How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: A case study using the alpine skypilot,Polemonium viscosum

Under climate change, shrubs encroaching into high altitude plant communities disrupt ecosystem processes. Yet effects of encroachment on pollination mutualisms are poorly understood. Here, we probe potential fitness impacts of interference from encroaching Salix (willows) on pollination quality of the alpine skypilot, Polemonium viscosum. Overlap in flowering time of Salix and Polemonium is a precondition for interference and was surveyed in four extant and 25 historic contact zones. Pollinator sharing was ascertained from observations of willow pollen on bumble bees visiting Polemonium flowers and on Polemonium pistils. We probed fitness effects of pollinator sharing by measuring the correlation between Salix pollen contamination and seed set in naturally pollinated Polemonium. To ascertain whether Salix interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and Salix pollen addition. In current and past contact zones Polemonium and Salix overlapped in flowering time. After accounting for variance in flowering date due to latitude, Salix and Polemonium showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. Salix pollen is carried by bumble bees when visiting Polemonium flowers and accounts for up to 25% of the grains on Polemonium pistils. Salix contamination correlates with reduced seed set in nature and when applied experimentally. Postpollination processes likely mediate these deleterious effects as seed set in nature was not limited by pollen delivery. Synthesis: As willows move higher with climate change, we predict that they will drive postpollination interference, reducing the fitness benefits of pollinator visitation for Polemonium and selecting for traits that reduce pollinator sharing.     

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

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

Interspecific pollen transfer as a mechanism of competition: Consequences of foreign pollen contamination for seed set in the alpine wildflower,Polemonium viscosum

Summary Earlier studies showed that under natural conditions foreign pollen receipt is correlated with loss of seed set in populations of P. viscosum. Here, we report on experimental pollinations using foreign pollen from two co-flowering species that share pollinators with P. viscosum. Experiments were designed to ascertain whether improper pollen transfer causes reduced seed set under controlled conditions and, if so, to determine the stage at which interference occurs. In flowers pre-treated by pollination with either Mertensia or Castilleja pollen, adherence of subsequently deposited conspecific pollen grains was unaffected. However, conspecific grains had significantly lower germination and ovule fertilization success in flowers receiving foreign pollen than in controls. These results suggest that pollinator infidelity in high alpine plant communities can reduce female reproductive success of P. viscosum by inhibiting the performance of conspecific pollen grains. Negative effects of Mertensia and Castilleja pollen were comparable despite large interspecific differences in pollen grain morphology. Accordingly, susceptibility to foreign pollen interference appears to be an intrinsic property of P. viscosum that is not dependent on the unique structural properties of pollen produced by competitive species.     

Bumble bee behavior and selection on flower size in the sky pilot,Polemonium viscosum

Summary In alpine Polemonium viscosum, plants having sweet-scented flowers are primarily pollinated by queens of the bumble bee species, Bombus kirbyellus. In this paper we ask whether two aspects of the pollination effectiveness of bumble bees, visitation rate and pollination efficiency, vary significantly with flower size in sweet-flowered P. viscosum.(i) Bumble bees visited plants with large flowers on 80–90% of encounters, but visited those with smaller flowers on only 49% of encounters. (ii) However, the gain in pollination that large-flowered plants obtained via increased visitation was countered in part because bumble bees deposited fewer outcross pollen grains per visit on stigmas of large flowers than on those of small ones. When both visitation rate and pollination efficiency are taken into account, the predicted value of a single bumble bee encounter declines from 1.06 seeds for flowers larger than 18 mm in diameter to 0.55 seeds for flowers smaller than 12 mm in diameter. Our results suggest that bumble bee pollinators of P. viscosum prefer flower morphologies that are poorly suited for precise pollination. Such behavioral complexities are likely to place constraints on the evolution of optimal floral design.     

Norditerpene alkaloid concentrations in tissues and floral rewards of larkspurs and impacts on pollinators

Plant secondary compounds mediate interactions with insects and other animals. The norditerpene alkaloids are significant secondary compounds in Delphinium (larkspur) species which are divided into two classes: the 7, 8-methylenedioxylycoctonine (MDL-type) and N-(methylsuccinimido) anthranoyllycoctonine (MSAL-type), and are known to be toxic to herbivorous insects and livestock. Alkaloid concentrations were measured in a whole plant context in vegetative and floral tissues as well as rewards (pollen and nectar) in Delphinium barbeyi and Delphinium nuttallianum. Alkaloid concentrations differed between vegetative tissues, floral tissues and floral rewards. Alkaloid concentrations in floral parts were consistent with optimal defense theory, with tissues more closely tied to plant fitness, such as fruits, being more heavily defended than foliage. However, alkaloid concentrations were significantly lower in nectar compared to other tissues. The norditerpene alkaloids influenced the activity of bumble bees, the dominant pollinator of larkspur, but the effects were concentration dependent. Alkaloids in nectar are found at concentrations that have no effect on bee activity; however, if alkaloid concentrations in nectar were similar to those in foliage bee activity would be reduced significantly. These results suggest that nectar with low alkaloid concentrations may be beneficial to plant fitness by limiting adverse effects on pollinator activity.     

The effect of pollinators,predators, and energy constraints on the floral ecology and evolution of Trillium erectum

Summary The reproductive success and energy budgets of single and multiflowered plants of Trillium erectum L. (Liliaceae) were assessed over a period of 4 years. Plants with more than one flower were found to attract more pollinators, to be less vulnerble to predation by Tortricid moths, and to produce more seeds than single flowered plants. Despite the reproductive superiority exhibited by multiflowered plants, most plants (85%) in the population studied were single flowered. It is shown that even though the insect interactions make it advantageous to produce more than one flower, most plants lack the energy to do so. The optimality approach which has guided previous studies of this kind is criticzied.     

The evolution of floral scent: the influence of olfactory learning by insect pollinators on the honest signalling of floral rewards

1.  The evolution of flowering plants has undoubtedly been influenced by a pollinator's ability to learn to associate floral signals with food. Here, we address the question of 'why' flowers produce scent by examining the ways in which olfactory learning by insect pollinators could influence how floral scent emission evolves in plant populations.
2.  Being provided with a floral scent signal allows pollinators to learn to be specific in their foraging habits, which could, in turn, produce a selective advantage for plants if sexual reproduction is limited by the income of compatible gametes. Learning studies with honeybees predict that pollinator-mediated selection for floral scent production should favour signals which are distinctive and exhibit low variation within species because these signals are learned faster. Social bees quickly learn to associate scent with the presence of nectar, and their ability to do this is generally faster and more reliable than their ability to learn visual cues.
3.  Pollinators rely on floral scent as a means of distinguishing honestly signalling flowers from deceptive ones. Furthermore, a pollinator's sensitivity to differences in nectar rewards can bias the way that it responds to floral scent. This mechanism may select for flowers that provide olfactory signals as an honest indicator of the presence of nectar or which select against the production of a detectable scent signal when no nectar is present.
4.  We expect that an important yet commonly overlooked function of floral scent is an improvement in short-term pollinator specificity which provides an advantage to both pollinator and plant over the use of a visual signal alone. This, in turn, impacts the evolution of plant mating systems via its influence on the species-specific patterns of floral visitation by pollinators.     

Testing the potential for conflicting selection on floral chemical traits by pollinators and herbivores: predictions and case study

1.  There are myriad ways in which pollinators and herbivores can interact via the evolutionary and behavioural responses of their host plants.
2.  Given that both herbivores and pollinators consume and are dependent upon plant-derived nutrients and secondary metabolites, and utilize plant signals, plant chemistry should be one of the major factors mediating these interactions.
3.  Here we build upon a conceptual framework for understanding plant-mediated interactions of pollinators and herbivores. We focus on plant chemistry, in particular plant volatiles and aim to unify hypotheses for plant defence and pollination. We make predictions for the evolutionary outcomes of these interactions by hypothesizing that conflicting selection pressures from herbivores and pollinators arise from the constraints imposed by plant chemistry.
4.  We further hypothesize that plants could avoid conflicts between pollinator attraction and herbivore defence through tissue-specific regulation of pollinator reward chemistry, as well as herbivore-induced changes in flower chemistry and morphology.
5.  Finally, we test aspects of our predictions in a case study using a wild tomato species, Solanum peruvianum , to illustrate the diversity of tissue-specific and herbivore-induced differences in plant chemistry that could influence herbivore and pollinator behaviour, and plant fitness.     

Mismatch in the distribution of floral ecotypes and pollinators: insights into the evolution of sexually deceptive orchids

Plants are predicted to show floral adaptation to geographic variation in the most effective pollinator, potentially leading to reproductive isolation and genetic divergence. Many sexually deceptive orchids attract just a single pollinator species, limiting opportunities to experimentally investigate pollinator switching. Here, we investigate Drakaea concolor, which attracts two pollinator species. Using pollinator choice tests, we detected two morphologically similar ecotypes within D. concolor. The common ecotype only attracted Zaspilothynnus gilesi, whereas the rare ecotype also attracted an undescribed species of Pogonothynnus. The rare ecotype occurred at populations nested within the distribution of the common ecotype, with no evidence of ecotypes occurring sympatrically. Surveying for pollinators at over 100 sites revealed that ecotype identity was not correlated with wasp availability, with most orchid populations only attracting the rare Z. gilesi. Using microsatellite markers, genetic differentiation among populations was very low (G_ST = 0.011) regardless of ecotype, suggestive of frequent gene flow. Taken together, these results may indicate that the ability to attract Pogonothynnus has evolved recently, but this ecotype is yet to spread. The nested distribution of ecotypes, rather than the more typical formation of ecotypes in allopatry, illustrates that in sexually deceptive orchids, pollinator switching could occur throughout a species' range, resulting from multiple potentially suitable but unexploited pollinators occurring in sympatry. This unusual case of sympatric pollinators highlights D. concolor as a promising study system for further understanding the process of pollinator switching from ecological, chemical and genetic perspectives.     

High and Dry: Drought Stress, Sex-Allocation Trade-offs, and Selection on Flower Size in the Alpine Wildflower Polemonium viscosum (Polemoniaceae)

Sex-allocation trade-offs may maintain variation in secondary sexual characteristics if such traits vary in their benefits or costs in association with different genders. In Polemonium viscosum, large flowers benefit both male and female aspects of reproduction. In this study, I explore how resource investment in flower size influences the cost of allocation to male and female function. Large flowers exact a water cost in P. viscosum under dry conditions. In an extreme drought in 1997, experimentally watered plants had higher survival and fecundity than controls. By comparing allocation patterns between plants dying from drought and survivors, I tested whether the demographic cost of large flowers increases with allocation to fecundity. Controls that died showed a positive relationship between flower size and fruit production, while survivors showed a negative relationship or trade-off. Watered plants showed no such trade-off. To test whether drought affects the relationship of corolla size to male function, I used leaf-water potential in 1998 to classify plants as stressed or unstressed. Corolla size showed positive correlations to pollen per flower regardless of drought stress. I conclude that under drought the demographic cost of producing large flowers is gender dependent, such that viability selection favors either small-flowered plants with female-biased reproduction or larger-flowered plants with male-biased reproduction.     

Patterns of pollen removal and deposition in Polemonium brandegeei (Polemoniaceae): the role of floral visitors,floral design and sexual interference

The arrangement, colour, shape and size of floral parts (collectively floral design) have evolved primarily to promote mating success via animal‐mediated pollen transfer. Although numerous studies have examined variation in pollinator assemblages, relatively few have examined patterns of pollen removal and deposition in the presence of fluctuating pollinators and ineffective floral visitors; therefore, net pollen removal and deposition by entire visitor assemblages are unclear. We studied the timing (diurnal or nocturnal) and effects of floral traits on pollen removal and deposition under a dynamic visitor assemblage of Polemonium brandegeei. We quantified pollen grains remaining in anthers (pollen removal) and deposited on stigmas (pollen deposition) of plants visited during either the day (07:30–20:00 h) or night (20:30–07:30 h) in natural populations over two flowering seasons. Pollen removal and deposition occurred both diurnally and nocturnally during our study. Increased diurnal removal and deposition coincided with peak floral visitations in 2006. This increase in pollen removal and deposition may reflect increased visits by pollen consumers, effective hawkmoth pollinators and increased self‐pollen deposition due to hot, dry weather. Nonlinear effects of style length significantly affected pollen removal, with less pollen remaining in flowers with intermediate style lengths. Pollen deposition was more complex, with herkogamy and anther height affecting deposition. Further, close proximity of stigmas and anthers increased the potential for sexual interference between pollen removal and deposition. Overall, flower visitations and pollen removal and deposition varied between years and populations, but sex organ placement consistently influenced the removal and deposition of pollen.     

Indirect effects of grazing intensity on pollinators and floral visitation

1. The introduction of livestock in natural areas is a common disturbance that affects both plant and pollinator diversity and might affect their interaction. Understanding whether livestock affect a food resource for pollinators (i.e. flower abundance) and/or a pollinator assemblage (i.e. abundance and richness) has important implications for plant–pollinator interactions and still needs deeper investigation. 2. This study investigated how pollinator communities and flower abundance determined floral visitation frequency along a grazing gradient, using seven large paddocks in Patagonian Monte Desert that varied in livestock densities. Pollinator visitation frequency was measured in five of the most abundant native plant species of the region, present in all the paddocks, but that differed in reproductive strategy ranging from insect‐pollinated self‐compatible and self‐incompatible to wind‐pollinated. The influence of livestock density, insect, and flower abundance on visitation frequency was evaluated using D‐separation hierarchical path models. 3. Intermediate stocking densities showed the highest insect richness and abundance. Livestock density showed a negative quadratic relationship with insect richness; hymenopterans being the main insect group in the region. Flower density decreased with the increase in livestock density. The five plant species shared several pollinator species although each one supported a distinct assemblage. 4. The path model showed that livestock was not directly associated with pollinator visitation frequency; however, this apparent lack of association was as a result of opposite forces acting together. An increase in livestock density reduced visitation frequency through a decrease in insect abundance, yet, livestock simultaneously increased the pollinator visitation rate through decreased flower abundance. 5. This study describes how changes in the density of exotic mammals can affect pollinator and flower abundance, resulting in contrasting effects on flower visitation rates with, apparently, neutral net consequences. This illustrates the complexity of responses to plant–pollinator interactions to anthropogenic disturbances that alter the ecological context.     

Temporal stability of pollinator preference in an alpine plant community and its implications for the evolution of floral traits

A traditional view of diverse floral traits is that they reflect differences in foraging preferences of pollinators. The role of pollinators in the evolution of floral traits has been questioned recently by broad community surveys, especially studies concerning variation in pollinator assemblages and visitation frequency, which suggest a diminished role of pollinators in floral evolution. Here, we investigate the relationships between six categories of floral traits of 29 species and 10 pollinator functional groups in an alpine meadow in the Hengduan Mountains of China, over three consecutive years. Simpson’s diversity index was used to estimate the level of pollinator generalization of each plant species by considering both pollinator groups and their relative visitation frequencies. Multivariate analyses indicated that eight of the ten pollinator groups showed constant preferences for at least two floral traits, leading to a relatively stable level of ecological generalization for most floral traits (two out of three categories), despite the fact that the level of generalization of the entire community varied across years. Shape preferences of butterflies, honeybees and beeflies varied such that open flowers exhibited a lower level of ecological generalization in 2007 than closed flowers, in contrast with the other 2 years. These results suggest that temporally stabilized preferences of diverse pollinators may contribute to the evolution of specialized versus generalized floral traits; however, their role may be moderated by variation in community structure, including both the composition and abundance of plants and pollinators.     

Morning floral heat as a reward to the pollinators of the Oncocyclus irises

Relationships between flowering plants and their pollinators are usually affected by the amount of reward, mainly pollen or nectar, offered to pollinators by flowers, with these amounts usually positively correlated with floral display. The large Oncocyclus iris flowers, despite being the largest flowers in the East Mediterranean flora, are nectarless and have hidden pollen. No pollinators visit the flowers during daytime, and these flowers are pollinated only by night-sheltering solitary male bees. These iris flowers are partially or fully dark-colored, suggesting that they gather heat by absorbing solar radiation. Here we test the hypothesis that the dark-colored flowers of the Oncocyclus irises offer heat reward to their male solitary bee pollinators. Floral temperature was higher by 2.5°C than ambient air after sunrise. Solitary male bees emerged earlier after sheltering in Oncocyclus flowers than from other experimental shelter types. Pollination tunnels facing east towards the rising sun hosted more male bees than other aspects. We suggest that floral heat reward can explain the evolution of dark floral colors in Oncocyclus irises, mediated by the pollinators’ behavior.     

The role of pollinators in floral diversification in a clade of generalist flowers

Pollinator‐mediated evolutionary divergence has seldom been explored in generalist clades because it is assumed that pollinators in those clades exert weak and conflicting selection. We investigate whether pollinators shape floral diversification in a pollination generalist plant genus, Erysimum. Species from this genus have flowers that appeal to broad assemblages of pollinators. Nevertheless, we recently reported that it is possible to sort plant species into pollination niches varying in the quantitative composition of pollinators. We test here whether floral characters of Erysimum have evolved as a consequence of shifts among pollination niches. For this, we quantified the evolutionary lability of the floral traits and their phylogenetic association with pollination niches. As with pollination niches, Erysimum floral traits show weak phylogenetic signal. Moreover, floral shape and color are phylogenetically associated with pollination niche. In particular, plants belonging to a pollination niche dominated by long‐tongued large bees have lilac corollas with parallel petals. Further analyses suggest, however, that changes in color preceded changes in pollination niche. Pollinators seem to have driven the evolution of corolla shape, whereas the association between pollination niche and corolla color has probably arisen by lilac‐flowered Erysimum moving toward certain pollination niches for other adaptive reasons.     

Interaction between flowers, ants and pollinators: additional evidence for floral repellence against ants

It has been commonly suggested that ants negatively affect plant pollination, particularly in the tropics. We studied ant–flower–pollinator interactions in a lowland rainforest in Borneo. Frequency and duration of pollinator visits were compared between flowers attended by ants and flowers from which ants were excluded. In all four plant species studied, the activity of ants decreased the rate and/or duration of the pollinators’ floral visits. For this and other reasons it is expected that plants repel ants from flowers during anthesis. We tested this prediction for a different set of plant species in which we observed the behaviour of Dolichoderus thoracicus ants when encountering flowers. In eight out of 18 plant species studied, ants showed a significantly higher rejection rate when they encountered flowers than when they encountered controls. Our results are thus consistent with the hypothesis that ants may negatively affect plant fitness by reduced intensity of pollinator visits and that ants are repelled from flowers of many tropical plant species, although this repellence is clearly not ubiquitous.     

Divergence on floral traits and vertebrate pollinators of two endemic Encholirium bromeliads

Shifts in pollen vectors favour diversification of floral traits, and differences in pollination strategies between congeneric sympatric species can contribute to reproductive isolation. Divergence in flowering phenology and selfing could also reduce interspecific crossing between self‐compatible species. We investigated floral traits and visitation rates of pollinators of two sympatric Encholirium species on rocky outcrops to evaluate whether prior knowledge of floral characters could indicate actual pollinators. Data on flowering phenology, visitation rates and breeding system were used to evaluate reproductive isolation. Flowering phenology overlapped between species, but there were differences in floral characters, nectar volume and concentration. Several hummingbird species visited flowers of both Encholirium spp., but the endemic bat Lonchophylla bokermanni and an unidentified sphingid only visited E. vogelii. Pollination treatments demonstrated that E. heloisae and E. vogelii were partially self‐compatible, with weak pollen limitation to seed set. Herbivores feeding on inflorescences decreased reproductive output of both species, but for E. vogelii the damage was higher. Our results indicate that actual pollinators can be known beforehand through floral traits, in agreement with pollination syndromes stating that a set of floral traits can be associated with the attraction of specific groups of pollinators. Divergence on floral traits and pollinator assemblage indicate that shifts in pollination strategies contribute to reproductive isolation between these Encholirium species, not divergence on flowering phenology or selfing. We suggest that hummingbird pollination might be the ancestral condition in Encholirium and that evolution of bat pollination made a substantial contribution to the diversification of this clade.