Insect preference for symmetrical artificial flowers

An insect preference for floral symmetry may be maintained because plants with symmetrical flowers, which are able to control developmental processes under given environmental conditions, also are able to provide more pollinator rewards than plants with asymmetrical flowers. Alternatively, insects may have an inherent preference for symmetrical structures and thereby impose selection for the maintenance of symmetry in flowers even in the absence of any pollinator rewards. We tested for an insect preference for radially symmetrical flowers by using horizontally placed units of four circular coloured flower models varying in size and symmetry. The shape and colour of the model flowers did not resemble any naturally occurring flowers in the environment. Insects and Hymenoptera, respectively (five species of Diptera and one species of Coleoptera) that visited the flower models clearly preferred symmetrical models over asymmetrical ones, and the ranking of visits to the models reflected a preference for large, symmetrical flowers. These results provide evidence for a preference for symmetrical flower models, even in the absence of pollinator rewards. Received: 11 September 1997 / Accepted: 2 November 1997     

Floral symmetry is associated with flower size and pollen production but not insect visitation rates in Geranium robertianum (Geraniaceae)

The degree to which fine‐scaled variation in floral symmetry is associated with variation in plant fitness remains unresolved, as does the question of whether floral symmetry is in itself a target of pollinator‐mediated selection. Geranium robertianum (Geraniaceae) is a broadly distributed species whose five‐petaled flowers vary widely with respect to their degree of rotational asymmetry. In this study, we used a naturally occurring population of plants to investigate whether floral rotational asymmetry and leaf bilateral symmetry were phenotypically correlated with a series of fitness‐related traits, and also used an experimental array with model flowers to investigate the preference of insect visitors for varying degrees of floral size and symmetry. We found that leaf asymmetry was not associated with any of the phenotypic traits measured, and that the degree of floral rotational asymmetry was strongly associated with decreased flower size and decreased pollen production. Our experimental arrays showed that insect visitors did not discriminate among model flowers on the basis of size or symmetry alone; however, insect visitors preferentially visited smaller, symmetric model flowers over larger, severely asymmetric model flowers. Taken together, our results suggest that floral and leaf symmetry in G. robertianum are not likely strong indicators of phenotypic quality, and that floral symmetry is unlikely to be a target of pollinator‐mediated selection. However, the relationship between floral asymmetry and pollen production may provide a role for fecundity selection on symmetry in this species. These data importantly add to the growing literature on the adaptive nature of floral symmetry in the wild.     

Evolution of pollination syndromes and corolla symmetry in Balsaminaceae reconstructed using phylogenetic comparative analyses

Background and AimsFloral diversity as a result of plant–pollinator interactions can evolve by two distinct processes: shifts between pollination systems or divergent use of the same pollinator. Although both are pollinator driven, the mode, relative importance and interdependence of these different processes are rarely studied simultaneously. Here we apply a phylogenetic approach using the Balsaminaceae (including the species-rich genus Impatiens) to simultaneously quantify shifts in pollination syndromes (as inferred from the shape and colour of the perianth), as well as divergent use of the same pollinator (inferred from corolla symmetry).MethodsFor 282 species we coded pollination syndromes based on associations between floral traits and known pollination systems, and assessed corolla symmetry. The evolution of these traits was reconstructed using parsimony- and model-based approaches, using phylogenetic trees derived from phylogenetic analyses of nuclear ribosomal and plastid DNA sequence data.Key ResultsA total of 71 % of studied species have a bee pollination syndrome, 22 % a bimodal syndrome (Lepidoptera and bees), 3 % a bird pollination syndrome and 5 % a syndrome of autogamy, while 19 % of species have an asymmetrical corolla. Although floral symmetry and pollination syndromes are both evolutionarily labile, the latter shifts more frequently. Shifts in floral symmetry occurred mainly in the direction towards asymmetry, but there was considerable uncertainty in the pattern of shift direction for pollination syndrome. Shifts towards asymmetrical flowers were associated with a bee pollination syndrome.ConclusionFloral evolution in Impatiens has occurred through both pollination syndrome shifts and divergent use of the same pollinator. Although the former appears more frequent, the latter is likely to be underestimated. Shifts in floral symmetry and pollination syndromes depend on each other but also partly on the region in which these shifts take place, suggesting that the occurrence of pollinator-driven evolution may be determined by the availability of pollinator species at large geographical scales.     

ECOLOGICAL COSTS OF PLANT RESISTANCE TO HERBIVORES IN THE CURRENCY OF POLLINATION

In this paper, we examine how ecological costs of resistance might be manifested through plant relationships with pollinators. If defensive compounds are incorporated into floral structures or if they are sufficiently costly that fewer rewards are offered to pollinators, pollinators may discriminate against more defended plants. Here we consider whether directional selection for increased resistance to herbivores could be constrained by opposing selection through pollinator discrimination against more defended plants. We used artificial selection to create two populations of Brassica rapa plants that had high and low myrosinase concentrations and, consequently, high and low resistance to flea beetle herbivores. We measured changes in floral characters of plants in both damaged and undamaged states from these populations with different resistances to flea beetle attack. We also measured pollinator visitation to plants, including numbers of pollinators and measures of visit quality (numbers of flowers visited and time spent per flower). Damage from herbivores resulted in reduced petal size, as did selection for high resistance to herbivores later in the plant lifetime. In addition, floral display (number of open flowers) was also altered by an interaction between these two effects. Changes in floral traits translated into overall greater use of low-resistance, undamaged plants based on total amount of time pollinators spent foraging on plants. Total numbers of pollinators attracted to plants did not differ among treatments; however, pollinators spent significantly more time per flower on plants from the low-resistance population and tended to visit more flowers on these plants as well. Previous work by other investigators on the same pollinator taxa has shown that longer visit times are associated with greater male and female plant fitness. Because initial numbers of pollinators did not differ between selection regimes, palatability and/or amount of rewards offered by high- and low-resistance populations are likely to be responsible for these patterns. During periods of pollinator limitation, less defended plants may have a selective advantage and pollinator preferences may mediate directional selection imposed by herbivores. In addition, if pollinator preferences limit seed set in highly defended plants, then lower seed set previously attributed to allocation costs of defense may also reflect greater pollinator limitation in these plants relative to less defended plants.     

Reproductive assurance varies with flower size in Collinsia parviflora (Scrophulariaceae)

A central question in plant evolutionary ecology is how mixed mating systems are maintained in the face of selection against self-pollination. Recently, attention has focused on the potential reproductive assurance (RA) benefit of selfing: the ability to produce seeds via autonomous selfing when the potential for outcrossing is reduced or absent. To date, there is little experimental support for this benefit under natural pollination conditions. In addition, the RA hypothesis has not been tested experimentally in a species displaying morphological variation for traits expected to influence the mating system, such as flower size, which affects both attractiveness to pollinators and ability to self autonomously. Here, we document significant among-population variation in flower size in Collinsia parviflora and show that pollinators preferred large flowers over small flowers in experimental arrays. The pollinator community varied among three study sites, and two small-flowered populations had lower pollinator visitation rates than one large-flowered population. We compared seed production between intact flowers (can self) and experimentally emasculated flowers (require a pollinator) on large- and small-flowered plants. As predicted by the RA hypothesis, small-flowered plants show a greater RA benefit of selfing than large-flowered plants; emasculated, small flowers produced very few seeds, relative to intact, small flowers or either emasculated or intact, large flowers. We also show that the RA benefit is pollination-context dependent, differing between small- and large-flowered test sites, likely due to a combination of pollinator discrimination against small flowers and differences between test sites in the pollinator community. This paper is the first experimental evidence showing a trait-dependent RA benefit of selfing under natural pollination conditions.     

Floral symmetry: pollinator-mediated stabilizing selection on flower size in bilateral species

Pollinator-mediated stabilizing selection (PMSS) has been proposed as the driver of the evolutionary shift from radial to bilateral symmetry of flowers. Studies have shown that variation in flower size is lower in bilateral than in radial species, but whether bilateral flowers experience more stabilizing selection pressures by employing fewer, more specialized pollinators than radial flowers remains unclear. To test the PMSS hypothesis, we investigate plant–pollinator interactions from a whole community in an alpine meadow in Hengduan Mountains, China, to examine: (i) variance in flower size and level of ecological generalization (pollinator diversity calculated using functional groups) in 14 bilateral and 13 radial species and (ii) the role pollinator diversity played in explaining the difference of variance in flower size between bilateral and radial species. Our data showed that bilateral species had less variance in flower size and were visited by fewer pollinator groups. Pollinator diversity accounted for up to 40 per cent of the difference in variance in flower size between bilateral and radial species. The mediator effect of pollinator diversity on the relationship between floral symmetry and variance in flower size in the community is consistent with the PMSS hypothesis.     

Nectar robbers deter legitimate pollinators by mutilating flowers

Nectar robbing – harvesting nectar illegitimately – can have a variety of outcomes for plant sexual reproduction and for the pollinator community. Nectar robbers can damage flowers while robbing nectar, which could affect the behavior of subsequent flower visitors and, consequently, plant reproduction. However, only nectar manipulation by nectar robbers has so far received attention. We found a short-tongued bee, Hoplonomia sp. (Halictidae), mutilating the conspicuous lower petal of the zygomorphic flowers of Leucas aspera (Lamiaceae) while robbing nectar. We hypothesized that the mutilation of the conspicuous lower petal deters legitimate pollinators on L. aspera flowers, which, in turn, might affect plant reproduction. We first assessed the proportion of naturally-robbed flowers in plant populations for three years to confirm that it was not a purely local phenomenon due to a few individual bees. We then studied diversity, community and visitation characteristics of pollinators, nectar dynamics and fruit set in unrobbed and robbed open flowers in naturally-robbed populations. The proportion of robbed flowers varied significantly across sites and years. Robbing did not affect nectar dynamics in flowers, but it did alter flower morphology, so much so that it reduced pollinator visitation and altered the pollinator community on robbed flowers. However, the maternal function of plant reproduction was not affected by nectar robbing. This study for the first time shows that a nectar robber can have an ecologically significant impact on floral morphology.     

Understanding the basis of pollinator selectivity in sexually dimorphic Fragaria virginiana

Pollinators of gender dimorphic (e.g., gynodioecious or dioecious) species often show a preference for hermaphrodite or male flowers over conspecific females, and such pollinator selectivity can have implications for the evolution of sexual dimorphism. However, because the sex morphs differ in several floral features, it is not readily apparent which specific traits generate differential pollinator service. In this study, we sought to understand the basis of pollinator preferences for hermaphrodites to females of gynodioecious Fragaria virginiana . We extend a previous study that revealed that lack of pollen-filled anthers and shorter petals were not solely responsible for small bee and fly discrimination against females, by investigating the roles of other morphological features, reward characters, and floral context in bringing about pollinator discrimination. Using floral manipulations we revealed that small bees preferentially visit flowers with wide petals, more nectar, intact stamens, as well as flowers located in hermaphrodite-dominated patches. Specifically, bees were able to remotely detect nectar presence. In addition, bees were attracted to both the anther sac and the filament regions of the stamen, indicating that not only anthers, but also filaments may function as visual or olfactory cues to pollen reward. Furthermore, pollinator preference for flowers in hermaphrodite-dominated patches serves to intensify their discrimination against female flowers which occur predominantly in female-dominated patches. For the most part, preferences of flies mirrored those of the bees with the exception that flies also showed a significant preference for flowers held high over those held closer to the ground. These results indicate that a suite of character differences contributes to pollinators' preferences for hermaphrodite to female flowers. Furthermore, they suggest pollinators could mediate selection on both floral morphology and reward in this species.     

The effect of petal-size manipulation on pollen removal,seed set,and insect-visitor behavior in Campanula americana

We investigated the effect of petal-size variation in tall bellflower, Campanula americana, on pollen removal rates, seed set, and pollinator behavior. Pollen removal, seed set, and the behavior of two bee visitors (Bombus and Halictus) were assessed in flowers that had 0%, 50%, or 100% of their petal lobes removed. Pollen removal rates did not differ significantly among the three treatment groups, probably due to the presence of an ineffective (ugly) pollinator (Halictus), which quickly removed pollen from completely reduced flowers. Seed set was significantly positively correlated with petal size. Bumblebees discriminated against reduced flowers, thereby lowering pollen movement to reduced flowers. Halictid bees rarely contacted stigmatic surfaces and thus pollen delivery to reduced flowers was limited. The presence of an ugly pollinator probably obscured the effect of petal reduction on pollen receipt, thus masking the impact of attractive structures on male fitness. Given the positive relationship between seed set and petal size, we caution against ascribing expenditure on attractive structures exclusively to male function. In plants with many olules, expenditure on attractive structures may affect seed set.     

The Roles of Colour and Shape in Pollinator Deception in the Orchid Mantis Hymenopus coronatus

The orchid mantis Hymenopus coronatus (Insecta: Mantodea) is a deceptive predator that attracts pollinators as prey. Their resemblance to a flower has given rise to the hypothesis that they are flower mimics. However, floral mimicry as a predatory strategy, and in particular, how predatory floral mimicry functions at a mechanistic level is poorly understood. Two main morphological characteristics are thought to make orchid mantises appear similar to flowers and thus attractive to pollinators: (1) their ‘flower‐like’ white colouration and (2) their ‘petal‐shaped’ expansions of exoskeleton on their mid‐femur and hind femur (femoral lobes). I investigated the contribution of these colour and shape characteristics to pollinator attraction using artificial orchid mantis models. Models with the ‘flower‐like’ white colouration of the orchid mantis had higher rates of pollinator inspection than brown models. Manipulating overall body shape by removing or changing the orientation of the ‘petal‐shaped’ femoral lobes did not affect the attractiveness of models. As certain flower‐like characteristics (symmetry and petals) did not affect the attractiveness of models, pollinators may not necessarily cognitively misclassify orchid mantises as flowers. Rather, mantises may be exploiting sensory biases of their pollinator prey, and their UV‐absorbing white colouration may be sufficient to lure pollinators. The effectiveness of using artificial models established here provides a basis for future research into orchid mantis morphology and the fine‐scale interactions between orchid mantises and pollinators.     

Effects of nectar robbing on pollinator behavior and plant reproductive success of Pitcairnia angustifolia (Bromeliaceae)

Nectar robbers may have direct and indirect effects on plant reproductive success but the presence of nectar robbing is not proof of negative fitness effects. We combined census data and field experiments to disentangle the complex effects of nectar robbing on nectar production rates, pollinator behavior, pollen export, and female reproductive success of Pitcairnia angustifolia. Under natural conditions flowers were visited by four different animal species including a robber‐like pollinator and a secondary robber. Natural levels of nectar robbing ranged from 40 to 100%. Natural variation in nectar robbing was not associated with fruit set in any year whereas seed set was weakly positively associated for 1 year only. Artificial nectar robbing did not increase nectar production or concentration, did not affect the behavior of long‐billed hummingbirds, and when faced with artificially robbed flowers, these visitors behaved as secondary nectar robbers. The number of stigmas within a patch that received pollen dye analogs and the average distance traveled by these analogs were not significantly different between robbing treatments (robbed flowers versus unrobbed flowers), but the maximum distance traveled by these pollen analogs was higher when nectar robbing was not prevented. Overall, the proportion of robbed flowers on an inflorescence had a neutral effect to a weak positive effect on the reproduction of individual plants (i.e. positive association between nectar robbing and fruit set in 2002) even when it clearly changed the behavior of its most efficient pollinator potentially increasing the frequency of nectar robbing within a plant.     

The reproductive assurance benefit of selfing: importance of flower size and population size

Autonomous selfing can provide reproductive assurance (RA) for flowering plants that are unattractive to pollinators or in environments that are pollen limited. Pollen limitation may result from the breakdown of once-continuous habitat into smaller, more isolated patches (habitat fragmentation) if fragmentation negatively impacts pollinator populations. Here we quantify the levels of pollen limitation and RA among large and small populations of Collinsia parviflora, a wildflower with inter-population variation in flower size. We found that none of the populations were pollen limited, as pollen-supplemented and intact flowers did not differ in seed production. There was a significant effect of flower size on RA; intact flowers (can self) produced significantly more seeds than emasculated flowers (require pollen delivery) in small-flowered plants but not large-flowered plants. Population size nested within flower size did not significantly affect RA, but there was a large difference between our two replicate populations for large-flowered, small populations and small-flowered, large populations that appears related to a more variable pollination environment under these conditions. In fact, levels of RA were strongly negatively correlated with rates of pollinator visitation, whereby infrequent visitation by pollinators yielded high levels of RA via autonomous selfing, but there was no benefit of autonomous selfing when visitation rates were high. These results suggest that autonomous selfing may be adaptive in fragmented habitats or other ecological circumstances that affect pollinator visitation rates.     

Pollinator visitation to mass-flowering courgette and co-flowering wild flowers: Implications for pollination and bee conservation on farms

Managing the complex relationship between pollinators and their habitat requirements is of particular concern to growers of pollinator-dependent crop species, such as courgette (Cucurbita pepo). Naturally occurring wild flowers (i.e. agricultural weeds) offer a free, sustainable, and often underappreciated resource for pollinators, however, they may compete with crop flowers for visits. To understand the extent to which floral resources mediate pollinator visitation to courgette flowers and courgette fields, plant community and pollinator visitation data were collected at two spatial scales: field scale (in margins, and in the cropped area) and farm scale (500 m and 2000 m radii) for nine courgette fields across the UK. Apis mellifera (honeybees) and Bombus spp. (bumblebees) were the only pollinators observed to visit courgette flowers. Bumblebees were significantly more abundant on courgette flowers in fields with a greater species richness of wild flowers in the crop, whilst honeybees were significantly more abundant on courgette flowers in areas with less semi-natural habitat. For both honeybees and bumblebees, their abundance in field margins did not significantly reduce their abundance on courgette flowers, suggesting that wild flowers were not competing with courgette flowers for pollinator visitation. Although solitary bees were not observed to visit courgette flowers, their abundance and species richness in courgette fields were significantly greater with more semi-natural habitat and a greater species richness of wild flowers. Therefore, allowing uncultivated areas around the crop to be colonised by species-rich wild flowers is an effective way of boosting the abundance of bumblebees, which are important visitors to courgette flowers, as well as the abundance and species richness of solitary bees, thereby benefitting pollinator conservation.     

Context-dependent reproductive isolation mediated by floral scent and color

Reproductive isolation due to pollinator behavior is considered a key mode of speciation in flowering plants. Although floral scent is thought to mediate pollinator behavior, little is known about its effects on pollinator attraction and floral visitation in the wild. We used field experiments with wild hawkmoths and laboratory experiments with naïve hawkmoths to investigate attraction to and probing of flowers in response to indole, a volatile emitted by Ipomopsis tenuituba but not its close relative I. aggregata, both alone and in combination with floral color differences. We demonstrated that indole attracts wild hawkmoths to flowers, but has little effect on the rate at which those attracted moths probe flowers. In contrast, white flower color did not influence hawkmoth attraction in the field, but caused more attracted moths to probe flowers. Thus, the moths require both scent and high visual contrast, in that order, to feed at flowers at dusk. Their preference for indole-scented flowers is innate, but species-specific preference is mitigated by previous experience and plant spatial patterning. This context-dependent behavior helps explain why these Ipomopsis species show geographical variation in the extent of hybridization and may potentially explain formation of hybrid bridges in other systems of hawkmoth-pollinated plants.     

Nectar guide fluctuating asymmetry does not relate to female fitness in Mimulus luteus

Most studies assessing the importance of developmental instability of floral characters for pollinator visits and plant fitness have focused on the fluctuating asymmetry (FA) of the corolla phenotype. The importance of stability process for nectar guide characters that signal floral reward has not been considered in the literature. In principle, flowers with symmetrical guides should be more successful at attracting pollinators, therefore increasing their reproductive success in comparison to asymmetrical flowers. In this paper we test this hypothesis in a population of 171 individuals of the Andean monkey flower, Mimulus luteus in northern Chile. This species shows a conspicuous red spot in the landing yellow petal, which permits assessment of the functional relationship between nectar guide FA and female fitness. Our results did not reveal a significant linear nor nonlinear relationship between nectar guide FA and fitness. This result was consistent after controlling the level of FA by guide and corolla size. Because the corolla of M. luteus did not show evidence of UV wavelength reflectance, our negative result could not be attributable to a confounding effect of UV guides. Even though we can not rule out that nectar guide FA correlates better with male than female fitness, the low fraction of the variance in female fitness accounted for nectar guide FA, suggests that other components of the floral phenotype as well as environmental factors may be more important to predict pollinator preference and reproductive success in this species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Changes in Sexual Expression as Result of Defoliation and Environment in a Monoecious Shrub in Mexico: Implications for Pollination

Folivory may indirectly impact plant reproduction through changes in sexual expression ( i.e. , number or proportion of male and female flowers produced), which influence plant–pollinator interactions via changes in pollinator preference or efficiency. This study is an experimental evaluation of the effect that defoliation has on sex expression in the monoecious shrub Cnidoscolus aconitifolius , how such effect varies across sites, as well as how such changes indirectly affect pollinator visitation rates. The present study used three populations of C. aconitifolius , each one located in a different site in Yucatán (México): pasture, deciduous forest and subdeciduous medium height forest and three levels of defoliation: 50 percent, 100 percent, and a control (no damage). Results showed that defoliation reduced significantly the total number of male flowers produced in two of the sites. Defoliation did not impact female flower production or the proportion of female flowers produced. Finally, floral visit rates were not affected by defoliation via changes in sexual expression and neither by site or by the interaction site × defoliation. Findings showed that defoliation had an effect on sex expression in C. aconitifolius , although apparently this change did not affect the plant–pollinator interactions.     

Sexual interference in two Chamerion species with contrasting modes of movement herkogamy

Evolution of herkogamy has been ascribed to preventing autonomous selfing within flowers, but this argument has received challenges due to the negative effects of sexual interference on reproductive success of plant species. We examined the effects of the two contrasting modes of herkogamy, detailed by the different patterns of style movement, on the foraging time of pollinators on flowers, pollen removal, and deposition in two Chamerion species. Our results suggested that the duration of foraging time of bumblebees was generally higher in C. angustifolium (L.) Holub than in C. conspersum (Hausskn.) Holub during the male-phase of flowers, but the reverse during the female-phase of flowers. As a result, stigma interfered with pollen removal more in C. conspersum than in C. angustifolium, based on examinations of pollen removal after one visit by a pollinator. However, this negative effect on total pollen removal was covered by the high pollinator diversity and the possible high visitation rate in the study sites. In contrast, we found that almost no difference existed in pollen deposition rate after one visit of a pollinator between the two Chamerion species, and emasculation did not increase the total pollen deposition in either species, indicating that anthers did not interfere with pollen deposition in the two Chamerion species. Our results suggested that, compared with C. conspersum, the derived mode of herkogamy in C. angustifolium reduced interference of stigmas in pollen removal. This might contribute to the wide distribution of C. angustifolium, to some degree, in environments of pollinator scarcity and low activity in newly colonized habitats.     

Modification of bumblebee behavior by floral color change and implications for pollen transfer in <Emphasis Type="Italic">Weigela middendorffiana</Emphasis>

Flowers of Weigela middendorffiana change the color from yellow to red. The previous study revealed that red-phase flowers no longer have sexual function and nectar, and bumblebees selectively visit yellow-phase flowers. The present study examined how retaining color-changed flowers can regulate the foraging behavior of bumblebees and pollen transport among flowers within (geitonogamous pollination) and between (outcrossing pollination) plants and how the behavior is influenced by display size (i.e., number of functional flowers) and visitation frequency. The visitation frequencies of bumblebees to plants and successive flower probes within plants were observed in the field using plants whose flower number and composition of the two color-phase flowers had been manipulated. To evaluate pollination efficiency over multiple pollinator visits, a pollen transport model was constructed based on the observed bumblebee behavior. In the simulation, three flowering patterns associated with display size and existence of color-changed flowers were postulated as follows: Type 1, large display (100 functional flowers) and no retention of color-changed flowers; Type 2, small display (50 functional flowers) and retention of color-changed flowers (50 old flowers), and; Type 3, large display (100 functional flowers) and retention of color-changed flowers (100 old flowers). Color-changed flowers did not contribute to increasing bumblebee attraction at a distance but reduced the number of successive flower probes within plants. Comparisons of pollen transfer between Types 1 and 3 revealed that the retention of color-changed flowers did not influence the total amount of pollen exported when pollinator visits were abundant (>100 visits) but decreased geitonogamous pollination. Comparisons between Types 2 and 3 revealed that the discouragement effect of floral color change on successive probes accelerated in plants with a large display size. Overall, the floral color change strategy contributed to reduce geitonogamous pollination, but its effectiveness was highly sensitive to display size and pollinator frequency.     

Floral bagging differentially affects handling behaviours and single-visit pollen deposition by honey bees and native bees

1. Measurements of pollinator performance are crucial to pollination studies, enabling researchers to quantify the relative value of different pollinator species to plant reproduction. One of the most widely employed measures of pollinator performance is single-visit pollen deposition, the number of conspecific pollen grains deposited to a stigma after one pollinator visit. To ensure a pollen-free stigma, experimenters must first bag flowers before exposing them to a pollinator. 2. Bagging flowers, however, may unintentionally manipulate floral characteristics to which pollinators respond. In this study, we quantified the effect of bagging on nectar volume in watermelon (Citrullus lanatus) flowers, and how this affects pollinator performance and behaviour. 3. Experimental bagging resulted in roughly 30-fold increases in nectar volume relative to unmanipulated, open-pollinated field flowers after only a few hours. Honey bees, but not native bees, consistently displayed elevated handling times and single-visit pollen deposition on unmanipulated bagged flowers relative to those from which we removed nectar to mimic volumes in open-pollinated flowers. 4. Furthermore, we identify specific bee foraging behaviours during a floral visit that account for differences in pollen deposition, and how these differ between honey bees and native bees. 5. Our findings suggest that experimental bagging of flowers, without accounting for artificially accumulated nectar, can lead to biased estimates of pollinator performance in pollinator taxa that respond strongly to nectar volume. We advise that pollination studies be attentive to nectar secretion dynamics in their focal plant species to ensure unbiased estimates of pollinator performance across multiple pollinator species.     

Floral size variation in Campanula rotundifolia (Campanulaceae) along altitudinal gradients: patterns and possible selective mechanisms

We investigated patterns of flower‐size variation along altitudinal gradients in the bee‐pollinated perennial Campanula rotundifolia (Campanulaceae) by examining 22 Norwegian populations at altitudes between 240 and 1100 m a.s.l. We explored potential mechanisms for the underlying pattern by quantifying pollinator–faunal composition, pollinator‐visitation rates and pollen limitation of seed set in subsets of the study populations. Despite a decrease in plant size, several measures of flower size increased with elevation. Bumble bees were the main pollinators at both alpine and lowland sites in the study area. However, species composition of the pollinator fauna differed, and pollinators were larger in higher‐elevation than in lower‐elevation sites. Pollinator visitation rates were lower at higher‐elevations than at lower elevations. Pollen limitation of seed set did not vary significantly with altitude. Our results are consistent with differences in bumble‐bee size and visitation rates as causal mechanisms for the relatively larger flowers at higher elevations, in three non‐mutually exclusive ways: 1) Larger flowers reflect selection for increased attractiveness where pollinators are rare. 2) Larger and fewer flowers represent a risk avoidance strategy where the probability of pollination is low on any given day. 3) Flower size variation reflects selection to improve the fit of pollinators with fertile structures by matching flower size to pollinator size across sites.