Consequences of multiple flower-insect interactions for subsequent plant-insect interactions and plant reproduction

Agricultural landscape simplification affects wild plant reproduction indirectly through herbivore-mediated changes in floral display

As natural landscapes are modified and converted into simplified agricultural landscapes, the community composition and interactions of organisms persisting in these modified landscapes are altered. While many studies examine the consequences of these changing interactions for crops, few have evaluated the effects on wild plants. Here, we examine how pollinator and herbivore interactions affect reproductive success for wild resident and phytometer plants at sites along a landscape gradient ranging from natural to highly simplified. We tested the direct and indirect effects of landscape composition on plant traits and reproduction mediated by insect interactions. For phytometer plants exposed to herbivores, we found that greater landscape complexity corresponded with elevated herbivore damage, which reduced total flower production but increased individual flower size. Though larger flowers increased pollination, the reduction in flowers ultimately reduced plant reproductive success. Herbivory was also higher in complex landscapes for resident plants, but overall damage was low and therefore did not have a cascading effect on floral display and reproduction. This work highlights that landscape composition directly affects patterns of herbivory with cascading effects on pollination and wild plant reproduction. Further, the absence of an effect on reproduction for resident plants suggests that they may be adapted to their local insect community.

Trade-off mitigation: a conceptual framework for understanding floral adaptation in multispecies interactions

Explanations of floral adaptation to diverse pollinator faunas have often invoked visitor‐mediated trade‐offs in which no intermediate, generalized floral phenotype is optimal for pollination success, i.e. fitness valleys are created. In such cases, plant species are expected to specialize on particular groups of flower visitors. Contrary to this expectation, it is commonly observed that flowers interact with various groups of visitors, while at the same time maintaining distinct phenotypes among ecotypes, subspecies, or congeners. This apparent paradox may be due to a gap in our understanding of how visitor‐mediated trade‐offs could affect floral adaptation. Here we provide a conceptual framework for analysing visitor‐mediated trade‐offs with the hope of stimulating empirical and theoretical studies to fill this gap. We propose two types of visitor‐mediated trade‐offs to address negative correlations among fitness contributions of different visitors: visitor‐mediated phenotypic trade‐offs (phenotypic trade‐offs) and visitor‐mediated opportunity trade‐offs (opportunity trade‐offs). Phenotypic trade‐offs occur when different groups of visitors impose conflicting selection pressures on a floral trait. By contrast, opportunity trade‐offs emerge only when some visitors’ actions (e.g. pollen collection) remove opportunities for fitness contribution by more beneficial visitors. Previous studies have observed disruptive selection due to phenotypic trade‐offs less often than expected. In addition to existing explanations, we propose that some flowers have achieved ‘adaptive generalization’ by evolving features to avoid or eliminate the fitness valleys that phenotypic trade‐offs tend to produce. The literature suggests a variety of pathways to such ‘trade‐off mitigation’. Trade‐off mitigation may also evolve as an adaptation to opportunity trade‐offs. We argue that active exclusion, or floral specialization, can be viewed as a trade‐off mitigation, occurring only when flowers cannot otherwise avoid strong opportunity trade‐offs. These considerations suggest that an evolutionary strategy for trade‐off mitigation is achieved often by acquiring novel combinations of traits. Thus, phenotypic diversification of flowers through convergent evolution of certain trait combinations may have been enhanced not only through adaptive specialization for particular visitors, but also through adaptive generalization for particular visitor communities. Explorations of how visitor‐mediated trade‐offs explain the recurrent patterns of floral phenotypes may help reconcile the long‐lasting controversy on the validity of pollination syndromes.

Plant ontogeny determines strength and associated plant fitness consequences of plant-mediated interactions between herbivores and flower visitors

Plants show ontogenetic variation in growth-defence strategies to maximize reproductive output within a community context. Most work on plant ontogenetic variation in growth-defence trade-offs has focussed on interactions with antagonistic insect herbivores. Plants respond to herbivore attack with phenotypic changes. Despite the knowledge that plant responses to herbivory affect plant mutualistic interactions with pollinators required for reproduction, indirect interactions between herbivores and pollinators have not been included in the evaluation of how ontogenetic growth-defence trajectories affect plant fitness.In a common garden experiment with the annual Brassica nigra, we investigated whether exposure to various herbivore species on different plant ontogenetic stages (vegetative, bud or flowering stage) affects plant flowering traits, interactions with flower visitors and results in fitness consequences for the plant.Effects of herbivory on flowering plant traits and interactions with flower visitors depended on plant ontogeny. Plant exposure in the vegetative stage to the caterpillar Pieris brassicae and aphid Brevicoryne brassicae led to reduced flowering time and flower production, and resulted in reduced pollinator attraction, pollen beetle colonization, total seed production and seed weight. When plants had buds, infestation by most herbivore species tested reduced flower production and pollen beetle colonization. Pollinator attraction was either increased or reduced. Plants infested in the flowering stage with P. brassicae or Lipaphis erysimi flowered longer, while infestation by any of the herbivore species tested increased the number of flower visits by pollinators.Our results show that the outcome of herbivore-flower visitor interactions in B. nigra is specific for the combination of herbivore species and plant ontogenetic stage. Consequences of herbivory for flowering traits and reproductive output were strongest when plants were attacked early in life. Such differences in selection pressures imposed by herbivores to specific plant ontogenetic stages may drive the evolution of distinct ontogenetic trajectories in growth-defence-reproduction strategies and include indirect interactions between herbivores and flower visitors. Synthesis. Plant ontogeny can define the direct and indirect consequences of herbivory. Our study shows that the ontogenetic stage of plant individuals determined the effects of herbivory on plant flowering traits, interactions with flower visitors and plant fitness.

Ecology of Plastic Flowers

Plant phenotypic plasticity in response to herbivore attack includes changes in flower traits. Such herbivore-induced changes in flower traits have consequences for interactions with flower visitors. We synthesize here current knowledge on the specificity of herbivore-induced changes in flower traits, the underlying molecular mechanisms, and the ecological consequences for flower-associated communities. Herbivore-induced changes in flower traits seem to be largely herbivore species-specific. The extensive plasticity observed in flowers influences a highly connected web of interactions within the flower-associated community. We argue that the adaptive value of herbivore-induced plant responses and flower plasticity can be fully understood only from a community perspective rather than from pairwise interactions.