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.     

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.     

Biocontrol attack increases pollen limitation under some circumstances in the invasive plant Centaurea solstitialis

Herbivore damage often deters pollinator visitation and many invasive plants in North America are pollinator-dependent. This has important implications for the biological control of invasive plants because it means that agents that deter pollinators may have a larger than expected impact on the plant. Yet interactions between pollinators and biocontrol agents are rarely evaluated. Centaurea solstitialis, one of the most problematic invasive species in California, is dependent on pollinators for reproduction. I factorially manipulated infection by a biocontrol pathogen and pollen supplementation to test for (1) pollen limitation in C. solstitialis, (2) whether infection increased pollen limitation, and (3) whether this varied across a soil moisture gradient. Plants growing on north-facing slopes where soil moisture was higher experienced mild pollen limitation in the absence of the pathogen and more pronounced pollen limitation when they were infected. Plants on drier south-facing slopes did not suffer from pollen limitation but instead appeared to suffer from resource limitation. Pathogen infection directly reduced seed set in C. solstitialis by 67–72 %. On north-facing slopes, infection had an additional, indirect effect by increasing the degree of pollen limitation plants experienced. The trait that mediates this indirect pathogen–pollinator interaction is the number of inflorescences plants produced: infected plants made fewer inflorescences which led to greater pollen limitation. Although in the present study this outcome is dependent on abiotic factors that vary over small spatial scales, exploiting other invasive plants’ dependence on pollinators by selecting agents that deter visitation may enhance agent impact.     

A modelling approach to estimate the effect of exotic pollinators on exotic weed population dynamics: bumblebees and broom in Australia

The role of mutualisms in contributing to species invasions is rarely considered, inhibiting effective risk analysis and management options. Potential ecological consequences of invasion of non‐native pollinators include increased pollination and seed set of invasive plants, with subsequent impacts on population growth rates and rates of spread. We outline a quantitative approach for evaluating the impact of a proposed introduction of an invasive pollinator on existing weed population dynamics and demonstrate the use of this approach on a relatively data‐rich case study: the impacts on Cytisus scoparius (Scotch broom) from proposed introduction of Bombus terrestris. Three models have been used to assess population growth (matrix model), spread speed (integrodifference equation), and equilibrium occupancy (lattice model) for C. scoparius. We use available demographic data for an Australian population to parameterize two of these models. Increased seed set due to more efficient pollination resulted in a higher population growth rate in the density‐independent matrix model, whereas simulations of enhanced pollination scenarios had a negligible effect on equilibrium weed occupancy in the lattice model. This is attributed to strong microsite limitation of recruitment in invasive C. scoparius populations observed in Australia and incorporated in the lattice model. A lack of information regarding secondary ant dispersal of C. scoparius prevents us from parameterizing the integrodifference equation model for Australia, but studies of invasive populations in California suggest that spread speed will also increase with higher seed set. For microsite‐limited C. scoparius populations, increased seed set has minimal effects on equilibrium site occupancy. However, for density‐independent rapidly invading populations, increased seed set is likely to lead to higher growth rates and spread speeds. The impacts of introduced pollinators on native flora and fauna and the potential for promoting range expansion in pollinator‐limited ‘sleeper weeds’ also remain substantial risks.     

Little evidence for negative effects of an invasive alien plant on pollinator services

Many invasive alien plants occur in large populations with abundant flowers which are highly attractive to pollinators, and thus might affect pollination of co-occurring native species. This study focuses on the invasive Heracleum mantegazzianum and distance-dependent effects on pollination of Mimulus guttatus in abandoned grassland over 2 years. First, we examined pollinator abundance in yellow traps at 0, 10, 30 and 60–200 m from H. mantegazzianum. We then placed M. guttatus plants at the same distances to monitor effects of the invasive species on pollinator visitation and seed set of neighbouring plants. Finally, we conducted a garden experiment to test if deposition of H. mantegazzianum pollen reduces seed set in M. guttatus. No distance effect was found for the number of bumblebees in traps, although the invasive species attracted a diverse assemblage of insects, and visitation of M. guttatus was enhanced close to H. mantegazzianum. This positive effect was not reflected by seed set of M. guttatus, and heterospecific pollen decreased seed set in these plants. Overall there is little evidence for negative effects of the invasive species on pollination of neighbouring plants, and flower visitation even increases close to the invaded patches. The functional role of the invader and suitable control strategies need further clarification, since removal of H. mantegazzianum may actually damage local pollinator populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Indirect positive effects of a parasitic plant on host pollination and seed dispersal

Parasitic plants often have a strong fitness‐impact on their plant hosts through increased host mortality and reduced or complete suppression of reproduction. Tristerix corymbosus (Loranthaceae) is a hemiparasitic mistletoe that infects a wide range of host species along its distribution range. Among such species, Rhaphithamnus spinosus (Verbenaceae) is a frequent host with a flowering and fruiting season partially synchronized with mistletoe reproductive phenology. As parasitized hosts have, in principle, a larger flower display and fruit crop size than non‐parasitized hosts, we examined whether host and parasite reproductive synchrony make infected hosts more attractive for pollinators and seed dispersers than uninfected hosts. Our results showed that pollinator visit rates did not differ between parasitized and non‐parasitized hosts. Conversely, seed rain was higher in parasitized than non‐parasitized individuals. The number of seeds fallen under non‐parasitized plants was spatially associated with crop size, while parasitized plants did not show such association. Finally, the number of seedlings of R. spinosus was significantly larger near parasitized than non‐parasitized hosts. Our results suggest that the presence of the mistletoe might be responsible of the higher reproductive success showed by the parasitized fraction of R. spinosus. This effect, however, seems to be related to seed dispersal processes rather than pollination effects.     

Effects of fungal endophytes on the seed and seedling biology of Lolium perenne and Festuca arundinacea

Summary Many grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.     

Evaluation of pollinator effectiveness based on pollen deposition and seed production in a gynodieocious alpine plant,Cyananthus delavayi

Examining variations in pollinator effectiveness can enhance our understanding of how pollinators and plants interact. Pollen deposition and seed production after a single visit by a pollinator are often used to estimate pollinator effectiveness. However, seed production is not always directly related to pollen deposition because not all pollen grains that are deposited on a stigma are compatible or conspecific. In the field, we tested pollinator effectiveness based on pollen deposition and the resulting seed production after single visits by different pollinator groups in a gynodieocious alpine plant Cyananthus delavayi (Campanulaceae). Our results showed that mean pollen deposition was generally inconsistent with mean seed production when comparisons were performed among different pollinator groups and sexes. In general, the correlations were not significant between pollen deposition and seed production in both perfect and female flowers after single visits by halictid bees, bumble bees, and hoverflies. We suggest seed set of virgin flowers after single visits is a more reliable indicator of pollinator effectiveness than pollen deposition and would be a better indicator of pollinator effectiveness for future studies.     

Direct and indirect effects of episodic frost on plant growth and reproduction in subalpine wildflowers

Frost is an important episodic event that damages plant tissues through the formation of ice crystals at or below freezing temperatures. In montane regions, where climate change is expected to cause earlier snow melt but may not change the last frost‐free day of the year, plants that bud earlier might be directly impacted by frost through damage to flower buds and reproductive structures. However, the indirect effects of frost mediated through changes in plant–pollinator interactions have rarely been explored. We examined the direct and pollinator‐mediated indirect effects of frost on three wildflower species in southwestern Colorado, USA, Delphinium barbeyi (Ranunculaceae), Erigeron speciosus (Asteraceae), and Polemonium foliosissimum (Polemoniaceae), by simulating moderate (?1 to ?5°C) frost events in early spring in plants in situ. Subsequently, we measured plant growth, and upon flowering measured flower morphology and phenology. Throughout the flowering season, we monitored pollinator visitation and collected seeds to measure plant reproduction. We found that frost had species‐specific direct and indirect effects. Frost had direct effects on two of the three species. Frost significantly reduced flower size, total flowers produced, and seed production of Erigeron. Furthermore, frost reduced aboveground plant survival and seed production for Polemonium. However, we found no direct effects of frost on Delphinium. When we considered the indirect impacts of frost mediated through changes in pollinator visitation, one species, Erigeron, incurred indirect, negative effects of frost on plant reproduction through changes in floral traits and pollinator visitation, along with direct effects. Overall, we found that flowering plants exhibited species‐specific direct and pollinator‐mediated indirect responses to frost, thus suggesting that frost may play an important role in affecting plant communities under climate change.     

Contrasting effects of invasive plants in plant–pollinator networks

The structural organization of mutualism networks, typified by interspecific positive interactions, is important to maintain community diversity. However, there is little information available about the effect of introduced species on the structure of such networks. We compared uninvaded and invaded ecological communities, to examine how two species of invasive plants with large and showy flowers (Carpobrotus affine acinaciformis and Opuntia stricta) affect the structure of Mediterranean plant–pollinator networks. To attribute differences in pollination to the direct presence of the invasive species, areas were surveyed that contained similar native plant species cover, diversity and floral composition, with or without the invaders. Both invasive plant species received significantly more pollinator visits than any native species and invaders interacted strongly with pollinators. Overall, the pollinator community richness was similar in invaded and uninvaded plots, and only a few generalist pollinators visited invasive species exclusively. Invasive plants acted as pollination super generalists. The two species studied were visited by 43% and 31% of the total insect taxa in the community, respectively, suggesting they play a central role in the plant–pollinator networks. Carpobrotus and Opuntia had contrasting effects on pollinator visitation rates to native plants: Carpobrotus facilitated the visit of pollinators to native species, whereas Opuntia competed for pollinators with native species, increasing the nestedness of the plant–pollinator network. These results indicate that the introduction of a new species to a community can have important consequences for the structure of the plant–pollinator network.     

Comparative population genetic structure in a plant-pollinator/seed predator system

Comparative analyses of spatial genetic structure of populations of plants and the insects they interact with provide an indication of how gene flow, natural selection and genetic drift may jointly influence the distribution of genetic variation and potential for local co‐adaptation for interacting species. Here, we analysed the spatial scale of genetic structure within and among nine populations of an interacting species pair, the white campion Silene latifolia and the moth Hadena bicruris, along a latitudinal gradient across Northern/Central Europe. This dioecious, short‐lived perennial plant inhabits patchy, often disturbed environments. The moth H. bicruris acts both as its pollinator and specialist seed predator that reproduces by laying eggs in S. latifolia flowers. We used nine microsatellite markers for S. latifolia and eight newly developed markers for H. bicruris. We found high levels of inbreeding in most populations of both plant and pollinator/seed predator. Among populations, significant genetic structure was observed for S. latifolia but not for its pollinator/seed predator, suggesting that despite migration among populations of H. bicruris, pollen is not, or only rarely, carried over between populations, thus maintaining genetic structure among plant populations. There was a weak positive correlation between genetic distances of S. latifolia and H. bicruris. These results indicate that while significant structure of S. latifolia populations creates the potential for differentiation at traits relevant for the interaction with the pollinator/seed predator, substantial gene flow in H. bicruris may counteract this process in at least some populations.     

Different effects of two exotic herbivores on the pollinator-mediated effect of an exotic plant on a native plant

Exotic plants can affect native plants indirectly through various biotic interactions. However, combinations of the multiple indirect effects of exotic plants on native plants have been rarely evaluated. Herbivory can either positively or negatively influence plant–pollinator interactions. Here, we addressed whether the pollinator-mediated plant interaction between exotic and native plants is altered through the introduction of exotic herbivores by conducting a 2-year common garden experiment. We compared the effects of pollinator-mediated indirect effects of an exotic plant, Solidago altissima, on the co-flowering native plant Aster microcephalus in geographically different populations reflecting differences in insect herbivore communities. We found a positive effect of co-flowering S. altissima on pollinator visitation of A. microcephalus, which varied between gardens and years. The co-flowering S. altissima did not significantly affect the seed set of A. microcephalus in the first year but had a negative effect in the second year. The facilitative effect of S. altissima on A. microcephalus pollination was suggested to be negatively affected by an exotic aphid, while it was not significantly affected by an exotic lace bug. Our study suggests that the phenology and feeding guilds of the herbivores may be critical for predicting the effect of exotic plants on native plants through herbivore–pollinator interactions. Integrated effects between plant interactions via multiple species interactions under different abiotic and biotic environments are necessary to understand the impact of exotic plants under complex interactions in nature.     

A non‐pollinating moth inflicts higher seed predation than two co‐pollinators in an obligate pollination mutualism

1. Mutualisms are relationships of mutual exploitation, in which interacting species receive a net benefit from their association. In obligate pollination mutualisms (OPMs), female pollinators move pollen between the flowers of a single plant species and oviposit eggs within the female flowers that they visit. 2. Competition between co‐occurring pollinator species is predicted to increase pollinator virulence, i.e. laying more eggs or consuming more seeds per fruit. Plants involved in OPMs frequently host various non‐pollinating seed parasites and parasitoids that may influence the outcome of the mutualism. Quantifying the prevalence of parasites and parasitoids and competition between pollinators is important for understanding the factors that influence OPM evolutionary stability. 3. This study investigated the pollination mutualism occurring between the leaf flower plant, Breynia oblongifolia, and its co‐pollinating Epicephala moths. A third moth, Herpystis, also occurs in B. oblongifolia fruits as a non‐pollinating seed parasite. 4. Breynia oblongifolia fruits were collected to quantify seed predation and compare seed predation costs between the three moth species. Results showed that the larvae of the two pollinator species consume similar numbers of seeds, and that adults deposit similar numbers of eggs per flower. As such, no evidence of increases in virulent behaviours was detected as a result of competition between co‐pollinators. 5. By contrast, the seed parasite Herpystis consumed more seeds than either pollinator species, and fruit crops with a high proportion of Herpystis had significantly lower net seed production. 6. This work adds to the growing understanding of the ecology and dynamics of plant–pollinator mutualisms.     

Effects of experimental manipulation of inflorescence size on pollination and pre-dispersal seed predation in the hummingbird-pollinated plant Ipomopsis aggregata

Large floral displays should theoretically provide advantages to plants through increased pollinator visitation and resulting fruit and seed set. However empirical tests of the response of pollinators to floral display size have been limited by a lack of direct experimentation, and the results of such studies have been equivocal. In addition, other selective agents such as pre-dispersal seed predators might modulate effects of floral display on pollination. By artificially altering flower number, we examined the direct effects of floral display in the monocarpic herb, Ipomopsis aggregata (Polemoniaceae), on visitation rates by broad-tailed and rufous hummingbird pollinators, as well destruction of fruits by a pre-dispersal seed predator (Hylemya: Anthomyiidae). In addition, we quantified the ultimate effects of flower number on female reproductive success. Plants with larger floral displays were most likely to be visited first in any given foraging bout (P < 0.01). As expected, plants with more flowers received more total flower visits. However, we found no gain in the proportion of flowers visited for many- versus few-flowered plants, or the total number of approaches/hour. In fact, a significantly greater percentage of flowers were visited on few-flowered plants. Plants did not compensate for our reduction in flowers by increasing investment in the number or proportion of flowers that set fruit, the number of seeds/fruit, or seed weight. Pre-dispersal seed predation was greater for many- than for few-flowered plants (P < 0.001), but this did not offset the potential fitness gains of producing large displays. Our data support the hypothesis that large floral displays function primarily in long-distance attraction of pollinators, and enhance maternal success. Received: 21 March 1996 / Accepted: 24 October 1996     

Post-establishment assessment of host plant specificity of Arytainilla spartiophila (Hemiptera: Psyllidae), an adventive biological control agent of Scotch broom,Cytisus scoparius

Scotch broom, Cytisus scoparius (Fabaceae), is a shrub native to Europe that is invasive in the USA, New Zealand and Australia. The psyllid Arytainilla spartiophila has been purposely introduced to Australia and New Zealand as a biological control agent of C. scoparius, but is an accidental introduction to California. Lupines (Lupinus spp.) are the closest native taxon to Cytisus in North America, and are therefore considered to be at the highest risk for non-target damage. However, because no lupines are native to Australia or New Zealand, only one imported forage species was evaluated during prior host specificity testing. We conducted a laboratory nymphal transfer experiment, a field choice experiment and a field survey to assess risk to three lupine species (Lupinus albifrons, Lupinus bicolor and Lupinus formosus). In the laboratory, 20% of third-instar nymphs were able to develop to adulthood on L. formosus but not on the other lupine species, while 40% completed development on C. scoparius. In the field experiment, potted lupine and C. scoparius plants were placed beside large infested C. scoparius plants; oviposition occurred on all the potted C. scoparius plants, but on none of the lupines. In the field survey, no A. spartiophila eggs or nymphs were found on naturally occurring lupines growing adjacent to infested C. scoparius. The results indicate that A. spartiophila is not likely to damage or reproduce on lupines in the field. This study provides an example of how field studies can help clarify the host specificity of biological control agents.     

Introduced Scotch broom (Cytisus scoparius) invades the genome of native populations in vulnerable heathland habitats

Cytisus scoparius is a global invasive species that affects local flora and fauna at the intercontinental level. Its natural distribution spans across Europe, but seeds have also been moved among countries, mixing plants of native and non‐native genetic origins. Hybridization between the introduced and native gene pool is likely to threaten both the native gene pool and the local flora. In this study, we address the potential threat of invasive C. scoparius to local gene pools in vulnerable heathlands. We used nuclear single nucleotide polymorphic (SNP) and simple sequence repeat (SSR) markers together with plastid SSR and indel markers to investigate the level and direction of gene flow between invasive and native heathland C. scoparius. Analyses of population structures confirmed the presence of two gene pools: one native and the other invasive. The nuclear genome of the native types was highly introgressed with the invasive genome, and we observed advanced‐generation hybrids, suggesting that hybridization has been occurring for several generations. There is asymmetrical gene flow from the invasive to the native gene pool, which can be attributed to higher fecundity in the invasive individuals, measured by the number of flowers and seed pods. Strong spatial genetic structure in plastid markers and weaker structure in nuclear markers suggest that seeds spread over relatively short distances and that gene flow over longer distances is mainly facilitated by pollen dispersal. We further show that the growth habits of heathland plants become more vigorous with increased introgression from the invaders. Implications of the findings are discussed in relation to future management of invading C. scoparius.     

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.     

Plant–pollinator interactions between an invasive and native plant vary between sites with different flowering phenology

Floral displays of invasive plants have positive and negative impacts on native plant pollination. Invasive plants may also decrease irradiance, which can lead to reduced pollination of native plants. The effects of shade and flowers of invasive plant species on native plant pollination will depend on overlap in flowering phenologies. We examined the effect of the invasive shrub Lonicera maackii on female reproductive success of the native herb Hydrophyllum macrophyllum at two sites: one with asynchronous flowering phenologies (slight overlap) and one with synchronous (complete overlap). At each site, we measured light availability, pollinator visitation, pollen deposition, and seed set of potted H. macrophyllum in the presence and absence of L. maackii. At both sites, understory light levels were lower in plots containing L. maackii. At the asynchronous site, H. macrophyllum received fewer pollinator visits in the presence of L. maackii, suggesting shade from L. maackii reduced visitation to H. macrophyllum. Despite reduced visitation, H. macrophyllum seed set did not differ between treatments. At the synchronous site, H. macrophyllum received more pollinator visits and produced more seeds per flower in the presence of co-flowering L. maackii compared to plots in which L. maackii was absent, and conspecific pollen deposition was positively associated with seed set. Our results support the hypothesis that co-flowering L. maackii shrubs facilitated pollination of H. macrophyllum, thereby mitigating the negative impacts of shade, leading to increased seed production. Phenological overlap appears to influence pollinator-mediated interactions between invasive and native plants and may alter the direction of impact of L. maackii on native plant pollination.     

Interactions between a pollinating seed predator and its host plant: the role of environmental context within a population

Plant–insect interactions often are important for plant reproduction, but the outcome of these interactions may vary with environmental context. Pollinating seed predators have positive and negative effects on host plant reproduction, and the interaction outcome is predicted to vary with density or abundance of the partners. We studied the interaction between Silene stellata, an herbaceous perennial, and Hadena ectypa, its specialized pollinating seed predator. Silene stellata is only facultatively dependent upon H. ectypa for pollination because other nocturnal moth co‐pollinators are equally effective at pollen transfer. We hypothesized that for plants without conspecific neighbors, H. ectypa would have higher visitation rates compared to co‐pollinators, and the plants would experience lower levels of H. ectypa pollen deposition. We predicted similar oviposition throughout the study site but greater H. ectypa predation in the area without conspecific neighbors compared to plants embedded in a naturally high density area. We found that H. ectypa had consistently higher visitation than moth co‐pollinators in all host plant contexts. However, H. ectypa pollinator importance declined in areas with low conspecific density because of reduced pollen deposition, resulting in lower seed set. Conversely, oviposition was similar across the study site independent of host plant density. Greater likelihood of very high fruit predation combined with lower pollination by H. ectypa resulted in reduced S. stellata female reproductive success in areas with low conspecific density. Our results demonstrate local context dependency of the outcomes of pollinating seed predator interactions with conspecific host plant density within a population.     

Pollination niche overlap between a parasitic plant and its host

Niche theory predicts that species which share resources should evolve strategies to minimise competition for those resources, or the less competitive species would be extirpated. Some plant species are constrained to co-occur, for example parasitic plants and their hosts, and may overlap in their pollination niche if they flower at the same time and attract the same pollinators. Using field observations and experiments between 1996 and 2006, we tested a series of hypotheses regarding pollination niche overlap between a specialist parasitic plant Orobanche elatior (Orobanchaceae) and its host Centaurea scabiosa (Asteraceae). These species flower more or less at the same time, with some year-to-year variation. The host is pollinated by a diverse range of insects, which vary in their effectiveness, whilst the parasite is pollinated by a single species of bumblebee, Bombus pascuorum, which is also an effective pollinator of the host plant. The two species therefore have partially overlapping pollination niches. These niches are not finely subdivided by differential pollen placement, or by diurnal segregation of the niches. We therefore found no evidence of character displacement within the pollination niches of these species, possibly because pollinators are not a limiting resource for these plants. Direct observation of pollinator movements, coupled with experimental manipulations of host plant inflorescence density, showed that Bombus pascuorum only rarely moves between inflorescences of the host and the parasite and therefore the presence of one plant is unlikely to be facilitating pollination in the other. This is the first detailed examination of pollination niche overlap in a plant parasite system and we suggest avenues for future research in relation to pollination and other shared interactions between parasitic plants and their hosts.