POLLEN DISPERSAL DYNAMICS IN AN ALPINE WILDFLOWER,POLEMONIUM VISCOSUM

The alpine wildflower, Polemonium viscosum, depends on insect visitors for effective pollination. Here, I examine experimentally the effects of pollinator visitation on pollen removal, pollen dispersal success, paternity, and gene flow. Bumble bee pollinators visited donor individuals homozygous for marker alleles at an isozyme (GOT-2) encoding locus and then were presented with arrays of recipient plants lacking the marker alleles. Four aspects of male fitness were estimated for each donor: the number of pollen grains dispersed to flowers of the first recipient visited, the number of offspring sired on that recipient, the proportion of offspring sired in the full array, and the proportion of mates in the array bearing seeds of the donor. Pollen removal was strongly influenced by the number of bee visits to donor flowers. The amount of pollen removed in turn significantly affected the number of pollen grains reaching flowers of the first recipient. However, because seed production decelerates with stigma pollen load, the relationship between pollen export and paternal success at this proximate scale showed diminishing returns. The probability of reaching mates within the array also increased with pollen export. These findings show that floral characters enhancing pollinator visitation rate in P. viscosum have positive effects on paternity and gene flow.     

CONSEQUENCES OF FLORAL VARIATION FOR MALE AND FEMALE REPRODUCTION IN EXPERIMENTAL POPULATIONS OF WILD RADISH,RAPHANUS SATIVUS L

We documented effects of floral variation on seed paternity and maternal fecundity in a series of small experimental populations of wild radish, R. sativus. Each population was composed of two competing pollen donor groups with contrasting floral morphologies and several designated maternal plants. Progeny testing with electrophoretic markers allowed us to measure paternal success. Realized fecundity by each maternal plant and the fraction of those seeds attributable to each pollen donor group were used as outcome variables in path analysis to explore relationships between floral characters (petal size, pollen grain number per flower, and modal pollen grain size), pollinator visitation patterns, and reproductive success. A wide range of pollinator taxa visited the experimental populations, and patterns of discrimination appeared to vary among them. The impact of visitation on male and female reproduction also varied among taxa; visits of small native bees significantly increased paternal success, while those of honey bees reduced male fitness. Only visits by large native bees had discernible effects on recipient fecundity, and, overall, fecundity was not limited by visitation. Maternal plants bearing large-petalled flowers produced fewer flowers during the experiment, reducing their total seed production. In these small populations, postpollination processes (at least in part, compatibility) significantly influenced male and female reproductive success. Variation in pollinator pools occurring on both spatial and temporal scales may act to preserve genetic variation for floral traits in this species.     

Behavioral responses by bumble bees to variation in pollen availability

Summary Pollen-collecting bumble bees (Bombus spp.) detect differences between individual flowers in pollen availability and alter their behavior to capitalize on rewarding flowers. Specific responses by bees to increased pollen availability included: longer visits to flowers; visits to more flowers within an inflorescence, including an increased frequency of revisits; an increased likelihood of grooming while the bee flow between flowers within the inflorescence; and more protracted inter-flower flights, probably because of longer grooming bouts. The particular suite of responses that a bee adopted depended on the pollen-dispensing mechanism of the plant species involved. Bees buzzed previously-unvisited Dode-catheon flowers longer than empty flowers. In contrast, pollen availability did not significantly affect the duration of visits to Lupinus flowers, which control the amount of pollen that can be removed during a single visit. Simulation results indicate that the observed movement patterns of bumble bees on Lupinus inflorescences would return the most pollen per unit of expended energy. The increased foraging efficiency resulting from facultative responses by bees to variation in pollen availability, especially changes in the frequency and intensity of grooming, could correspondingly decrease pollen dispersal between plants.     

Experimental studies of pollen carryover: Hummingbirds and Ipomopsis aggregata

Summary We present results of experiments designed to identify floral characteristics that influence patterns of pollen carryover by hummingbirds visiting Ipomopsis aggregata flowers. We used fluorescent dye powders as pollen analogues. For all four experimental treatments considered, amounts of dye deposited on recipient stigmas declined linearly as a function of flower position in a visitation sequence. The decline was significantly steeper when recipient flowers had pollen-carrying anthers than when they did not; whereas degree of stigma clogging and presence or absence of empty anthers did not influence carryover. From this we conclude that presence of pollen on recipient flowers significantly reduces the average number of subsequent flowers reached by donor pollen. We discuss mechanisms for this effect and its significance for the evolution of floral structure.     

Pollinator parasites and the evolution of floral traits

The main selective force driving floral evolution and diversity is plant–pollinator interactions. Pollinators use floral signals and indirect cues to assess flower reward, and the ensuing flower choice has major implications for plant fitness. While many pollinator behaviors have been described, the impact of parasites on pollinator foraging decisions and plant–pollinator interactions have been largely overlooked. Growing evidence of the transmission of parasites through the shared‐use of flowers by pollinators demonstrate the importance of behavioral immunity (altered behaviors that enhance parasite resistance) to pollinator health. During foraging bouts, pollinators can protect themselves against parasites through self‐medication, disease avoidance, and grooming. Recent studies have documented immune behaviors in foraging pollinators, as well as the impacts of such behaviors on flower visitation. Because pollinator parasites can affect flower choice and pollen dispersal, they may ultimately impact flower fitness. Here, we discuss how pollinator immune behaviors and floral traits may affect the presence and transmission of pollinator parasites, as well as how pollinator parasites, through these immune behaviors, can impact plant–pollinator interactions. We further discuss how pollinator immune behaviors can impact plant fitness, and how floral traits may adapt to optimize plant fitness in response to pollinator parasites. We propose future research directions to assess the role of pollinator parasites in plant–pollinator interactions and evolution, and we propose better integration of the role of pollinator parasites into research related to pollinator optimal foraging theory, floral diversity and agricultural practices.     

Pollen consumption by flower mites in three hummingbird-pollinated plant species

Laboratory studies suggest that pollen consumption by flower mites may decrease the male fitness of the plant by reducing the available pollen for dispersal. Here we assessed pollen consumption by flower mites under natural conditions in three plant species with long-lived, protandrous flowers, Moussonia deppeana (Gesneriaceae), Lobelia laxiflora and L. cardinalis (Lobeliaceae). Total pollen mass was measured after 24 and 48 h in flowers exposed to flower mites and excluded from hummingbirds, flowers exposed to mites and hummingbird visitation, and in flowers recently opened with dehisced anthers. Compared with recently opened flowers, pollen availability was reduced about half in the presence of flower mites and the same effect was observed in the three plant species. Our results suggest that flower mites are removing a great deal of pollen and the reduction of pollen implies the possibility of direct impact on pollen transfer.     

Ants in your plants: effects of nectar-thieves on pollen fertility and seed-siring capacity in the alpine wildflower, Polemonium viscosum

Flowers of the alpine skypilot, Polemonium viscosum , are attacked by nectar thieving ants of Formica neorufibarbus gelida . Ants exert selection on flower scent, size and shape in skypilots by damaging the pistils. Here, I report on the frequency and nature of contact between ants and pollen-bearing anthers and determine the consequences of such contact for pollen performance and pollen donor paternity. In laboratory trials, ants entered flowers with full intact anthers and emasculated (female) flowers equivalently. Similarly, flower visitation rates of ants foraging naturally were not affected by the frequency of male phase flowers per plant. Ants actively interacted with the pollen-bearing anthers during 21% of flower visits, on average. The rate at which such interactions occurred was predicted by the proportion of flowers in the male phase, under a random foraging model. The effect of ants on pollen fertility was tested experimentally by enclosing ants in male-phase flowers on intact inflorescences. Adjacent control flowers were left un-occupied. Pollen from flowers with a history of ant occupancy had significantly lower germination on virgin recipient stigmas than pollen from unoccupied control flowers. With hand-pollination, sufficient pollen was transferred from ant-occupied flowers to saturate seed set. However, a model based on the relationship between seed set and compatible pollen delivery by natural pollinators indicated that ant damage to pollen should reduce paternity accruing per flower visit by 20–26% on average, in nature. Results support the hypothesis that in P. viscosum , selection on floral traits by nectar-thieving ants operates through male as well as female function.     

Multiple pollinator visits to Mimulus ringens (Phrymaceae) flowers increase mate number and seed set within fruits

The timing and effectiveness of pollinator visitation to flowers is an important factor influencing mating patterns and reproductive success. Multiple pollinator probes to a flower may increase both the quantity and genetic diversity of progeny, especially if single probes deposit insufficient pollen for maximal seed set or if the interval between probes is brief. When pollen carryover is limited, sequential pollen loads may also differ markedly in sire representation. We hypothesized that these conditions help explain high levels of multiple paternity in Mimulus ringens fruits. We documented all bee visits to individual flowers, quantified resulting seed set, and determined paternity for 20 seeds per fruit. Most (76%) flowers received multiple probes, and the interval between probes was usually <30 min. Flowers probed multiple times produced 44% more seeds than flowers probed once. All fruits were multiply sired. Flowers receiving a single probe averaged 3.12 outcross sires per fruit, indicating that single probes deposit pollen from several donors. Multiple paternity was even greater after three or more probes (4.92 outcross sires), demonstrating that sequential visits bring pollen from donors not represented in the initial probe.     

PATERNITY ANALYSIS IN A NATURAL POPULATION OF ASCLEPIAS EXALTATA: MULTIPLE PATERNITY,FUNCTIONAL GENDER,AND THE “POLLEN-DONATION HYPOTHESIS”

Recently, some evolutionary biologists have argued that selection on the male component of fitness shapes the evolution of reproductive characters in angiosperms. Floral features, such as inflorescence size, that lead to increased insect visitation without a concomitant increase in seed production are viewed as adaptations to enhance the probability of fathering seeds on other plants. In tests of this “pollen donation hypothesis,” male reproductive success has usually been measured indirectly by flower production, pollinator visitation, or pollen removal. We tested the pollen donation hypothesis directly by quantifying the number of seeds sired by individual genotypes in a natural population of poke milkweed, Asclepias exaltata, in southwestern Virginia. Multiple paternity was low within fruits, a fact which allowed us to use genotypes of progeny arrays to identify a unique pollen parent for 85% of the fruits produced in the population. Seeds sired (male success) and seeds produced (female success) were significantly correlated with flower number per plant (for male success, r = 0.32, P > 0.05; for female success, r = 0.66, P > 0.001). While the number of pollinaria removed, the usual estimator of male success in milkweeds, was highly correlated with numbers of seeds sired (r = 0.47; P > 0.001), it was even more highly correlated with numbers of seeds produced (r = 0.71, P > 0.001). Analysis of functional gender indicated that plants with many flowers did not behave primarily as males. In fact, individuals with the highest total reproductive success contributed equally as males and females. Furthermore, estimates of gender based on numbers of flowers produced or pollinaria removed overestimated the number of functional males in the population. In pollen-limited species, such as many milkweeds, proportional increases in both male and female reproductive success indicate the potential for selection to shape the evolution of large floral displays through both male and female functions.     

Evaluating pollen flow indicators for an insect-pollinated plant species

For insect-pollinated plant species, reproductive success and genetic exchange via the transfer of pollen between flowers depends (i.a.) on the efficiency, abundance and behaviour of floral visitors. These in turn are expected to respond to plant population size and flower density. High floral densities for example usually attract large numbers of pollinators that visit more flowers per plant or patch, which increases pollen deposition at short distances. Thus, population characteristics might serve as indicators for pollen dispersal patterns and help to identify suitable habitat size and quality for conservation measures. To test this hypothesis, we observed floral visitors of a generalist, entomophilous species, Comarum palustre, and compared their abundance and visitation rates in populations of different sizes and flower densities. At the same time, we mimicked pollen flow using fluorescent dye. In the large and dense populations, pollinator abundance and visitation rates were high and dye was dispersed to the edges of the populations (up to 200 m). In the medium-sized population with high flower density, insect abundance and visitation rates were unexpectedly low and dye dispersal declined very quickly. On the contrary, in the smallest population with scattered flowers, especially bumble bee abundance was similar to the large populations and dye dispersal mirrored this high bumble bee activity. Thus, our results indicate that in smaller habitat fragments, the mere size of a population might be insufficient to suggest pollen flow for a plant species. Instead, the abundance of its major pollinators should be considered.     

EFFECTS OF SOIL NITROGEN ON POLLEN PRODUCTION,POLLEN GRAIN SIZE,AND POLLEN PERFORMANCE IN CUCURBITA PEPO (CUCURBITACEAE)

To determine the effects of soil nitrogen on pollen production, pollen size, and pollen performance, two cultivars of zucchini (Cucurbita pepo) were grown under two nitrogen regimes in an experimental garden. The two cultivars were true breeding for alternative alleles for a one gene trait, ovary color. The soil nitrogen treatment had a significant effect on most measures of reproductive output through the female function. The nitrogen treatment did not affect the number of staminate flowers per plant but did have an effect on the number of pollen grains per staminate flower and the mean pollen grain size. A pollen mixture experiment revealed that pollen produced by plants in the high nitrogen treatment sired significantly more seeds than pollen from low nitrogen plants. Moreover, we found that the high nitrogen pollen sired even a greater percentage of seeds in the region of the fruit (ovary) previously shown to be fertilized by the fastest growing pollen tubes. Thus, the difference in the number of seeds sired by pollen from the two nitrogen treatments is due to differences in pollen performance. We conclude that spatial heterogeneity in soil nitrogen can influence the paternity of seeds in a plant population.     

POLLEN-TUBE COMPETITION AND MALE FITNESS IN HIBISCUS MOSCHEUTOS

The stigmas of animal-pollinated flowers often capture more pollen than is needed to fertilize all available ovules, and mixed-donor pollen loads are probably common. When this is the case, variation in average pollen-tube growth rates can potentially affect the number of seeds sired by a given plant. Despite considerable interest in effects of postpollination processes on male fitness, little is known about the extent of variation in pollen performance among plants from natural populations. To examine this question in Hibiscus moscheutos (rose mallow), we conducted mixed-donor hand-pollination experiments with 39 pollen donors bearing distinctive isozyme markers. Pairs of competing donors were compared on sets of 11 to 15 recipient plants per pair. These donors often differed in the proportions of seeds they sired, with the maximum deviation from an expected ratio of 50:50 being 68:32. Furthermore, three intensively studied plants exhibited consistent trends in relative pollen performance when each was tested against (1) the same three competitors, and (2) groups of 14 competitors chosen at random from the study population. In a separate experiment, we investigated the effects of salinity stress and high soil nutrients on pollen performance. These environmental factors had anticipated effects on leaf production, flower production, and petal length, but style length and (most importantly) the number of seeds sired relative to a standard pollen donor were not affected. In summary, this study provides the strongest evidence to date that pollen-tube competitive ability varies among coexisting plants and may be an important component of male fitness in plants.     

VARIATION IN POLLEN SIZE,FERTILIZATION ABILITY,AND POSTFERTILIZATION SIRING ABILITY IN ERYTHRONIUM GRANDIFLORUM

The mean volume of pollen grains and total pollen production varied both within and among plants of Erythronium grandiflorum. The second flowers of two-flowered plants tended to produce smaller and fewer grains than first flowers, but there was no overall relationship between mean pollen grain size and production per flower. I evaluated the effects of pollen size differences within and among plants on two components of male reproductive success: pollen tube growth and postfertilization siring ability. Pollen tubes grown in media were longer for second flowers, but were not correlated with the mean size of pollen grains, suggesting that (1) internal resource content of pollen (i.e., carbohydrates plus lipids) was not associated with the hydrated size of pollen, and that (2) pollen from second flowers contained more resources. I analyzed the growth rate and the fertilization ability of pollen growing in styles. Growth rate differed among donors and recipients, but no effects of pollen or donor characters (i.e., pollen production, grain size, and flower position) were detected. In single donor pollinations, pollen size was negatively correlated with fertilization ability across donors, and positively correlated with postfertilization siring ability of donors. A second experiment used pairs of donors; within-plant differences in pollen size and flower position had effects similar to the single donor experiment on fertilization ability, but among-plant differences were not significant. The results corroborate earlier experiments that suggest that the growth of pollen tubes in the style is probably controlled by the recipient, since donor characters had minimal effects on pollen fertilization ability. Postfertilization siring ability was not affected by within-plant differences in mean grain size and production. For among-donor differences, the number of seeds set for each donor was positively correlated with the mean grain volume, and when a donor producing large pollen fertilized ovules in an ovary, there was increased seed abortion for seeds in the same ovary sired by a second donor. In addition, the total number of seeds produced by a fruit was decreased when both donors had large pollen, apparently due to increased postfertilization abortion. Postfertilization processes appear to be influenced by paternal differences that are expressed through competition among developing seeds for maternal resources.     

BUMBLE BEE POLLINATION AND FLORAL MORPHOLOGY: FACTORS INFLUENCING POLLEN DISPERSAL IN THE ALPINE SKY PILOT,POLEMONIUM VISCOSUM (POLEMONIACEAE)

Pollen dispersal success in entomophilous plants is influenced by the amount of pollen produced per flower, the fraction of pollen that is exported to other flowers during a pollinator visit, visitation frequency, and the complementarity between pollen donor and recipients. For bumble bee-pollinated Polemonium viscosum the first three determinants of male function are correlated with morphometric floral traits. Pollen production is positively related to corolla and style length, whereas pollen removal per visit by bumble bee pollinators is a positive function of corolla flare. Larger-flowered plants receive more bumble bee visits than small-flowered individuals. We found no evidence of tradeoffs between pollen export efficiency and per visit accumulation of outcross pollen; each was influenced by unique aspects of flower morphology. Individual queen bumble bees of the principal pollinator species, Bombus kirbyellus, were similar in male, female, and absolute measures of pollination effectiveness. An estimated 2.9% of the pollen that bumble bees removed from flowers during a foraging bout was, on average, deposited on stigmas of compatible recipients. Significant plant-to-plant differences in pollen production, pollen export per visit, and outcross pollen receipt were found for co-occurring individuals of P. viscosum indicating that variation in these fitness related traits can be seen by pollinator-mediated selection.     

ADAPTIVE SIGNIFICANCE OF FLOWER COLOR AND INTER-TRAIT CORRELATIONS IN AN IPOMOPSIS HYBRID ZONE

Flower color is often viewed as a trait that signals rewards to pollinators, such that the relationship between flower color and plant fitness might result from its association with another trait. We used experimental manipulations of flower color and nectar reward to dissociate the natural character correlations present in a hybrid zone between Ipomopsis aggregata and Ipomopsis tenuituba. Isozyme markers were used to follow the male and female reproductive success of these engineered phenotypes. One field experiment compared fitnesses of I. aggregata plants that varied only in flower color. Plants with flowers painted red received more hummingbird visits and sired more seeds than did plants with flowers painted pink or white to match those of hybrids and I. tenuituba. Our second field experiment compared fitnesses of I. aggregata, I. tenuituba, and hybrid plants in an unmanipulated array and in a second array where all flowers were painted red. In the unmanipulated array, I. aggregata received more hummingbird visits, set more seeds per flower, and sired more seeds per flower. These fitness differences largely disappeared when the color differences were eliminated. The higher male fitness of I. aggregata was due to its very high success at siring seeds on conspecific recipients. On both I. tenuituba and hybrid recipients, hybrid plants sired the most seeds, despite showing lower pollen fertility than I. aggregata in mixed donor pollinations in the greenhouse. Ipomopsis tenuituba had a fitness of only 13% relative to I. aggregata when traits varied naturally, compared to a fitness of 36% for white relative to red flowers when other traits were held constant.     

Relative impact of mate versus pollinator availability on pollen limitation and outcrossing rates in a mass‐flowering species

Plant mating systems are driven by several pre‐pollination factors, including pollinator availability, mate availability and reproductive traits. We investigated the relative contributions of these factors to pollination and to realized outcrossing rates in the patchily distributed mass‐flowering shrub Rhododendron ferrugineum. We jointly monitored pollen limitation (comparing seed set from intact and pollen‐supplemented flowers), reproductive traits (herkogamy, flower size and autofertility) and mating patterns (progeny array analysis) in 28 natural patches varying in the level of pollinator availability (flower visitation rates) and of mate availability (patch floral display estimated as the total number of inflorescences per patch). Our results showed that patch floral display was the strongest determinant of pollination and of the realized outcrossing rates in this mass‐flowering species. We found an increase in pollen limitation and in outcrossing rates with increasing patch floral display. Reproductive traits were not significantly related to patch floral display, while autofertility was negatively correlated to outcrossing rates. These findings suggest that mate limitation, arising from high flower visitation rates in small plant patches, resulted in low pollen limitation and high selfing rates, while pollinator limitation, arising from low flower visitation rates in large plant patches, resulted in higher pollen limitation and outcrossing rates. Pollinator‐mediated selfing and geitonogamy likely alleviates pollen limitation in the case of reduced mate availability, while reduced pollinator availability (intraspecific competition for pollinator services) may result in the maintenance of high outcrossing rates despite reduced seed production.     

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.     

Nectar secretion dynamic links pollinator behavior to consequences for plant reproductive success in the ornithophilous mistletoe Psittacanthus robustus

The mistletoe Psittacanthus robustus was studied as a model to link flower phenology and nectar secretion strategy to pollinator behaviour and the reproductive consequences for the plant. The bright‐coloured flowers presented diurnal anthesis, opened asynchronously throughout the rainy season and produced copious dilute nectar as the main reward for pollinators. Most nectar was secreted just after flower opening, with little sugar replenishment after experimental removals. During the second day of anthesis in bagged flowers, the flowers quickly reabsorbed the offered nectar. Low values of nectar standing crop recorded in open flowers can be linked with high visitation rates by bird pollinators. Eight hummingbirds and two passerines were observed as potential pollinators. The most frequent flower visitors were the hummingbirds Eupetomena macroura and Colibri serrirostris, which actively defended flowering mistletoes. The spatial separation between anthers, stigma and nectar chamber promotes pollen deposition on flapping wings of hovering hummingbirds that usually probe many flowers per visit. Seed set did not differ between hand‐, self‐ and cross‐pollinated flowers, but these treatments set significantly more seeds than flowers naturally exposed to flower visitors. We suggest that the limitation observed in the reproductive success of this plant is not related to pollinator scarcity, but probably to the extreme frequency of visitation by territorial hummingbirds. We conclude that the costs and benefits of plant reproduction depend on the interaction strength between flowers and pollinators, and the assessment of nectar secretion dynamics, pollinator behaviour and plant breeding system allows clarification of the complexity of such associations.     

Intra‐floral resource partitioning between endemic and invasive flower visitors: consequences for pollinator effectiveness

1. Sympatric flower visitor species often partition nectar and pollen and thus affect each other's foraging pattern. Consequently, their pollination service may also be influenced by the presence of other flower visiting species. Ants are solely interested in nectar and frequent flower visitors of some plant species but usually provide no pollination service. Obligate flower visitors such as bees depend on both nectar and pollen and are often more effective pollinators. 2. In Hawaii, we studied the complex interactions between flowers of the endemic tree Metrosideros polymorpha (Myrtaceae) and both, endemic and introduced flower‐visiting insects. The former main‐pollinators of M. polymorpha were birds, which, however, became rare. We evaluated the pollinator effectiveness of endemic and invasive bees and whether it is affected by the type of resource collected and the presence of ants on flowers. 3. Ants were dominant nectar‐consumers that mostly depleted the nectar of visited inflorescences. Accordingly, the visitation frequency, duration, and consequently the pollinator effectiveness of nectar‐foraging honeybees (Apis mellifera) strongly decreased on ant‐visited flowers, whereas pollen‐collecting bees remained largely unaffected by ants. Overall, endemic bees (Hylaeus spp.) were ineffective pollinators. 4. The average net effect of ants on pollination of M. polymorpha was neutral, corresponding to a similar fruit set of ant‐visited and ant‐free inflorescences. 5. Our results suggest that invasive social hymenopterans that often have negative impacts on the Hawaiian flora and fauna may occasionally provide neutral (ants) or even beneficial net effects (honeybees), especially in the absence of native birds.     

Relationships among nectar‐dwelling yeasts,flowers and ants: patterns and incidence on nectar traits

Nectar‐dwelling yeasts are emerging as widely distributed organisms playing a potentially significant and barely unexplored ecological role in plant pollinator mutualisms. Previous efforts at understanding nectar–pollinator–yeast interactions have focused on bee‐pollinated plants, while the importance of nectarivorous ants as vectors for yeast dispersal remains unexplored so far. Here we assess the abundance and composition of the nectar fungal microbiota of the ant‐pollinated plant Cytinus hypocistis, study whether yeast transmission is coupled with ant visitation, and discern whether ant‐ transported yeasts promote changes in nectar characteristics. Our results show that a high percentage of flowers (77%) and plants (94%) contained yeasts, with yeast cell density in nectar reaching up to 6.2 × 10⁴ cells mm^?3, being the highest densities associated with the presence of the nectar‐specialist yeast Metschnikowia reukaufii. The establishment of fungal microbiota in nectar required flower visitation by ants, with 70% of yeast species transported by them being also detected in nectar. Ant‐vectored yeasts diminished the nutritional quality of nectar, with flowers exposed to pollinators and yeasts containing significantly lower nectar sugar concentration than virgin flowers (13.4% and 22.8%, respectively). Nectar of flowers that harbored M. reukaufii showed the lowest quality, with nectar concentration declining significantly with increasing yeast density. Additionally, yeasts modified patterns of interpopulation variation in nectar traits, homo genizing differences between populations in some nectar attributes. We show for the first time that the outcome of the tripartite pollinator–flower–yeast interaction is highly dependent on the identity and inherent properties of the participants, even to the extent of influencing the species composition of this ternary system, and can be mediated by ecological characteristics of plant populations. Through their influence on plant functional traits, yeasts have the potential to alter nectar consumption, pollinator foraging behavior and ultimately plant reproduction.