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.     

Honey bees and wild pollinators differ in their preference for and use of introduced floral resources

Introduced plants may be important foraging resources for honey bees and wild pollinators, but how often and why pollinators visit introduced plants across an entire plant community is not well understood. Understanding the importance of introduced plants for pollinators could help guide management of these plants and conservation of pollinator habitat. We assessed how floral abundance and pollinator preference influence pollinator visitation rate and diversity on 30 introduced versus 24 native plants in central New York. Honey bees visited introduced and native plants at similar rates regardless of floral abundance. In contrast, as floral abundance increased, wild pollinator visitation rate decreased more strongly for introduced plants than native plants. Introduced plants as a group and native plants as a group did not differ in bee diversity or preference, but honey bees and wild pollinators preferred different plant species. As a case study, we then focused on knapweed (Centaurea spp.), an introduced plant that was the most preferred plant by honey bees, and that beekeepers value as a late‐summer foraging resource. We compared the extent to which honey bees versus wild pollinators visited knapweed relative to coflowering plants, and we quantified knapweed pollen and nectar collection by honey bees across 22 New York apiaries. Honey bees visited knapweed more frequently than coflowering plants and at a similar rate as all wild pollinators combined. All apiaries contained knapweed pollen in nectar, 86% of apiaries contained knapweed pollen in bee bread, and knapweed was sometimes a main pollen or nectar source for honey bees in late summer. Our results suggest that because of diverging responses to floral abundance and preferences for different plants, honey bees and wild pollinators differ in their use of introduced plants. Depending on the plant and its abundance, removing an introduced plant may impact honey bees more than wild pollinators.     

Local plant density,pollination and trait–fitness relationships in a perennial herb

Both differences in local plant density and phenotypic traits may affect pollination and plant reproduction, but little is known about how density affects trait–fitness relationships via changes in pollinator activity. In this study we examined how plant density and traits interact to determine pollinator behaviour and female reproductive success in the self‐incompatible, perennial herb Phyteuma spicatum. Specifically, we hypothesised that limited pollination service in more isolated plants would lead to increased selection for traits that attract pollinators. We conducted pollinator observations and assessed trait–fitness relationships in a natural population, whose individuals were surrounded by a variable number of inflorescences. Both local plant density and plant phenotypic traits affected pollinator foraging behaviour. At low densities, pollinator visitation rates were low, but increased with increasing inflorescence size, while this relationship disappeared at high densities, where visitation rates were higher. Plant fitness, in terms of seed production per plant and per capsule, was related to both floral display size and flowering time. Seed production increased with increasing inflorescence size and was highest at peak flowering. However, trait–fitness relationships were not density‐dependent, and differences in seed production did not appear to be related to differences in pollination. The reasons for this remain unclear, and additional studies are needed to fully understand and explain the observed patterns.     

Geographic variation of reproductive traits and competition for pollinators in a bird‐pollinated plant

Geographic variation in the reproductive traits of animal‐pollinated plants can be shaped by spatially variable selection imposed by differences in the local pollination environment. We investigated this process in Babiana ringens (Iridaceae), an enigmatic species from the Western Cape region of South Africa. B. ringens has evolved a specialized perch facilitating cross‐pollination by sunbirds and displays striking geographic variation in perch size and floral traits. Here, we investigate whether this variation can be explained by geographic differences in the pollinator communities. We measured floral and inflorescence traits, and abiotic variables (N, P, C, and rainfall) and made observations of sunbirds in populations spanning the range of B. ringens. In each population, we recorded sunbird species identity and measured visitation rates, interfloral pollen transfer, and whether the seed set of flowers was pollen limited. To evaluate whether competition from co‐occurring sunbird‐pollinated species might reduce visitation, we quantified nectar rewards in B. ringens and of other co‐flowering bird‐pollinated species in local communities in which populations occurred. Variation in abiotic variables was not associated with geographical variation of traits in B. ringens. Malachite sunbirds were the dominant visitor (97% of visits) and populations with larger‐sized traits exhibited higher visitation rates, more between‐flower pollen transfer and set more seed. No sunbirds were observed in four populations, all with smaller‐sized traits. Sunbird visitation to B. ringens was not associated with local sunbird activity in communities, but sunbird visitation was negatively associated with the amount of B. ringens sugar relative to the availability of alternative nectar sources. Our study provides evidence that B. ringens populations with larger floral traits are visited more frequently by sunbirds, and we propose that visitation rates to B. ringens may be influenced, in part, by competition with other sunbird‐pollinated species.     

Optimal pollinator attraction strategies in Trollius ranunculoides Hemsl. (Ranunculaceae) at different altitudes: increased floral display or promotion of nectar output?

For alpine plant species, patterns of resource allocation to functional floral traits for pollinator attraction can be highly significant in adaptation to low pollinator abundance and consequent pollen limitation. Increased pollination can be achieved either through a larger floral display or production of more pollen rewards. In this study, variation in resource allocation to different components for pollinator attraction was studied along an altitudinal gradient in Trollius ranunculoides, an obligate self‐incompatible out‐crosser of the Qinghai–Tibet Plateau. We compared resource allocation to conspicuous yellow sepals (which mainly provide visual attraction) and degenerate petals (which provide the major nectar reward) between populations at four altitudes. Furthermore, we investigated the contribution of sepals and petals to pollinator attraction and female reproductive success in an experiment with sepal or petal removal at sites at different altitudes. At the level of single flowers, resource allocation increased to sepals but decreased to petals with increasing altitude. Consistent with these results, sepals contributed much more to visitation rate and seed set than petals, as confirmed in the sepal or petal removal experiment. Sepals and petals contributed to female reproductive success by ensuring visitation rate rather than visitation duration. To alleviate increasing pollen limitation with increasing altitude, resource allocation patterns of T. ranunculoides altered to favour development of sepals rather than petals. This strategy may improve pollination and reproductive success through visual attraction (sepal) rather than nectar reward (petal) over a gradient of decreasing pollinator abundance.     

Factors affecting pollinator movement and plant fitness in a specialized pollination system

The rate of pollen exchange within and among flowers may depend on pollinator attraction traits such as floral display size and flowering plant density. Variations in these traits may influence pollinator movements, pollen receipt, and seed number. To assess how floral display size and flowering plant density affect parameters of pollinator visitation rate, pollen receipt per flower, seed number per fruit and the between-plant pollinator movements, we studied the self-incompatible plant, Nierembergia linariifolia. Per-flower pollinator visitation rate and bout length increased linearly with increasing floral display size. Pollen receipt per flower increased linearly with increasing flowering plant density. For seed number per fruit, a polynomial model describing an increased seed number per fruit at low density and a decreased seed number per fruit at high density provided a significant fit. Per-flower pollinator visitation rate was not associated with pollen receipt per flower and seed number per fruit. Bees visited plants located near to the center of the population more frequently than plants located at the periphery. Increases in both floral display size and flowering plant density led to an increased chance of a plant being chosen as the center of the pollinator foraging area. These results suggest that even though large floral displays and high flowering plant density are traits that attract more pollinators, they may also reduce potential mate diversity by restricting pollen movement to conspecific mates that are closely located.     

Sugar landscapes and pollinator‐mediated interactions in plant communities

Pollinator‐mediated interactions between plants can play an important role for the dynamics of plant communities. Pollination services depend on the abundance and the foraging behaviour of pollinators, which in turn respond to the availability and distribution of floral resources (notably nectar sugar). However, it is still insufficiently understood how the ‘sugar landscapes’ provided by flowering plant communities shape pollinator‐mediated interactions between multiple plant species and across different spatial scales. A better understanding of pollinator‐mediated interactions requires an integrative approach that quantifies different aspects of sugar landscapes and investigates their relative importance for pollinator behaviour and plant reproductive success. In this study, we quantified such sugar landscapes from individual‐based maps of Protea shrub communities in the Cape Floristic Region, South Africa. The 27 study sites of 4 ha each jointly comprise 127 993 individuals of 19 species. We analysed how rates of visitation by key bird pollinators and the seed set of plants respond to different aspects of sugar landscapes: the distribution of nectar sugar amounts, as well as their quality, taxonomic purity and phenology. We found that pollinator visitation rates strongly depended on phenological variation of site‐scale sugar amounts. The seed set of focal plants increased with nectar sugar amounts of conspecific neighbours and with site‐scale sugar amounts. Seed set increased particularly strongly if site‐scale sugar amounts were provided by plants that offer less sugar per inflorescence. These combined effects of the amount, quality, purity and phenological variation of nectar sugar show that nectar sugar is a common interaction currency that determines how multiple plant species interact via shared pollinators. The responses of pollinator‐mediated interactions to different aspects of this interaction currency alter conditions for species coexistence in Protea communities and may cause community‐level Allee effects that promote extinction cascades.     

Responses of nectar‐feeding birds to floral resources at multiple spatial scales

The responses of animal pollinators to the spatially heterogeneous distribution of floral resources are important for plant reproduction, especially in species‐rich plant communities. We explore how responses of pollinators to floral resources varied across multiple spatial scales and studied the responses of two nectarivorous bird species (Cape sugarbird Promerops cafer, orange‐breasted sunbird Anthobaphes violacea) to resource distributions provided by communities of co‐flowering Protea species (Proteaceae) in South African fynbos. We used highly resolved maps of about 125 000 Protea plants at 27 sites and estimated the seasonal dynamics of standing crop of nectar sugar for each plant to describe the spatiotemporal distribution of floral resources. We recorded avian population sizes and the rates of bird visits to > 1300 focal plants to assess the responses of nectarivorous birds to floral resources at different spatial scales. The population sizes of the two bird species responded positively to the amount of sugar resources at the site scale. Within sites, the effects of floral resources on pollinator visits to plants varied across scales and depended on the resources provided by individual plants. At large scales (radii > 25 m around focal plants), high sugar density decreased per‐plant visitation rates, i.e. plants competed for animal pollinators. At small scales (radii < 5 m around focal plants), we observed either competition or facilitation for pollinators between plants, depending on the sugar amount offered by individual focal plants. In plants with copious sugar, per‐plant visitation rates increased with increasing local sugar density, but visitation rates decreased in plants with little sugar. Our study underlines the importance of scale‐dependent responses of pollinators to floral resources and reveals that pollinators’ responses depend on the interplay between individual floral resources and local resource neighbourhood.     

Relationships between species’ floral traits and pollinator visitation in a temperate grassland

Knowledge about plant–plant interactions for pollinator service at the plant community level is still scarce, although such interactions may be important to seed production and hence the population dynamics of individual plant species and the species compositions of communities. An important step towards a better understanding of pollination interactions at the community level is to assess if the variation in floral traits among plant species explain the variation in flower visitation frequency among those species. We investigated the relative importance of various floral traits for the visitation frequency of all insects, and bumblebees and flies separately, to plant species by measuring the visitation frequency to all insect-pollinated species in a community during an entire flowering season. Visitation frequency was identified to be strongly positive related to the visual display area and the date of peak flowering of plant species. Categorical variables, such as flower form and symmetry, were important to the visitation frequency of flies only. We constructed floral similarity measures based on the species’ floral traits and found that the floral similarity for all species’ traits combined and the continuous traits separately were positively related to individual visitation frequency. On the other hand, plant species with similar categorical floral traits did not have similar visitation frequencies. In conclusion, our results show that continuous traits, such as flower size and/or density, are more important for the variation in visitation frequency among plant species than thought earlier. Furthermore, differences in visitation frequency among pollinator groups give a poor support to the expectations derived from the classical pollination syndromes.     

Pollination context alters female advantage in gynodioecious Silene vulgaris

Gynodioecy, the co‐occurrence of females and hermaphrodites, is arguably the most common angiosperm gender polymorphism in many florae. Females’ ability to invade and persist among hermaphrodites depends, in part, on pollinators providing adequate pollination to females. We directly measured diurnal and nocturnal pollinators’ contributions to female and hermaphrodite seed production in artificial populations of gynodioecious Silene vulgaris by experimentally restricting pollinator access. We found that female relative seed production in this system depended strongly on pollination context: females produced more than twice as many seeds as hermaphrodites in the context of abundant, nectar‐collecting moths. Conversely, females showed no seed production advantage in the context of pollen‐collecting syrphid flies and bees due to acutely hermaphrodite‐biased visitation. We infer that variation in pollinator type, behaviour and abundance may be important for achieving the female relative fitness thresholds necessary for the maintenance of gynodioecy. Generally, our study illustrates how pollinator‐mediated mechanisms may influence the evolution of breeding systems and associated suites of floral traits. Segments of a pollinator community may facilitate gynodioecy by selecting for plant characteristics that increase the attractiveness of both sexes to pollinators, such as nectar rewards. Conversely, discriminating visitors in search of pollen may restrict gynodioecy in associated plant lineages by reducing male steriles’ fitness below threshold levels.     

Local adaptation: Mechanical fit between floral ecotypes of Nerine humilis (Amaryllidaceae) and pollinator communities

Geographic variation in floral morphology is often assumed to reflect geographic variation in pollinator communities and associated divergence in selective pressures. We studied populations of Nerine humilis (Amaryllidaceae) to assess whether geographic variation in floral form is the result of local adaptation to different pollinator communities. We first tested for associations between floral traits and visitor communities, and found that populations with similar floral morphologies were visited by similar insect communities. Mean style length in each population was also closely associated with the mean body length of the local visitor community. A reciprocal translocation experiment demonstrated that native phenotypes set more seed than translocated phenotypes. Single visitation experiments showed that native flowers received more pollen, and set more seed per visit, than introduced phenotypes in both populations. This suggests that the effectiveness of pollinator visits is determined by the degree of mechanical fit between flowers and visitors. We provide strong evidence that the observed among‐population variation in floral traits is an adaptive response to geographic variation in the pollinator community.     

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.     

Florally rich habitats reduce insect pollination and the reproductive success of isolated plants

Landscape heterogeneity in floral communities has the potential to modify pollinator behavior. Pollinator foraging varies with the diversity, abundance, and spatial configuration of floral resources. However, the implications of this variation for pollen transfer and ultimately the reproductive success of insect pollinated plants remains unclear, especially for species which are rare or isolated in the landscape. We used a landscape‐scale experiment, coupled with microsatellite genotyping, to explore how the floral richness of habitats affected pollinator behavior and pollination effectiveness. Small arrays of the partially self‐compatible plant Californian poppy (Eschscholzia californica) were introduced across a landscape gradient to simulate rare, spatially isolated populations. The effects on pollinator activity, outcrossing, and plant reproduction were measured. In florally rich habitats, we found reduced pollen movement between plants, leading to fewer long‐distance pollination events, lower plant outcrossing, and a higher incidence of pollen limitation. This pattern indicates a potential reduction in per capita pollinator visitation, as suggested by the lower activity densities and richness of pollinators observed within florally rich habitats. In addition, seed production reduced by a factor of 1.8 in plants within florally rich habitats and progeny germination reduced by a factor of 1.2. We show this to be a consequence of self‐fertilization within the partially self‐compatible plant, E. californica. These findings indicate that locally rare plants are at a competitive disadvantage within florally rich habitats because neighboring plant species disrupt conspecific mating by co‐opting pollinators. Ultimately, this Allee effect may play an important role in determining the long‐term persistence of rarer plants in the landscape, both in terms of seed production and viability. Community context therefore requires consideration when designing and implementing conservation management for plants which are comparatively rare in the landscape.     

Pollinator visitation in populations of tristylous Eichhornia paniculata in northeastern Brazil

Summary The frequencies of floral morphs in populations of tristylous Eichhornia paniculata often deviate from the theoretical expectation of equality. This variation is associated with the breakdown of tristyly and the evolution of self-fertilization. Differences in morph frequencies could result from selection pressures due to variable levels of insect visitation to populations and contrasting foraging behavior among the floral morphs. We estimated pollinator densities in 16 populations and quantified visitation sequences to morphs in five populations of E. paniculata in northeastern Brazil. Foraging behavior among floral morphs was measured as the frequency of visits to morphs relative to their frequency in the population (preference) and number of flights between inflorescences of the same versus different morphs (constancy). Pollinator density (number/m²/minute) was not correlated with population size, plant density or morph diversity. Pollinator densities varied most among populations of less than 200 plants. Whether pollinators discriminated among the morphs, depended on whether they primarily collected nectar or pollen. In four populations, nectar-feeding bees (Ancyloscelis and Florilegus spp.) and butterflies showed no consistent preference or constancy among the morphs. In contrast, pollen-collecting bees (Trigona sp.) visited a lower proportion of longstyled inflorescences than expected and tended to visit more mid-and short-styled inflorescences in succession, once they were encountered. Pollinator constancy for morphs did not result from differences in inflorescence production or spatial patchiness among the morphs. Although non-random pollinator visitation to morphs in heterostylous populations could potentially affect mating and hence morph frequencies, the observed visitation patterns in this study do not provide evidence that pollinators play a major role in influencing floral morph frequencies.     

Variation in composition of two bumble bee species across communities affects nectar robbing but maintains pollinator visitation rate to an alpine plant,Salvia przewalskii

1. In many flowering plants, bumble bees may forage as both pollinators and nectar robbers. This mixed foraging behaviour may be influenced by community context and consequently, potentially affect pollination of the focal plant. 2. Salvia przewalskii is both pollinated and robbed exclusively by bumble bees. In the present study area, it was legitimately visited by two species of bumble bees with different tongue length, Bombus friseanus and Bombus religiosus, but it was only robbed by Bombus friseanus, the shorter‐tongued bumble bee. The intensity of nectar robbing and pollinator visitation rate to the plant were investigated across 26 communities in the Hengduan Mountains in East Himalaya during a 2‐year project. For each of these communities, the floral diversity, and the population size and floral resource of S. przewalskii were quantified. The abundances of the two bumble bee species were also recorded. 3. Both nectar robbing and pollinator visitation rate were influenced by floral diversity. However, pollinator visitation rate was not affected by nectar robbing. The results revealed that relative abundance of the two bumble bee species significantly influenced the incidence of nectar robbing but not the pollinator visitation rate. Increased abundance of B. religiosus, the legitimate visitors, exacerbated nectar robbing, possibly by causing B. friseanus to shift to robbing; however, pollinator visitation remained at a relatively high level. 4. The results may help to explain the persistence of both nectar robbing and pollination, and suggest that, in comparison to pollination, nectar robbing is a more unstable event in a community.     

Variation in floral morphology and plant reproductive success in four Ipomoea species (Convolvulaceae) with contrasting breeding systems

• This study tested the hypothesis that self‐compatibility would be associated with floral traits that facilitate autonomous self‐pollination to ensure reproduction under low pollinator visitation. In a comparison of two pairs of Ipomoea species with contrasting breeding systems, we predicted that self‐compatible (SC) species would have smaller, less variable flowers, reduced herkogamy, lower pollinator visitation and higher reproductive success than their self‐incompatible (SI) congeners.
• We studied sympatric species pairs, I. hederacea (SC)– I. mitchellae (SI) and I. purpurea (SC)–I. indica (SI), in Mexico, over two years. We quantified variation in floral traits and nectar production, documented pollinator visitation, and determined natural fruit and seed set. Hand‐pollination and bagging experiments were conducted to determine potential for autonomous self‐pollination and apomixis.
• Self‐compatible Ipomoea species had smaller flowers and lower nectar production than SI species; however, floral variation and integration did not vary according to breeding system. Bees were primary pollinators of all species, but visitation rates were seven times lower in SC than SI species. SC species had a high capacity for autonomous self‐pollination due to reduced herkogamy at the highest anther levels. Self‐compatible species had two to six times higher fruit set than SI species.
• Results generally support the hypothesis that self‐compatibility and autonomous self‐pollination ensure reproduction under low pollinator visitation. However, high variation in morphological traits of SC Ipomoea species suggests they maintain variation through outcrossing. Furthermore, reduced herkogamy was associated with high potential for autonomous self‐pollination, providing a reproductive advantage that possibly underlies transitions to self‐compatibility in Ipomoea.

     

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

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

Neighborhood‐contingent indirect interactions between native and exotic plants: multiple shared pollinators mediate reproductive success during invasions

Native and exotic plants can influence one another's fecundity through their influence on shared pollinators. Specifically, invasion may alter abundance and composition of local floral resources, affecting pollinator visitation and ultimately causing seedset of natives in more‐invaded and less‐invaded floral neighborhoods to differ. Such pollinator‐mediated effects of exotic plants on natives are common, but native and exotic plants often share multiple pollinators, which may differ in their responses to altered floral neighborhoods. We quantified pollinator‐mediated interactions between three common forbs of western Washington prairies (native Microseris laciniata and Eriophyllum lanatum and European Hypochaeris radicata) in three floral neighborhoods: 1) high native and low exotic floral density, 2) high exotic floral density and low native density, and 3) experimentally manipulated low exotic floral density. Pollinator visitation rates varied by floral neighborhood, plant species identity, and their interaction for all three plant species. Similarly, pollinator functional groups (eusocial bees, solitary bees, and syrphid flies) contributed differing proportions of total visitation to each species depending upon neighborhood context. Consequently, in exotic neighborhoods H. radicata competed with native M. laciniata, reducing seed set, while simultaneously facilitating visitation and seed set for native E. lanatum. Seed set of H. radicata was also highest in exotic neighborhoods (with high densities of conspecifics), raising the possibility of a positive feedback between exotic abundance and success. Our results suggest that the outcome of indirect interactions between native and exotic plants depends on the density and the composition of the floral neighborhood and of the pollinator fauna, and on context‐dependent pollinator foraging.     

Effects of pollen reward removal on fecundity in a self-incompatible hermaphrodite plant

The pollen of hermaphrodite plants is often utilised by flower-visiting animals. While pollen production has obvious benefits for plant male fitness, its consequences for plant female fitness, especially in self-incompatible hermaphrodite species, are less certain. Pollen production could either enhance seed production though increased pollinator attraction, or reduce it if ovules are discounted by deposition of self pollen, as can occur in species with late-acting self-incompatibility. To test the effects of pollen reward provision on female fitness, we artificially emasculated flowers in two populations of the succulent Aloe maculata (Asphodelaceae), which has a late-acting self-incompatibility system, over the course of its flowering period. Flowers of this species are visited by sunbirds (for nectar) and native bees (for pollen and nectar). We measured floral visitation rates, floral rejection rates, pollen deposition on stigmas and fruit and seed set in both emasculated and non-emasculated plants. We found that flowers of emasculated plants suffered reduced visitation and increased rejection (arrival without visitation) by bees, but not by sunbirds; had fewer pollen grains deposited on stigmas and showed an overall decrease in fruit set and seed set. Rates of seed abortion were, however, greatly reduced in emasculated flowers. This study shows that pollen rewards can be important for seed set, even in self-incompatible plants, which have been assumed to rely on nectar rewards for pollinator attraction. Seed abortion was, however, increased by pollen production, a result that highlights the complexity of selection on pollen production in hermaphrodite flowers.     

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

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