Bottom-up effects of nutrient availability on flower production, pollinator visitation, and seed output in a high-Andean shrub

Soil nutrient availability directly enhances vegetative growth, flowering, and fruiting in alpine ecosystems. However, the impacts of nutrient addition on pollinator visitation, which could affect seed output indirectly, are unknown. In a nutrient addition experiment, we tested the hypothesis that seed output in the insect-pollinated, self-incompatible shrub, Chuquiraga oppositifolia (Asteraceae) of the Andes of central Chile, is enhanced by soil nitrogen (N) availability. We aimed to monitor total shrub floral display, size of flower heads (capitula), pollinator visitation patterns, and seed output during three growing seasons on control and N addition shrubs. N addition did not augment floral display, size of capitula, pollinator visitation, or seed output during the first growing season. Seed mass and viability were 25–40% lower in fertilised shrubs. During the second growing season only 33% of the N addition shrubs flowered compared to 71% of controls, and a significant (50%) enhancement in vegetative growth occurred in fertilised shrubs. During the third growing season, floral display in N addition shrubs was more than double that of controls, received more than twice the number of insect pollinator visits, and seed output was three- to four-fold higher compared to controls. A significant (50%) enhancement in vegetative growth again occurred in N addition shrubs. Results of this study strongly suggest that soil N availability produces strong positive bottom-up effects on the reproductive output of the alpine shrub C. oppositifolia. Despite taking considerably longer to be manifest in comparison to the previously reported top-down indirect negative effects of lizard predators in the same study system, our results suggest that both bottom-up and top-down forces are important in controlling the reproductive output of an alpine shrub.     

Pollination of Euphorbia dendroides by lizards and insects: Spatio-temporal variation in patterns of flower visitation

The patterns of flower visitation by lizards (Podarcis lilfordi, Lacertidae) and insects (mainly flies, bees and wasps) on the shrub Euphorbia dendroides, were studied in the island of Cabrera (Balearic Islands) during the flowering seasons of 1995 and 1996. Lizards act as true pollinators of the plant, moving large quantities of pollen within and among shrubs. To our knowledge, this is the first time that pollination by lizards has been empirically demonstrated. Variation in the quantitative component of pollination (frequency of visits × flower visitation rate) by the two groups of pollinators (lizards and insects) is documented at both spatial (within a plant population) and temporal scales (throughout the flowering season and between seasons). Variation in lizard density on a small spatial scale (within c. 200 m), presumably due to differences in vegetation cover, strongly affected their frequency of flower visitation. Insects were rather scarce, mainly because the plant flowers at a time (mid-March) when temperatures are still low. At the site where lizards were abundant, their frequency of flower visits was more than 3 times that of insects, they stayed on the shrubs about 3 times longer and visited about 8 times more cyathia per minute than did insects. Fruit and seed set were greater at this site, and this is attributed to the different frequency of flower visits by lizards, as shrubs are similar in size and produce similar amounts of cyathia in the two sites compared. Both, lizards and insects went more frequently to plants with large flower crops. However, flower crop was not associated with seed viability. We found no evidence for pollinator-mediated selection on plant traits related to fitness. Received: 28 August 1996 / Accepted: 26 February 1997     

Experimental reduction of pollinator visitation modifies plant–plant interactions for pollination

The strength of interactions between plants for pollination depends on the abundance of plants and pollinators in the community. The abundance of pollinators may influence plant associations and densities at which individual fitness is maximized. Reduced pollinator visitation may therefore affect the way plant species interact for pollination. We experimentally reduced pollinator visitation to six pollinator‐dependent species (three from an alpine and three from a lowland community in Norway) to study how interactions for pollination were modified by reduced pollinator availability. We related flower visitation, pollen limitation and seed set to density of conspecifics and pollinator‐sharing heterospecifics inside 30 dome‐shaped cages partially covered with fishnet (experimental plots) and in 30 control plots. We expected to find stronger interactions between plants in experimental compared to controls plots. The experiment modified plant–plant interactions for pollination in all the six species; although for two of them neighbourhood interactions did not affect seed set. The pollen limitation and seed set data showed that reduction of pollinator visits most frequently resulted in novel and/or stronger interactions between plants in the experimental plots that did not occur in the controls. Although the responses were species‐specific, there was a tendency for increasing facilitative interactions with conspecific neighbours in experimental plots where pollinator availability was reduced. Heterospecifics only influenced pollination and fecundity in species from the alpine community and in the experimental plots, where they competed with the focal species for pollination. The patterns observed for visitation rates differed from those for fecundity, with more significant interactions between plants in the controls in both communities. This study warns against the exclusive use of visitation data to interpret plant–plant interactions for pollination, and helps to understand how plant aggregations may buffer or intensify the effects of a pollinator loss on plant fitness.     

The relative importance of solitary bees and syrphid flies as pollinators of two outcrossing plant species in the New Zealand alpine

Pollinators vary in their relative contribution to the conspecific pollen deposited onto receptive stigmas, because of variation in both visitation rate and effectiveness of pollen transfer. Syrphid flies and short‐tongued solitary bees are common flower visitors in alpine New Zealand, yet their relative importance as pollinators is unknown. We measured pollinator performance of the New Zealand alpine endemics Hylaeus matamoko (Hymenoptera: Colletidae) and Allograpta spp. (Diptera: Syrphidae) on two New Zealand alpine herbs, Ourisia glandulosa (Plantaginaceae) and Wahlenbergia albomarginata (Campanulaceae). Ourisia glandulosa received visits by solitary bees and syrphid flies at equal frequencies, whereas W. albomarginata was mostly visited by H. matamoko. Based on single‐visit pollen deposition to virgin stigmas, H. matamoko was a much more effective pollinator than Allograpta spp., delivering 10 times as much pollen per visit to O. glandulosa stigmas and 3 times as much to W. albomarginata stigmas. By multiplying visitation frequency by single‐visit pollen deposition, we estimated that H. matamoko performed 90% and 95% of the pollination of O. glandulosa and W. albomarginata, respectively. Although H. matamoko bees are short‐tongued and small in size, they are critically important to plant reproductive success in the New Zealand alpine. These bees contributed most of the pollination, even to a species that received just as many visits by flies, underscoring the need to consider per‐visit effectiveness as well as visitation rate in assessing the importance of different pollinators.     

Plant connectivity underlies plant‐pollinator‐exploiter distributions in Ficus petiolaris and associated pollinating and non‐pollinating fig wasps

Mutualism is ubiquitous in nature, and nursery pollination mutualisms provide a system well suited to quantifying the benefits and costs of symbiotic interactions. In nursery pollination mutualisms, pollinators reproduce within the inflorescence they pollinate, with benefits and costs being measured in the numbers of pollinator offspring and seeds produced. This type of mutualism is also typically exploited by seed‐consuming non‐pollinators that obtain resources from plants without providing pollination services. Theory predicts that the rate at which pollen‐bearing ‘foundresses’ visit a plant will strongly affect the plant's production of pollinator offspring, non‐pollinator offspring, and seeds. Spatially aggregated plants are predicted to have high rates of foundress visitation, increasing pollinator and seed production, and decreasing non‐pollinator production; very high foundress visitation may also decrease seed production indirectly through the production of pollinators. Working with a nursery mutualism comprised of the Sonoran Desert rock fig, Ficus petiolaris, and host‐specific pollinating and non‐pollinating fig wasps, we use linear models to evaluate four hypotheses linking species interactions to benefits and costs: 1) foundress density increases with host‐tree connectivity, 2) pollinator production increases with foundress density, and 3) non‐pollinator production and 4) seed production decrease with pollinator production. We also directly test how tree connectivity affects non‐pollinator production. We find strong support for our four hypotheses, and we conclude that tree connectivity is a key driver of foundress visitation, thereby strongly affecting spatial distributions in the F. petiolaris community. We also find that foundress visitation decreases at the northernmost edge of the F. petiolaris range. Finally, we find species‐specific effects of tree connectivity on non‐pollinators to be strongly correlated with previously estimated non‐pollinator dispersal abilities. We conclude that plant connectivity is highly important for predicting plant‐pollinator‐exploiter dynamics, and discuss the implications of our results for species coexistence and adaptation.     

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.     

Within-population spatial variation in pollinator visitation rates, pollen limitation on seed set, and flower longevity in an alpine species

Pollen limitation through insufficient pollen deposition on stigmas caused by too infrequent pollinator visitation may influence the reproductive outcome of plants. In this study we investigated how pollinator visitation rate, the degree of pollen limitation, and flower longevity varied spatially among three sites at different altitudes within a population of the dwarf shrub Dryas octopetala L. in alpine southern Norway. Significant pollen limitation on seed set only occurred at the mid-elevation site, while seed set at the other sites appeared to be mainly resource limited, thus indicating a spatial variation in pollen limitation. There was no association between the spatial variation in the extent of pollen limitation and pollinator visitation rate to flowers. However, pollinator visitation rates were related to flower longevity of Dryas; sites with low visitation rates had long-lived flowers and vice versa. Thus, our results suggest within-population spatial co-variation between pollinator visitation rates, pollen limitation, and a developmental response to these factors, flower longevity.     

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

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

Pollinators and seed predators generate conflicting selection on Dalechampia blossoms

Plant–pollinator relationships are often mediated by floral traits that advertise the presence or amount of rewards. However, herbivores may also use these traits to find their hosts. In Dalechampia scandens, we tested whether floral advertisements that attract pollinators were also used by seed predators, and whether this generated conflicting selection pressures. We studied the influence of natural variation in the size of showy bracts, amount of reward, and two shape traits on pollinator visitation, pollen arrival on stigmas, seed production and seed predation. We then built a multivariate fitness function for these traits to estimate selection generated by pollinators and seed predators. Blossoms with larger bracts were visited by bees more frequently and received more pollen on their stigmas. Seed predators laid more eggs on blossoms with larger bracts and also on blossoms later producing more seeds. Consequently, selection for larger bract size exerted by pollinators was counteracted by the selection exerted by seed predators. As a result, net selection on bract size tended to be stabilizing. Additionally, we found selection on traits that increased the rate of self‐pollination (assuming uniform seed quality). These results illustrate the importance of both mutualists and antagonists in floral evolution, as well as the value of taking an integrative approach to assessing selection on floral traits.     

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

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

Intrinsic and extrinsic factors acting on the reproductive process in alpine‐snowbed plants: roles of phenology,biological interaction,and breeding system

Pollinator activity and competition for pollinators lead to quantitative and qualitative pollen limitations on seed production and affect the reproductive success of plant species, depending on their breeding system (e.g., self‐compatibility and heterospecific compatibility) and genetic load (e.g., inbreeding depression and hybrid inviability). In alpine ecosystems, snowmelt regimes determine the distribution and phenology of plant communities. Plant species growing widely along a snowmelt gradient often grow with different species among local populations. Their pollinators also vary in their abundance, activity, and behavior during the season. These variations may modify plant–pollinator and plant–plant interactions. We integrated a series of our studies on the alpine dwarf shrub, Phyllodoce aleutica (Ericaceae), to elucidate the full set of intrinsic (species‐specific breeding system) and extrinsic factors (snow condition, pollinator activity, and interspecific competition) acting on their reproductive process. Seasonality of pollinator activity led to quantitative pollen limitation in the early‐blooming populations, whereas in the late‐blooming populations, high pollinator activity ensured pollination service, but interspecific competition for pollinators led to qualitative and quantitative pollen limitation in less competitive species. However, negative effects of illegitimate pollen receipt on seed‐set success might be reduced when cryptic incompatibility systems (i.e., outcross pollen grains took priority over self‐ and heterospecific pollen grains) could effectively prevent ovule and seed discounting. Our studies highlight the importance of species‐specific responses of plant reproduction to changing pollinator availability along environmental gradients to understand the general features of pollination networks in alpine ecosystems.     

Temporal variations in the linkages between plants and flower visitors and the pollination success of Primula modesta along the snowmelt gradient

Although pollination networks between plants and flower visitors are diverse and flexible, seed production of many plant species is restricted by pollen limitation. Obligate outcrossers often suffer from low pollinator activity or severe interspecific competition for pollinator acquisition among co-flowering species. This study focused on seasonal changes in plant–flower visitor linkages in an alpine ecosystem and examined whether and how this seasonality affected the seed-set of Primula modesta, a self-incompatible distylous herb having long-tubed flowers. First, we recorded the linkages between plants and flower visitors along the snowmelt gradient. Then, pollination experiment was conducted to estimate the degree of pollen limitation over the course of flowering season of P. modesta. Flower visitors were classified by their tongue length based on the morphological matching with P. modesta flowers. As the season progressed, plant–visitor linkages became more diverse and generalized, and the visitation frequency to P. modesta flowers increased. In the later part of the season, however, the seed set of P. modesta was significantly reduced due to severe pollen limitation, presumably because of increased competition for long-tongued pollinators among co-flowering species. The present study revealed that pollinator availability for specialist species may be restricted even when plant–visitor linkages are diverse and generalized as a whole. In the case of P. modesta, morphological matching and competition for pollinators might be the main factors explaining this discrepancy.     

POLLEN DISPERSAL DYNAMICS IN AN ALPINE WILDFLOWER,POLEMONIUM VISCOSUM

The alpine wildflower, Polemonium viscosum, depends on insect visitors for effective pollination. Here, I examine experimentally the effects of pollinator visitation on pollen removal, pollen dispersal success, paternity, and gene flow. Bumble bee pollinators visited donor individuals homozygous for marker alleles at an isozyme (GOT-2) encoding locus and then were presented with arrays of recipient plants lacking the marker alleles. Four aspects of male fitness were estimated for each donor: the number of pollen grains dispersed to flowers of the first recipient visited, the number of offspring sired on that recipient, the proportion of offspring sired in the full array, and the proportion of mates in the array bearing seeds of the donor. Pollen removal was strongly influenced by the number of bee visits to donor flowers. The amount of pollen removed in turn significantly affected the number of pollen grains reaching flowers of the first recipient. However, because seed production decelerates with stigma pollen load, the relationship between pollen export and paternal success at this proximate scale showed diminishing returns. The probability of reaching mates within the array also increased with pollen export. These findings show that floral characters enhancing pollinator visitation rate in P. viscosum have positive effects on paternity and gene flow.     

Fly pollination of Linum lewish (Linaceae)

This study examines the reproductive biology of Linum lewisii Pursh. (Linaceae), a polyphilic species visited by small bees and generalist flies in montane Colorado. L. lewisii plants growing at different sites experience large temporal and spatial variations in pollinator visits. Their ability to attract both dipteran and hymenopteran pollinators allows pollination under varying conditions as pollinator pool composition changes. Although L. lewisii is self-compatible, hand-pollination studies indicate that insects are required for seed production. The relative effectiveness of fly and bee pollinators is assessed in terms of per-visit pollen deposition. Insect visitation patterns are combined with per-visit effectiveness data to evaluate the relative importance of different pollinator groups. Overall, bees tend to be more effective than flies in depositing pollen. However, in many instances flies appear to be responsible for more pollen deposition due to their higher visitation rates.     

Effect of elevation on sexual reproduction in alpine populations of Saxifraga oppositifolia (Saxifragaceae)

Self-compatibility in high arctic and alpine areas is regarded as an adaptation to low pollinator abundance. However, high genetic variability as a consequence of outcrossing is, with regard to population persistence, favorable in highly stochastic environments such as tundra habitats. To evaluate these contradictory scenarios, I performed in situ pollination experiments to examine the breeding system of the predominant outcrosser Saxifraga oppositifolia in ten populations at two different elevations in the Swiss Alps. Pollinator limitation was detected at both elevations, but fruit set in naturally pollinated flowers was only slightly less at the higher elevation. Increased pollinator limitation at high compared with low elevation thus could not be demonstrated in this experiment. Hand-crossings yielded equal mean proportion seed set at both elevations, and so did hand-selfings. This constant pattern of the breeding system in S. oppositifolia indicates selective factors that lead to the maintenance of a high level of outcrossing even in high-elevation populations. Based on sex allocation models, it was expected that a high ovule number should be selectively advantageous in a plant-pollinator system where chance visitation or selfing play important roles. However, female reproductive offer in terms of ovule number per flower did not change from low to high elevation. Since neither increased pollinator limitation nor increased seed set in selfed flowers was found at high compared with low elevation, the prerequisites for testing the hypothesis were not given. This study contradicts the hypothesis that inimical environmental factors in alpine or arctic habitats necessarily select for increased selfing rates in a preferentially outcrossing species like S. oppositifolia. Received: 28 April 1997 / Accepted: 20 October 1997     

Variation in pollinator assemblages in a fragmented landscape and its effects on reproductive stages of a self-incompatible treelet, <Emphasis Type="Italic">Psychotria suterella</Emphasis> (Rubiaceae)

Few studies of plant–pollinator interactions in fragmented landscapes evaluate the consequences of floral visitor variation on multiple stages of plant reproduction. Given that fragmentation potentially has positive or negative effects on different organisms, and that self-incompatible plant species depend on pollinators for sexual reproduction, differences in floral visitor assemblages may affect certain plant reproductive stages. We evaluated how pollinator assemblage, availability of floral resources, pollination, reproductive output, and seed and seedling performance of Psychotria suterella Muell. Arg. varied among three fragmentation categories: non-fragmented habitats, fragments connected by corridors, and isolated fragments. Richness and frequency of floral visitors were greater in fragments than in non-fragmented sites, resulting mainly from the addition of species typically found in disturbed areas. Although 24 species visited Psychotria suterella flowers, bumblebees were considered the most important pollinators, because they showed the highest frequency of visits and were present in eight out of ten sites. Additionally, the number of pollen tubes per flower per visit was lower in areas without bumblebees. The increased visitation in fragments seemed to enhance pollination slightly. However, fruit and seed output, germination, and seed and seedling mass were similar in non-fragmented sites, connected sites, and isolated fragments. Our results suggested that, even for a self-incompatible species, responses to habitat fragmentation at different stages of plant reproduction might be decoupled from the responses observed in floral visitors, if fruit set is not pollen limited. If all reproductive stages were considered, variation on the small scale was more important than the variation explained by fragmentation category. In spite of its self-incompatible breeding system, this plant–pollinator system showed resilience to habitat fragmentation, mainly as a result of high availability of potential mates to P. suterella individuals, absence of pollen limitation, and the presence of bumblebees (Bombus spp.) throughout this highly connected landscape.     

Shading by invasive shrub reduces seed production and pollinator services in a native herb

Plant invasions disrupt native plant reproduction directly via competition for light and other resources and indirectly via competition for pollination. Furthermore, shading by an invasive plant may reduce pollinator visitation and therefore reproduction in native plants. Our study quantifies and identifies mechanisms of these direct and indirect effects of an invasive shrub on pollination and reproductive success of a native herb. We measured pollinator visitation rate, pollen deposition, and female reproductive success in potted arrays of native Geranium maculatum in deciduous forest plots invaded by the non-native shrub Lonicera maackii and in two removal treatments: removal of aboveground L. maackii biomass and removal of flowers. We compared fruit and seed production between open-pollinated and pollen-supplemented plants to test for pollen and light limitation of reproduction. Plots with L. maackii had significantly lower light, pollinator visitation rate, and conspecific pollen deposition to G. maculatum than biomass removal plots. Lonicera maackii flower removal did not increase pollinator visitation or pollen deposition compared to unmanipulated invaded plots, refuting the hypothesis of competition for pollinators. Thus, pollinator-mediated impacts of invasive plants are not limited to periods of co-flowering or pollinator sharing between potential competitors. Geranium maculatum plants produced significantly fewer seeds in plots containing L. maackii than in plant removal plots. Seed set was similar between pollen-supplemented and open-pollinated plants, but pollen-supplemented plants exhibited higher seed set in plant removal plots compared to invaded plots. Therefore, we conclude that the mechanism of impact of L. maackii on G. maculatum reproduction was increased understory shade.     

Positive interactions among plant species for pollinator service: assessing the ‘magnet species’ concept with invasive species

Plants with poorly attractive flowers or with little floral rewards may have inadequate pollinator service, which in turn reduces seed output. However, pollinator service of less attractive species could be enhanced when they are associated with species with highly attractive flowers (so called ‘magnet‐species’). Although several studies have reported the magnet species effect, few of them have evaluated whether this positive interaction result in an enhancement of the seed output for the beneficiary species. Here, we compared pollinator visitation rates and seed output of the invasive annual species Carduus pycnocephalus when grow associated with shrubs of the invasive Lupinus arboreus and when grow alone, and hypothesized that L. arboreus acts as a magnet species for C. pycnocephalus. Results showed that C. pycnocephalus individuals associated with L. arboreus had higher pollinator visitation rates and higher seed output than individuals growing alone. The higher visitation rates of C. pycnocephalus associated to L. arboreus were maintained after accounting for flower density, which consistently supports our hypothesis on the magnet species effect of L. arboreus. Given that both species are invasives, the facilitated pollination and reproduction of C. pycnocephalus by L. arboreus could promote its naturalization in the community, suggesting a synergistic invasional process contributing to an ‘invasional meltdown’. The magnet effect of Lupinus on Carduus found in this study seems to be one the first examples of indirect facilitative interactions via increased pollination among invasive species.     

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.