Floral antagonists counteract pollinator‐mediated selection on attractiveness traits in the hummingbird‐pollinated Collaea cipoensis (Fabaceae)

Pollinator‐mediated selection toward larger and abundant flowers is common in naturally pollen‐limited populations. However, floral antagonists may counteract this effect, maintaining smaller‐ and few‐flowered individuals within populations. We quantified pollinator and antagonist visit rates and determined a multiplicative female fitness component from attacked and non‐attacked flowers of the Brazilian hummingbird‐pollinated shrub Collaea cipoensis to determine the selective effects of pollinators and floral antagonists on flower size and number. We predicted that floral antagonists reduce the female fitness component and thus exert negative selective pressures on flower size and number, counteracting the positive effects of pollinators. Pollinators, mainly hummingbirds, comprised 4% of total floral visitation, whereas antagonist ants and bees accounted for 90% of visitation. Nectar‐robbers involved about 99% of floral antagonist visit rates, whereas florivores comprised the remaining 1%. Larger and abundant flowers increased both pollinator and antagonist visit rates and the female fitness component significantly decreased in flowers attacked by nectar‐robbers and florivores in comparison to non‐attacked flowers. We detected that pollinators favored larger‐ and many‐flowered individuals, whereas floral antagonists exerted negative selection on flower size and number. This study confirms that floral antagonists reduce female plant fitness and this pattern directly exerts negative selective pressures on flower size and number, counteracting pollinator‐mediated selection on floral attractiveness traits.     

Selection by Pollinators on Floral Traits in Generalized Trollius ranunculoides (Ranunculaceae) along Altitudinal Gradients

Abundance and visitation of pollinator assemblages tend to decrease with altitude, leading to an increase in pollen limitation. Thus increased competition for pollinators may generate stronger selection on attractive traits of flowers at high elevations and cause floral adaptive evolution. Few studies have related geographically variable selection from pollinators and intraspecific floral differentiation. We investigated the variation of Trollius ranunculoides flowers and its pollinators along an altitudinal gradient on the eastern Qinghai-Tibet Plateau, and measured phenotypic selection by pollinators on floral traits across populations. The results showed significant decline of visitation rate of bees along altitudinal gradients, while flies was unchanged. When fitness is estimated by the visitation rate rather than the seed number per plant, phenotypic selection on the sepal length and width shows a significant correlation between the selection strength and the altitude, with stronger selection at higher altitudes. However, significant decreases in the sepal length and width of T. ranunculoides along the altitudinal gradient did not correspond to stronger selection of pollinators. In contrast to the pollinator visitation, mean annual precipitation negatively affected the sepal length and width, and contributed more to geographical variation in measured floral traits than the visitation rate of pollinators. Therefore, the sepal size may have been influenced by conflicting selection pressures from biotic and abiotic selective agents. This study supports the hypothesis that lower pollinator availability at high altitude can intensify selection on flower attractive traits, but abiotic selection is preventing a response to selection from pollinators.     

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 population size and pollination ecosystem service responses to enhancing floral and nesting resources

Modeling pollination ecosystem services requires a spatially explicit, process‐based approach because they depend on both the behavioral responses of pollinators to the amount and spatial arrangement of habitat and on the within‐ and between‐season dynamics of pollinator populations in response to land use. We describe a novel pollinator model predicting flower visitation rates by wild central‐place foragers (e.g., nesting bees) in spatially explicit landscapes. The model goes beyond existing approaches by: (1) integrating preferential use of more rewarding floral and nesting resources; (2) considering population growth over time; (3) allowing different dispersal distances for workers and reproductives; (4) providing visitation rates for use in crop pollination models. We use the model to estimate the effect of establishing grassy field margins offering nesting resources and a low quantity of flower resources, and/or late‐flowering flower strips offering no nesting resources but abundant flowers, on bumble bee populations and visitation rates to flowers in landscapes that differ in amounts of linear seminatural habitats and early mass‐flowering crops. Flower strips were three times more effective in increasing pollinator populations and visitation rates than field margins, and this effect increased over time. Late‐blooming flower strips increased early‐season visitation rates, but decreased visitation rates in other late‐season flowers. Increases in population size over time in response to flower strips and amounts of linear seminatural habitats reduced this apparent competition for pollinators. Our spatially explicit, process‐based model generates emergent patterns reflecting empirical observations, such that adding flower resources may have contrasting short‐ and long‐term effects due to apparent competition for pollinators and pollinator population size increase. It allows exploring these effects and comparing effect sizes in ways not possible with other existing models. Future applications include species comparisons, analysis of the sensitivity of predictions to life‐history traits, as well as large‐scale management intervention and policy assessment.     

No evidence for a role of pollinator discrimination in causing selection on flower size through female reproduction

The preference for certain floral phenotypes by flower visiting animals may fuel the evolution of floral traits because variation in flower visitation rates may lead to fitness variation within a population. Here, I examine the importance of flower size for pollinator visitation rate, seed set, and seed mass in two alpine populations of the insect-pollinated herb Ranunculus acris L. during two seasons. There was no pollen limitation of seed set or mass. Pollinators discriminated strongly against flowers experimentally reduced in size. Despite this, there were no signs of any significant impact of flower size on female reproductive success. The results show that although pollinators discriminate strongly among floral phenotypes, this may not always result in female fitness differences within a population because seed set or mass is not limited by pollen availability alone. Probably abiotic environmental constraints prevent plants with high pollinator visitation from capitalizing on the high pollen deposition.     

High levels of fecundity in small and isolated populations of a self‐compatible Aloe pollinated by opportunistic birds and bees

• Plant species that are effective colonisers of transient habitats are expected to have a capacity for uniparental reproduction and show flexibility in pollination systems. Such traits may enable populations to be established from a small number of founding individuals without these populations succumbing to reductions in fecundity arising from pollinator limitation.
• We tested these predictions for Aloe thraskii (Xanthorrhoeaceae), a succulent treelet that colonises shifting coastal dunes and has both bird and bee pollinators. We performed hand‐pollination experiments, and selectively excluded bird visitors to determine differences in pollinator effectiveness. We measured pollinator visitation rates and fecundity in populations varying in their size, density and isolation distance.
• Controlled hand‐pollinations revealed that unlike most other Aloe species, A. thraskii is self‐compatible and thus capable of uniparental reproduction. The species does however depend on pollinators and is visited by various bird species as well as by bees. Fruit and seed set are not affected by selective exclusion of birds, thus indicating that bees are effective pollinators. Bird visitation rates increased with increasing plant height and population size, while bee visitation rates increased with increasing population size and density. We found that seed set per flower was lower in large populations than in small populations.
• These results suggest that establishment of populations of A. thraskii from a small number of individuals is unlikely to be limited by the fecundity of individual plants.

     

Within-population spatial variation in pollinator visitation rates,pollen limitation on seed set,and flower longevity in an alpine species

Pollen limitation through insufficient pollen deposition on stigmas caused by too infrequent pollinator visitation may influence the reproductive outcome of plants. In this study we investigated how pollinator visitation rate, the degree of pollen limitation, and flower longevity varied spatially among three sites at different altitudes within a population of the dwarf shrub Dryas octopetala L. in alpine southern Norway. Significant pollen limitation on seed set only occurred at the mid-elevation site, while seed set at the other sites appeared to be mainly resource limited, thus indicating a spatial variation in pollen limitation. There was no association between the spatial variation in the extent of pollen limitation and pollinator visitation rate to flowers. However, pollinator visitation rates were related to flower longevity of Dryas; sites with low visitation rates had long-lived flowers and vice versa. Thus, our results suggest within-population spatial co-variation between pollinator visitation rates, pollen limitation, and a developmental response to these factors, flower longevity.     

Within-population spatial variation in pollinator visitation rates, pollen limitation on seed set, and flower longevity in an alpine species

Pollen limitation through insufficient pollen deposition on stigmas caused by too infrequent pollinator visitation may influence the reproductive outcome of plants. In this study we investigated how pollinator visitation rate, the degree of pollen limitation, and flower longevity varied spatially among three sites at different altitudes within a population of the dwarf shrub Dryas octopetala L. in alpine southern Norway. Significant pollen limitation on seed set only occurred at the mid-elevation site, while seed set at the other sites appeared to be mainly resource limited, thus indicating a spatial variation in pollen limitation. There was no association between the spatial variation in the extent of pollen limitation and pollinator visitation rate to flowers. However, pollinator visitation rates were related to flower longevity of Dryas; sites with low visitation rates had long-lived flowers and vice versa. Thus, our results suggest within-population spatial co-variation between pollinator visitation rates, pollen limitation, and a developmental response to these factors, flower longevity.     

Pollinator‐mediated phenotypic selection does not always modulate flower size and number in the large‐flowered Mediterranean shrub Cistus ladanifer (Cistaceae)

Flower size and number usually evolve under pollinator‐mediated selection. However, hot, dry environments can also modulate display, counteracting pollinator attraction. Increased pollen deposition on larger flower displays may not involve higher female fitness. Consequently, stressful conditions may constrain flower size, favouring smaller‐sized flowers. The large‐flowered, self‐incompatible Mediterranean shrub Cistus ladanifer was used to test that: (1) this species suffers pollen limitation; (2) pollinators are spatially–temporally variable and differentially visit plants with more/larger flowers; (3) increased visits enhance reproduction under pollen limitation; (4) stressful conditions reduce female fitness of larger displays; and (5) phenotypic selection on floral display is not just pollinator‐mediated. We evaluated pollen limitation, related floral display to pollinator visits and fruit and seed production and estimated phenotypic selection. Flower size was 7.2–10.5 cm and varied spatially–temporally. Visitation rates (total visits/50 min) ranged from 0.26 to 0.43 and increased with flower size. Fruit set averaged 80% and seed number averaged 855, but only fruit set varied between populations and years. Selection towards larger flowers was detected under conditions of pollen limitation. Otherwise, we detected stabilizing selection on flower size and negative selection on flower number. Our results suggest that selection on floral display is not only pollinator‐dependent through female fitness in C. ladanifer. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 540–555.     

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.     

Breeding system of Macromeria viridiflora (Boraginaceae) and geographic variation in pollinator assemblages

This study explores the association between variation in pollinator type and flower size in Macromeria viridiflora (Boraginaceae) by studying the breeding system of the plant and the pollinator effectiveness of floral visitors. Studies were conducted at two sites where plants differ in flower size and floral visitors. Breeding system studies showed that while plants are self-compatible and occasionally produce seed autogamously, pollinators are important for reproductive success in the plants. However, plants are not pollinator-limited at these sites. Combining visitation rate and pollen deposition as measures of pollinator effectiveness, I found hummingbirds to be the most effective pollinators at both sites. Although hawkmoths also pollinate the flowers, they visit the flowers less frequently and, at one of the two sites, deposit less pollen. These results are consistent with the hypothesis that geographic variation in corolla size is the result of selection by different hummingbird species.     

The effects of floral display on pollinator visitation vary among populations ofPhacelia linearis (Hydrophyllaceae)

Summary Individual plants in gynodioecious populations ofPhacelia linearis (Hydrophyllaceae) vary in flower gender, flower size, and flower number. This paper reports the effects of variation in floral display on the visitation behaviour of this species' pollinators (mainly pollen-collecting solitary bees) in several natural and three experimental plant populations, and discusses the results in terms of the consequences for plant fitness. The working hypotheses were: (1) that because female plants do not produce pollen, pollen-collecting insects would visit hermaphrodite plants at a higher rate than female plants and would visit more flowers per hermaphrodite than per female; and (2) that pollinator arrival rate would increase with flower size and flower number, the two main components of visual display. These hypotheses were generally supported, but the effects of floral display on pollinator visitation varied substantially among plant populations. Hermaphrodites received significantly higher rates of pollinator arrivals and significantly higher rates of visits to flowers than did females in all experimental populations. Flower size affected arrival rate and flower visit rate positively in natural populations and in two of the three experimental populations. The flower size effect was significant only among female plants in one experimental population, and only among hermaphrodites in another. The effect of flower number on arrival rate was positive and highly significant in natural populations and in all experimental populations. In two out of three experimental populations, insects visited significantly more flowers per hermaphrodite than per female and visited more flowers on many-flowered plants than on few-flowered plants, but neither effect was detected in the third experimental population. Because seed production is not pollen-limited in this species, variation in pollinator visitation behaviour should mainly affect the male reproductive success of hermaphrodite plants. These findings suggest that pollinator-mediated natural selection for floral display inP. linearis varies in space and time.     

How do visitation patterns vary among pollinators in relation to floral display and floral design in a generalist pollination system?

Diverse pollinator assemblages may impose complex selection and thus limit specialisation to particular pollinators. Previous work has concentrated on how visitation rates of different pollinators vary in space and time and how pollinators may vary in efficiency. In this study I quantify variation in visitation rates and foraging behaviour of different insect types (1) in space and time and (2) in relation to variation in floral design (flower size and form) and floral display (number of open flowers) for the distylous clonal shrub Jasminum fruticans. Mean visitation rate showed a significant interaction between insect type and population for seven populations in one year, and between insect types and years for a single population over 3 years. There was also a significant interaction between insect type and population for the proportion of flowers visited. In general the number of visits was positively related to the number of open flowers in a patch, but analyses by insect type showed that this was only true for bee flies and butterflies. Short-tongued bees showed a positive relationship between visitation rate and the number of open flowers on the focal stem, and hawkmoths and butterflies made more visits to plants with larger flowers. Hawkmoths were the only insect type to show a positive relation between the number of flowers visited per foraging bout and flower size. The significant differences between different insect types in patterns of variation in visitation rates in response to floral design and display may act to diversify selection on floral traits, and thereby constrain specialisation of the plant to particular pollinators.     

Pollen limitation,species’ floral traits and pollinator visitation: different relationships in contrasting communities

Failures in the process of pollen transfer among conspecific plants can severely impact female reproductive success. Thus, pollen limitation can cause selection on plant mating systems and floral traits. The relationships between pollen limitation and floral traits might be partly mediated by the quantity and identity of pollinator visits. However, very little is known about the relationship between pollinator visits and pollen limitation. We examined the relationships between pollen limitation and floral traits at the community level to connect them to community ecology processes. We used 48 plant species from two contrasting communities: one species‐rich lowland community and one species‐poor alpine community. In addition, we calculated visitation rates and ecological pollination generalization for 38 of the species to examine the relationship between pollinator visitation and pollen limitation at the community level. We found low overall levels of pollen limitation that did not differ significantly between the alpine and the lowland community. In both communities, species with evolutionary specialized flowers were more pollen limited than species with unspecialized flowers. Species’ visitation rates and selfing capability were negatively related to pollen limitation in the alpine community, where pollinators are scarcer. However, flower size/number, ecological generalization of plants and flowering onset had greater effects on pollen limitation levels at the lowland community, indicating that the identity of the visitors and plant‐plant competitive interactions are more decisive for plant reproduction in this species‐rich community. There, pollen limitation increased with flower size and flowering onset, and decreased with ecological generalization, but only in species with evolutionary specialized flowers. Our study suggests that selection on plant mating system and floral traits may be idiosyncratic to each particular community and highlights the benefits of conducting community‐level studies for a better understanding of the processes underlying evolutionary responses to pollen limitation.     

Pollination ecology of Silene acutifolia (Caryophyllaceae): floral traits variation and pollinator attraction

BACKGROUND AND AIMS: The floral display influences the composition of pollinators interacting with a plant species. Geographic and temporal variation in pollinator composition complicates the understanding of the evolutionary consequences of floral display variation. This paper analyses the relationships between Silene acutifolia, a hermaphroditic perennial herb, and its pollinators, based on field studies in the north-west of Spain. METHODS: Studies were conducted over three years (1997-1999). Firstly, the main pollinators of this species were determined for two years in one population. Secondly, pollen limitation in fruit and seed production was analysed by supplementary hand pollinations, and counting the pollen grains and tubes growing in styles for two different-sized populations. Finally, the effect of flower size and number on the rate of visitation and total seed number was examined for 15 marked plants. RESULTS AND CONCLUSIONS: The primary pollinators were long-tongued insects, including Hymenoptera, Lepidoptera and Diptera, but the composition and visitation frequencies differed between years. Pollen limitation occurred in one of the years of study. There was between-population variation in the number of pollen grains and pollen tubes found in styles, suggesting pollen limitation in one population. Overall, pollinators visited plants with more open flowers more frequently, and pollinated more flowers within these plants. Conversely, petal and calyx sizes had no effect on insect visitation. Plants with higher rates of visits produced higher number of seeds, suggesting that pollinator-mediated limitation of seed and fruit production may be important in some years.     

Where have all the blue flowers gone: pollinator responses and selection on flower colour in New Zealand Wahlenbergia albomarginata

Although pollinators are thought to select on flower colour, few studies have experimentally decoupled effects of colour from correlated traits on pollinator visitation and pollen transfer. We combined selection analysis and phenotypic manipulations to measure the effect of petal colour on visitation and pollen export at two spatial scales in Wahlenbergia albomarginata. This species is representative of many New Zealand alpine herbs that have secondarily evolved white or pale flowers. The major pollinators, solitary bees, exerted phenotypic selection on flower size but not colour, quantified by bee vision. When presented with manipulated flowers, bees visited flowers painted blue to resemble a congener over white flowers in large, but not small, experimental arrays. Pollen export was higher for blue flowers in large arrays. Pollinator preference does not explain the pale colouration of W. albomarginata, as commonly hypothesized. Absence of bright blue could be driven instead by indirect selection of correlated characters.     

Flower colours along an alpine altitude gradient, seen through the eyes of fly and bee pollinators

Alpine flowers face multiple challenges in terms of abiotic and biotic factors, some of which may result in selection for certain colours at increasing altitude, in particular the changing pollinator species composition, which tends to move from bee-dominated at lower elevations to fly-dominated in high-alpine regions. To evaluate whether growing at altitude—and the associated change in the dominant pollinator groups present—has an effect on the colour of flowers, we analysed data collected from the Dovrefjell National Park in Norway. Unlike previous studies, however, we considered the flower colours according to ecologically relevant models of bee and fly colour vision and also their physical spectral properties independently of any colour vision system, rather than merely looking at human colour categories. The shift from bee to fly pollination with elevation might, according to the pollination syndrome hypothesis, lead to the prediction that flower colours should shift from more bee-blue and UV-blue flowers (blue/violet to humans, i.e. colours traditionally associated with large bee pollinators) at low elevations to more bee-blue-green and green (yellow and white to humans—colours often linked to fly pollination) flowers at higher altitude. However, although there was a slight increase in bee-blue-green flowers and a decrease in bee-blue flowers with increasing elevation, there were no statistically significant effects of altitude on flower colour as seen either by bees or by flies. Although flower colour is known to be constrained by evolutionary history, in this sample we also did not find evidence that phylogeny and elevation interact to determine flower colours in alpine areas. Handling editor: Neal Williams     

Pollinators exert positive selection on flower size on urban,but not on rural Scotch broom (Cytisus scoparius L. Link)

Aims Adaptive evolution of invasive species is both particularly exciting for the evolutionary biologist and worrisome for those interested in controlling or halting spread. Invasive species often have a distinct timeline and well-recorded population expansion. As invaders encounter new environments, they undergo rapid adaptive evolution. Our aim in this study was to measure variation of floral size in the invasive shrub Cytisus scoparius (Scotch broom) and measure natural selection by pollinators on that trait. Past research has found that this invasive plant is pollinator limited in Washington State and that declines in pollinator populations can contribute to local extinction in another invaded range (New Zealand). This plant is pollinated by both native and introduced species of bees, representing a broad range of pollinator sizes. Cytisus scoparius has a flower structure that is highly conducive to studies on pollinator choice, even in the absence of direct pollinator observations.Methods We surveyed urban and rural sites in and around the city of Olympia in Washington State. Measuring banner width, we were able to show that flower size varies substantially between plants but minimally within plants. By measuring the proportion of flowers that were 'tripped', we could determine pollinator visitation rates and thus determine the level of selection due to pollinator choice alone.Important findings We found that C. scoparius is under natural selection by pollinators for increased flower size. However, such positive natural selection was only seen in urban populations although it was consistent across two flowering seasons. Rural populations of Scotch broom do not appear to be under selection on flower size. The natural selection by pollinators on broom flowers could result in adaptive evolution into a new pollination niche by an invading species. A higher level of variation in broom flowers seen here than seen in previous works in native regions suggests that C. scoparius may be highly diverse and primed for adaptive evolution.     

Effect of elevation on sexual reproduction in alpine populations of Saxifraga oppositifolia (Saxifragaceae)

Self-compatibility in high arctic and alpine areas is regarded as an adaptation to low pollinator abundance. However, high genetic variability as a consequence of outcrossing is, with regard to population persistence, favorable in highly stochastic environments such as tundra habitats. To evaluate these contradictory scenarios, I performed in situ pollination experiments to examine the breeding system of the predominant outcrosser Saxifraga oppositifolia in ten populations at two different elevations in the Swiss Alps. Pollinator limitation was detected at both elevations, but fruit set in naturally pollinated flowers was only slightly less at the higher elevation. Increased pollinator limitation at high compared with low elevation thus could not be demonstrated in this experiment. Hand-crossings yielded equal mean proportion seed set at both elevations, and so did hand-selfings. This constant pattern of the breeding system in S. oppositifolia indicates selective factors that lead to the maintenance of a high level of outcrossing even in high-elevation populations. Based on sex allocation models, it was expected that a high ovule number should be selectively advantageous in a plant-pollinator system where chance visitation or selfing play important roles. However, female reproductive offer in terms of ovule number per flower did not change from low to high elevation. Since neither increased pollinator limitation nor increased seed set in selfed flowers was found at high compared with low elevation, the prerequisites for testing the hypothesis were not given. This study contradicts the hypothesis that inimical environmental factors in alpine or arctic habitats necessarily select for increased selfing rates in a preferentially outcrossing species like S. oppositifolia. Received: 28 April 1997 / Accepted: 20 October 1997     

Plant reproductive strategies vary under low and high pollinator densities

Long‐term variation in the population density of honey bees Apis mellifera across landscapes has been shown to correlate with variation in the floral traits of plant populations in these landscapes, suggesting that variations in pollinator population density and foraging rates can drive floral trait evolution of their host plants. However, it remained to be determined whether this variation in plant traits is associated with adaptive variation in plant reproductive strategies under conditions of high and low pollinator densities. Here we conducted a reciprocal transplant experiment to examine how this variation in floral traits, under conditions of either high and low pollinator density, impacted seed production in the Tibetan lotus Saussurea nigrescens. In 2014 and 2015, we recorded the floral traits, pollinator visitation rates, and seed production of S. nigrescens populations grown in both home sites and foreign sites, where sites varied in honey bee population density. Our results demonstrated that the floral traits reflected those of their original population, regardless of their current location. However, seed production varied with both population origin and transplant site. Seed number was positively correlated with flower abundance in the pollinator‐rich sites, but with nectar production in the pollinator‐poor sites. Pollinator visitation rate was also positively correlated with flower number at pollinator‐rich sites, and with nectar volume at pollinator‐poor sites. Overall, the local genotype had higher seed production than nonlocal genotypes in home sites. However, when pollen is hand‐supplemented, plants from pollinator‐rich populations had higher seed production than plants from pollinator‐poor populations, regardless of whether they were transplanted to pollinator‐rich or ‐poor sites. These results suggest that the plant genotypic differences primarily drive variation in pollinator attraction, and this ultimately drives variation in seed: ovule ratio. Thus, our results suggest that flowering plant species use different reproductive strategies to respond to high or low pollinator densities.