The relative importance of birds and bees in the pollination of Metrosideros excelsa (Myrtaceae)

Exclusion experiments were used to assess the effect of different pollinator groups on outcrossing and seed production in Metrosideros excelsa. The main study site was Little Barrier Island, New Zealand where indigenous bird and native solitary bees are the main flower visitors. Our results showed that native birds were more important pollinators of M. excelsa than native bees. Seed production was much higher in open pollination than in two exclusion experiments where either birds were excluded and native bees only had access to flowers, or where all pollinators had been excluded. The number of fertile seeds per capsule was 45% higher after open pollination than in treatments with bee visitation only and 28% higher than in treatments where all flower visitors were excluded. Estimated outcrossing rates were significantly higher (t_m = 0.71) for open pollination in the upper canopy (>4 m above‐ground level) where bird visitation is presumed to be more frequent than for a treatment with native bee access only (t_m = 0.40). Our results also suggest that a large proportion of seeds (66%) arise from autonomous self‐pollination when all pollinators are excluded. In four trees of a modified mainland population with predominantly introduced birds and a mixture of introduced and native bees there was no decrease in seed production for the treatment allowing bee access only, indicating that – in contrast to native bees – honeybees may be more efficient pollinators of M. excelsa. Observation of the foraging behaviour of both groups of bees showed that native bees contact the stigma of flowers less frequently than honeybees. This is likely to be a consequence of their smaller body size relative to honeybees.     

Floral size variation in Campanula rotundifolia (Campanulaceae) along altitudinal gradients: patterns and possible selective mechanisms

We investigated patterns of flower‐size variation along altitudinal gradients in the bee‐pollinated perennial Campanula rotundifolia (Campanulaceae) by examining 22 Norwegian populations at altitudes between 240 and 1100 m a.s.l. We explored potential mechanisms for the underlying pattern by quantifying pollinator–faunal composition, pollinator‐visitation rates and pollen limitation of seed set in subsets of the study populations. Despite a decrease in plant size, several measures of flower size increased with elevation. Bumble bees were the main pollinators at both alpine and lowland sites in the study area. However, species composition of the pollinator fauna differed, and pollinators were larger in higher‐elevation than in lower‐elevation sites. Pollinator visitation rates were lower at higher‐elevations than at lower elevations. Pollen limitation of seed set did not vary significantly with altitude. Our results are consistent with differences in bumble‐bee size and visitation rates as causal mechanisms for the relatively larger flowers at higher elevations, in three non‐mutually exclusive ways: 1) Larger flowers reflect selection for increased attractiveness where pollinators are rare. 2) Larger and fewer flowers represent a risk avoidance strategy where the probability of pollination is low on any given day. 3) Flower size variation reflects selection to improve the fit of pollinators with fertile structures by matching flower size to pollinator size across sites.     

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.

     

Pollination efficiency of cockroaches and other floral visitors of Clusia blattophila

• Cockroaches have rarely been documented as pollinators. In this paper we examine whether this is because they might be inefficient at pollination compared to other pollinators. Clusia blattophila, a dioecious shrub growing on isolated rocky outcrops in French Guiana, is pollinated by Amazonina platystylata cockroaches and provides a valuable system for the study of cockroach pollination efficiency.
• We examined the species composition of the visitor guild and visitation rates by means of camcorder recordings and visitor sampling. Then, we investigated the capacity for pollen transfer of principal visitors and found correlations between visitation rates and pollen loads on stigmas. In an exclusion experiment we determined the contributions of individual species to pollination success.
• Amazonina platystylata, crickets and two species of Diptera transferred pollen, but the number of transferred pollen grains was only related to visitation rates in the case of cockroaches. Crickets visited and rarely carried pollen. Dipterans were as frequent as cockroaches, carried similar pollen loads, but transferred much less pollen. An estimated 41% and 17% of ovules were pollinated by cockroaches and dipterans, respectively. The remaining ovules were not pollinated. There was no spatial variation in pollinator guild composition, but cockroaches visited flowers less frequently at the smaller study site.
• We demonstrate that cockroaches pollinate a large proportion of ovules. Their pollination service is not confined to one study site and, unlike that provided by dipterans, is not limited to certain years. We suggest that cockroach pollination has been overlooked and that cockroach‐pollinated plants, which share certain floral features, possess adaptations to pollination by cockroaches.

     

Seasonal and annual variations in the pollination efficiency of a pollinator community of Dictamnus albus L.

The interplay between insect and plant traits outlines the patterns of pollen transfer and the subsequent plant reproductive fitness. We studied the factors that affect the pollination efficiency of a pollinator community of Dictamnus albus L. by evaluating insect behaviour and morphological characteristics in relation to flowering phenology. In order to extrapolate the pollinator importance of single taxa and of the whole pollinator guild, we calculated an index distinguishing between potential (PPI) and realized (RPI) pollinator importance. Although the pollinator species spectrum appeared rather constant, we found high intra‐ and inter‐annual variability of pollinator frequency and importance within the insect community. Flower visitation rate strictly depended on insect abundance and on the overlap between their flying period and flower blooming. All the pollinators visited flowers from the bottom to the top of the racemes, excluding intra‐plant geitonogamous pollination, and most of them showed high pollen fidelity. Only medium large‐sized bees could contact the upward bending stiles while feeding on nectar, highlighting a specialisation of the plant towards bigger pollinators. Moreover, we found evidence of functional specialisation, since all pollinators were restricted to a single taxonomic group (order: Hymenoptera; superfamily: Apoidea). Both the PPI and RPI indices indicate Habropoda tarsata as the most important pollinator of D. albus. Following hand cross‐pollination experiments we revealed the presence of pollination limitation in 1 of the 3 years of field study. We discuss this result in relation to flowering abundance and to possible mismatches of phenological periods between plants and insects.     

Flower specialisation: the occluded corolla of snapdragons (Antirrhinum) exhibits two pollinator niches of large long‐tongued bees

• Flower specialisation of angiosperms includes the occluded corollas of snapdragons (Antirrhinum and some relatives), which have been postulated to be one of the most efficient structures to physical limit access to pollinators. The Iberian Peninsula harbours the highest number of species (18 Iberian of the 20 species of Antirrhinum) that potentially share similar pollinator fauna.
• Crossing experiments with 18 Iberian species from this study and literature revealed a general pattern of self‐incompatibility (SI) – failure in this SI system has been also observed in a few plants – which indicates the need for pollinator agents in Antirrhinum pollination.
• Field surveys in natural conditions (304 h) found flower visitation (>85%) almost exclusively by 11 species of bee (Anthophora fulvitarsis, Anthophora plumipes, Anthidium sticticum, Apis mellifera, Bombus hortorum, Bombus pascuorum, Bombus ruderatus, Bombus terrestris, Chalicodoma lefebvrei, Chalicodoma pyrenaica and Xylocopa violacea). This result covering the majority of Antirrhinum species suggests that large bees of the two long‐tongued bee families (Megachilidae, Apidae) are the major pollinators of Antirrhinum.
• A bipartite modularity analysis revealed two pollinator systems of long‐tongued bees: (i) the long‐studied system of bumblebees (Bombus spp.) associated with nine primarily northern species of Antirrhinum; and (ii) a newly proposed pollinator system involving other large bees associated with seven species primarily distributed in southern Mediterranean areas.

     

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.     

Pollination of Campomanesia phaea (Myrtaceae) by night‐active bees: a new nocturnal pollination system mediated by floral scent

• Bees are the most important diurnal pollinators of angiosperms. In several groups of bees a nocturnal/crepuscular habit developed, yet little is known about their role in pollination and whether some plants are adapted specifically to these bees. We used a multidisciplinary approach to investigate the reproductive biology and to understand the role of nocturnal/crepuscular bees in pollination of Campomanesia phaea (Myrtaceae), popularly named cambuci.
• We studied the floral biology and breeding system of C. phaea. We collected the floral visitors and tested the pollinators' effectiveness. We also determined the floral scents released at night and during daytime, and studied behavioural responses of crepuscular/nocturnal bees towards these scents.
• The flowers of cambuci were self‐incompatible and had pollen as the only resource for flower visitors. Anthesis lasted around 14 h, beginning at 04:30 h at night. The flowers released 14 volatile compounds, mainly aliphatic and aromatic compounds. We collected 52 species of floral visitors, mainly bees. Nocturnal and crepuscular bees (four species) were among the most frequent species and the only effective pollinators. In field bioassays performed at night, nocturnal/crepuscular bees were attracted by a synthetic scent blend consisting of the six most abundant compounds.
• This study describes the first scent‐mediated pollination system between a plant and its nocturnal bee pollinators. Further, C. phaea has several floral traits that do not allow classification into other nocturnal pollination syndromes (e.g. pollinator attraction already before sunrise, with pollen as the only reward), instead it is a plant specifically adapted to nocturnal bees.

     

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.     

Does intraspecific behavioural variation of pollinator species influence pollination? A quantitative study with hummingbirds and a Neotropical shrub

• Floral visitors differ in their efficacy as pollinators, and the impact of different pollinator species on pollen flow and plant reproduction has been frequently evaluated. In contrast, the impact of intraspecific behavioural changes on their efficacy as pollinators has seldom been quantified.
• We studied a self‐incompatible shrub Palicourea rigida (Rubiaceae) and its hummingbird pollinators, which adjust their behaviour according to floral resource availability. Fluorescence microscopy was used to access pollen tube growth and incompatibility reaction in pistils after a single visit of territorial or intruder hummingbirds in two populations. To characterise the plant populations and possible differences in resource availability between areas we used a three‐term quadrat variance method to detect clusters of floral resources.
• Within‐species variation in foraging behaviour, but not species identity, affected pollinator efficacy. Effectively, hummingbirds intruding into territories deposited more compatible pollen grains on P. rigida stigmas than territory holders in both study areas. Additionally, territory holders deposited more incompatible than compatible pollen grains.
• Our results imply that intraspecific foraging behaviour variation has consequences for pollination success. Quantifying such variation and addressing the implications of intraspecific variability contribute to a better understanding of the dynamics and consequences of plant–pollinator interactions.

     

Comparative reproductive biology and pollinator specificity among sympatric Gomesa (Orchidaceae: Oncidiinae)

• The incredible pollination mechanisms displayed by orchid flowers has inspired biologists over the centuries. Based on the intriguing flower structures, the relationship among orchid species and their pollinators has been frequently regarded as very specialised.
• Given that visits on flowers pollinated by oil‐collecting bees are regularly rare, and in Oncidiinae the flowers frequently attractexclusively species that act as effective pollinators, the comparative reproductive biology and pollinator specificity of two sympatric Gomesa (G. varicosa and G. montana; Oncidiinae) were analysedbased on records of floral morphology, production of floral rewards, pollinators and pollination mechanisms. Furthermore, experimental pollinations were carried out in order to examine the breeding systems.
• The results have show that in the studied population, both Gomesa are visited by several bee species, but these orchids present a specific pollination system.Pollinaria are deposited on the head of Centridini (G. varicosa and G. montana) and Epicharitini (G. varicosa) bees when landed on the central callus of the labellumto collect lipoidal substances produced by glandular elaiophores on lateral lobes of the labellum. Both species are dependent on a biotic pollen vector to set fruits. Gomesamontana is completely self‐incompatible, while G. varicosa is partially self‐compatible.
• Our results indicate that although the occurrence of self‐sterile species seems to be common in Oncidiinae, in partially self‐incompatible species, as is the case of G. varicosa, self‐compatibility has been considered as an important factor favouring reproductive assurance in populations with low visitation frequencies, despite occurrence of inbreeding depression.

     

Bees provide pollination service to Campsis radicans (Bignoniaceae), a primarily ornithophilous trumpet flowering vine

1. Pollination syndromes refer to stereotyped floral characteristics (flower colour, shape, etc.) that are associated with a functional group of pollinators (bee, bird, etc.).
2. The trumpet creeper Campsis radicans, endemic to the southeast and mid‐west United States, has been assigned to the hummingbird‐pollination syndrome, due mainly to its red, trumpet‐shaped flowers.
3. Previous studies demonstrated that the ruby‐throated hummingbird Archilochus colubris is C. radicans' primary pollinator, but anecdotal data suggest various bee species may provide pollination service when hummingbirds are absent.
4. This study characterised C. radicans nectar volume and concentration by time of day. Nectar volume was suitable for hummingbirds, but concentration was higher than typical hummingbird‐pollinated plants (～20% w/w); at ～30% w/w, it approached the concentration expected in bee‐pollinated plants (～50% w/w). We also found substantial amounts of nectar at night.
5. Two C. radicans populations received virtually no hummingbird visits, but the number of bees were markedly higher than in the populations previously described. Interestingly, there were no night‐time visitors despite the large quantity of nocturnal nectar.
6. Based on previously published pollen delivery per visit by various species, this study estimated that cumulative deposition by bees routinely reached pollen deposition thresholds for setting fruit in C. radicans. They are, unequivocally, the predominant pollinators in these populations, thus providing pollination service in the absence of hummingbirds.
7. These results highlight C. radicans as a food source for native bees and add to the understanding of how floral phenotypes can facilitate pollination by disparate functional groups.

     

Pollination efficiency of wild bees and hoverflies provided to oilseed rape

• 1 Declining numbers in honeybees and various wild bee species pose a threat to global pollination services. The identification and quantification of the pollination service provided by different taxa within the pollinator guild is a prerequisite for the successful establishment of nature conservation and crop management regimes.
• 2 Wild bees and hoverflies are considered to be valuable pollinators in agricultural and natural systems. Although some information on pollination efficiency of individual pollinator species is available, comparative studies of both taxa at different densities are rare. In the present study, the efficiency of the solitary mason bee Osmia rufa and two hoverfly species (Eristalis tenax and Episyrphus balteatus) as pollinators of oilseed rape Brassica napus was examined in a standardized caged plant breeding regime. Honeybee Apis mellifera colonies were used as a reference pollinator taxon.
• 3 Yield parameters responded differently to pollinator density and identity. Fruit set and number of seeds per pod increased with increasing pollinator density, although these were stronger in the mason bee than the hoverfly treatment. Weight per 1000 seeds did not respond to any pollinator treatment, indicating that seed quality was not affected. Oilseed rape yield in the highest tested densities of both pollinator taxa resulted in yield values close to the efficiency of small honeybee colonies.
• 4 Hoverflies required approximately five‐fold densities of the red mason bees to reach a similar fruit set and yield. Thus, mason bees are more efficient in plant breeding and managed pollination systems. Both natural pollinator taxa, however, are of potential value in open and closed crop production systems.

     

Bees of the Victorian Alps: Network structure and interactions of introduced species

Bees are considered the most important plant pollinators in many ecosystems, yet little is known about pollination of native plants by bees in many Australian ecosystems including the alpine region. Here we consider bee pollination in this region by constructing a bee visitation network and investigating the degree of specialism and network ‘nestedness’, which are related to the robustness of the network to perturbations. Bees and flowers were collected and observed from 10 sites across the Bogong High Plains/Mt Hotham region in Victoria. Low nestedness and a low degree of specialism were detected, consistent with patterns in other alpine regions. Twenty‐one native and one non‐indigenous bee species were observed visiting 46 of the 67 flower species recorded. The introduced Apis mellifera had a large floral overlap with native bees, which may reduce fecundity of native bees through competition. The introduced plant, Hypochaeris radicata (Asteraceae), had the largest and most sustained coverage of any flower and had the most visitations and bee species of any flower. The network developed in this study is a first step in understanding pollination patterns in the alpine/subalpine region and serves as a baseline for future comparisons.     

Reproductive biology of a hummingbird‐pollinated Billbergia: light influence on pollinator behaviour and specificity in a Brazilian semi‐deciduous forest

• Ornithophily has evolved in parallel several times during evolution of angiosperms. Bird pollination is reported for 65 families, including Bromeliaceae. One of the most diverse bromeliad is Billbergia, which comprises species pollinated mainly by hummingbirds.
• Based on investigations on flowering phenology, morpho‐anatomy, volume and concentration of nectar, pollinators and breeding system, this paper explores the reproductive biology and pollinator specificity of B. distachia in a mesophytic semi‐deciduous forest of southeastern Brazil.
• The results have show that B. distachia is pollinated by a single species of hermit hummingbird, Phaethornis eurynome, which search for nectar produced by a septal nectary, where the secretory tissue is located above the placenta. The species is self‐incompatible. The combination of pollinator specificity, due to long corolla tubes that exclude visitation of short‐billed hummingbirds, complete self‐incompatibility and non‐territorial behaviour of pollinators, it is very important to reduce pollen loss and increase gene flow within population.
• Our results indicate that studies on pollination biology and reproduction are essential to understand the evolutionary history of pollination systems of plants since, at least in Billbergia, variation in the pollinator spectrum has been recorded for different habitats among Brazilian forests. Furthermore, according to our data, foraging of Phaethornis on flowers is independent of air temperature and humidity, while the main factor influencing hummingbird visitation is daylight. Considering current knowledge on climatic parameters influencing hummingbird foraging, pollination and reproductive biology of Neotropical flora and environment of the hermit hummingbird in tropical forests, new insights on plant–pollinator interaction are provided.

     

Pollination efficiency and foraging behaviour of honey bees and non‐Apis bees to sweet cherry

1. Crop pollination generally increases with pollinator diversity and wild pollinator visitation. To optimize crop pollination, it is necessary to investigate the pollination contribution of different pollinator species. In the present study, we examined this contribution of honey bees and non‐Apis bees (bumble bees, mason bees and other solitary bees) in sweet cherry.
2. We assessed the pollination efficiency (fruit set of flowers receiving only one visit) and foraging behaviour (flower visitation rate, probability of tree change, probability of row change and contact with the stigma) of honey bees and different types of non‐Apis bees.
3. Single visit pollination efficiency on sweet cherry was higher for both mason bees and solitary bees compared with bumble bees and honey bees. The different measures of foraging behaviour were variable among non‐Apis bees and honey bees. Adding to their high single visit efficiency, mason bees also visited significantly more flower per minute, and they had a high probability of tree change and a high probability to contact the stigma.
4. The results of the present study highlight the higher pollination performance of solitary bees and especially mason bees compared with bumble bees and honey bees. Management to support species with high pollination efficiency and effective foraging behaviour will promote crop pollination.

     

Nectar robbers deter legitimate pollinators by mutilating flowers

Nectar robbing – harvesting nectar illegitimately – can have a variety of outcomes for plant sexual reproduction and for the pollinator community. Nectar robbers can damage flowers while robbing nectar, which could affect the behavior of subsequent flower visitors and, consequently, plant reproduction. However, only nectar manipulation by nectar robbers has so far received attention. We found a short-tongued bee, Hoplonomia sp. (Halictidae), mutilating the conspicuous lower petal of the zygomorphic flowers of Leucas aspera (Lamiaceae) while robbing nectar. We hypothesized that the mutilation of the conspicuous lower petal deters legitimate pollinators on L. aspera flowers, which, in turn, might affect plant reproduction. We first assessed the proportion of naturally-robbed flowers in plant populations for three years to confirm that it was not a purely local phenomenon due to a few individual bees. We then studied diversity, community and visitation characteristics of pollinators, nectar dynamics and fruit set in unrobbed and robbed open flowers in naturally-robbed populations. The proportion of robbed flowers varied significantly across sites and years. Robbing did not affect nectar dynamics in flowers, but it did alter flower morphology, so much so that it reduced pollinator visitation and altered the pollinator community on robbed flowers. However, the maternal function of plant reproduction was not affected by nectar robbing. This study for the first time shows that a nectar robber can have an ecologically significant impact on floral morphology.     

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

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

Heteranthery as a solution to the demand for pollen as food and for pollination – Legitimate flower visitors reject flowers without feeding anthers

• Heteranthery, the presence of feeding and pollinating anthers in the same flower, seems to mediate the evolutionary dilemma for plants to protect their gametes and yet provide food for pollinators. This study aims to elucidate the role of heteranthery in the buzz‐pollinated Senna reniformis.
• The fecundity of pollen from long‐, medium‐ and short‐sized anthers was determined by hand cross‐pollination experiments, and the quantity, size, ornamentation and viability of pollen of different anthers were compared. Rates of flower rejection by bees were measured in anther removal experiments to assess the preferences of flower visitors for feeding or pollinating anthers.
• Large bees, which were the effective pollinators of self‐incompatible S. reniformis, avoided flowers without short feeding anthers, but not those without medium or long anthers. Illegitimate small and medium‐sized bees were unresponsive to anther exclusion experiments. Long anthers deposited pollen on the back and short anthers on the venter of large bees. Pollen from long anthers had higher in vitro viability and higher fruit and seed set after cross‐pollination than pollen from other sized anthers.
• Short anthers produce feeding pollen to effective pollinators and long anthers are related to pollination of S. reniformis. Bee behaviour and size was found to directly influence the role of anthers in the ‘division of labour’. Only large bee pollinators that carry the pollinating pollen from long anthers in ‘safe sites’ associated short anthers with the presence of food. In the absence of these larger bee pollinators, the role of heteranthery in S. reniformis would be strongly compromised and its function would be lost.

     

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.