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Photographs of the Claytonia virginica pollination system. (A) A C. virginica female-phase flower. (B) A C. virginica male-phase flower. (C) The bee-fly Bombylius major visiting C. virginica. (D) The oligolectic bee Andrena erigeniae visiting C. virginica.
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Pollinators that collect pollen – and specifically, pollen-specialist bees – are often considered to be the best pollinators of a (host) plant. Although pollen collectors and pollen specialists often benefit host plants, especially in the pollen that they deliver (their pollination “effectiveness”), they can also exact substantial costs because the...
Contexts in source publication
Context 1
... virginica L., "Spring Beauty" (Portulacaceae) is a spring ephemeral wildflower native to North American eastern woodlands, ranging from Georgia to Ontario and from the East Coast to Kansas and Nebraska. Flowers are protandrous; pollen and nectar are offered on the first day, in the male phase, and only nectar is produced in the female phase ( Fig. 1). On the second day, as the flower opens the three lobes of the stigma unfold, indicat- ing that the stigma is receptive ( Motten et al. 1981). The nectar in pollinator-excluded second day flowers con- tained twice the sugar of first day flowers, indicating that nectar production rates of male-and female-phase flowers are approximately ...
Context 2
... not self-pollinating (Motten et al. 1981), and self-pollinated flowers produce fewer seeds than outcrossed flowers (Schemske 1977). The flow- ers are visited by a variety of insects, among them the pollen-specialist solitary bee Andrena erigeniae, which col- lects pollen exclusively from C. virginica and the closely related Claytonia carolinana ( Fig. 1, Davis and LaBerge 1975). The geographic range and phenology of A. erige- niae match that of C. virginica (Davis and LaBerge 1975). A number of generalist insect species also visit, collecting pollen, nectar, or both. The most frequent generalists are the bee-fly Bombylius major, which does not actively col- lect pollen, and generalist ...
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Citations
... Plants pollinated by insects suffer from high pollen loss during transfer, resulting in only a tiny fraction of the produced pollen reaching conspecific stigmas (Harder and Thomson, 1989;Rademaker et al., 1997;Gong and Huang, 2014). Pollen loss due to pollen consumption (Parker et al., 2016), grooming behavior (Holmquist et al., 2012;Koch et al., 2017), or inefficient pollen transfer (Parker et al., 2016) can even lead to an almost complete depletion of pollen within a plant population (Stanghellini et al., 2002b;Parker et al., 2016;Koski et al., 2018) and consequently limit plant sexual reproduction. Facing such a threat, plants are under strong selective pressure to develop adaptations to prevent pollen losses by limiting the amount of pollen removed by a single pollen-wasting pollinator and increasing the number of pollen carriers (Harder and Thomson, 1989;LeBuhn and Holsinger, 1998;Castellanos et al., 2004). ...
... Plants pollinated by insects suffer from high pollen loss during transfer, resulting in only a tiny fraction of the produced pollen reaching conspecific stigmas (Harder and Thomson, 1989;Rademaker et al., 1997;Gong and Huang, 2014). Pollen loss due to pollen consumption (Parker et al., 2016), grooming behavior (Holmquist et al., 2012;Koch et al., 2017), or inefficient pollen transfer (Parker et al., 2016) can even lead to an almost complete depletion of pollen within a plant population (Stanghellini et al., 2002b;Parker et al., 2016;Koski et al., 2018) and consequently limit plant sexual reproduction. Facing such a threat, plants are under strong selective pressure to develop adaptations to prevent pollen losses by limiting the amount of pollen removed by a single pollen-wasting pollinator and increasing the number of pollen carriers (Harder and Thomson, 1989;LeBuhn and Holsinger, 1998;Castellanos et al., 2004). ...
... Plants pollinated by insects suffer from high pollen loss during transfer, resulting in only a tiny fraction of the produced pollen reaching conspecific stigmas (Harder and Thomson, 1989;Rademaker et al., 1997;Gong and Huang, 2014). Pollen loss due to pollen consumption (Parker et al., 2016), grooming behavior (Holmquist et al., 2012;Koch et al., 2017), or inefficient pollen transfer (Parker et al., 2016) can even lead to an almost complete depletion of pollen within a plant population (Stanghellini et al., 2002b;Parker et al., 2016;Koski et al., 2018) and consequently limit plant sexual reproduction. Facing such a threat, plants are under strong selective pressure to develop adaptations to prevent pollen losses by limiting the amount of pollen removed by a single pollen-wasting pollinator and increasing the number of pollen carriers (Harder and Thomson, 1989;LeBuhn and Holsinger, 1998;Castellanos et al., 2004). ...
Premise:
Animal-pollinated plants face a high risk of pollen loss during its transfer. To limit the negative effect of pollen losses by pollen consumption and heterospecific transfer, plant species may adjust and stratify their pollen availability during the day (i.e., "schedule" their pollen presentation) and attract pollinators in specific time frames.
Methods:
We investigated diurnal patterns of pollen availability and pollinator visitation in three coflowering plant species: Succisa pratensis with open flowers and accessible pollen, pollinated mainly by pollen-feeding hoverflies; Centaurea jacea with open flowers and less accessible pollen, pollinated mainly by pollen-collecting bees; and Trifolium hybridum with closed flowers and pollen accessible only after the active opening of the flower, pollinated exclusively by bees.
Results:
The three plant species differed in the peak pollen availability, tracked by the visitation activity of their pollinators. Succisa pratensis released all pollen in the morning, while pollinator activity was still low and peaked with a slight delay. In contrast, C. jacea and T. hybridum had distinct pollen presentation schedules, peaking in the early afternoon. The pollinator visitation to both of these species closely matched their pollen availability.
Conclusions:
Stratifying pollen availability to pollinators during the day may be one of several mechanisms that allow coflowering plants to share their pollinators and decrease the probability of heterospecific pollen transfer.
... The reproduction of specialised plants may be negatively affected by fluctuations of the abundance of their pollinators (Waser et al., 1996). On the other hand, specialised pollinators may be more effective than generalists by providing higher single visit contribution to plant reproductive fitness (Larsson, 2005;McIntosh, 2005), although it was reported that specialised solitary bees may remove more pollen per flower visit than generalists, which increases the costs for the plants (Larsson, 2005;Parker, Williams & Thomson, 2016). However, it is important to emphasise that specialisation specifically at the level of individual foraging bouts, i.e., high flower constancy, matters for pollination because it ensures that pollen is transferred between flowers of the same plant species (Brosi, 2016) and it minimises heterospecific pollen transfer which may decrease both the male fitness of the donor plant and the female fitness of the recipient plant (Waser, 1978;Morales & Traveset, 2008). ...
It is increasingly recognised that intraspecific variation in traits, such as morphology, behaviour, or diet is both ubiquitous and ecologically important. While many species of predators and herbivores are known to display high levels of between-individual diet variation, there is a lack of studies on pollinators. It is important to fill in this gap because individual-level specialisation of flower-visiting insects is expected to affect their efficiency as pollinators with consequences for plant reproduction. Accordingly, the aim of our study was to quantify the level of individual-level specialisation and foraging preferences, as well as interspecific resource partitioning, in three co-occurring species of bees of the genus Ceratina (Hymenoptera: Apidae: Xylocopinae), C. chalybea , C. nigrolabiata , and C. cucurbitina . We conducted a field experiment where we provided artificial nesting opportunities for the bees and combined a short-term mark-recapture study with the dissection of the bees’ nests to obtain repeated samples from individual foraging females and complete pollen provisions from their nests. We used DNA metabarcoding based on the ITS2 locus to identify the composition of the pollen samples. We found that the composition of pollen carried on the bodies of female bees and stored in the brood provisions in their nests significantly differed among the three co-occurring species. At the intraspecific level, individual females consistently differed in their level of specialisation and in the composition of pollen carried on their bodies and stored in their nests. We also demonstrate that higher generalisation at the species level stemmed from larger among-individual variation in diets, as observed in other types of consumers, such as predators. Our study thus reveals how specialisation and foraging preferences of bees change from the scale of individual foraging bouts to complete pollen provisions accumulated in their nests over many days. Such a multi-scale view of foraging behaviour is necessary to improve our understanding of the functioning of plant-flower visitor communities.
... In the northern areas, C. virginica is near-exclusively pollinated by A. erigeniae, but it can also be pollinated by bee flies (Bombyliidae) in the south (Parker et al., 2018), which may be able to keep pace with advances in flowering. However, A. erigeniae females only collect pollen from C. virginica (Parker et al., 2016). Given the short life span and univoltine nature of A. erigeniae and its obligate reliance upon C. virginica, the negative impacts of phenological mismatch may be more immediate and severe for the bees. ...
Plant-pollinator mutualisms rely upon the synchrony of interacting taxa. Climate change can disrupt this synchrony as phenological responses to climate vary within and across species. However, intra- and interspecific variation in phenological responses is seldom considered simultaneously, limiting our understanding of climate change impacts on interactions among taxa across their ranges. ● We investigated how variation in phenological sensitivity to climate can alter ecological interactions simultaneously within and among species using natural history collections and citizen science data. We focus on a unique system, comprising a wide-ranged spring ephemeral with varying color morphs (Claytonia virginica) and its specialist bee pollinator (Andrena erigeniae). ● We found strongly opposing trends in the phenological sensitivities of plants versus their pollinators. Flowering phenology was more sensitive to temperature in warmer regions, whereas bee phenology was more responsive in colder regions. Phenological sensitivity varied across flower color morphs. Temporal synchrony between flowering and pollinator activity were predicted to change heterogeneously across the species' ranges in the future. ● Our work demonstrates the complexity and fragility of ecological interactions in time and the necessity of incorporating variation in phenological responses across multiple axes to understand how such interactions will change in the future.
... The relevant pollinator responses fundamentally involve influences of plant characteristics on foraging benefits and costs, which have been demonstrated experimentally and within plant populations (Harder et al., 2001). The foraging benefits and costs also depend on environmental circumstance, including the frequency of pollinator visitation, which can differ among sites (e.g., Cuevas and Rosas-Guerrero, 2016;Nakamura and Kudo, 2016;Parker et al., 2016;Waser and Price, 2016). The resulting context-dependent plantpollinator interaction determines the local characteristics of pollinator-mediated pollen dispersal, including overall pollen removal and receipt, the relative incidence of self-and cross-pollination, and the representation of pollen from different plants on stigmas (Barrett and Harder, 2017). ...
Premise:
Fecundity and mating outcomes commonly differ among plant populations occupying contrasting environments. If self-pollination occurs primarily among flowers within plants, contrasting reproductive outcomes among populations must reflect environmental effects on plant-pollinator interaction. Specifically, local conditions could affect features of plant phenotypes that influence pollinator behavior, in turn modifying plant reproductive outcomes.
Methods:
We compared phenotypes, pollinator abundance and behavior, and female fecundity and mating of two meadow populations and two forest populations of Aconitum kusnezoffii within 3 km of each other. Mating outcomes were assessed using microsatellites.
Results:
Meadow plants generally produced more, shorter ramets with more, larger flowers, but less nectar per flower than forest plants. These differences likely largely represent phenotypic plasticity. Individual bumble bees visited more flowers on forest plants, likely because the more abundant bees in the meadows depleted nectar availability, as indicated by briefer visits to individual flowers. Despite similar fruit set in both habitats, forest plants set more seeds per fruit. Nevertheless, meadow plants produced more seeds overall, owing to seven-fold greater flower production. Consistent with individual bees visiting fewer flowers on meadow plants, more of their seeds were outcrossed. However, the outcrossed seeds of forest plants included more male mates.
Conclusions:
Reproductive outcomes can vary among populations of animal-pollinated plants owing to differences in the availability of effective pollinators and environmental effects on plant phenotypes and their functional consequences for pollinator behavior that governs pollen dispersal. This article is protected by copyright. All rights reserved.
... The lack of relationship between fruit weight and visit duration after an initial stingless bee visit, as well as in stingless bee only visitation sequences, is suggestive of a strong pre-emptive priority effect for pollination success as subsequent visitation had no effect on fruit weight (MacInnis & Forrest, 2019;Parker et al., 2016). In contrast, when honeybees are the initial visitor or the only visitor in a visitation sequence, the effect of additional visits is additive; the cumulative effect of longer total visitation sequences, including those involving both bee species, is heavier fruit. ...
Priority effects occur when the order of species arrival affects subsequent ecological processes. The order that pollinator species visit flowers may affect pollination through a priority effect, whereby the first visitor reduces or modifies the contribution of subsequent visits. We observed floral visitation to blueberry flowers from honeybees, stingless bees or a mixture of both species and investigated how (i) the initial visits differed in duration to later visits; and (ii) how the visit sequences from different pollinator taxa influenced fruit weight. Stingless bees visited blueberry flowers for significantly longer than honeybees and maintained their floral visit duration, irrespective of the number of preceding visits. In contrast, honeybee visit duration declined significantly with an increasing number of preceding visits. Fruit weight was positively associated with longer floral visit duration by honeybees but not from stingless bee or mixed species visitation. Fruit from mixed species visits were heavier overall than single species visits, because of a strong priority effect. An initial visit by a stingless bee fully pollinated the flower, limiting the pollination contribution of future visitors. However, after an initial honeybee visit, flowers were not fully pollinated and additional visitation had an additive effect upon fruit weight. Blueberries from flowers visited first by stingless bees were 60% heavier than those visited first by honeybees when total floral visitation was short (∼1 min). However, when total visitation time was long (∼ 8 min), blueberry fruit were 24% heavier when initial visits were from honeybees. Our findings highlight that the initial floral visit can have a disproportionate effect on pollination outcomes. Considering priority effects alongside traditional measures of pollinator effectiveness will provide a greater mechanistic understanding of how pollinator communities influence plant reproductive success.
... Although the pollen loads of visitors do not always adequately predict effective pollination (Adler and Irwin, 2006), high conspecific pollen transport likely predisposes visitors to higher pollination effectiveness on average. Another possible explanation is that, for pollencollecting visitors, more frequent visitors could be more efficient at extracting large quantities of pollen (e.g., Parker et al., 2016) and might therefore transfer more pollen depending on how well pollen is groomed. Addressing whether more frequent visitors transport more conspecific pollen or deliver fewer heterospecific pollen grains are ripe questions for further study. ...
... First, we only included measures of female reproductive success in assessing pollination effectiveness (e.g., pollen deposition, seed set). The proportion of extracted pollen that is successfully transferred to stigmas may be a better assessment of the overall reproductive contribution of different taxa (Parker et al., 2016), because pollen that is removed-but not successfully transferred-represents a loss to male fitness (Harder and Thomson, 1989;Minnaar et al., 2019). Unfortunately, data on such transfer dynamics are much rarer in the literature. ...
Premise:
Many animals provide ecosystem services in the form of pollination, including honeybees which have become globally dominant floral visitors. A rich literature documents considerable variation in single visit pollination effectiveness, but this literature has yet to be extensively synthesized to address whether honeybees are effective pollinators.
Methods:
We conducted a hierarchical meta-analysis of 168 studies and extracted 1564 single visit effectiveness (SVE) measures for 240 plant species. We paired SVE data with visitation frequency data for 69 of these studies. We used these data to ask: 1) Do honeybees (Apis mellifera) and other floral visitors differ in their SVE?; 2) To what extent do plant and pollinator attributes predict differences in SVE between honeybees and other visitors?; and 3) Is there a correlation between visitation frequency and SVE?
Key results:
Honeybees were significantly less effective than the most effective non-honeybee pollinators but as effective as the average pollinator. The type of pollinator moderated these effects. Honeybees were less effective compared to the most effective and average bird and bee pollinators but were as effective as other taxa. Visitation frequency and SVE were positively correlated, but this trend was largely driven by data from communities where honeybees were absent.
Conclusions:
Although high visitation frequencies make honeybees important pollinators, they were less effective than the average bee and rarely the most effective pollinator of the plants they visit. As such, honeybees may be imperfect substitutes for the loss of wild pollinators and safeguarding pollination will benefit from conservation of non-honeybee taxa. This article is protected by copyright. All rights reserved.
... In Solanum rostratum (Solanaceae), several bee species are legitimate pollinators while others are thieves (e.g. Apidae; Solís-Montero et al., 2015;Quinalha et al., 2017), and for Claytonia virginica (Montiaceae) even a single, highly specialized taxon can operate both as a legitimate pollinator and a thief (Parker et al., 2016). ...
Background and aims:
Most angiosperms rely on pollinators to transport pollen and effect fertilization. While some floral visitors are effective pollinators, others act as thieves-consuming pollen but effecting little pollination in return. The importance of pollen theft to male and female reproductive success has received little attention. Here, we examined if pollen consumption by flies altered pollen receipt and exacerbated pollen limitation for a bumblebee-pollinated plant, Polemonium foliosissimum (Polemoniaceae).
Methods:
To examine the effect of pollen-thieving flies, we took a three-pronged approach. First, we used single-visit observations to quantify pollen removal and pollen deposition by flies and bumblebees. Second, we manipulated pollen in the neighborhood around focal plants in two years to test whether pollen reduction reduced pollen receipt. Third, we combined pollen reduction with hand-pollination to test whether pollen thieving exacerbated pollen limitation. Polemonium foliosissimum is gynodioecious in most populations in the Elk Mountains of central Colorado, USA. Thus, we also tested whether pollen theft affected hermaphrodites and females differently.
Results:
Flies removed significantly more pollen and deposited less pollen/visit than did bumblebees. Reduction of pollen in the neighborhood around focal plants reduced pollen receipt in both years but only nearly-significantly so in 2015. In 2016, plants were significantly pollen limited; hand-pollination significantly increased seeds/fruit for both hermaphrodites and females. However, the reduction of pollen around focal plants did not exacerbate pollen limitation for either hermaphrodites or females.
Conclusions:
Our results suggest that plants tolerate significant consumption of pollen by thieves and pollinators by producing ample pollen to feed both and fertilize available ovules. Our results demonstrate that pollen limitation in P. foliosissimum is driven by lack of effective pollinators rather than lack of pollen. Teasing out these effects elucidates the relative importance of drivers of reproductive success and thus the expected response to selection by different floral visitors.
... Although the pollen loads of visitors do not always adequately predict effective pollination (Adler and Irwin, 2006), high conspecific pollen transport likely predisposes visitors to higher pollination effectiveness on average. Another possible explanation is that, for pollencollecting visitors, more frequent visitors could be more efficient at extracting large quantities of pollen (e.g., Parker et al., 2016) and might therefore transfer more pollen depending on how well pollen is groomed. Addressing whether more frequent visitors transport more conspecific pollen or deliver fewer heterospecific pollen grains are ripe questions for further study. ...
... First, we only included measures of female reproductive success in assessing pollination effectiveness (e.g., pollen deposition, seed set). The proportion of extracted pollen that is successfully transferred to stigmas may be a better assessment of the overall reproductive contribution of different taxa (Parker et al., 2016), because pollen that is removed-but not successfully transferred-represents a loss to male fitness (Harder and Thomson, 1989;Minnaar et al., 2019). Unfortunately, data on such transfer dynamics are much rarer in the literature. ...
Many animals provide essential ecosystem services in the form of plant pollination. A rich literature documents considerable variation in the single visit pollination effectiveness of different plant visitors, but this literature has yet to be comprehensively synthesized. We conducted a hierarchical meta-analysis of 193 studies and extracted 1716 single visit effectiveness (SVE) comparisons for 252 plant species. We paired SVE data with visitation frequency data for 75 of these studies. Given the global dominance of honeybees in pollinator communities, we used these data to ask: 1) Do honeybees ( Apis mellifera ) and other floral visitors vary in their SVE?; 2) To what extent do plant and pollinator attributes predict the difference in SVE between honeybees and other visitors?; and 3) Is there a correlation between floral visitation frequency and SVE? We found that honeybees were significantly less effective than the most effective non-honeybee pollinator. Although not significantly different, honeybees also tended to be less effective than the mean community effectiveness. Honeybees were less effective as pollinators of crop plants and when compared to birds and other bees. Visitation frequency and pollination effectiveness were positively correlated, but this trend was largely driven by data from communities where honeybees were absent, suggesting that honeybees generally combine high visitation frequency and lower SVE. Our study demonstrates that non-honeybee floral visitors are highly effective pollinators of many crop and non-crop plants. While the high visitation frequency typically displayed by honeybees undoubtably makes them important pollinators, we show that honeybees are slightly less effective than the average pollinator and rarely the most effective pollinator of the plants they visit. As such, honeybees may be imperfect substitutes for the loss of wild pollinators and safeguarding global crop production will benefit from conservation of non-honeybee taxa.
Open Research Statement
Although we are fully committed to data transparency, we are also aware of different research teams working on related meta-analyses. As such, we prefer to wait until our paper is accepted to make data publicly available but are happy to share data upon request. Data will be permanently archived on Figshare following acceptance.
... A bee whose diet is specialized on certain spring ephemeral flowers, e.g. Andrena erigeniae (Parker et al. 2016) may similarly be more often found in the understory. In contrast, bees foraging in the early spring canopy may be seeking nonfloral plant resources (Requier and Leonhardt 2020) or have generalist diets (Ramalho 2004). ...
Temperate hardwood deciduous forest is the dominant landcover in the Northeastern US, yet its canopy is usually ignored as pollinator habitat due to the abundance of wind-pollinated trees. We describe the vertical stratification of spring bee communities in this habitat and explore associations with bee traits, canopy cover, and coarse woody debris. For three years, we sampled second-growth woodlots and apple orchard-adjacent forest sites from late March to early June every 7–10 days with paired sets of tri-colored pan traps in the canopy (20–25 m above ground) and understory (<1m). Roughly one fifth of the known New York state bee fauna were caught at each height, and 90 of 417 species overall, with many species shared across the strata. We found equal species richness, higher diversity, and a much higher proportion of female bees in the canopy compared to the understory. Female solitary, social, soil- and wood-nesting bees were all abundant in the canopy while soil-nesting and solitary bees of both sexes dominated the understory. Canopy cover increased with leaf-out, and was negatively associated with understory but not canopy bee abundance. Site-level volume of coarse woody debris (CWD) did not predict bee abundance, while approximated landscape-scale availability of woody debris was positively correlated with understory wood-nesting and solitary-bee abundance. This work expands our understanding of habitats where bees are likely foraging and reveals vertically stratified behavior. We emphasize deciduous forests as an important habitat for wild bee conservation and recommend further research into the behavior and diets of bees occupying the canopy, speculating that females forage for anemophilous tree pollen. Forest management plans that conserve above-ground deadwood may provide nest sites for wood-nesting bees.
... Pollination mutualisms are often complicated by the fact that the agents of pollen dispersal are usually attracted to flowers by the prospect of nourishment. Floral visitors and plants consequently have conflicting agendas, especially when the nourishment sought by the floral visitors is pollen [1][2][3]. Bees have evolved various structural and behavioural adaptations to promote pollen collection, as the development of their larvae relies on the pollen protein [4]. For example, harvested pollen is often packed on the corbiculae of bees where it is not available for stigmatic deposition or ovule fertilization. ...
Angiosperm pollen grain diameter varies greatly from a few microns to over 100, but the selective forces driving the interspecific variation in pollen size remain unclear. Although both pre- and post-pollination hypotheses have been proposed, empirical evidence remains scarce. Here we propose that visits by pollen-foraging pollinators have selected against large pollen grains. An association between pollinator behaviour and pollen grain size was confirmed by field studies of 80 flowering species in natural communities, showing that pollinators positively collected pollen in those species with relatively smaller pollen grains but rarely did so in species with larger ones. Allowing for the confounding effects of pollinator type, flower size or style length and pollen grain number, we found a significant effect of pollen-foraging behaviour on variation in pollen grain size, particularly in bee-pollinated plants. While these results suggest that many plant species whose pollen is collected or consumed by pollinators produce small pollen grains, it remains unclear whether pollen grain size is directly affected by pollinator foraging habit or indirectly mediated by pollen number trade-offs.
