pollutants (e.g., smog from mobile emissions). Our study suggests that the interactions of foliar uptake, plant defence and herbivory could have significant implications for understanding the environmental consequences of reactive nitrogen.

Exposure to Guava Affects Citrus Olfactory Cues and Attractiveness to Diaphorina citri (Hemiptera: Psyllidae).

Intercropping can reduce agricultural pest incidence, and represents an important sustainable alternative to conventional pest control methods. Understanding the ecological mechanisms for intercropping could help optimize its use, particularly in tropical systems which present a large number of intercropping possibilities. Citrus is threatened worldwide by greening disease (huanglongbing, HLB) vectored by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae). Control of HLB and citrus psyllid can be partially achieved through intercropping with guava, Psidium guajava L., but the mechanisms remain unclear. We tested the hypothesis that guava olfactory cues affect psyllid behavior by altering the attractiveness of citrus through plant-plant interactions. In choice and no-choice cage experiments, psyllid settlement was reduced on citrus shoots that had been exposed to guava shoot odors for at least 2 h. In Y-tube olfactometer experiments, psyllids oriented to odors of unexposed, compared with guava-exposed, citrus shoots. These behavioral results indicate that a mechanism for the success of guava intercropping for sustainable, ecological disease management may be the indirect effect of guava on citrus attractiveness.

Ecological mechanisms for the coevolution of mating systems and defence.

The diversity of flowering plants is evident in two seemingly unrelated aspects of life history: sexual reproduction, exemplified by the stunning variation in flower form and function, and defence, often in the form of an impressive arsenal of secondary chemistry. Researchers are beginning to appreciate that plant defence and reproduction do not evolve independently, but, instead, may have reciprocal and interactive (coevolutionary) effects on each other. Understanding the mechanisms for mating-defence interactions promises to broaden our understanding of how ecological processes can generate these two rich sources of angiosperm diversity. Here, I review current research on the role of herbivory as a driver of mating system evolution, and the role of mating systems in the evolution of defence strategies. I outline different ecological mechanisms and processes that could generate these coevolutionary patterns, and summarize theoretical and empirical support for each. I provide a conceptual framework for linking plant defence with mating system theory to better integrate these two research fields.

Benefits of gregarious feeding by aposematic caterpillars depend on group age structure.

Gregarious feeding is a common feature of herbivorous insects and can range from beneficial (e.g. dilution of predation risk) to costly (e.g. competition). Group age structure should influence these costs and benefits, particularly when old and young larvae differ in their feeding mode or apparency to predators. We investigated the relative value of gregarious feeding by aposematic larvae of Uresiphita reversalis that we observed feeding in groups of mixed ages and variable densities on wild Lupinus diffusus. In a manipulative field experiment, the survivorship and growth of young larvae were enhanced in the presence of older conspecifics, but not in large groups of similarly aged larvae. Estimates of insect damage and induced plant responses suggest that mixed-age groups enhance plant quality for young larvae while avoiding competition. We conclude that benefits of gregariousness in this species are contingent on group age structure, a finding of significance for the ecology and evolution of gregariousness and other social behaviours.

Plant mating systems affect adaptive plasticity in response to herbivory.

The fitness consequences of mating system variation (e.g. inbreeding) have been studied for at least 200 years, yet the ecological consequences of this variation remain poorly understood. Most plants are capable of inbreeding, and also exhibit a remarkable suite of adaptive phenotypic responses to ecological stresses such as herbivory. We tested the consequences of experimental inbreeding on phenotypic plasticity in resistance and growth (tolerance) traits in Solanum carolinense (Solanaceae). Inbreeding reduced the ability of plants to up-regulate resistance traits following damage. Moreover, inbreeding disrupted growth trait responses to damage, indicating the presence of deleterious mutations at loci regulating growth under stress. Production of the phytohormones abscisic and indole acetic acid, and wounding-induced up-regulation of the defence signalling phytohormone jasmonic acid were all significantly reduced under inbreeding, indicating a phytohormonal basis for inbreeding effects on growth and defence trait regulation. We conclude that the plasticity of induced responses is negatively affected by inbreeding, with implications for fragmented populations facing mate limitation and stress as a consequence of environmental change.

Plant mating system transitions drive the macroevolution of defense strategies.

Understanding the factors that shape macroevolutionary patterns in functional traits is a central goal of evolutionary biology. Alternative strategies of sexual reproduction (inbreeding vs. outcrossing) have divergent effects on population genetic structure and could thereby broadly influence trait evolution. However, the broader evolutionary consequences of mating system transitions remain poorly understood, with the exception of traits related to reproduction itself (e.g., pollination). Across a phylogeny of 56 wild species of Solanaceae (nightshades), we show here that the repeated, unidirectional transition from ancestral self-incompatibility (obligate outcrossing) to self-compatibility (increased inbreeding) leads to the evolution of an inducible (vs. constitutive) strategy of plant resistance to herbivores. We demonstrate that inducible and constitutive defense strategies represent evolutionary alternatives and that the magnitude of the resulting macroevolutionary tradeoff is dependent on the mating system. Loss of self-incompatibility is also associated with the evolution of increased specificity in induced plant resistance. We conclude that the evolution of sexual reproductive variation may have profound effects on plant-herbivore interactions, suggesting a new hypothesis for the evolution of two primary strategies of plant defense.

Plant chemistry underlies herbivore-mediated inbreeding depression in nature.

The cost of inbreeding (inbreeding depression, ID) is an important variable in the maintenance of reproductive variation. Ecological interactions such as herbivory could modulate this cost, provided that defence traits harbour deleterious mutations and herbivores are responsible for differences in fitness. In the field, we manipulated the presence of herbivores on experimentally inbred and outcrossed plants of Solanum carolinense (horsenettle) for three years. Damage was greater on inbred plants, and ID for growth and fitness was significantly greater under herbivory. Inbreeding reduced phenolic expression both qualitatively (phytochemical diversity) and quantitatively, indicating deleterious load at loci related to the biosynthesis of defence compounds. Our results indicate that inbreeding effects on plant-herbivore interactions are mediated by changes to functional plant metabolites, suggesting that variation in inbreeding could be a predictor of defence trait variation. The magnitude of herbivore-mediated, ecological ID indicates that herbivores could maintain outcrossing mating systems in nature.

Lobelia siphilitica plants that escape herbivory in time also have reduced latex production.

Flowering phenology is an important determinant of a plant's reproductive success. Both assortative mating and niche construction can result in the evolution of correlations between phenology and other reproductive, functional, and life history traits. Correlations between phenology and herbivore defence traits are particularly likely because the timing of flowering can allow a plant to escape herbivory. To test whether herbivore escape and defence are correlated, we estimated phenotypic and genetic correlations between flowering phenology and latex production in greenhouse-grown Lobelia siphilitica L. (Lobeliaceae). Lobelia siphilitica plants that flower later escape herbivory by a specialist pre-dispersal seed predator, and thus should invest fewer resources in defence. Consistent with this prediction, we found that later flowering was phenotypically and genetically correlated with reduced latex production. To test whether herbivore escape and latex production were costly, we also measured four fitness correlates. Flowering phenology was negatively genetically correlated with three out of four fitness estimates, suggesting that herbivore escape can be costly. In contrast, we did not find evidence for costs of latex production. Generally, our results suggest that herbivore escape and defence traits will not evolve independently in L. siphilitica.

Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms.

The impact of exotic species on native organisms is widely acknowledged, but poorly understood. Very few studies have empirically investigated how invading plants may alter delicate ecological interactions among resident species in the invaded range. We present novel evidence that antifungal phytochemistry of the invasive plant, Alliaria petiolata, a European invader of North American forests, suppresses native plant growth by disrupting mutualistic associations between native canopy tree seedlings and belowground arbuscular mycorrhizal fungi. Our results elucidate an indirect mechanism by which invasive plants can impact native flora, and may help explain how this plant successfully invades relatively undisturbed forest habitat.