Effects of local density on pollination and reproduction in Delphinium nuttallianum and Aconitum columbianum (Ranunculaceae)

Plant populations vary in density both naturally and as a consequence of anthropogenic impacts. Density in turn can influence pollination by animals. For example, plants in dense populations might enjoy more frequent visitation if pollinators forage most efficiently in such populations. We explored effects of plant density on pollination and seed set in the larkspur Delphinium nuttallianum and monkshood Aconitum columbianum. At our site in the Colorado Rocky Mountains, flowers of D. nuttallianum are pollinated primarily by queen bumble bees, solitary bees, and hummingbirds, whereas those of A. columbianum are pollinated primarily by queen and worker bumble bees. We found that the quantity of pollination service to both species (pollinator visitation rate and pollen deposition) was at best weakly related to density. In contrast, seed set declined by approximately one-third in sparse populations relative to nearby dense populations. This decline may stem from the receipt of low-quality pollen, for example, inbred pollen. Alternatively, sparsity may indicate poor environmental conditions that lower seed set for reasons unrelated to pollination. Our results demonstrate the value of simultaneously exploring pollinator behavior, pollen receipt, and seed set in attempting to understand how the population context influences plant reproductive success.     

Experimental manipulation of plant density and its effect on pollination and reproduction of two confamilial montane herbs

The density of a plant population is expected to influence reproductive success through changes in the quantity and quality of pollination service, or because both density and reproduction respond to quality of the local environment. We reported previously that seed set in sparse natural populations of Delphinium nuttallianum and Aconitum columbianum was lower than in nearby dense populations, whereas quantity of pollination service was equivalent. To explore the hypotheses that environmental quality or pollination quality are lower in sparse natural populations, leading to lower seed set, we manipulated density using arrays of potted plants. In three replicate experiments with D. nuttallianum, pollinator visitation rate and seed set were indistinguishable in sparse and dense arrays, consistent with the interpretation that environmental quality contributed to the earlier result in natural populations of this species. In three replicates with A. columbianum, visitation rate tended to increase with density, and seed set increased significantly, in contrast to our earlier result. One element of pollination quality, the degree of within-plant selfing, did not vary between sparse and dense arrays. These results highlight the complexity of mechanisms by which population parameters may influence plant reproductive success, and the temporal variation that characterizes pollination service.     

Do pollen carryover and pollinator constancy mitigate effects of competition for pollination?

Pollinator constancy and pollen carryover are both thought to mitigate competitive effects that result when shared pollinators cause loss of pollen to heterospecific flowers. I present analytical and simulation models to investigate how pollinator constancy and pollen carryover interact with each other and with the relationship between pollen receipt and seed set to determine pollination success in competitive environments. With inconstant pollinators, increased pollen carryover reduces variance in pollen receipt without affecting average pollen receipt. Consequently, for flowers requiring at least a threshold quantity of pollen for success, rare flowers with inconstant pollinators benefit from reduced carryover, especially for high pollen receipt thresholds, whereas common flowers benefit from increased carryover, especially for low receipt thresholds. Pollinator constancy is predicted to increase pollen receipt, especially if pollen carryover rates are low. As a result, increased pollinator constancy reduces the range of pollen receipt thresholds for which carryover is beneficial. Similarly, for flowers whose pollination success is a convex function of pollen receipt, carryover is expected to increase fecundity if pollinators are inconstant, but with even a low degree of pollinator constancy, carryover reduces fecundity. These results predict that rare plants with many ovules per flower benefit from dispersing aggregations of pollen, especially if their pollinators exhibit constancy, whereas plants with inconstant pollinators and low thresholds of pollen receipt benefit from pollen grains dispersing individually to increase the number of flowers reached by the pollen.     

POLLEN AND GENE DISPERSAL: THE INFLUENCES OF COMPETITION FOR POLLINATION

Pollinators that forage indiscriminately can transfer pollen from one species to another, reducing the amount that reaches conspecific flowers. I present evidence that the presence of another plant species visited by the same pollinators can also reduce pollen dispersal distances and outcrossing. This has the potential to influence gene flow and reproductive success. Pollen carryover and movement patterns were measured for the shared insect pollinators of Stellaria pubera and Claytonia virginica in North Carolina. Bee flies deposited similar amounts of Stellaria pollen on a series of pistillate Claytonia flowers as on a series of pistillate Stellaria flowers. In arrays of potted plants, flies and solitary bees visited most flowers on a plant before leaving and then flew to a nearby plant chosen independently of species; 95% of moves were to one of 12 nearest neighbors. These measures of pollen carryover and movement patterns were used in a set of computer simulations to predict pollen dispersal distances. The simulations suggested that C. virginica substantially reduces outcrossing and pollen flow in S. pubera. These predictions were tested by tracking dye movement from anthers in populations of potted plants. Addition of C. virginica reduced the mean squared distance moved by dye to receptive S. pubera flowers by 23% and reduced the amount of dye moved by 51%. The estimated pollen component of gene flow was also much lower in a natural population of 5. pubera mixed with C. virginica than in the synthetic single-species populations.     

Predicting patterns of mating and potential hybridization from pollinator behavior

Hybridization in flowering plants is determined in part by the rate at which animal pollinators move between species and by the effectiveness of such movements in transferring pollen. Pollinator behavior can also influence hybrid fitness by determining receipt and export of pollen. We incorporated information on pollinator effectiveness and visitation behavior into a simulation model that predicts pollen transfer between Ipomopsis aggregata, Ipomopsis tenuituba, and hybrids. These predictions were compared with estimates of pollen transfer derived from movement of fluorescent dyes in experimental plant arrays. Interspecific pollen transfer was relatively uncommon in these arrays, whereas transfer between hybrids and the parental species was at least as common as conspecific transfer. Backcrossing was asymmetrical; I. aggregata flowers frequently received mixed loads of hybrid and conspecific pollen. The simulation suggests that these patterns of pollen transfer are largely explained by the visitation sequences of hummingbird and insect pollinators, with little contribution from mechanical isolation. Pollen receipt by hybrids exceeded that of both parental species in a year when pollinators preferred to visit F(1) and F(2) hybrids and was intermediate in another year when they preferred to visit I. aggregata. This suggests that natural variation in pollination may produce spatiotemporal variation in hybridization and hybrid fitness.     

Pollen dispersal is driven by pollinator response to plant disease and plant spatial aggregation

Most plant species are pollinated by animals, mainly insects, who adjust their foraging behaviour to the spatial distribution of rewards. Any changes in rewards of individual plants could then affect pollen dispersal at the level of plant patches or populations. Such change in floral rewards often results from infection by plant pathogens, for example by anther smuts (i.e. no pollen and reduced nectar in diseased flowers). Here, we tested the hypothesis that the infection of plant populations by anther smuts affects the pattern of pollen dispersal.We investigated the patterns of pollen dispersal in experimental arrays of potted plants differing in the presence of diseased plants and the degree of plant spatial aggregation. We tracked pollen dispersal using a fluorescent dye powder as a pollen analogue, while we simultaneously observed pollinator foraging behaviour.We found that the dispersal of the pollen analogue increased in the presence of diseased plants in experimental arrays, but this effect was strongly dependant on plant spatial aggregation. The parallel observations of pollinator behaviour suggest that this pattern resulted from pollinator discrimination against diseased plants and increased movement in arrays with intermingled diseased plants, provided that plant clusters were close to each other.Our study indicates that pollinators respond to diseased plants in a similar way as to healthy plants with low rewards. Consequently, diseased plants should be treated not only as a potential source of infection but also as a factor influencing pollen dispersal in plant populations.     

Effects of low-efficiency pollinators on plant fitness and floral trait evolution in <Emphasis Type="Italic">Campanula americana</Emphasis> (Campanulaceae)

Floral visitors vary in their pollination efficiency and their preferences for floral traits. If low-efficiency pollinators decrease the amount of pollen available to higher efficiency visitors, then low-efficiency visitors may actually have negative fitness consequences for the plants that they visit. We used experimental arrays in two populations to determine the floral preferences and the fitness effects of low-efficiency (or ugly) pollinators on Campanula americana. These ugly pollinators (halictid bees) preferentially visited flowers with pollen over flowers that had had their pollen removed. C. americana pollen color varies quantitatively from light tan to dark purple, and we found that natural variation in pollen color influenced the magnitude of halictid preferences for flowers with pollen. In general, preferences for flowers with pollen were stronger when the ugly pollinators foraged in arrays of flowers with tan-colored pollen than in arrays with purple-colored pollen. When plants received few visits by efficient Bombus pollinators, visits by ugly pollinators significantly decreased siring success relative to plants where visits by ugly pollinators were prevented. In contrast, ugly pollinators did not influence siring success when higher efficiency pollinators were more abundant. Thus, the relationship between low-efficiency pollinators and the plants that they visit varies from commensalistic to antagonistic depending on the presence of other pollinators in the community. Our findings suggest that the negative fitness effects and floral preferences of low-efficiency or ugly pollinators may contribute to the maintenance of a pollen color polymorphism in C. americana.     

Adaptive plasticity of floral display size in animal-pollinated plants

Plants need not participate passively in their own mating, despite their immobility and reliance on pollen vectors. Instead, plants may respond to their recent pollination experience by adjusting the number of flowers that they display simultaneously. Such responsiveness could arise from the dependence of floral display size on the longevity of individual flowers, which varies with pollination rate in many plant species. By hand-pollinating some inflorescences, but not others, we demonstrate plasticity in display size of the orchid Satyrium longicauda. Pollination induced flower wilting, but did not affect the opening of new flowers, so that within a few days pollinated inflorescences displayed fewer flowers than unpollinated inflorescences. During subsequent exposure to intensive natural pollination, pollen removal and receipt increased proportionally with increasing display size, whereas pollen-removal failure and self-pollination accelerated. Such benefit-cost relations allow plants that adjust display size in response to the prevailing pollination rate to increase their attractiveness when pollinators are rare (large displays), or to limit mating costs when pollinators are abundant (small displays). Seen from this perspective, pollination-induced flower wilting serves the entire plant by allowing it to display the number of flowers that is appropriate for the current pollination environment.     

Attracting pollinators and avoiding herbivores: insects influence plant traits within and across years

Perennial plants interact with herbivores and pollinators across multiple growing seasons, and thus may respond to herbivores and pollinators both within and across years. Joint effects of herbivores and pollinators influence plant traits, but while some of the potential interactions among herbivory, pollination, plant size, and plant reproductive traits have been well studied, others are poorly understood. This is particularly true for perennial plants where effects of herbivores and pollinators may manifest across years. Here, we describe two experiments addressing the reciprocal interactions of plant traits with herbivore damage and pollination across 2 years using the perennial plant Chamerion angustifolium. We measured (1) plant responses to manipulation of damage and pollination in the year of treatment and the subsequent season, (2) damage and pollination responses to manipulation of plant size and flowering traits in the year of treatment, and (3) plant-mediated indirect interactions between herbivores and pollinators. We found that plant traits had little effect on damage and pollination, but damage and pollination affected plant traits in both the treatment year and the subsequent year. We found evidence of indirect effects between leaf herbivores and pollinators in both directions; indirect effects of pollinators on leaf herbivores have not been previously demonstrated. Our results indicate that pollen receipt results in shorter plants with fewer stems but does not change flower number, while leaf herbivory results in taller plants with fewer flowers. Together, herbivory and pollination may contribute to intermediate plant height and plants with fewer stems and flowers in our system.     

Sex in advertising: dioecy alters the net benefits of attractiveness in Sagittaria latifolia (Alismataceae)

The flowers and inflorescences of animal-pollinated dioecious plants are generally small and inconspicuous in comparison with outcrossing cosexual species. The net benefits of an attractive floral display may be different for dioecious compared to cosexual populations because dioecious species experience a more severe reduction in pollen delivery when pollinators forage longer on fewer individuals. Here, we develop a model that predicts the decrease in pollen delivery in dioecious relative to cosexual populations from female-female, female-male and male-male visit sequences as the number of individuals visited varies. To evaluate the predictions of our model we conducted a common garden experiment with dioecious and monoecious (cosexual) arrays of the insect-pollinated herb Sagittaria latifolia. We find that, although increasing the advertisements of floral rewards (i.e. increasing floral display) attracts more pollinators to individuals, the probability that these pollinators subsequently deliver pollen to neighbouring plants depends on sexual system. Because the number of individual plants visited per foraging trip did not increase significantly with floral display, the relative pollination success of dioecious versus monoecious populations decreases with increased floral display. We propose that this could explain why dioecy is strongly correlated with reduced floral display among angiosperm species.     

Effects of herkogamy and inbreeding on the mating system of <Emphasis Type="Italic">Mimulus luteus</Emphasis> in the absence of pollinators

Self-pollination mechanisms are often invoked to explain plant reproduction in environments devoid of pollinators. However, populations may evolve a diverse variety of strategies depending on the ecological context of pollination. In this study we examined the pollination environment, inbreeding depression, and herkogamy of Mimulus luteus (Phrymaceae) in two consecutive years in an attempt to evaluate the extent to which these factors determine the current level of autogamy in this population. Results indicate that pollinators were almost absent in the study site in 2005 and 2006. Likewise, no evidence of significant inbreeding depression for seed production and seed germination was detected in the 2 years. Because M. luteus presents hermaphrodite flowers with ‘movement herkogamy’ (touch-sensitive stigmata that close upon contact), and anther–stigma separation, we evaluated the role of these traits in self-fertilization. First, we assessed the effects of pollen source (self- and xenogamous pollen, and control solution) on stigmata dynamics after pollen deposition, and seed production. We observed that stigmata that received pollen (self- and xenogamous hand-pollinated) remained closed for a longer time than flowers that received no pollen (control flowers). Seed production, however, was unaffected by pollen source, indicating that movement herkogamy does not prevent self-fertilization in this population. Second, a phenotypic selection analysis revealed that seed production increased with a reduction of anther–stigma separation in absence of pollinators (bagged flowers), suggesting that low herkogamy levels are promoted in this population. Our results indicate that lack of inbreeding depression, ineffective movement herkogamy, and selection against anther–stigma separation are factors that may contribute to the prevalence of autogamous reproductive mechanisms when M. luteus faces environments with scarce mating opportunity.     

Extremely high proportions of male flowers and geographic variation in floral ratios within male figs of Ficus tikoua despite pollinators displaying active pollen collection

Most plants are pollinated passively, but active pollination has evolved among insects that depend on ovule fertilization for larval development. Anther‐to‐ovule ratios (A/O ratios, a coarse indicator of pollen‐to‐ovule ratios) are strong indicators of pollination mode in fig trees and are consistent within most species. However, unusually high values and high variation of A/O ratios (0.096–10.0) were detected among male plants from 41 natural populations of Ficus tikoua in China. Higher proportions of male (staminate) flowers were associated with a change in their distribution within the figs, from circum‐ostiolar to scattered. Plants bearing figs with ostiolar or scattered male flowers were geographically separated, with scattered male flowers found mainly on the Yungui Plateau in the southwest of our sample area. The A/O ratios of most F. tikoua figs were indicative of passive pollination, but its Ceratosolen fig wasp pollinator actively loads pollen into its pollen pockets. Additional pollen was also carried on their body surface and pollinators emerging from scattered‐flower figs had more surface pollen. Large amounts of pollen grains on the insects' body surface are usually indicative of a passive pollinator. This is the first recorded case of an actively pollinated Ficus species producing large amounts of pollen. Overall high A/O ratios, particularly in some populations, in combination with actively pollinating pollinators, may reflect a response by the plant to insufficient quantities of pollen transported in the wasps’ pollen pockets, together with geographic variation in this pollen limitation. This suggests an unstable scenario that could lead to eventual loss of wasp active pollination behavior.     

INCOMPATIBILITY AND THE POLLEN ECONOMY OF JEPSONIA PARRYI

Jepsonia parryi (Saxifragaceae) has heterostylous flowers and is strongly self-incompatible. Pin flowers have long styles, large stigmas, short stamens, and numerous, small pollen grains with finely sculptured walls. Thrum flowers have short styles, small stigmas, long stamens, and fewer, larger pollen grains with coarsely sculptured walls. Pin plants and thrum plants occur in a 1:1 ratio in field populations. Although the insect pollinators of J. parryi transfer ample compatible pollen to pin and thrum stigmas to account for full seed production, much of the pollen deposited on stigmas is incompatible. Analysis of the pollen deposits on stigmas collected from field populations indicates that compatible “legitimate” pollination of pin and thrum flowers is essentially random and is not obviously aided by floral dimorphism. It is suggested that although heterostyly had a positive adaptive value in the past evolutionary history of Jepsonia it is no longer adaptive under the present pollination regime, although it is maintained because of its strong genetic fixity.     

Effects of pollination and pollen source on the seed set ofPedicularis palustris

Habitat alteration can deteriorate plant-pollinator interactions and thereby increase the risk of population extinction. As part of a larger study on the effects of changes in land use on fen grassland vegetation, factors influencing the seed set of a short-lived, endangered wetland plant,Pedicularis palustris, were studied. We conducted field pollination experiments in one large and one small population. To investigate the effect of pollen source on seed set, individual flowers of caged plants were left unpollinated or were pollinated with pollen from the same flower, the same population or another population. To study pollen limitation and flower display, whole plants were subjected to pollinator exclosure, hand pollination or natural pollination. Self-compatibility was high, but differed between populations (61% and 97% of seed set after cross-pollination within populations). Cross-pollination between populations did not significantly alter seed number per capsule. Pollinator exclosure resulted in a very low seed set (<15% of natural seed set), despite high self-compatibility. The most likely explanations for high self-compatibility in combination with low autofertility are geitonogamy as reproductive assurance, selective neutrality of self-compatibility and phylogenetic constraints. Because of low autofertility, the seed set inP. palustris depends on pollinators. In the study populations, natural pollination was clearly sufficient for maximum seed production per plant, but seed set per capsule was significantly pollen-limited in the smaller population. Plants in this population also had a higher maximum percentage of simultaneously open flowers than those of the large population (31% vs. 13%), while flower longevity was generally extended without pollination. It is concluded thatP. palustris may influence pollinator behaviour and therefore the risk of pollen limitation by flower display.P. palustris showed a flexible reaction to differing pollination regimes without losses in overall seed set in the study populations.     

Pollination biology of fruit-bearing hedgerow plants and the role of flower-visiting insects in fruit-set

Background and Aims

In the UK, the flowers of fruit-bearing hedgerow plants provide a succession of pollen and nectar for flower-visiting insects for much of the year. The fruits of hedgerow plants are a source of winter food for frugivorous birds on farmland. It is unclear whether recent declines in pollinator populations are likely to threaten fruit-set and hence food supply for birds. The present study investigates the pollination biology of five common hedgerow plants: blackthorn (Prunus spinosa), hawthorn (Crataegus monogyna), dog rose (Rosa canina), bramble (Rubus fruticosus) and ivy (Hedera helix).

Methods

The requirement for insect pollination was investigated initially by excluding insects from flowers by using mesh bags and comparing immature and mature fruit-set with those of open-pollinated flowers. Those plants that showed a requirement for insect pollination were then tested to compare fruit-set under two additional pollination service scenarios: (1) reduced pollination, with insects excluded from flowers bagged for part of the flowering period, and (2) supplemental pollination, with flowers hand cross-pollinated to test for pollen limitation.

Key Results

The proportions of flowers setting fruit in blackthorn, hawthorn and ivy were significantly reduced when insects were excluded from flowers by using mesh bags, whereas fruit-set in bramble and dog rose were unaffected. Restricting the exposure of flowers to pollinators had no significant effect on fruit-set. However, blackthorn and hawthorn were found to be pollen-limited, suggesting that the pollination service was inadequate in the study area.

Conclusions

Ensuring strong populations of insect pollinators may be essential to guarantee a winter fruit supply for birds in UK hedgerows.Key words: Blackthorn, bramble, Crataegus monogyna, frugivorous birds, hawthorn, Hedera helix, hedgerows, ivy, insect pollination, Prunus spinosa, Rubus fruticosus, Rosa canina     

FLORAL COLOR CHANGE IN LUPINUS ARGENTEUS (FABACEAE): WHY SHOULD PLANTS ADVERTISE THE LOCATION OF UNREWARDING FLOWERS TO POLLINATORS?

I examined the adaptive significance of two floral traits in the perennial herb, Lupinus argenteus: 1) the retention of corollas on “spent” flowers, i.e., flowers containing inviable pollen, unreceptive stigmas, and negligible pollinator rewards and 2) a change in corolla color of retained “spent” flowers, which is restricted to a spot on the banner petal. At anthesis, this spot is yellow, and approximately four days later, it changes to purple. After the change, purple flowers remain on plants an additional 5–7 days before corolla abscission occurs; purple flowers were avoided by pollinators, presumably because they contained less pollen (rewards) than yellow ones. I experimentally tested the hypothesis that purple flowers contribute to the floral display of the plant by removing varying numbers of spent flowers and assessing the effect on pollination visitation. Pollinators preferentially approached and foraged on plants with greater numbers of flowers per inflorescence; they did not discriminate between yellow (rewarding) and purple (nonrewarding) flowers at interplant distances greater than 0.4 meters but would preferentially forage on plants with more total flowers, even if these individuals contained fewer rewarding flowers. Thus, spent flowers increased the overall attractiveness of plants to pollinators. In theory, color change may benefit plants in two ways. First, by directing pollinators to rewarding flowers, the change may increase pollinator foraging efficiency, with the result that pollinators visit more flowers before leaving plants (pollinator-tenure mechanism). Second, by directing pollinators to receptive flowers, the color change may prevent incoming pollen from being wasted on unreceptive stigmas and may prevent collection of inviable pollen (pollination-efficiency mechanism). I tested the pollinator-tenure mechanism experimentally by removing pollen from yellow flowers, thereby reducing the reliability of the color-reward signal. Pollinators visited fewer total flowers on experimental plants than on controls, resulting in reduced seed production in one year.     

Pollen transfer in fragmented plant populations: insight from the pollen loads of pollinators and stigmas in a mass‐flowering species

Pollinator and/or mate scarcity affects pollen transfer, with important ecological and evolutionary consequences for plant reproduction. However, the way in which the pollen loads transported by pollinators and deposited on stigmas are affected by pollination context has been little studied. We investigated the impacts of plant mate and visiting insect availabilities on pollen transport and receipt in a mass‐flowering and facultative autogamous shrub (Rhododendron ferrugineum). First, we recorded insect visits to R. ferrugineum in plant patches of diverse densities and sizes. Second, we analyzed the pollen loads transported by R. ferrugineum pollinators and deposited on stigmas of emasculated and intact flowers, in the same patches. Overall, pollinators (bumblebees) transported much larger pollen loads than the ones found on stigmas, and the pollen deposited on stigmas included a high proportion of conspecific pollen. However, comparing pollen loads of emasculated and intact flowers indicated that pollinators contributed only half the conspecific pollen present on the stigma. At low plant density, we found the highest visitation rate and the lowest proportion of conspecific pollen transported and deposited by pollinators. By contrast, at higher plant density and lower visitation rate, pollinators deposited larger proportion of conspecific pollen, although still far from sufficient to ensure that all the ovules were fertilized. Finally, self‐pollen completely buffered the detrimental effects on pollination of patch fragmentation and pollinator failure. Our results indicate that pollen loads from pollinators and emasculated flowers should be quantified for an accurate understanding of the relative impacts of pollinator and mate limitation on pollen transfer in facultative autogamous species.     

Post-pollination emission of a repellent compound in a sexually deceptive orchid: a new mechanism for maximising reproductive success?

The flowers of the sexually deceptive orchid Ophrys sphegodes are pollinated by pseudocopulating males of the solitary bee Andrena nigroaenea. We investigated the changes in odor emission and reduced attractiveness that occur after pollination in these plants. We analyzed floral odor of unpollinated and pollinated flowers by gas chromatography and compared relative and absolute amounts of electrophysiologically active compounds. Headspace odor samples of O. sphegodes flowers showed a significant increase in absolute and relative amounts of all-trans-farnesyl hexanoate after pollination. Flower extracts also indicated an increase of farnesyl hexanoate after pollination. The total amount of the other physiologically active odor compounds decreased slightly. Farnesyl hexanoate is a major constituent of the Dufour's gland secretion in females of the pollinator bees, A. nigroaenea, where it functions in the lining of the brood cells. Furthermore, this compound lowers the number of copulation attempts in males. In dual-choice tests, we showed that flowers artificially scented with an amount of farnesyl hexanoate equal to the increased amount after pollination were significantly less attractive than flowers treated with solvent only. We propose that the increased production of farnesyl hexanoate in pollinated flowers is a signal to guide pollinators to unpollinated flowers of the inflorescence, which represents a new mechanism in this pollination system.     

Pollination in Verbascum thapsus (Scrophulariaceae): the advantage of being tall

According to the "effective pollination" hypothesis, tall stature resulting from strong apical dominance attracts greater pollinator visitation, thus allowing larger pollen loads and/or greater outcrossing rates, which in turn produces more vigorous offspring with greater genotypic variability and/or less inbreeding depression. Components of this hypothesis were tested in Verbascum thapsus, which commonly grows unbranched to over 2 m tall with strong apical dominance suppressing all axillary meristems. A natural population survey indicated that plants with visiting pollinators were significantly taller than their nearest neighboring individuals not possessing a visiting pollinator. Plants in natural populations with excluded pollinators produced seeds via a delayed selfing mechanism. However, delayed selfing under pollinator exclusion resulted in only 75% of the seed set obtained with natural pollinators. Under natural pollination, emasculated flowers experienced a 50% reduction in pollen deposition by the time of flower closure but only a 5% reduction in seed set relative to intact flowers. Hence, taller plants attracted more pollinators and maximum seed set could not be achieved without pollinators. Comparison of seed set and seed mass in plants that were artificially selfed and artificially crossed (in both the greenhouse and in natural populations) indicated that plants were fully self-compatible with no evidence of early-acting inbreeding depression. However, this does not exclude the possibility that inbreeding depression is manifested in later life stages. The results suggest that V. thapsus has a mixed mating system with potential for reproductive assurance and various levels of outcrossing depending on variables affecting pollinator availability (e.g., population size).     

Patterns and sources of variation in pollen deposition and pollen tube formation in flowers of the endemic monoecious shrub Cnidoscolus souzae (Euphorbiaceae)

Pollen deposition and pollen tube formation are key components of angiosperm reproduction but intraspecific variation in these has rarely been quantified. Documenting and partitioning (populations, plants and flowers) natural variation in these two aspects of plant reproduction can help uncover spatial mosaics of reproductive success and underlying causes. In this study, we assess variation in pollen deposition and pollen tube formation for the endemic monoecious shrub Cnidoscolus souzae throughout its distribution range in Mexico, and determine how this variation is structured among populations, plants and flowers. We also infer the relative importance of pollen quantity and quality in determining pollination success in this species. While we found no evidence suggesting that pollen receipt limits C. souzae reproduction across 19 populations, we did find extensive variation in pollen load size and pollen tube number per flower. Total variation in pollen receipt and pollen tube number was mostly explained by intra‐individual and among‐population variance. Furthermore, pollen load size had a stronger effect on the number of pollen tubes at the base of the style than pollen germination rate, suggesting that pollen quantity may be more important than quality for pollen tube success in C. souzae. Our results suggest that both small within‐plant flower differences and broad‐scale differences in community attributes can play an important role in determining pollination success. We emphasise the need to evaluate patterns and sources of variation in pollen deposition and pollen tube formation as a first step in understanding the causes of variation in pollination success over broad spatial scales.