Pollen Competition Research Articles

Interspecific competition in pollination systems: costs to male fitness via pollen misplacement

Summary1. Although competition for pollination is often invoked as a driver of broad‐scale evolutionary and ecological patterns, we still lack a clear understanding of the mechanics of such competition. When flower visitors alternate between two species of flower, heterospecific pollen transfer takes place. The impact of these mixed loads on the female reproductive success of a recipient has received considerable attention, but the concomitant loss of male reproductive success – because of pollens grains being lost to foreign stigmas – has received less. Furthermore, pollen losses are not limited to grains that land on stigmas, but can also include deposition on non‐stigmatic surfaces of the intervening flowers, or loss from the animal’s body through passive detachment or active grooming. We collectively term these losses because of competition ‘pollen misplacement’.2. Here, we quantify pollen transferred by nectar bats between focal flowers (Aphelandra acanthus) with and without intervening visits to one of two competitor species. One competitor (Centropogon nigricans) places its pollen in the same region of bats’ heads as the focal species, while the other (Burmeistera sodiroana) places its pollen farther forward.3. We found that (i) any intervening visit caused some reduction in the number of pollen grains transferred, (ii) competitor flowers with similar pollen‐placement locations caused greater reductions in pollen transfer and (iii) of these competitors, those in male phase (dispensing pollen) caused greater pollen loss than those in female phase (without pollen).4. This study provides rare empirical support for the detrimental effects of competition for pollination on male fitness via pollen misplacement and is the first to show an added cost imposed by male‐phase competitors. Although this competition is especially strong when competitors overlap in pollen placement, diverging in pollen placement will not completely eliminate pollen loss during visits to foreign flowers, simply because pollen sheds or is groomed from pollinator’s bodies at some background rate over time. This suggests that any angiosperms that share pollinators face pervasive selection through male fitness to diverge in floral traits, alleviating competition by attracting different pollinators, altering floral phenology or encouraging floral constancy.

RECONCILING EXTREMELY STRONG BARRIERS WITH HIGH LEVELS OF GENE EXCHANGE IN ANNUAL SUNFLOWERS

In several cases, estimates of gene flow between species appear to be higher than we might predict given the strength of interspecific barriers separating these species pairs. However, as far as we are aware, detailed measurements of reproductive isolation have not previously been compared with a coalescent-based assessment of gene flow. Here, we contrast these two measures in two species of sunflower, Helianthus annuus and H. petiolaris. We quantified the total reproductive barrier strength between these species by compounding the contributions of the following prezygotic and postzygotic barriers: ecogeographic isolation, reproductive asynchrony, niche differentiation, pollen competition, hybrid seed formation, hybrid seed germination, hybrid fertility, and extrinsic postzygotic isolation. From this estimate, we calculated the probability that a reproductively successful hybrid is produced: estimates of P(hyb) range from 10(-4) to 10(-6) depending on the direction of the cross and the degree of independence among reproductive barriers. We then compared this probability with population genetic estimates of the per generation migration rate (m). We showed that the relatively high levels of gene flow estimated between these sunflower species (N(e) m= 0.34-0.76) are mainly due to their large effective population sizes (N(e) > 10(6)). The interspecific migration rate (m) is very small (<10(-7)) and an order of magnitude lower than that expected based on our reproductive barrier strength estimates. Thus, even high levels of reproductive isolation (>0.999) may produce genomic mosaics.

Unravelling the role of mate density and sex ratio in competition for pollen

Mate density and sex ratio are commonly used concepts in pollination biology, but are not always clearly distinguished. Here we propose that mate density should only capture the number of male‐phase flowers in a defined area and ignore female‐phase flowers. Sex ratio is the ratio of male and female‐phase flowers in a defined area and captures female–female competition. We use a spatially explicit simulation model to quantify the effect of variation in mate density and sex ratio for plant–pollinator systems characterized by combinations of high or low rates of pollen loss and deposition from pollinator to stigma and then assess the size of pollination neighbourhoods. In efficient systems with relatively little pollen loss, female–female competition is of overriding importance. In contrast, in wasteful systems with high pollen loss rates, mate density becomes the dominant factor and sex ratio is no longer consequential. These patterns were observed at both landscape and local scales. Systems with low deposition and low pollen loss rates are associated with large pollination neighbourhoods, which decline with increasing deposition and pollen loss rates. These results show that mate density and sex ratio should carefully be distinguished and highlight the complex way in which pollen loss interacts with deposition rate, which has not previously been appreciated.

Pollen competition as a reproductive isolation barrier represses transgene flow between compatible and co-flowering citrus genotypes.

Background/ObjectiveDespite potential benefits granted by genetically modified (GM) fruit trees, their release and commercialization raises concerns about their potential environmental impact, and the transfer via pollen of transgenes to cross-compatible cultivars is deemed to be the greatest source for environmental exposure. Information compiled from field trials on GM trees is essential to propose measures to minimize the transgene dispersal. We have conducted a field trial of seven consecutive years to investigate the maximum frequency of pollen-mediated crop-to-crop transgene flow in a citrus orchard, and its relation to the genetic, phenological and environmental factors involved.Methodology/Principal FindingsThree different citrus genotypes carrying the uidA (GUS) tracer marker gene (pollen donors) and a non-GM self-incompatible contiguous citrus genotype (recipient) were used in conditions allowing natural entomophilous pollination to occur. The examination of 603 to 2990 seeds per year showed unexpectedly low frequencies (0.17–2.86%) of transgene flow. Paternity analyses of the progeny of subsets of recipient plants using 10 microsatellite (SSR) loci demonstrated a higher mating competence of trees from another non-GM pollen source population that greatly limited the mating chance of the contiguous cross-compatible and flowering-synchronized transgenic pollen source. This mating superiority could be explained by a much higher pollen competition capacity of the non-GM genotypes, as was confirmed through mixed-hand pollinations.Conclusions/SignificancePollen competition strongly contributed to transgene confinement. Based on this finding, suitable isolation measures are proposed for the first time to prevent transgene outflow between contiguous plantings of citrus types that may be extendible to other entomophilous transgenic fruit tree species.

Pollinator guild organization and its consequences for reproduction in three synchronopatric species of Tibouchina (Melastomataceae)

Pollinator guild organization and its consequences for reproduction in three synchronopatric species of Tibouchina (Melastomataceae). In co-flowering plant species, pollinator sharing can result in interspecific pollen transfer and fecundity reduction. Competition will be relaxed whenever there is a large amount of initial pollen supply or if each plant species occupies different habitat patches. Reproduction in Tibouchina cerastifolia (Naudin) Cogn., T. clinopodifolia (DC.) Cogn. and T. gracilis (Bonpl.) Cogn. was studied in an area of Atlantic rainforest to examine whether synchronopatry induces time partitioning among pollinator species. Eleven bee species comprised the pollinator guild. Among pollinators, there were overlaps in bee species composition and in flower visitation time. Direct competition for pollen in Tibouchina Aubl. at the study site seems to lead to different activity periods among the bee species, in which Bombus pauloensis Friese,1913 was most active earlier, while the other species were active later in the day. Bombus pauloensis, the largest bee species recorded on Tibouchina flowers, was the most important and efficient pollinator. This species harvested pollen before the other species and had the shortest handling time. The plants reproduced sexually by selfing or outcrossing, and hybridization was not avoided by incompatibility reactions at the style. The avoidance of direct competition for pollen and no pollinator partitioning among the synchronopatric species of Tibouchina may reflect a facilitative interaction among these pioneer plants.

Competition versus facilitation: conspecific effects on pollinator visitation and seed set in the iris Lapeirousia oreogena

Ecological interactions between conspecific plants can range from facilitative to competitive depending on the spacing and abundance of individuals. Competitive interactions are expected when plants flower en‐masse and availability of pollinators is limited. We tested this prediction using Lapeirousia oreogena, a mass‐flowering South African iris that is pollinated by a single species of long‐proboscid fly. Controlled hand‐pollination experiments showed that L. oreogena is self‐compatible, but reliant on pollinator visits for seed set. Seed production per flower showed a significant negative relationship with patch size (and the correlated measure of number of individuals per patch), but was not affected by flower density or distance to neighbouring patches. There was a tendency for fly abundance to increase with patch size, but the rate of visits to individual flowers by flies was not affected by patch size. Seed set of hand‐pollinated flowers did not differ for plants in and out of dense patches, indicating that the large differences in seed set among patches were likely to reflect pollinator visits, rather than the genetic or physiological capacity of plants to produce seeds. The reduced fecundity of L. oreogena in large patches with a greater numbers of flowers is consistent with the idea that plants with highly specialized pollination systems can experience intra‐specific competition for pollination.

Twenty-five years of progress in understanding pollination mechanisms in palms (Arecaceae).

With more than 90 published studies of pollination mechanisms, the palm family is one of the better studied tropical families of angiosperms. Understanding palm-pollinator interactions has implications for tropical silviculture, agroforestry and horticulture, as well as for our understanding of palm evolution and diversification. We review the rich literature on pollination mechanisms in palms that has appeared since the last review of palm pollination studies was published 25 years ago. Visitors to palm inflorescences are attracted by rewards such as food, shelter and oviposition sites. The interaction between the palm and its visiting fauna represents a trade-off between the services provided by the potential pollinators and the antagonistic activities of other insect visitors. Evidence suggests that beetles constitute the most important group of pollinators in palms, followed by bees and flies. Occasional pollinators include mammals (e.g. bats and marsupials) and even crabs. Comparative studies of palm-pollinator interactions in closely related palm species document transitions in floral morphology, phenology and anatomy correlated with shifts in pollination vectors. Synecological studies show that asynchronous flowering and partitioning of pollinator guilds may be important regulators of gene flow between closely related sympatric taxa and potential drivers of speciation processes. Studies of larger plant-pollinator networks point out the importance of competition for pollinators between palms and other flowering plants and document how the insect communities in tropical forest canopies probably influence the reproductive success of palms. However, published studies have a strong geographical bias towards the South American region and a taxonomic bias towards the tribe Cocoseae. Future studies should try to correct this imbalance to provide a more representative picture of pollination mechanisms and their evolutionary implications across the entire family.

Flower number and distance to neighbours affect the fecundity of Goetzea elegans (Solanaceae)

Abstract:Pollinator visits to flowers of self-incompatible tropical trees are critical for plant fecundity. However, few studies have examined how much of the variance in tropical tree fecundity is explained by individual attributes of trees (e.g. number of flowers), and how much is due to contextual variables such as distances to nearest flowering neighbours. Using multiple regressions we examined the relative contributions of these factors to the pollination and fecundity of Goetzea elegans, a mainly self-incompatible tree endemic to Puerto Rico. We studied the largest wild population of the species during the peak flowering and collected data on the frequency of pollinator visits (N = 25 trees), and the fecundity of the whole population (N = 105), including the visitation rate of two pollinators (the honey bee Apis mellifera and the bananaquit Coereba flaveola), the total number of flowers produced by each tree, and the total fruit set and seed viability per tree. We also recorded the distance to flowering conspecifics and heterospecifics, and the height for each tree. Flower number had a strong positive effect on pollinator visitation, but distance to nearest neighbours was equally or more important than flower number in influencing fecundity. Also, competition for limited pollinators between G. elegans and other species has a stronger effect than the facilitation that conspecifics may provide. Our results suggest that pollinator visits and aspects of fecundity of G. elegans depend both on the attributes of individual plants, and on those of the community of other nearby plants.

Enhancing pollen competition by delaying stigma receptivity: Pollen deposition schedules affect siring ability, paternal diversity, and seed production inCollinsia heterophylla(Plantaginaceae)

Even though pollen deposition schedules may have profound effects on the evolutionary outcome of pollen competition, few studies have investigated such effects in relation to pistil traits such as delayed stigma receptivity that enhance pollen competition. In Collinsia heterophylla, a largely outcrossing species with delayed stigma receptivity, we performed a series of controlled crosses involving several donors to understand how timing of pollen deposition influences siring ability, paternal diversity, and offspring fitness. Pollen was applied to fully receptive stigmas either as mixtures or consecutively with or without a time lag to mimic cases with early or delayed stigma receptivity. We used a genetic marker to assess offspring paternity. As expected, siring ability was affected by application order in crosses without a time lag, providing a first-donor advantage for pollen arriving on unreceptive stigmas. However, because pollen donor identity influenced siring ability, delaying stigma receptivity may still favor pollen of high competitive ability. In crosses on fully receptive pistils with a time lag of 24 h, a surprisingly high proportion of seeds (12-47%) were sired by pollen applied last. A novel finding was that pollen applied only once (as a mixture), mimicking delayed stigma receptivity, led to higher paternal diversity within progeny families, which was associated with increased seed production. Our results suggest fitness advantages of enhancing pollen competition by delaying stigma receptivity in C. heterophylla, particularly in relation to increased paternal diversity.

Stylish lengths: Mate choice in flowers

The styles of flowers may represent an arena for pollen competition in the race to fertilize ovules. Accordingly, selection should favour a longer 'race' to better discriminate among variable pollen by increasing style length. Sampling across a taxonomically diverse range of wild and outcrossed species, we found that the distribution of style lengths within plants were skewed towards longer styles, as predicted. In self-pollinated domesticated species, where discrimination among pollen is less important, we found no such pattern. We conclude that style length is under directional selection towards longer styles as a mechanism for mate choice among pollen of variable quality.

Effects of floral neighborhood on seed set and degree of outbreeding in a high-alpine cushion plant

Plants flowering together may influence each other's pollination and fecundity over a range of physical distances. Their effects on one another can be competitive, neutral, or facilitative. We manipulated the floral neighborhood of the high-alpine cushion plant Eritrichium nanum in the Swiss Alps and measured the effects of co-flowering neighbors on both the number of seeds produced and the degree of inbreeding and outbreeding in the offspring, as deduced from nuclear microsatellite markers. Seed set of E. nanum did not vary significantly with the presence or absence of two Saxifraga species growing as near neighbors, but it was higher in E. nanum cushions growing at low conspecific density than in those growing at high density. In addition, floral neighborhood had no detectable effect on the degree of selfing of E. nanum, but seeds from cushions growing at low conspecific density were more highly outbred than seeds from cushions at high density. Thus, there was no evidence of either competition or facilitation between E. nanum and Saxifraga spp. as mediated by pollinators at the spatial scale of our experimental manipulation. In contrast, the greater fecundity of E. nanum cushions at low density was consistent with reduced intraspecific competition for pollinators and might also represent a beneficial effect of highly outbred seeds as brought about by more long-distance pollinator flights under low-density conditions.

Expansion of mass-flowering crops leads to transient pollinator dilution and reduced wild plant pollination

Agricultural land use results in direct biodiversity decline through loss of natural habitat, but may also cause indirect cross-habitat effects on conservation areas. We conducted three landscape-scale field studies on 67 sites to test the hypothesis that mass flowering of oilseed rape (Brassica napus) results in a transient dilution of bees in crop fields, and in increased competition between crop plants and grassland plants for pollinators. Abundances of bumble-bees, which are the main pollinators of the grassland plant Primula veris, but also pollinate oilseed rape (OSR), decreased with increasing amount of OSR. This landscape-scale dilution affected bumble-bee abundances strongly in OSR fields and marginally in grasslands, where bumble-bee abundances were generally low at the time of Primula flowering. Seed set of Primula veris, which flowers during OSR bloom, was reduced by 20 per cent when the amount of OSR within 1 km radius increased from 0 to 15 per cent. Hence, the current expansion of bee-attractive biofuel crops results in transient dilution of crop pollinators, which means an increased competition for pollinators between crops and wild plants. In conclusion, mass-flowering crops potentially threaten fitness of concurrently flowering wild plants in conservation areas, despite the fact that, in the long run, mass-flowering crops can enhance abundances of generalist pollinators and their pollination service.

A phylogenetically controlled analysis of the roles of reproductive traits in plant invasions

Reproductive traits are tightly linked to plant fitness and may therefore be mechanisms driving biological invasions, including the greater success of more phylogenetically novel introduced species in some systems. We present a phylogenetic comparative analysis of "Baker's law'', that introduced plants with the ability to reproduce autogamous or asexually may be better able to establish on introduction. We gathered data from both published and unpublished sources on pollen limitation of 141 species, including 26 introduced species and 115 native species. Our analysis compared differences in the proportion of autonomous autogamy, asexual reproduction, and pollen limitation among native, introduced noninvasive, and introduced invasive plant species, and included the phylogenetic novelty of the introduced species to the native species in that community. Introduced species were more likely to be autogamous than native species, consistent with Baker's law. On the other hand, introduced species were less likely to have the ability to reproduce asexually. Further, among species with no autonomous autogamy, pollen limitation was greater for introduced compared to native species. Such a result is consistent with the idea that plants entering a new continent receive lower quality or quantity of services from resident pollinators than species native to that continent. Finally, more phylogenetically novel invasive species had lower pollen limitation than less novel invasive species, potentially because they experience less competition for pollinators. This is the first evidence that enhanced pollination may be one mechanism driving the greater invasiveness of phylogenetically novel introduced species observed in some systems.

Effect of local community phylogenetic structure on pollen limitation in an obligately insect‐pollinated plant

Pollination is a key aspect of ecosystem function in the majority of land plant communities. It is well established that many animal-pollinated plants suffer lower seed set than they are capable of, likely because of competition for pollinators. Previously, competition for pollinator services has been shown to be most intense in communities with the greatest plant diversity. In spite of the fact that community evolutionary relations have a demonstrated impact on many ecological processes, their role in competition for pollinator services has rarely been examined. In this study, we explore relations among several aspects of the surrounding plant community, including species richness, phylodiversity, evolutionary distance from a focal species, and pollen limitation in an annual insect-pollinated plant. We did not find a significant effect of species richness on competition for pollination. However, consistent with a greater role for facilitation than competition, we found that a focal species occurring in communities composed of species of close relatives, especially other members of the Asteraceae, was less pollen limited than when it occurred in communities composed of more distant relatives. Our results demonstrate that community phylodiversity is an important correlate of pollen limitation in this system and that it has greater explanatory power than species richness alone.

Variable responses of hawkmoths to nectar-depleted plants in two native Petunia axillaris (Solanaceae) populations

Pollination success of deceptive orchids is affected by the density and distribution of nectar providing plant species and overall plant density. Here we extended the framework of how plant density can affect pollination to examine how it may promote the success of plant intraspecific cheaters. We compared hawkmoth behaviour in two native populations of Petunia axillaris, where we simultaneously offered rewarding and manually depleted P. axillaris. We asked whether pollinator foraging strategies change as a function of plant density and whether such changes may differentially affect nectarless plants. We observed the first choice and number of flowers visited by pollinators and found that in the dense population, pollinators visited more flowers on rewarding plants than on nectar-depleted plants. In the sparse population, such discrimination was absent. As we found no differences in nectar volume between plants of the two populations, the observed differences in plant density may be temporal. We reason that if differences were more permanent, an equivalent of the remote habitat hypothesis prevails: in a sparse population, cheating plants benefit from the absence of inter- and intraspecific competitors because pollinators tend to visit all potential resources. In a denser population, a pollinator’s optimal foraging strategy involves more selectivity. This would cause between-plant competition for pollinators in a pollinator-limited context, which applies to most hawkmoth-pollinated systems. We propose that nectar-provisioning of plants can be density-dependant, with cheaters able to persist in low density areas.

First Come, First Served: Delayed Fertilization Does Not Enhance Pollen Competition in a Wind-Pollinated Tree,Fraxinus excelsiorL. (Oleaceae)

Under natural conditions pollen vectors act stochastically, and the order in which pollen arrives can influence seed paternity. In a number of wind-pollinated taxa, however, fertilization does not occur immediately. It has been proposed that delayed fertilization promotes pollen competition by providing a “fair start” for pollen tubes. To test this hypothesis we compared paternity after simultaneous and consecutive pollinations with multiple donors in Fraxinus excelsior. Additionally, we investigated pollen tube growth in naturally pollinated flowers. Under natural conditions pollen grains arrived on the stigma one by one, and their mean number reached 6 after 10 d of anthesis. A pollen tube from the first-arriving pollen grain entered the style after 24 h and reached the base on the second day following pollination. Fertilization by nuclei from the same first-arriving pollen tube occurred after stigma necrosis about 3 wk later. Paternity tests confirmed that pollen applied first in the experimental pollinations had an overwhelming advantage over pollen applied 24 h later. As delayed fertilization does not play a role in prezygotic selection in this species, flowering phenology might be more important for paternity success than pollen tube growth performance.

Investigation of pollen competition between wild and cultivated sorghums (Sorghum bicolor (L.) Moench) using simple sequence repeats markers

In self-compatible plant species stigmata receive a mixture of self and outcrossed pollen and competition between them is expected to play a major role in determining the pollen-mediated gene flow. The use of male sterile bait plants in field trials to demonstrate the rate of gene flow is questionable due to lack of pollination competition. However, little direct evidence has been published. A field experiment of male sterile and male fertile sorghum pollen recipient bait plants was conducted to evaluate pollen competition between wild and cultivated sorghums and the effects of pollen competition on gene flow assessment. Pollen competition between wild and cultivated sorghums was estimated from two-component pollen mixtures of wild and cultivated sorghum (1:1 ratio) applied to wild, cultivated and male-sterile maternal bait plants. Paternity was determined in the progeny using two diagnostic Simple Sequence Repeat markers. The study found that self pollen has higher seed-siring success. Maternal genotype influences the siring ability of the pollen donor components which significantly deviated from the 1:1 pollen loads. The study showed that published estimates of gene flow derived from studies using male-sterile bait plants seriously overestimate gene flow and that pollen competition may be a significant factor influencing outcrossing rates. The results suggest that the predominant direction of gene flow is from cultivated to wild sorghum, potentially leading to introgression of crop genes into wild sorghum. Pollen competition should be taken into account in gene flow estimation, since presence of self-pollen can account for over half of seed produced irrespective of maternal genotype.

Why do stigmas move in a flexistylous plant?

Flexistyly is a recently documented stylar polymorphism involving both spatial and temporal segregation of sex roles within hermaphroditic flowers. Using the experimental manipulation of stigma movement in self-compatible Alpinia mutica, we tested the hypothesis that selection for reducing interference between male and female function drives the evolution and/or maintenance of stigma movement. In experimental arrays, anaflexistylous (protogynous) flowers served as pollen donors competing for mating opportunities on cataflexistylous (protandrous) flowers. The pollen donors were either manipulated so their stigmas could not move or were left intact, and their success was determined using allozymes to assess the paternity of recipient seeds. We found that manipulated flowers sired a significantly smaller proportion of seeds, showing that stigma movement in unmanipulated plants increased male fitness. This result was strongest under conditions in which pollen competition was expected to be highest, specifically when pollinators visited multiple donor plants before visiting recipient flowers.

Pollination strategies in Cretan Arum lilies

Flowers of the genus Arum are known to attract dung-breeding flies and beetles through olfactory deceit. In addition to this strategy, the genus has evolved several other pollination mechanisms. The present study aimed to characterize the pollination strategies of the Cretan Arum species by investigating the flowering phenology, thermogeny, inflorescence odours, and the pollinating fauna. The results obtained show that Arum cyrenaicum and Arum concinnatum emit a strong dung smell and exhibit the distinctive features associated with this pollination syndrome. Both species are highly thermogenic, have a similar odour profile and attract small-bodied Diptera. Although sharing the same habitat, these two plant species are never found growing sympatrically as a result of the early blooming period of A. cyrenaicum. By contrast, Arum creticum and Arum idaeum have evolved a more traditional and mutually beneficial pollination mechanism. The stinking smell has been replaced by a more flower-like odour that attracts bees (Lasioglossum sp.) and, occasionally, bugs (Dionconotus cruentatus). Although attracting the same pollinator, the main compound present in the odour of A. creticum is different from that of A. idaeum. Principal component analysis (PCA), based on physiologically active components of the flower odours determined by testing on the antenna of the Lasioglossum bee, revealed two different clusters, indicating that pollinators can potentially discriminate between the odours of the two species. A further PCA on the main floral odour volatiles as identified by gas chroatography-mass spectroscopy from all the Arum species under investigation displayed odour-based similarities and differences among the species. The PCA-gas chomotographyelectroantennographic detection active peaks analysis showed that the two species, A. creticum and A. idaeum, form two groups and are clearly separated from A. cyrenaicum and A. concinnatum, which, conversely, cluster together. The evolutionary forces and selective pressures leading to diversification of pollination mechanisms in the Cretan Arum spp. are discussed. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 991–1001. ADDITIONAL KEYWORDS: Araceae – diversification – flower constancy – flowering time – odour – pollinator competition.

Effects of multiple competitors for pollination on bumblebee foraging patterns and Mimulus ringens reproductive success

When co‐occurring plant species overlap in flowering phenology they may compete for the service of shared pollinators. Competition for pollination may lower plant reproductive success by reducing the number of pollinator probes or by decreasing the quality of pollen transport to or from a focal species. Pair‐wise interactions between plants sharing pollinators have been well documented. However, relatively few studies have examined interactions for pollination among three or more plant species, and little is known about how the outcomes and mechanisms of competition for pollination may vary with competitor species composition. To better understand how the dynamics of competition for pollination may be influenced by changes in the number of competitors, we manipulated the presence of two competitors, Lythrum salicaria and Lobelia siphilitica, and quantified reproductive success for a third species, Mimulus ringens. Patterns of pollinator preference and interspecific transitions in mixed‐species arrays were significantly influenced by the species composition of competitor plants present. Both pair‐wise and three‐species competition treatments led to a similar ∼ 40% reduction in Mimulus ringens seed set. However, the patterns of pollinator foraging we observed suggest that the relative importance of different mechanisms of competition for pollination may vary with the identity and number of competitors present. This variation in mechanisms of competition for pollination may be especially important in diverse plant communities where many species interact through shared pollinators.