Reproductive biology of two Himalayan alpine gingers (Roscoea spp., Zingiberaceae) in China: pollination syndrome and compensatory floral mechanisms

According to the concept of pollination syndromes, floral traits reflect specialisation to a particular pollinator or set of pollinators. However, the reproductive biology of endemic, and often specialised, plants may require increased attention as climate change accelerates worldwide. Species of Roscoea endemic to the Himalayan region have striking orchid-like flowers with long corolla tubes, suggesting pollination by long-tongued insects. Until now, the reproductive biology of species of Roscoea has been poorly documented. We investigated the floral biology, breeding system and pollination ecology of R. cautleoides and R. humeana, from Hengduan Mountains, a global biodiversity hotspot in southwest China. We also tested whether floral longevity increases pollination success. Pollination experiments showed that the two species were self-compatible and depended on insects for fruit production. Over several flowering seasons we did not observe any potential pollinators with long tongues that matched the corolla tube visiting flowers in centres of distribution. The principal pollinators observed were pollen-collecting generalist bees, with low visitation frequencies. In general, members of the ginger family are characterised by short-lived (usually 1 day) flowers, but flowers of R. cautleoides and R. humeana last 8 and 6 days, respectively. Removing stigmas decreased fruit set in both study populations. Our results suggest that the original pollinators may have been long-tongued insects that are now absent from the Chinese Himalayas because habitats have responded to climate change. However, long-lived and self-compatible flowers, coupled with the presence of generalist pollinators, are traits that have allowed these gingers to reproduce and continue to persist in the alpine habitats.     

Phenotypic selection on flowering phenology and pollination efficiency traits between Primula populations with different pollinator assemblages

Floral traits have largely been attributed to phenotypic selection in plant–pollinator interactions. However, the strength of this link has rarely been ascertained with real pollinators. We conducted pollinator observations and estimated selection through female fitness on flowering phenology and floral traits between two Primula secundiflora populations. We quantified pollinator‐mediated selection by subtracting estimates of selection gradients of plants receiving supplemental hand pollination from those of plants receiving open pollination. There was net directional selection for an earlier flowering start date at populations where the dominant pollinators were syrphid flies, and flowering phenology was also subjected to stabilized quadratic selection. However, a later flowering start date was significantly selected at populations where the dominant pollinators were legitimate (normal pollination through the corolla tube entrance) and illegitimate bumblebees (abnormal pollination through nectar robbing hole which located at the corolla tube), and flowering phenology was subjected to disruptive quadratic selection. Wider corolla tube entrance diameter was selected at both populations. Furthermore, the strength of net directional selection on flowering start date and corolla tube entrance diameter was stronger at the population where the dominant pollinators were syrphid flies. Pollinator‐mediated selection explained most of the between‐population variations in the net directional selection on flowering phenology and corolla tube entrance diameter. Our results suggested the important influence of pollinator‐mediated selection on floral evolution. Variations in pollinator assemblages not only resulted in variation in the direction of selection but also the strength of selection on floral traits.     

Mast fruiting in a hawkmoth-pollinated orchid Habenaria glaucifolia: an 8-year survey

Aims The hypothesis of predator satiation has been proposed to explain mast fruiting in various flowering plants. It considers that the simultaneous production of large numbers of seeds by a plant population reduces the risk of seed predation for each individual. Orchids produce huge numbers of seeds per fruit and rarely experience seed predation. It remains unclear which factors may affect fluctuating fruit production in orchids, which generally suffer a widespread pollen limitation. To explore the temporal pattern of fruiting and potential factors related to fluctuation in fruit production, we investigated reproductive success of a long-spurred orchid (Habenaria glaucifolia) in an alpine meadow with thousands of individuals over 8 years.Methods To estimate reproductive success, pollinator observation was conducted by day and at night, and pollinia removal and receipt were recorded in the field population for 8 years. To examine whether fruit set and seed set are pollen limited, we conducted supplementary pollination experiments and compared fruit set, seed set and pollinia movement of open-pollinated flowers from 2011 to 2013. We measured lengths of spurs and pollinator proboscises, and nectar volume and concentration, to identify potential pollinators.Important findings Hawkmoths were seen to be effective pollinators for H. glaucifolia in 3 years, whereas in the remaining 5 years no pollinators were observed, and consequently pollinia were rarely transferred. Numerous pollinia movements were observed in 2012, 2013 and 2014 (pollinia removal: 48, 59 and 85%; pollinia receipt 51, 70 and 80%), and correspondingly fruit set was significantly higher in 2012 and 2013 (59 and 46%) than in 2011 (25%). It was fruit set, rather than seed set, that was pollen limited in this orchid in the 3 years, in that supplementary pollination increased fruit set but did not increase seed set per fruit compared to natural. Three species of hawkmoths had proboscis lengths that matched the spur length of H. glaucifolia. Fruit set in this long spurred orchid depends on the activity of long-tongued hawkmoths, resulting in significant temporal variation in fruit production. Mast fruiting in this alpine orchid could be attributed to a 'sit and wait' strategy, awaiting an abundance of effective pollinators.     

Variation in native pollinators in the absence of honeybees: implications for reproductive success of an Australian generalist-pollinated herb Trachymene incisa (Apiaceae)

Most plants are generalist in terms of pollination, with the potential for significant spatial and temporal variation in their pollinators. Few studies have investigated how variable pollinators are in a generalist pollination system or how this variation affects plant reproduction. We investigated the degree of variation in pollinators and resulting reproductive success among populations of a widespread generalist-pollinated herb, Trachymene incisa (Apiaceae). The European honeybee was unexpectedly absent from the pollinator assemblages, providing the unique opportunity to study the native Australian pollinators. Insect visitation rates and the taxonomic composition of the pollinator assemblage varied significantly across populations, indicating that populations of T. incisa are not equally serviced and are not equally generalist. This highlights that sampling one population would not characterize the extent of species generalization. There was no positive correlation between insect visitation rate and reproductive success, with the Agnes Banks population receiving the highest visitation rate but producing the lowest reproductive output, and the Myall Lakes population receiving the lowest visitation rate and producing the highest seedling emergence. This study shows that variation in pollinators can have measurable effects on populations of generalist-pollinated plants, therefore there is potential for large-scale change in all plant–pollinator interactions.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 479–490.     

Geographic variation in reproductive success of Stenocereus thurberi (Cactaceae): Effects of pollination timing and pollinator guild

? Premise of the study: It has been proposed that species of columnar cacti from dry tropical areas depend on bats for their reproduction, whereas species from dry subtropical areas are also pollinated by other species. To test this hypothesis, we examined the effects of pollinator guild and of variation in time and space on the reproductive success of a widespread species. ? Methods: Changes in fruit set, seed set, and pollinator activity through time were recorded in three widely separated populations of Stenocereus thurberi. Breeding system and sources of pollination limitation were determined by controlled pollinator exclusions in each population. ? Key results: Significant differences were found in the timing of activity and in the effectiveness of pollinators among sites. In the northern and central populations, reproductive success depends on bats, whereas in the southern population a combination of pollinators was more effective. No difference between open and hand cross-pollination treatments was found in the northern and central populations, which suggests no pollen limitation. However, significant differences were detected in the southern population, which indicates temporal differences in pollinator abundance or arrival time. ? Conclusions: Local variation in pollinator assemblages and reproductive success could greatly affect the evolution of pollination systems. The pattern of generalist pollination in the southernmost populations and specialized pollination in the central and northern populations contradicts the hypothesis of latitudinal variation. In the absence of nocturnal pollinators, the accumulated nectar can sustain visits by diurnal pollinators, a bet-hedging strategy that increases the chances of fruit set in some populations.     

Pollinator‐mediated interactions between cultivated papaya and co‐flowering plant species

Many modern crop varieties rely on animal pollination to set fruit and seeds. Intensive crop plantations usually do not provide suitable habitats for pollinators so crop yield may depend on the surrounding vegetation to maintain pollination services. However, little is known about the effect of pollinator‐mediated interactions among co‐flowering plants on crop yield or the underlying mechanisms. Plant reproductive success is complex, involving several pre‐ and post‐pollination events; however, the current literature has mainly focused on pre‐pollination events in natural plant communities. We assessed pollinator sharing and the contribution to pollinator diet in a community of wild and cultivated plants that co‐flower with a focal papaya plantation. In addition, we assessed heterospecific pollen transfer to the stigmatic loads of papaya and its effect on fruit and seed production. We found that papaya shared at least one pollinator species with the majority of the co‐flowering plants. Despite this, heterospecific pollen transfer in cultivated papaya was low in open‐pollinated flowers. Hand‐pollination experiments suggest that heterospecific pollen transfer has no negative effect on fruit production or weight, but does reduce seed production. These results suggest that co‐flowering plants offer valuable floral resources to pollinators that are shared with cultivated papaya with little or no cost in terms of heterospecific pollen transfer. Although HP reduced seed production, a reduced number of seeds per se are not negative, given that from an agronomic perspective the number of seeds does not affect the monetary value of the papaya fruit.     

Pollinator attraction in Cornus capitata (Cornaceae): the relative role of visual and olfactory cues

Aims It is generally accepted that visual displays and floral scent play important roles in communication between flowering plants and their pollinators. However, the relative role of visual and olfactory cues in pollinator attraction is largely unknown. In this study, we determined the roles of both types of cue in attracting pollinators to Cornus capitata, a medium sized tree with each capitulum surrounded by four large, white, petaloid bracts.Methods Pollinator observations and pollination experiments were conducted in a natural population; the inflorescences' visual and olfactory signals were characterized by spectral and chemical analyses; the responses of pollinators to visual and olfactory cues were tested using dual-choice behavioural bioassays; the relative roles of visual and olfactory cues in pollinator attraction were tested by comparing the responses of pollinators to inflorescences subjected to three experimental treatments (intact, all bracts removed, and capitulum removed) within the natural population.Important findings For fruit set, C. capitata is entirely dependent on pollinators, with a bee, Anthophora sp., being the main pollinator. Bracts present high colour distance and green contrast against the leaves. Twelve volatile compounds in the floral scent were detected, most of which have previously been reported to be attractive to a broad spectrum of bee species. Behavioural bioassays showed that both, visual cues alone and olfactory cues alone, are attractive to pollinating bees. However, visual cues alone attracted significantly more approaches than olfactory cues alone, while olfactory cues alone elicited a significantly higher landing percentage than visual cues alone. The finding suggests that, in the C. capitata – Anthophora sp. interaction, visual cues are mainly used for location from long distances, while olfactory cues mainly aid landing from short distances. Our results indicate that different modalities of floral cues should be considered together to understand fully the communication between flowering plant and pollinators.     

Floral display and habitat fragmentation: Effects on the reproductive success of the threatened mass‐flowering Conospermum undulatum (Proteaceae)

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How generalists coexist: the role of floral phenotype and spatial factors in the pollination systems of two Ranunculus species

Aims Competition for pollinators between phenotypically similar flowers is believed to play an important role in floral trait diversification in the angiosperms. However, in many plant communities, species with apparently similar floral phenotypes and generalist pollination systems co-flower. Here, the pollination systems of Ranunculus acris L. and Ranunculus repens L. were investigated to determine the factors enabling the species to coexist within apparently overlapping pollination niches.Methods Sympatrically flowering populations of R. acris and R. repens were investigated at three study sites in West Wales. The floral phenotypes of the two species were compared using measurements of floral morphology and spectral analyses of petal reflectance, using principal component analysis and bee and fly colour-space models. Evidence of inter-specific discrimination by foraging insects was tested for in the field and using floral arrays. The relative roles of behavioural constancy and spatial patchiness in maintaining pollinator fidelity were estimated.Important findings The floral phenotypes of R. acris and R. repens differed significantly. Social bees were highly constant when foraging at flowers of the two species and patchy floral distribution explained some of the observed fidelity. Dipterans visiting mixed floral arrays appeared to discriminate between the species, visiting more R. acris than R. repens flowers, but there was no difference in the number of visits to single-species arrays. Social bees were more likely to display constancy to flowers of R. repens in the field. Patchiness in floral distribution, subtle differences in floral phenotype, pollinator preferences and behavioural constancy are all likely to contribute to the continued coexistence of R. acris and R. repens, despite apparent overlap in their pollination niches. Such differences have the potential to facilitate the maintenance of species diversity in plant communities, even where plants appear to share similar floral phenotypes.     

The role of individual variation in flowering and pollination in the reproductive success of a crepuscular buzz-pollinated plant

Background and AimsPlant individuals within a population differ in their phenology and interactions with pollinators. However, it is still unknown how individual differences affect the reproductive success of plants that have functionally specialized pollination systems. Here, we evaluated whether plant individual specialization in phenology (temporal specialization) and in pollination (pollinator specialization) affect the reproductive success of the crepuscular-bee-pollinated plant Trembleya laniflora (Melastomataceae).MethodsWe quantified flowering activity (amplitude, duration and overlap), plant–pollinator interactions (number of flowers visited by pollinators) and reproductive success (fruit set) of T. laniflora individuals from three distinct locations in rupestrian grasslands of southeastern Brazil. We estimated the degree of individual temporal specialization in flowering phenology and of individual specialization in plant–pollinator interactions, and tested their relationship with plant reproductive success.Key Results Trembleya laniflora presented overlapping flowering, a temporal generalization and specialized pollinator interactions. Flowering overlap among individuals and populations was higher than expected by chance but did not affect the individual interactions with pollinators and nor their reproductive success. In contrast, higher individual generalization in the interactions with pollinators was related to higher individual reproductive success.ConclusionsOur findings suggest that individual generalization in plant–pollinator interaction reduces the potential costs of specialization at the species level, ensuring reproductive success. Altogether, our results highlight the complexity of specialization/generalization of plant–pollinator interactions at distinct levels of organization, from individuals to populations, to species.     

Effects of distance from models on the fitness of floral mimics

• Rewardless plants can attract pollinators by mimicking floral traits of rewarding heterospecific plants. This should result in the pollination success of floral mimics being dependent on the relative abundance of their models, as pollinator abundance and conditioning on model signals should be higher in the vicinity of the models. However, the attraction of pollinators to signals of the models may be partially innate, such that spatial isolation of mimics from model species may not strongly affect pollination success of mimics.
• We tested whether pollination rates and fruit set of the rewardless orchid Disa pulchra were influenced by proximity and abundance of its rewarding model species, Watsonia lepida.
• Pollination success of the orchid increased with proximity to the model species, while fruit set of the orchid increased with local abundance of the model species. Orchids that were experimentally translocated outside the model population experienced reduced pollinaria removal and increased pollinator‐mediated self‐pollination.
• These results confirm predictions that the pollination success of floral mimics should be dependent on the proximity and abundance of model taxa, and thus highlight the importance of ecological facilitation among species involved in mimicry systems.

     

Sex‐specific phenotypic selection and geographic variation in gender divergence in a gynodioecious shrub

In sexually polymorphic plant species the extent of gender divergence in floral morphology and phenology may be influenced by gender‐specific selection patterns imposed by pollinators, which may change geographically. Distribution margins are areas where changes in the pollinator fauna, and thus variation in gender divergence of floral traits, are expected. We tested for pollination‐driven geographic variation in the gender divergence in floral and phenological traits in the gynodioecious shrub Daphne laureola, in core and marginal areas differing in the identity of the main pollinator. Pollinators selected for longer corolla tubes in hermaphrodite individuals only in core populations, which in turn recorded higher fruit set. Consistent with these phenotypic selection patterns, gender divergence in flower corolla length was higher in core populations. Moreover, pollinators selected towards delayed flowering on hermaphrodite individuals only in marginal populations, where the two sexes differed more in flowering time. Our results support that a shift in main pollinators is able to contribute to geographic variation in the gender divergence of sexually polymorphic plant species.     

Pollinator coupling can induce synchronized flowering in different plant species

Synchronous and intermittent plant reproduction has been identified widely in diverse biomes. While synchronous flowering is normally observed within the same species, different species also flower in synchrony. A well-known example of interspecific synchrony is “general flowering" in tropical rain forests of Southeast Asia. Environmental factors, such as low temperature and drought, have been considered as major trigger of general flowering. However, environmental cues are not enough to explain general flowering because some trees do not flower even when they encounter favorable environmental cues. We propose alternative explanation of general flowering; “pollinator coupling”. When species flower synchronously, the elevated pollen and nectar resource may attract increased numbers of generalist pollinators, with a concomitant enhancement of pollination success (facilitation). However, under these circumstances, plants of different species may compete with one another for limited pollinator services, resulting in declines in pollination success for individual species (competition). Here, we present a model describing resource dynamics of individual trees serviced by generalist pollinators. We analyze combinations of conditions under which plants reproduce intermittently with synchronization within species, and/or (sometimes) between different species. We show that plants synchronize flowering when the number of pollinators attracted to an area increases at an accelerating rate with increasing numbers of flowers. In this case, facilitation of flowering by different species exceeds the negative influence of interspecific plant competition. We demonstrate mathematically that co-flowering of different species occurs under a much narrower range of circumstances than intraspecific co-flowering.     

Night and day service: Distyly and mixed pollination system in Faramea cyanea (Rubiaceae)

Currently, pollination is seen as involving more generalist interactions than specialized ones. Supporting this trend, some nocturnal distylous flowers may also receive floral visitors during the day, and since the latter contribute to fruit set, the pollination system is mixed and less specialized. Common among the Rubiaceae, distyly has been regarded as a reproductive strategy which requires a precise and specialized pollination system, and in this important tropical family, environmental disturbance and pollination failure have been used to explain anomalies in distylous features. Faramea cyanea Müll. Arg. is a common tree species in forest formations in the increasingly threatened Cerrado biome, the Neotropical savannas in Central Brazil. We evaluated the floral morphology, pollination biology and breeding system of a population of F. cyanea. Despite their moth pollination features, flowers were visited by diurnal (bees) as well as nocturnal (moths) pollinators. Experimental results showed that both pollinator groups contributed equally to pollen flow and legitimate pollination. The population presented distyly, isoplethy and heteromorphic self-incompatibility. Although F. cyanea did not present exact reciprocal herkogamy between floral morphs, pollination and reproductive success were not impaired. Floral features, which allowed pollination by complementary groups of pollinators, may explain the absence of anomalies in the isoplethy and distylous features in the studied population, anomalies which have been observed in other sympatric distylous Rubiaceae.     

Co-flowering plants support diverse pollinator populations and facilitate pollinator visitation to sweet cherry crops

Many food crops depend on animal pollination to set fruit. In light of pollinator declines there is growing recognition of the need for agro-ecosystems that can sustain wild pollinator populations, ensuring fruit production and pollinator conservation into the future. One method of supporting resident wild pollinator populations within agricultural landscapes is to encourage and maintain floral diversity. However, pollinator visitation to crop plants can be affected either positively (facilitation) or negatively (competition) by the presence of co-flowering plants. The strength and direction of the facilitative/competitive relationship is driven by multiple factors, including floral abundance and the degree of overlap in pollinator visitation networks. We sought to determine how plant-pollinator networks, within and surrounding sweet cherry (Prunus avium) orchards, change across key time points during the cherry flowering season, in three growing regions in Australia. We found significant overlap in the suite of flower visitors, with seven taxa (including native bees, flies, hoverflies and introduced honey bees, Apis mellifera) observed visiting cherry and other co-flowering species within the orchard and/or the wider surrounding matrix. We found evidence of pollinator facilitation with significantly more total cherry flower visits with increasing percent cover of co-flowering plants within the wider landscape matrix and increased visitation to cherry by honey bees with increasing co-flowering plant richness within the orchard. During the cherry flowering period there was a significant positive relationship between pollinator richness on cherry and pollinator richness on co-flowering plants within the orchard and the area of native vegetation surrounding orchards. Outside of the crop flowering season, co-flowering plants within the orchard and wider landscape matrix supported the same pollinator taxa that were recorded visiting cherry when the crop was flowering. This shows wild plants help support the pollinators important to crop pollination, outside of the crop flowering season, highlighting the role of co-flowering plants within pollinator-dependent cropping systems.     

Pollination efficiency and reproductive patterns in relation to local plant density,population size,and floral display in the rewarding Listera ovata (Orchidaceae)

Pollination efficiency and reproductive success vary strongly among populations of most animal‐pollinated plant species, depending on their size and local density, whereas individual plants within populations experience varying levels of reproductive output as a result of differences in floral display. Although most orchid species have been shown to be severely pollination limited, few studies have investigated the impact of the above‐mentioned factors on pollination success and reproduction, especially in rewarding species. In this study, the impact of population size, local density of flowering plants, and floral display on the rates of pollinia export and fruit production was investigated in 13 natural populations of the rewarding terrestrial orchid Listera ovata. In addition, an emasculation experiment was set up to examine how floral display and local density of flowering plants affected the relative importance of cross‐ vs. geitonogamous pollination in determining fruit set. In the studied populations, pollination efficiency, pollen removal, and fruit set increased with increasing population size until a threshold value of 30–40 flowering plants was reached, above which pollination efficiency and reproductive output decreased again. On average, plants with large floral displays showed higher proportional pollinia removal and fruit set compared with smaller plants. Fruit production was also significantly and positively related to local plant density, whereas emasculation did not affect the relationship between local plant density and fruit set, suggesting that geitonogamous pollination did not affect the outcome of female function. The results of this study are discussed in the light of the flowering mechanism of the species and its generalized pollination system. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157 , 713–721.     

Phylogenetic Community Context Influences Pollen Delivery to <Emphasis Type="Italic">Allium cernuum</Emphasis>

Few studies have examined how the number and identity of species in the neighbouring community influences the reproductive success of particular focal species. Pollen delivery, an important component of fitness of sexual plants, is a function of not just the floral traits of any particular individual, but of features of the population and community as it depends on pollinator abundance and preferences. Many pollinators in flowering communities will prefer patches with high floral abundance or diversity yet may exhibit lower floral constancy when more flowering species are present. Thus, pollination efficiency could increase or decrease with increased species richness and this will alter the selection pressures placed upon the floral traits (such as floral colour or reward) of any member of a particular community. Moreover, recent studies have indicated that plant-pollinator networks are phylogenetically structured (pollinators visit related plant species more than expected by chance) and this may be an important factor structuring flowering plant communities. Thus, the sheer number of species within a patch may be less important than the number of closely-related species. We investigate whether species richness or phylogenetic proximity of coflowering species influences the amount and proportion of conspecific pollen delivered to nodding onion, Allium cernuum, in fragment patches of Garry Oak meadows in South Western British Columbia, Canada. We find that pollen delivery depended upon the presence of close relatives far more than on species richness or population density, indicating a central role of the community structure on pollination in flowering plant communities. Insofar as pollen delivery relates to the relative number of seeds produced by members of the community, pollination may in turn determine the community structure of the next generation.     

Does community‐level floral abundance affect the pollination success of a rewardless orchid,Calanthe reflexa Maxim.?

Decreases in pollinator abundance may particularly constrain plants that lack floral rewards, since they are poor competitors for pollinators in the plant community. Here, we documented the pollination ecology of a rewardless orchid, Calanthe reflexa Maxim., and examined effects of forest understory degradation by deer browsing on pollination success of the species in the light of a change in the abundance of neighboring flowering plants in 2010 and 2011. Bombus species were the only pollinators at each site and the flowering phenology of C. reflexa did not overlap with that of other rewarding plants. Pollinator visit rates (assessed by time‐lapse photography), and pollinia removal rate were higher in the undegraded understory site than the degraded site in both years, while the fruit set ratio did not differ between the sites in 2011. Coverage by neighboring flowering plants was extremely low in the degraded site. Our results suggest that, although its flowering phenology and consequently lower interspecific competition of C. reflexa with rewarding plants for attracting bumblebees, neighboring flowering plants may play an important role for maintaining the visitation frequency of bumblebees of C. reflexa and contribute to its pollination success.     

A test for Allee effects in the self‐incompatible wasp‐pollinated milkweed Gomphocarpus physocarpus

It has been suggested that plants that are good colonizers will generally have either an ability to self‐fertilize or a generalist pollination system. This prediction is based on the idea that these reproductive traits should confer resistance to Allee effects in founder populations and was tested using Gomphocarpus physocarpus (Asclepiadoideae: Apocynaceae), a species native to South Africa that is invasive in other parts of the world. We found no significant relationships between the size of G. physocarpus populations and various measures of pollination success (pollen deposition, pollen removal and pollen transfer efficiency) and fruit set. A breeding system experiment showed that plants in a South African population are genetically self‐incompatible and thus obligate outcrossers. Outcrossing is further enhanced by mechanical reconfiguration of removed pollinaria before the pollinia can be deposited. Self‐pollination is reduced when such reconfiguration exceeds the average duration of pollinator visits to a plant. Observations suggest that a wide variety of wasp species in the genera Belonogaster and Polistes (Vespidae) are the primary pollinators. We conclude that efficient pollination of plants in small founding populations, resulting from their generalist wasp‐pollination system, contributes in part to the colonizing success of G. physocarpus. The presence of similar wasps in other parts of the world has evidently facilitated the expansion of the range of this milkweed.     

Pollinators exert positive selection on flower size on urban,but not on rural Scotch broom (Cytisus scoparius L. Link)

Aims Adaptive evolution of invasive species is both particularly exciting for the evolutionary biologist and worrisome for those interested in controlling or halting spread. Invasive species often have a distinct timeline and well-recorded population expansion. As invaders encounter new environments, they undergo rapid adaptive evolution. Our aim in this study was to measure variation of floral size in the invasive shrub Cytisus scoparius (Scotch broom) and measure natural selection by pollinators on that trait. Past research has found that this invasive plant is pollinator limited in Washington State and that declines in pollinator populations can contribute to local extinction in another invaded range (New Zealand). This plant is pollinated by both native and introduced species of bees, representing a broad range of pollinator sizes. Cytisus scoparius has a flower structure that is highly conducive to studies on pollinator choice, even in the absence of direct pollinator observations.Methods We surveyed urban and rural sites in and around the city of Olympia in Washington State. Measuring banner width, we were able to show that flower size varies substantially between plants but minimally within plants. By measuring the proportion of flowers that were 'tripped', we could determine pollinator visitation rates and thus determine the level of selection due to pollinator choice alone.Important findings We found that C. scoparius is under natural selection by pollinators for increased flower size. However, such positive natural selection was only seen in urban populations although it was consistent across two flowering seasons. Rural populations of Scotch broom do not appear to be under selection on flower size. The natural selection by pollinators on broom flowers could result in adaptive evolution into a new pollination niche by an invading species. A higher level of variation in broom flowers seen here than seen in previous works in native regions suggests that C. scoparius may be highly diverse and primed for adaptive evolution.