Earlier snow melt and reduced summer precipitation alter floral traits important to pollination

Climate change can cause changes in expression of organismal traits that influence fitness. In flowering plants, floral traits can respond to drought, and that phenotypic plasticity has the potential to affect pollination and plant reproductive success. Global climate change is leading to earlier snow melt in snow-dominated ecosystems as well as affecting precipitation during the growing season, but the effects of snow melt timing on floral morphology and rewards remain unknown. We conducted crossed manipulations of spring snow melt timing (early vs. control) and summer monsoon precipitation (addition, control, and reduction) that mimicked recent natural variation, and examined plastic responses in floral traits of Ipomopsis aggregata over 3 years in the Rocky Mountains. We tested whether increased summer precipitation compensated for earlier snow melt, and if plasticity was associated with changes in soil moisture and/or leaf gas exchange. Lower summer precipitation decreased corolla length, style length, corolla width, sepal width, and nectar production, and increased nectar concentration. Earlier snow melt (taking into account natural and experimental variation) had the same effects on those traits and decreased inflorescence height. The effect of reduced summer precipitation was stronger in earlier snow melt years for corolla length and sepal width. Trait reductions were explained by drier soil during the flowering period, but this effect was only partially explained by how drier soils affected plant water stress, as measured by leaf gas exchange. We predicted the effects of plastic trait changes on pollinator visitation rates, pollination success, and seed production using prior studies on I. aggregata. The largest predicted effect of drier soil on relative fitness components via plasticity was a decrease in male fitness caused by reduced pollinator rewards (nectar production). Early snow melt and reduced precipitation are strong drivers of phenotypic plasticity, and both should be considered when predicting effects of climate change on plant traits in snow-dominated ecosystems.

Highest Defoliation Tolerance in Amaranthus cruentus Plants at Panicle Development Is Associated With Sugar Starvation Responses

Defoliation tolerance (DT) in Amaranthus cruentus is known to reach its apex at the panicle emergence (PE) phase and to decline to minimal levels at flowering (FL). In this study, defoliation-induced changes were recorded in the content of non-structural carbohydrates and raffinose family oligosaccharides (RFOs), and in the expression and/or activity of sugar starvation response-associated genes in plants defoliated at different vegetative and reproductive stages. This strategy identified sugar-starvation-related factors that explained the opposite DT observed at these key developmental stages. Peak DT at PE was associated with increased cytosolic invertase (CI) activity in all organs and with the extensive induction of various class II trehalose-phosphate synthase (TPS) genes. Contrariwise, least DT at FL coincided with a sharp depletion of starch reserves and with sucrose (Suc) accumulation, in leaves and stems, the latter of which was consistent with very low levels of CI and vacuolar invertase activities that were not further modified by defoliation. Increased Suc suggested growth-inhibiting conditions associated with altered cytosolic Suc-to-hexose ratios in plants defoliated at FL. Augmented cell wall invertase activity in leaves and roots, probably acting in a regulatory rather than hydrolytic role, was also associated with minimal DT observed at FL. The widespread contrast in gene expression patterns in panicles also matched the opposite DT observed at PE and FL. These results reinforce the concept that a localized sugar starvation response caused by C partitioning is crucial for DT in grain amaranth.

A systematic review of the direct and indirect effects of herbivory on plant reproduction mediated by pollination

BACKGROUND

Plant reproduction is influenced by the net outcome of plant-herbivore and plant-pollinator interactions. While both herbivore impacts and pollinator impacts on plant reproduction have been widely studied, few studies examine them in concert.

METHODOLOGY

Here, we review the contemporary literature that examines the net outcomes of herbivory and pollination on plant reproduction and the impacts of herbivores on pollination through damage to shared host plants using systematic review tools. The direct or indirect effects of herbivores on floral tissue and reported mechanisms were compiled including the taxonomic breadth of herbivores, plants and pollinators.

RESULTS

A total of 4,304 studies were examined producing 59 relevant studies for synthesis that reported both pollinator and herbivore measures. A total of 49% of studies examined the impact of direct damage to floral tissue through partial florivory while 36% of studies also examined the impact of vegetative damage on pollination through folivory, root herbivory, and stem damage. Only three studies examined the effects of both direct and indirect damage to pollination outcomes within the same study.

CONCLUSIONS

It is not unreasonable to assume that plants often sustain simultaneous forms of damage to different tissues and that the net effects can be assessed through differences in reproductive output. Further research that controls for other relative drivers of reproductive output but examines more than one pathway of damage simultaneously will inform our understanding of the mechanistic relevance of herbivore impacts on pollination and also highlight interactions between herbivores and pollinators through plants. It is clear that herbivory can impact plant fitness through pollination; however, the relative importance of direct and indirect damage to floral tissue on plant reproduction is still largely unknown.

Evidence that a herbivore tolerance response affects selection on floral traits and inflorescence architecture in purple loosestrife (Lythrum salicaria)

BACKGROUND AND AIMS

The study of the evolution of floral traits has generally focused on pollination as the primary driver of selection. However, herbivores can also impose selection on floral traits through a variety of mechanisms, including florivory and parasitism. Less well understood is whether floral and inflorescence architecture traits that influence a plant's tolerance to herbivory, such as compensatory regrowth, alter pollinator-mediated selection.

METHODS

Because herbivore damage to Lythrum salicaria meristems typically leads to an increase in the number of inflorescences and the size of the floral display, an experiment was conducted to test whether simulated herbivory (i.e. clipping the developing meristem) could alter the magnitude or direction of pollinator-mediated selection on a suite of floral and inflorescence architecture traits. Using a pollen supplementation protocol, pollen limitation was compared in the presence and absence of meristem damage in order to quantify any interaction between pollinator and herbivore-mediated selection on floral traits.

KEY RESULTS

Surprisingly, in spite of an obvious impact on floral display and architecture, with clipped plants producing more inflorescences and more flowers, there was no difference in pollen limitation between clipped and unclipped plants. Correspondingly, there was no evidence that imposing herbivore damage altered pollinator-mediated selection in this system. Rather, the herbivory treatment alone was found to alter direct selection on floral display, with clipped plants experiencing greater selection for earlier flowering and weaker selection for number of inflorescences when compared with unclipped plants.

CONCLUSIONS

These findings imply that herbivory on its own can drive selection on plant floral traits and inflorescence architecture in this species, even more so than pollinators. Specifically, herbivory can impose selection on floral traits if such traits influence a plant's tolerance to herbivory, such as through the timing of flowering and/or the compensatory regrowth response.

Flower color preferences of insects and livestock: effects on Gentiana lutea reproductive success

Angiosperms diversification was primarily driven by pollinator agents, but non-pollinator agents also promoted floral evolution. Gentiana lutea shows pollinator driven flower color variation in NW Spain. We test whether insect herbivores and livestock, which frequently feed in G.lutea, play a role in G. lutea flower color variation, by answering the following questions: (i) Do insect herbivores and grazing livestock show flower color preferences when feeding on G. lutea? (ii) Do mutualists (pollinators) and antagonists (seed predators, insect herbivores and livestock) jointly affect G. lutea reproductive success? Insect herbivores fed more often on yellow flowering individuals but they did not affect seed production, whereas livestock affected seed production but did not show clear color preferences. Our data indicate that flower color variation of G. lutea is not affected by insect herbivores or grazing livestock.

Modest Pollen Limitation of Lifetime Seed Production Is in Good Agreement with Modest Uncertainty in Whole-Plant Pollen Receipt

We recently introduced a model that predicts the degree to which a plant's lifetime seed production may be constrained by unpredictable shortfalls of pollen receipt ("pollen limitation"). Burd's comment in this issue criticized our analysis, first by arguing that the empirical literature documents much higher levels of pollen limitation than our model predicts and then suggesting that the apparent discrepancy stemmed from our (1) underestimating the costs of securing a fertilized ovule and (2) assuming too little unpredictability in whole-plant pollen receipt. We reply as follows. First, the empirical literature must be consulted carefully. Burd relies on pollen supplementation experiments performed on parts of plants or on whole plants but during only one reproductive season for polycarpic perennials; in both cases, resource reallocation often leads to gross overestimates of pollen limitation. We comprehensively review pollen limitation estimates that are free of these estimation problems and find strong agreement with our model predictions. Second, although cost estimates for different components of seed production are imprecise, errors are likely to be small relative to the >1,000-fold differences observed across plant species, the primary focus of our article. Finally, contrary to Burd's argument, pollen receipt by entire plants is much more predictable than that by individual flowers because the flower-to-flower variation "averages out" when summed across many flowers. Our model uses parameter values that are in broad agreement with the empirical record of modest plant-to-plant variation in pollen receipt and thus predicts the generally modest pollen limitation that is observed in nature.

Potential impacts of tolerance to herbivory on population dynamics of a monocarpic herb

PREMISE OF THE STUDY

Mammalian herbivores, particularly white-tailed deer, can have a major impact on plant abundance and distribution. However, plants can tolerate herbivory by increasing seed production or seed quality. We used the monocarpic perennial Prenanthes roanensis to examine tolerance to mammalian herbivory through seed quality and modeled the effects of tolerance on population growth rate.

METHODS

We examined seed quality (proportion of viable seeds, seed mass, germination, and seedling size) on damaged and undamaged plants to determine the extent to which plants tolerate herbivory. We then varied seed quality parameters over a range of values in population models to compare population growth rates under "no-tolerance" conditions (herbivory, but no tolerance) to those under "tolerance" conditions.

KEY RESULTS

In most populations, plants damaged by herbivores had a greater proportion of viable seeds per plant or a greater probability of seed germination. Incorporating observed tolerance into population models did not significantly increase population growth rate. However, at low germination rates, increased germination of seeds from damaged plants has the potential to significantly increase population growth rate.

CONCLUSIONS

Damaged plants can compensate for loss of reproductive heads by increasing seed viability and germination rates in the remaining seeds. This study is one of the first to demonstrate that tolerance through seed quality has the potential to affect population growth rate. Our results suggest that incorporating tolerance into population models may help elucidate mechanisms by which plant populations persist despite herbivory.

Additive effects of pollinators and herbivores result in both conflicting and reinforcing selection on floral traits

Mutualists and antagonists are known to respond to similar floral cues, and may thus cause opposing selection on floral traits. However, we lack a quantitative understanding of their independent and interactive effects. In a population of the orchid Gymnadenia conopsea, we manipulated the intensity of pollination and herbivory in a factorial design to examine whether both interactions influence selection on flowering phenology, floral display, and morphology. Supplemental hand-pollination increased female fitness by 31% and one-quarter of all plants were damaged by herbivores. Both interactions contributed to selection. Pollinators mediated selection for later flowering and herbivores for earlier flowering, while both selected for longer spurs. The strength of selection was similar for both agents, and their effects were additive. As a consequence, there was no. net selection on phenology, whereas selection on spur length was strong. The experimental results demonstrate that both pollinators and herbivores can markedly influence the strength of selection on flowering phenology and floral morphology, and cause both conflicting and reinforcing selection. They also indicate that the direction of selection on phenology will vary with the relative intensity of the mutualistic and antagonistic interaction, potentially resulting in both temporal and among-population variation in optimal flowering time.

The tolerance of grain amaranth (Amaranthus cruentus L.) to defoliation during vegetative growth is compromised during flowering

The biochemical processes underlying variations of tolerance are often accompanied by source-sink transitions affecting carbon (C) metabolism. We investigated the tolerance of Amaranthus cruentus L. to total mechanical defoliation through development and in different growing seasons. Defoliated A. cruentus recovered ∼80% of their above-ground biomass and ∼100% of grain yield compared to intact plants if defoliation occurred early during ontogeny, but could not compensate when defoliation occurred during flowering. Tolerance index was higher in the summer season (-0.3) than in the winter season (-0.7). Overall, defoliation tolerance was closely related to phosphoenolpyruvate carboxylase (PEPC) activity in leaves and the subsequent accumulation of starch (∼500 μmol/gDW) and sucrose (∼140 μmol/gDW) in stems and roots. Thus, A. cruentus accumulated sufficient C in roots and stem to allow branching and shoot re-growth after defoliation, but it only possessed sufficient C reserves to maintain <19% seed yield in the absence of new vegetative tissue. Seed size was larger during the warm season but it was not affected by foliar damage. Seed chemical composition was altered by defoliation at flowering. We conclude that A. cruentus defoliation tolerance depends on both, the re-allocation of starch from stem and roots, and the activation of dormant meristems before flowering to generate new photosynthetic capacity to sustain seed filling.

Comparison of ultrastructure, pollen tube growth pattern and starch content in developing and abortive ovaries during the progamic phase in hazel

HIGHLIGHTSIn an abortive ovary of hazel, an integument seldom differentiated and a mature embryo sac never developed.In an abortive ovary of hazel, pollen tube growth was arrested at the style base about 40 days after blooming. Thus, fertilization of the ovule was precluded.Ovary abortion in the four hybrid cultivars was indicated to be associated with insufficient resource availability to support fruit set by all flowers, whereas ovary abortion in C. heterophylla was at least partly determined by pollen availability. In Northeast China, a high frequency of ovary abortion contributes to serious losses in yield of hazelnut. The development of pistillate inflorescences and fruit clusters of four large-fruited hybrid hazel cultivars and the small-fruited Corylus heterophylla were used to study ovary abortion and its possible causes during the progamic phase in hazel. The average number of pistillate (ANP) flowers per inflorescence and average number of fruit (ANF) per cluster of four hybrid hazel cultivars were 7.6-8.5 and 2.4-3.0 respectively; in C. heterophylla, its ANP and ANF was 5.8-6.2 and 3.5, respectively. The total drop varied from 50 to 67%. Ovary abortion in hazel initiated from about 30 days after blooming. The percentage of abortive ovaries (PAO) in the four hybrid hazel cultivars ranged from 63 to 72%, and was significantly higher than that of C. heterophylla (29-42%). Only the abortive ovary ratio of C. heterophylla was significantly reduced after artificial pollination. Fruit number per cluster was positively and negatively correlated with yield and nut mass, respectively. In abortive ovaries, the diameter remained less than 2 mm during the entire fruit development, an integument seldom differentiated and a mature embryo sac never developed. In addition, pollen tube growth was arrested at the style base about 40 days after blooming. Thus, fertilization of the ovule was precluded. Compared with abortive ovary, starch content in developing ovary of four hybrid hazel cultivars and C. heterophylla were significantly higher. This study suggests that abortive ovary was incapable to finish fertilization process due to the absence of mature embryo sac and arrested pollen tubes, and this is likely associate with insufficient resource availability to support fruit set by all flowers in four hybrid hazel cultivars, whereas ovary abortion in C. heterophylla is at least partly determined by pollen availability.

Secondary reproduction in the herbaceous monocarp Lobelia inflata: time-constrained primary reproduction does not result in increased deferral of reproductive effort

BACKGROUND

Although semelparity is a life history characterized by a single reproductive episode within a single reproductive season, some semelparous organisms facultatively express a second bout of reproduction, either in a subsequent season ("facultative iteroparity") or later within the same season as the primary bout ("secondary reproduction"). Secondary reproduction has been explained as the adaptive deferral of reproductive potential under circumstances in which some fraction of reproductive success would otherwise have been lost (due, for example, to inopportune timing). This deferral hypothesis predicts a positive relationship between constraints on primary reproduction and expression of secondary reproduction. The herbaceous monocarp Lobelia inflata has been observed occasionally to express a secondary reproductive episode in the field. However, it is unknown whether secondary reproduction is an example of adaptive reproductive deferral, or is more parsimoniously explained as the vestigial expression of iteroparity after a recent transition to semelparity. Here, we experimentally manipulate effective season length in each of three years to test whether secondary reproduction is a form of adaptive plasticity consistent with the deferral hypothesis.

RESULTS

Our results were found to be inconsistent with the adaptive deferral explanation: first, plants whose primary reproduction was time-constrained exhibited decreased (not increased) allocation to subsequent secondary reproduction, a result that was consistent across all three years; second, secondary offspring-although viable in the laboratory-would not have the opportunity for expression under field conditions, and would thus not contribute to reproductive success.

CONCLUSIONS

Although alternative adaptive explanations for secondary reproduction cannot be precluded, we conclude that the characteristics of secondary reproduction found in L. inflata are consistent with predictions of incomplete or transitional evolution to annual semelparity.

Biocontrol attack increases pollen limitation under some circumstances in the invasive plant Centaurea solstitialis

Herbivore damage often deters pollinator visitation and many invasive plants in North America are pollinator-dependent. This has important implications for the biological control of invasive plants because it means that agents that deter pollinators may have a larger than expected impact on the plant. Yet interactions between pollinators and biocontrol agents are rarely evaluated. Centaurea solstitialis, one of the most problematic invasive species in California, is dependent on pollinators for reproduction. I factorially manipulated infection by a biocontrol pathogen and pollen supplementation to test for (1) pollen limitation in C. solstitialis, (2) whether infection increased pollen limitation, and (3) whether this varied across a soil moisture gradient. Plants growing on north-facing slopes where soil moisture was higher experienced mild pollen limitation in the absence of the pathogen and more pronounced pollen limitation when they were infected. Plants on drier south-facing slopes did not suffer from pollen limitation but instead appeared to suffer from resource limitation. Pathogen infection directly reduced seed set in C. solstitialis by 67-72%. On north-facing slopes, infection had an additional, indirect effect by increasing the degree of pollen limitation plants experienced. The trait that mediates this indirect pathogen-pollinator interaction is the number of inflorescences plants produced: infected plants made fewer inflorescences which led to greater pollen limitation. Although in the present study this outcome is dependent on abiotic factors that vary over small spatial scales, exploiting other invasive plants' dependence on pollinators by selecting agents that deter visitation may enhance agent impact.

When ecosystem services interact: crop pollination benefits depend on the level of pest control

Pollination is a key ecosystem service which most often has been studied in isolation although effects of pollination on seed set might depend on, and interact with, other services important for crop production. We tested three competing hypotheses on how insect pollination and pest control might jointly affect seed set: independent, compensatory or synergistic effects. For this, we performed a cage experiment with two levels of insect pollination and simulated pest control in red clover (Trifolium pratense L.) grown for seed. There was a synergistic interaction between the two services: the gain in seed set obtained when simultaneously increasing pollination and pest control outweighed the sum of seed set gains obtained when increasing each service separately. This study shows that interactions can alter the benefits obtained from service-providing organisms, and this needs to be considered to properly manage multiple ecosystem services.

Noise pollution alters ecological services: enhanced pollination and disrupted seed dispersal

Noise pollution is a novel, widespread environmental force that has recently been shown to alter the behaviour and distribution of birds and other vertebrates, yet whether noise has cumulative, community-level consequences by changing critical ecological services is unknown. Herein, we examined the effects of noise pollution on pollination and seed dispersal and seedling establishment within a study system that isolated the effects of noise from confounding stimuli common to human-altered landscapes. Using observations, vegetation surveys and pollen transfer and seed removal experiments, we found that effects of noise pollution can reverberate through communities by disrupting or enhancing these ecological services. Specifically, noise pollution indirectly increased artificial flower pollination by hummingbirds, but altered the community of animals that prey upon and disperse Pinus edulis seeds, potentially explaining reduced P. edulis seedling recruitment in noisy areas. Despite evidence that some ecological services, such as pollination, may benefit indirectly owing to noise, declines in seedling recruitment for key-dominant species such as P. edulis may have dramatic long-term effects on ecosystem structure and diversity. Because the extent of noise pollution is growing, this study emphasizes that investigators should evaluate the ecological consequences of noise alongside other human-induced environmental changes that are reshaping human-altered landscapes worldwide.

The good, the bad and the flexible: plant interactions with pollinators and herbivores over space and time are moderated by plant compensatory responses

BACKGROUND AND AIMS

Plants are sessile organisms that face selection by both herbivores and pollinators. Herbivores and pollinators may select on the same traits and/or mediate each others' effects. Erysimum capitatum (Brassicaceae) is a widespread and variable plant species with generalized pollination that is attacked by a number of herbivores. The following questions were addressed. (a) Are pollinators and herbivores attracted by similar plant traits? (b) Does herbivory affect pollinator preferences? (c) Do pollinators and/or herbivores affect fitness and select on plant traits? (d) Do plant compensatory responses affect the outcome of interactions among plants, pollinators and herbivores? (e) Do interactions among E. capitatum and its pollinators and herbivores differ among sites and years?

METHODS

In 2005 and 2006, observational and experimental studies were combined in four populations at different elevations to examine selection by pollinators and herbivores on floral traits of E. capitatum.

KEY RESULTS

Pollinator and herbivore assemblages varied spatially and temporally, as did their effects on plant fitness and selection. Both pollinators and herbivores preferred plants with more flowers, and herbivory sometimes reduced pollinator visitation. Pollinators did not select on plant traits in any year or population and E. capitatum was not pollen limited; however, supplemental pollen resulted in altered plant resource allocation. Herbivores reduced fitness and selected for plant traits in some populations, and these effects were mediated by plant compensatory responses.

CONCLUSIONS

Individuals of Erysimum capitatum are visited by diverse groups of pollinators and herbivores that shift in abundance and importance in time and space. Compensatory reproductive mechanisms mediate interactions with both pollinators and herbivores and may allow E. capitatum to succeed in this complex selective environment.

Performance of Apis mellifera, Bombus impatiens, and Peponapis pruinosa (Hymenoptera: Apidae) as pollinators of pumpkin

Pollination services of pumpkin, Cucurbita pepo L., provided by the European honey bee, Apis mellifera L., were compared with two native bee species, the common eastern bumble bee, Bombus impatiens (Cresson), and Peponapis pruinosa Say, in New York from 2008 to 2010. Performance of each species was determined by comparing single-visit pollen deposition, percentage of visits that contacted the stigma, flower-handling time, fruit and seed set, and fruit weight per number of visits. Fruit yield from small fields (0.6 ha) supplemented with commercial B. impatiens colonies was compared with yield from those not supplemented. A. mellifera spent nearly 2 and 3 times longer foraging on each pistillate flower compared with B. impatiens and P. pruinosa, respectively. A. mellifera also visited pistillate flowers 10-20 times more frequently than B. impatiens and P. pruinosa, respectively. Yet, B. impatiens deposited 3 times more pollen grains per stigma and contacted stigmas significantly more often than either A. mellifera or P. pruinosa. Fruit set and weight from flowers visited four to eight times by B. impatiens were similar to those from open-pollinated flowers, whereas flowers pollinated by A. mellifera and P. pruinosa produced fewer fruit and smaller fruit compared with those from open-pollinated flowers. Fields supplemented with B. impatiens produced significantly more pumpkins per plant than nonsupplemented fields. B. impatiens was a better pollinator of pumpkin than P. pruinosa and should be considered as a promising alternative to A. mellifera for pollinating this crop.

The role of pollen limitation on the coexistence of two dioecious, wind-pollinated, closely related shrubs in a fluctuating environment

Elucidating the mechanisms of species coexistence is a crucial goal in ecology. Theory suggests that, when resource abundance fluctuates, coexistence can be achieved if each species in a competing pair is better at exploiting resources at opposite extremes of a fluctuating resource spectrum. Nonetheless, the proximal mechanisms allowing coexistence remain largely unexplored. In a previous paper, we showed that the coexistence of two Atriplex species was facilitated by their varying demographic response (in survival, growth and recruitment) to fluctuation in water availability. Here we explore the effect of spatial distribution, and pollen and resource limitation on the reproductive success (production of viable seeds) of the same two species. An analysis of their spatial distribution showed that Atriplex acanthocarpa had a clumped distribution, which is thought to increase the effectiveness of pollination in wind-pollinated plants, while Atriplex canescens had a random distribution, a pattern expected to restrict wind-pollination success. A pollen and resource (water and nutrients) addition experiment implemented through a repeated-measures design demonstrated that seed viability of A. canescens was both pollen and resource limited, but that these effects were negligible in A. acanthocarpa. Under natural conditions, pollen limitation restricted seed number in A. canescens to only one-third of that recorded when manual pollination was performed. By decreasing its fecundity (and consequent potential seedling recruitment), pollen limitation reverses the competitive advantage of A. canescens over A. acanthocarpa when the limiting resource (water) is abundant and seedling recruitment takes place. To our knowledge, our study of this congeneric pair in the Chihuahuan Desert is the first to document a link between pollen limitation and species coexistence.

Realized tolerance to nectar robbing: compensation to floral enemies in Ipomopsis aggregata

BACKGROUND AND AIMS

Although the ecological and evolutionary consequences of foliar herbivory are well understood, how plants cope with floral damage is less well explored. Here the concept of tolerance, typically studied within the context of plant defence to foliar herbivores and pathogens, is extended to floral damage. Variation in tolerance to floral damage is examined, together with some of the mechanisms involved.

METHODS

The study was conducted on Ipomopsis aggregata, which experiences floral damage and nectar removal by nectar-robbing bees. High levels of robbing can reduce seeds sired and produced by up to 50 %, an indirect effect mediated through pollinator avoidance of robbed plants. Using an experimental common garden with groups of I. aggregata, realized tolerance to robbing was measured. Realized tolerance included both genetic and environmental components of tolerance. It was hypothesized that both resource acquisition and storage traits, and traits involved in pollination would mitigate the negative effects of robbers.

KEY RESULTS

Groups of I. aggregata varied in their ability to tolerate nectar robbing. Realized tolerance was observed only through a component of male plant reproduction (pollen donation) and not through components of female plant reproduction. Some groups fully compensated for robbing while others under- or overcompensated. Evidence was found only for a pollination-related trait, flower production, associated with realized tolerance. Plants that produced more flowers and that had a higher inducibility of flower production following robbing were more able to compensate through male function.

CONCLUSIONS

Variation in realized tolerance to nectar robbing was found in I. aggregata, but only through an estimate of male reproduction, and traits associated with pollination may confer realized tolerance to robbing. By linking concepts and techniques from studies of plant-pollinator and plant-herbivore interactions, this work provides insight into the role of floral traits in pollinator attraction as well as plant defence.

Resistance and tolerance to herbivory changes with inbreeding and ontogeny in a wild gourd (Cucurbitaceae)

Herbivory is a ubiquitous component of terrestrial communities that reduces plant growth and reproduction. Consequently, a goal of evolutionary ecology is to identify the causes and consequences of variation in herbivory within plant populations. This three-year study examined the effects of inbreeding on the resistance of wild gourd plants (Cucurbita pepo subsp. texana) to herbivory by cucumber beetles and the impact of the timing of herbivory on reproduction. We grew families of inbred and outbred gourds and recorded beetle damage at three developmental stages, incidence of beetle-vectored wilt disease, survival, and reproduction. While total beetle damage significantly depressed flower and fruit production, damage until mid-July did not depress any measure of reproduction, indicating that these gourds are tolerant of moderate levels of herbivory for most of the growing season. However, beetle damage accumulating after mid-July significantly depressed reproduction, indicating that plants have reduced tolerance during peak reproduction. Early damage, however, did increase the probability of contracting a deadly wilt disease that is vectored by the beetles, suggesting that tolerance and resistance are not alternative defense strategies. Inbreeding significantly reduced resistance to herbivory and, independently of beetle damage, reproductive output. Finally, we found additive genetic variation for both resistance and tolerance that varies with ontogeny.

Flower vs. leaf feeding by Pieris brassicae: glucosinolate-rich flower tissues are preferred and sustain higher growth rate

Interactions between butterflies and caterpillars in the genus Pieris and plants in the family Brassicaceae are among the best explored in the field of insect-plant biology. However, we report here for the first time that Pieris brassicae, commonly assumed to be a typical folivore, actually prefers to feed on flowers of three Brassica nigra genotypes rather than on their leaves. First- and second-instar caterpillars were observed to feed primarily on leaves, whereas late second and early third instars migrated via the small leaves of the flower branches to the flower buds and flowers. Once flower feeding began, no further leaf feeding was observed. We investigated growth rates of caterpillars having access exclusively to either leaves of flowering plants or flowers. In addition, we analyzed glucosinolate concentrations in leaves and flowers. Late-second- and early-third-instar P. brassicae caterpillars moved upward into the inflorescences of B. nigra and fed on buds and flowers until the end of the final (fifth) instar, after which they entered into the wandering stage, leaving the plant in search of a pupation site. Flower feeding sustained a significantly higher growth rate than leaf feeding. Flowers contained levels of glucosinolates up to five times higher than those of leaves. Five glucosinolates were identified: the aliphatic sinigrin, the aromatic phenylethylglucosinolate, and three indole glucosinolates: glucobrassicin, 4-methoxyglucobrassicin, and 4-hydroxyglucobrassicin. Tissue type and genotype were the most important factors affecting levels of identified glucosinolates. Sinigrin was by far the most abundant compound in all three genotypes. Sinigrin, 4-hydroxyglucobrassicin, and phenylethylglucosinolate were present at significantly higher levels in flowers than in leaves. In response to caterpillar feeding, sinigrin levels in both leaves and flowers were significantly higher than in undamaged plants, whereas 4-hydroxyglucobrassicin leaf levels were lower. Our results show that feeding on flower tissues, containing higher concentrations of glucosinolates, provides P. brassicae with a nutritional benefit in terms of higher growth rate. This preference appears to be in contrast to published negative effects of volatile glucosinolate breakdown products on the closely related Pieris rapae.

Effects of Resource Availability on Tolerance of Herbivory: A Review and Assessment of Three Opposing Models

Although it is widely acknowledged that a plant's tolerance of herbivore damage depends on resource availability in the plant's environment, there is no consensus on whether higher resource levels lead to greater or to lower tolerance. The prevailing model, the compensatory continuum hypothesis (CCH), predicts that tolerance of herbivory should be greater in high-resource or low-competition conditions. The main rival hypothesis, the growth rate model (GRM), makes the opposite prediction: tolerance of herbivory should be greater in more stressful conditions. The tolerance predictions of a recently introduced model, the limiting resource model (LRM), are more flexible and depend on the type of resource and herbivore under consideration. We reviewed 48 studies (from 40 published articles) of plant tolerance of leaf damage in conditions differing in levels of light, inorganic nutrients, water stress, or competition. The results of 31%, 48%, and 95% of the studies were consistent with the predictions of the CCH, GRM, and LRM, respectively. Thus, by considering which resource is primarily affected by herbivory and which resource is limiting a plant's fitness, the LRM offers a substantial advance in predicting how tolerance will be affected by environmental differences in resource availability.

Limits to reproductive success of Sarracenia purpurea (Sarraceniaceae)

Plant biologists have an enduring interest in assessing components of plant fitness and determining limits to seed set. Consequently, the relative contributions of resource and pollinator availability have been documented for a large number of plant species. We experimentally examined the roles of resource and pollen availability on seed set by the northern pitcher plant Sarracenia purpurea. We were able to distinguish the relative contributions of carbon (photosynthate) and mineral nutrients (nitrogen) to reproductive success. We also determined potential pollinators of this species. The bees Bombus affinis and Augochlorella aurata and the fly Fletcherimyia fletcheri were the only floral visitors to S. purpurea that collected pollen. Supplemental pollination increased seed set by <10%, a much lower percentage than would be expected, given data from noncarnivorous, animal-pollinated taxa. Seed set was reduced by 14% in plants that could not capture prey and by another 23% in plants whose pitcher-shaped leaves were also prevented from photosynthesizing. We conclude that resources are more important than pollen availability in determining seed set by this pitcher plant and that reproductive output may be another "cost" of the carnivorous habit.

Pollen and water limitation in Astragalus scaphoides, a plant that flowers in alternate years

Mast seeding is common in plant populations, but its causes have rarely been tested experimentally. We tested mechanisms of alternate-year flowering and fruit set in an iteroparous, bee-pollinated, herbaceous plant, Astragalus scaphoides, in semi-arid sagebrush steppe. Patterns of reproduction from 1986 to 1999 indicated that spring precipitation was a cue for synchronous flowering, and that increased pollination in high-flowering years was a fitness advantage of synchrony. We tested these patterns by adding supplemental water and pollen to plants in high- and low-flowering sites and years. Supplemental water had no effect on flowering or seed set, so water is not a proximate cue for reproduction, though it could be important over longer (>3 year) time scales. Supplemental pollination increased fruit set in low- but not high-flowering years, indicating that synchronous flowering increases pollination success. Many shorter-term studies also report increased fruit set after pollen supplementation, but not after resource addition. This pattern may reflect the fact that plants can store and reallocate resources, but not pollen, across multiple years. For animal-pollinated herbs such as these, uniting theories about pollination ecology and mast seeding may promote an understanding of the mechanisms that determine patterns of reproduction over time.

FOLIAR DAMAGE TO PARENTAL PLANTS INTERACTS TO INFLUENCE MATING SUCCESS OFIPOMOEA PURPUREA

Studies of how herbivory affects plant fitness often determine whether damage to one parent alters reproductive output (i.e., seed set or paternity) but ignore the possibility that the outcome may be different if both parents were damaged (i.e., the presence of maternal x paternal damage interactions). Using inbred lines of the common morning glory, Ipomoea purpurea, I conducted a series of greenhouse experiments to test whether foliar damage from a generalist insect herbivore, Trichoplusia ni, alters male and female fitness components when neither parent, one parent, or both I. purpurea parents had been damaged. In a single-donor experiment, flowers on both damaged and undamaged maternal plants received pollen from either damaged or undamaged paternal plants. I. purpurea flowers were more likely to be aborted when they received pollen from damaged paternal plants, or when maternal plants were both damaged and grown under low-resource conditions. Foliar damaged plants also produced less seed and pollen than undamaged plants, although seed mass and pollen viability were not affected by damage. In a multiple-donor experiment, flowers on damaged and undamaged maternal plants simultaneously received pollen from damaged and undamaged paternal plants, and F1 seeds were analyzed for paternity. Damaged paternal plants had reduced siring success compared to undamaged paternal plants, and this discrepancy was most pronounced when competition occurred on damaged maternal plants. Thus, damaged maternal plants were more "selective" than undamaged maternal plants. Although previous studies have demonstrated that herbivory can alter fruit and seed production and paternity patterns, this is the first study to show that the magnitude of herbivore damage experienced by both parents can interact to influence maternal and paternal mating success.

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.

Variation in resource limitation of plant reproduction influences natural selection on floral traits of Asclepias syriaca

The availability of both pollen and resources can influence natural selection on floral traits, but their relative importance in shaping floral evolution is unclear. We experimentally manipulated pollinator and resource (fertilizer and water) availability in the perennial wildflower Asclepias syriaca L. Nine floral traits, one male fitness component (number of pollinia removed), and two female fitness components (number of pollinia inserted and number of fruits initiated) were measured for plants in each of three treatments (unmanipulated control, decreased pollinator access, and resource supplementation). Although decreasing pollinators' access to flowers did result in fewer pollinia inserted and removed, fruit set and phenotypic selection on floral traits via female and male fitness did not differ from the control. In contrast, resource supplementation increased fruit set, and phenotypic selection on seven out of nine floral traits was stronger via female than male fitness, consistent with the prediction that selection via female fitness would be greater when reproduction was less resource-limited. Our results support the hypothesis that abiotic resource availability can influence floral evolution by altering gender-specific selection.

Herbivory differentially affects male and female reproductive traits of Cucumis sativus

Herbivory is an important selection pressure in the life history of plants. Most studies use seed or fruit production as an indication of plant fitness, but the impact of herbivory on male reproductive success is usually ignored. It is possible that plants compensate for resources lost to herbivory by shifting the allocation from seed production to pollen production and export, or vice versa. This study examined the impact of herbivory by Helix aspersa on both male and female reproductive traits of a monoecious plant, Cucumis sativus. The effects of herbivory on the relative allocation to male and female flowers were assessed through measurements of the number and size of flowers of both sexes, and the amount of pollinator visitation. We performed two glasshouse experiments; the first looked at the impact of three levels of pre-flowering herbivory, and the second looked at four levels of herbivory after the plants had started to flower. We found that herbivory during the flowering phase led to a significant increase in the number of plants without male flowers. As a consequence there was significantly less pollen export from this population, as estimated by movement of a pollen analog. The size of female flowers was reduced by severe herbivory, but there was no affect on pollen receipt by the female flowers of damaged plants. The decrease in allocation to male function after severe herbivory may be adaptive when male reproductive success is very unpredictable.

Herbivory alters the expression of a mixed-mating system

The direct and indirect effects of vegetative herbivory on the mating system of Impatiens capensis were analyzed through a survey of herbivory in natural I. capensis populations and manipulation of leaf damage in the field. Across 10 wild populations of I. capensis proportion of cleistogamous flowers had a significant positive exponential relationship with natural levels of herbivory. Similarly, experimental leaf damage increased the proportion of flowers and seeds that were cleistogamous. Leaf damage also reduced the biomass of cleistogamous progeny more severely relative to that of chasmogamous progeny. The cumulative effect of leaf damage was to increase plant reliance on fitness derived from cleistogamous progeny. Leaf damage indirectly affected mating system traits by reducing chasmogamous flower size, leading to a reduction in pollinator visitation. Under these experimental conditions, herbivory did not significantly reduce the number of simultaneously open flowers and potential for geitonogamy, nor did it result in significant changes in the composition of the pollinator fauna. These findings are among the first to demonstrate that herbivory has consequences for mating system and should be considered a factor shaping mating system evolution.