Divergent responses of generalist and specialist pollinators to experimental drought: Outcomes for plant reproduction

Drought is an increasingly important consequence of climate change. Drought often causes plants to alter patterns of resource allocation, which in turn can affect how plants interact with other species. How these altered interactions subsequently influence plant reproductive success remains incompletely understood and may depend on the degree of specialization exhibited by antagonists and mutualists. Specialist pollinators, for example, are dependent on floral resources from their obligate hosts and under drought conditions may thus indiscriminately visit these hosts (at least in certain circumstances). Generalist pollinators, in contrast, may only forage on host plants in good condition, given that they can forage on other plant species. We tested this hypothesis and its consequences for plant reproduction in squash (Cucurbita pepo) grown along an experimental moisture gradient ranging from dry (growth and flowering compromised) to wet conditions. Floral visitation increased with plant soil moisture for generalist honey bees but was independent of plant soil moisture for specialist squash bees. Pollen production increased with plant soil moisture, and fluorescent pigments placed on flowers revealed that pollinators primarily moved pollen from male flowers on well-watered plants to the stigmas of female flowers on well-watered plants. Seed set increased with increasing plant soil moisture but, notably, was higher in bee-pollinated plants compared to plants pollinated by hand with an even mix of pollen from plants grown at either end of the experimental moisture gradient. These results suggest that superior pollen rewards, perhaps combined with selective foraging by generalists, enhanced reproductive success in C. pepo when plant soil moisture was high and more generally illustrate that pollinator behavior may contribute to how drought conditions affect plant reproduction.

Water availability and plant-herbivore interactions

Water is essential to plant growth and drives plant evolution and interactions with other organisms such as herbivores. However, water availability fluctuates, and these fluctuations are intensified by climate change. How plant water availability influences plant-herbivore interactions in the future is an important question in basic and applied ecology. Here we summarize and synthesize the recent discoveries on the impact of water availability on plant antiherbivore defense ecology and the underlying physiological processes. Water deficit tends to enhance plant resistance and escape traits (i.e. early phenology) against herbivory but negatively affects other defense strategies, including indirect defense and tolerance. However, exceptions are sometimes observed in specific plant-herbivore species pairs. We discuss the effect of water availability on species interactions associated with plants and herbivores from individual to community levels and how these interactions drive plant evolution. Although water stress and many other abiotic stresses are predicted to increase in intensity and frequency due to climate change, we identify a significant lack of study on the interactive impact of additional abiotic stressors on water-plant-herbivore interactions. This review summarizes critical knowledge gaps and informs possible future research directions in water-plant-herbivore interactions.

Herbivory Amplifies Adverse Effects of Drought on Seedling Recruitment in a Keystone Species of Western North American Rangelands

Biotic interactions can affect a plant's ability to withstand drought. Such an effect may impact the restoration of the imperiled western North American sagebrush steppe, where seedlings are exposed to summer drought. This study investigated the impact of herbivory on seedlings' drought tolerance for a keystone species in this steppe, the shrub Artemisia tridentata. Herbivory effects were investigated in two field experiments where seedlings were without tree protectors or within plastic or metal-mesh tree protectors. Treatment effects were statistically evaluated on herbivory, survival, leaf water potential, and inflorescence development. Herbivory occurrence was 80% higher in seedlings without protectors. This damage occurred in early spring and was likely caused by ground squirrels. Most plants recovered, but herbivory was associated with higher mortality during the summer when seedlings experienced water potentials between -2.5 and -7 MPa. However, there were no differences in water potential between treatments, suggesting that the browsed plants were less tolerant of the low water potentials experienced. Twenty months after outplanting, the survival of plants without protectors was 40 to 60% lower than those with protectors. The percentage of live plants developing inflorescences was approximately threefold higher in plants with protectors. Overall, spring herbivory amplified susceptibility to drought and delayed reproductive development.

Plant-pollinator interaction niche broadens in response to severe drought perturbations

The composition of plant-pollinator interactions-i.e., who interacts with whom in diverse communities-is highly dynamic, and we have a very limited understanding of how interaction identities change in response to perturbations in nature. One prediction from niche and diet theory is that resource niches will broaden to compensate for resource reductions driven by perturbations, yet this has not been empirically tested in plant-pollinator systems in response to real-world perturbations in the field. Here, we use a long-term dataset of floral visitation to Ipomopsis aggregata, a montane perennial herb, to test whether the breadth of its floral visitation niche (i.e., flower visitor richness) changed in response to naturally occurring drought perturbations. Fewer floral resources are available in drought years, which could drive pollinators to expand their foraging niches, thereby expanding plants' floral visitation niches. We compared two drought years to three non-drought years to analyze changes in niche breadth and community composition of floral visitors to I. aggregata, predicting broadened niche breadth and distinct visitor community composition in drought years compared to non-drought years. We found statistically significant increases in niche breadth in drought years as compared to non-drought conditions, but no statistically distinguishable changes in community composition of flower visitors. Our findings suggest that plants' floral visitation niches may exhibit considerable plasticity in response to disturbance. This may have widespread consequences for community-level stability as well as functional consequences if increased niche overlap affects pollination services.

The Compensatory Tillering in the Forage Grass Hordeum brevisubulatum After Simulated Grazing of Different Severity

The response of compensatory growth is an important adaptive strategy for plants to grazing. However, most previous studies on compensatory growth of plants focused on the compensation of the biomass or the number of sexual reproductive offspring and neglected the compensatory growth of vegetative reproduction (VR). This is important not only for plant compensatory growth studies, but also for theoretical and practical studies of grassland production. The clonal tussock grass Hordeum brevisubulatum was selected as the research object. Four different clipping severities (unclipping and clipping stubble at heights of 15, 10, and 5 cm) at the jointing stage and flowering stage were implemented to study the effect of simulated grazing. To explore the effect of recovery growth time on plant growth after simulated grazing, three sampling times were used at different recovery times after simulated grazing (1, 3, and 7 weeks). We found that light and moderate grazing severity significantly increased the number of vegetative reproduction modules, the promotion of simulated grazing on the number of vegetative reproduction modules was higher in the jointing stage than the flowering stage, and the increase in simulated grazing severity decreased with prolonged recovery growth time. The number of tillers significantly decreased with the increase in simulated grazing in both the jointing and flowering stages at 1 week after damage, and the decreasing effect weakened with the prolonged recovery growth time. The bud number mainly showed over-compensation, the juvenile tiller number showed complete compensation, and the tiller number showed under-compensation at 1 and 3 weeks after recovery growth. The number of tillers showed complete compensation under different grazing severities in the jointing stage, while it showed under-compensation in the flowering stage at 7 weeks after recovery growth. Our results indicated that different grazing severities in the jointing stage could promote the output of tillers with matter production capacity from vegetative reproduction modules, as well as improve the capability of compensatory growth. Therefore, in plant production, there will be a sustainable development effect on the renewal and productivity of the H. brevisubulatum population, resulting in different grazing severities in the jointing stage.

Effects of Simulated Herbivory on the Vegetative Reproduction and Compensatory Growth of Hordeum brevisubulatum at Different Ontogenic Stages

The response of plant vegetative reproduction and compensatory growth to herbivory has been widely discussed in biological and ecological research. Most previous research has supported the idea that both vegetative reproduction and compensatory growth are affected by their ontogenic stage. However, in many studies, the effects of foraging at different ontogenic stages was often confounded with the effects of foraging at different phenological periods for perennials. Our experiment was conducted in a natural meadow with a perennial grass, Hordeum brevisubulatum, and four ontogenic stages were chosen as our experimental objects. Three different clipping intensities during three phenological periods were implemented to explore the effects of simulating animal foraging on vegetative reproduction and compensatory plant growth. The results indicated that there were significant effects of ontogenic stage, phenological period, and clipping intensity on vegetative reproduction and compensatory growth. Moderate clipping intensities significantly increased the number of vegetative tillers, the total number of juvenile tillers and buds, and the aboveground biomass at early phenological periods for individuals at early ontogenic stages. Our results suggested that moderate clipping intensities could induce only an over-compensation response in perennial grasses at both the early ontogenic stage and phenological period, and the ability of compensatory growth gradually decreased with the progression of the ontogenic stage. This is of great significance to the primary production of grasslands subjected to herbivory.

Overcompensation: a 30-year perspective

Biomass removal by herbivores usually incurs a fitness cost for the attacked plants, with the total cost per unit lost tissue depending on the value of the removed tissue (i.e., how costly it is to be replaced by regrowth). Optimal defense theory, first outlined in the 1960s and 1970s, predicted that these fitness costs result in an arms race between plants and herbivores, in which selection favors resistance strategies that either repel herbivores through morphological and chemical resistance traits in order to reduce their consumption, or result in enemy escape through rapid growth or by timing the growth or flowering to the periods when herbivores are absent. Such resistance against herbivores would most likely evolve when herbivores are abundant, cause extensive damage, and consume valuable plant tissues. The purpose of this Special Feature is to celebrate the 30th anniversary of the phenomenon of overcompensation, specifically, where the finding has brought us and where it is leading us 30 yr later. We first provide a short overview of how the phenomenon of overcompensation has led to broader studies on plant tolerance to herbivory, summarize key findings, and then discuss some promising new directions in light of six featured research papers.

Interactive effects of pests increase seed yield

Loss in seed yield and therefore decrease in plant fitness due to simultaneous attacks by multiple herbivores is not necessarily additive, as demonstrated in evolutionary studies on wild plants. However, it is not clear how this transfers to crop plants that grow in very different conditions compared to wild plants. Nevertheless, loss in crop seed yield caused by any single pest is most often studied in isolation although crop plants are attacked by many pests that can cause substantial yield losses. This is especially important for crops able to compensate and even overcompensate for the damage. We investigated the interactive impacts on crop yield of four insect pests attacking different plant parts at different times during the cropping season. In 15 oilseed rape fields in Sweden, we estimated the damage caused by seed and stem weevils, pollen beetles, and pod midges. Pest pressure varied drastically among fields with very low correlation among pests, allowing us to explore interactive impacts on yield from attacks by multiple species. The plant damage caused by each pest species individually had, as expected, either no, or a negative impact on seed yield and the strongest negative effect was caused by pollen beetles. However, seed yield increased when plant damage caused by both seed and stem weevils was high, presumably due to the joint plant compensatory reaction to insect attack leading to overcompensation. Hence, attacks by several pests can change the impact on yield of individual pest species. Economic thresholds based on single species, on which pest management decisions currently rely, may therefore result in economically suboptimal choices being made and unnecessary excessive use of insecticides.

The evolution of halophytes, glycophytes and crops, and its implications for food security under saline conditions

The effective development of salt tolerant crops requires an understanding that the evolution of halophytes, glycophytes and our major grain crops has involved significantly different processes. Halophytes (and other edaphic endemics) generally arose through colonization of habitats in severe disequilibrium by pre-adapted individuals, rather than by gradual adaptation from populations of 'glycophytes'. Glycophytes, by contrast, occur in low sodium ecosystems, where sodium was and is the major limiting nutrient in herbivore diets, suggesting that their evolution reflects the fact that low sodium individuals experienced lower herbivory and had higher fitness. For domestication/evolution of crop plants, the selective pressure was human imposed and involved humans co-opting functions of defense and reproductive security. Unintended consequences of this included loss of tolerance to various stresses and loss of the genetic variability needed to correct that. Understanding, combining and manipulating all three modes of evolution are now critical to the development of salt tolerant crops, particularly those that will offer food security in countries with few economic resources and limited infrastructure. Such efforts will require exploiting the genetic structures of recently evolved halophytes, the genetic variability of model plants, and endemic halophytes and 'minor' crops that already exist.

Plasticity in ploidy underlies plant fitness compensation to herbivore damage

How plants mitigate damage by animal herbivores is a fundamental ecological and evolutionary question of plant-animal interactions. Some plants can increase their fitness when damaged in a phenomenon termed 'overcompensation'. Despite overcompensation being observed in a variety of plant species, its mechanistic basis remains elusive. Recent research has shown that the Arabidopsis thaliana genotype Columbia-4 employs endoreduplication, the replication of the genome without mitosis, following damage and that it overcompensates for seed yield. The related genotype Landsberg erecta, in contrast, does not increase its endoreduplication following damage and suffers reduced seed yield. While these results suggest that a plant's ability to plastically increase its ploidy during regrowth may promote its mitigation of damage, no studies have explicitly linked the endoreduplication genetic pathway to the regrowth and fitness of damaged plants. By comparing fitness and ploidy between undamaged and damaged plants of Columbia-4, Landsberg erecta and their offspring, we provide evidence that endoreduplication is directly involved in compensatory performance. We then overexpressed an endoreduplication regulator and compared this mutant's endoreduplication and compensation with its background genotype Columbia-0, an undercompensator. Enhancing Columbia-0's ability to endoreduplicate during regrowth led to the complete mitigation of the otherwise detrimental effects of damage on its fitness. These results suggest that the ability of these plants to increase their ploidy via endoreduplication directly impacts their abilities to compensate for damage, providing a novel mechanism by which some plants can mitigate or even benefit from apical damage with potential across the wide range of plant taxa that endoreduplicate.

Ontogenetic differences of herbivory on woody and herbaceous plants: a meta-analysis demonstrating unique effects of herbivory on the young and the old, the slow and the fast

The effect of herbivory on plant performance is the subject of a large number of ecological studies, and plant responses to herbivory range from reduced reproduction to overcompensation. Because plant defenses, stored resources, and allocation demands change throughout a plant's lifetime, it can be hypothesized the effects of herbivory also vary with development. The present work extends previous analyses to incorporate hundreds of studies in a new meta-analysis addressing this topic. Herbivores had an overall negative effect on plant growth and reproduction, and, in contrast to a previous meta-analysis, this work shows the timing of herbivory is relevant. Differences in the effects of herbivory between life stages existed for woody plant reproduction and perennial herb growth. In addition, tree and shrub growth was reduced by herbivore damage at early ontogenetic stages, and perennial herb reproduction was limited by adult stage herbivory. These results partially support the continuum of an ontogenetic response model. Finally, consideration of this synthesis in conjunction with other work led to the conclusion that different plant groups optimize their defense investments in unique ways. Slow-growing plants may strongly chemically defend young tissues, supporting the plant-age hypothesis, because early herbivory is detrimental to growth. Faster-growing herbs may invest more in antiherbivore defense when they are older, supporting the growth-differentiation balance hypothesis, because later herbivory limits their reproduction.

Environmental variation mediates the deleterious effects of Coleosporium ipomoeae on Ipomoea purpurea

Variation in the environment is common within and between natural populations and may influence selection on plant resistance by altering the level of damage or the fitness consequences of damage from plant enemies. While much is known about how environmental variation influences the amount of damage a plant experiences, few studies have attempted to determine how variation in the environment may alter the fitness consequences of damage, particularly in plant-pathogen interactions. In this work we manipulated a rust pathogen, Coleosporium ipomoeae, in field experiments and showed that this pathogen reduced several components of fitness in its natural host plant, Ipomoea purpurea. Furthermore, we showed that the deleterious effects of C. ipomoeae were variable. We identified variation in the quality of a plant's microenvironment, the abundance of secondary enemy damage, and the length of a growing season as variable components of the environment that may influence the magnitude of damage and tolerance, causing the interaction between C. ipomoeae and I. purpurea to vary from parasitism to commensalism. Considering how environmental variation impacts the magnitude and negative fitness effects of pathogen damage is important to understanding spatially variable selection and coevolution in this and other plant-pathogen interactions.