Pollination Ecology and Seed Production of Rhododendron Ponticum in Native and Exotic Habitats

Alien plants may be reproductively limited in exotic habitats because of a lack of mutualistic pollinators. However, if plants are adequately served by generalist pollinators, successful reproduction, naturalisation and expansion into exotic habitats may occur. Rhododendron ponticum is very successful, ecologically damaging invasive plant in Britain and Ireland, but is in decline in its native Iberian habitat. It spreads locally by sending out lateral branches, but for longer distance dispersal it relies on sexually produced seeds. Little is known about R. ponticum's pollination ecology and breeding biology in invaded habitats. We examined the flower-visiting communities and maternal reproductive success of R. ponticum in native populations in southern Spain and in exotic ones in Ireland. R. ponticum in flowers are visited by various generalist (polylectic) pollinator species in both native and exotic habitats. Although different species visited flowers in Ireland and Spain, the flower visitation rate was not significantly different. Insects foraging on R. ponticum in Spain carried less R. ponticum pollen than their Irish counterparts, and carried fewer pollen types. Fruit production per inflorescence varied greatly within all populations but was significantly correlated with visitation at the population level. Nectar was significantly depleted by insects in some exotic populations, suggesting that this invasive species is providing a floral resource for native insects in some parts of Ireland. The generality of the pollination system may be factor contributing to R.ponticum's success in exotic habitats.     

Effects of conspecific and heterospecific floral density on the pollination of two related rewarding orchids

Variation in within-population floral density can affect interactions between plants and pollinators, resulting in variable pollen export for plants. We investigated the effects of conspecific and heterospecific floral densities on pollination success both of two related, self-compatible, nectar-rewarding orchid species in Ireland, Spiranthes romanzoffiana (rare and listed as endangered) and its congener, S. spiralis (more abundant and not of conservation concern). Floral densities, insect visitation rates, and orchid pollen transport were recorded in multiple quadrats in four populations of both orchid species over their flowering season. We found that conspecific and heterospecific co-flowering plant density affected pollination in both orchid species. For S. romanzoffiana, higher heterospecific density increased pollen removal. For S. spiralis, higher conspecific visitation increased pollen removal and increased heterospecific density decreased pollen deposition. In addition, increased conspecific density increased pollen deposition in both species. This study shows that plants may interact to facilitate or compete for different components of the pollination process, namely; pollinator attraction, pollen removal and deposition. Such interactions have immediate consequences for endangered plant species, as increases in both conspecific and heterospecific coflowering density may ameliorate the negative effects of rarity on pollination, hence overall reproductive success.     

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.     

The impact of an alien plant on a native plant-pollinator network: an experimental approach

Studies of pairwise interactions have shown that an alien plant can affect the pollination of a native plant, this effect being mediated by shared pollinators. Here we use a manipulative field experiment, to investigate the impact of the alien plant Impatiens glandulifera on an entire community of coflowering native plants. Visitation and pollen transport networks were constructed to compare replicated I. glandulifera invaded and I. glandulifera removal plots. Invaded plots had significantly higher visitor species richness, visitor abundance and flower visitation. However, the pollen transport networks were dominated by alien pollen grains in the invaded plots and consequently higher visitation may not translate in facilitation for pollination. The more generalized insects were more likely to visit the alien plant, and Hymenoptera and Hemiptera were more likely to visit the alien than Coleoptera. Our data indicate that generalized native pollinators can provide a pathway of integration for alien plants into native visitation systems.     

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.     

The cobblers stick to their lasts: pollinators prefer native over alien plant species in a multi-species experiment

The majority of plant species rely, at least partly, on animals for pollination. Our knowledge on whether pollinator visitation differs between native and alien plant species, and between invasive and non-invasive alien species is still limited. Additionally, because numerous invasive plant species are escapees from horticulture, the transition from human-assisted occurrence in urbanized habitats to unassisted persistence and spread in (semi-)natural habitats requires study. To address whether pollinator visitation differs between native, invasive alien and non-invasive alien species, we did pollinator observations for a total of 17 plant species representing five plant families. To test whether pollinator visitation to the three groups of species during the initial stage of invasion depends on habitat type, we did the study in three urbanized habitats and three semi-natural grasslands, using single potted plants. Native plants had more but smaller flower units than alien plants, and invasive alien plants had more but smaller flowers than non-invasive alien plants. After accounting for these differences in floral display, pollinator visitation was higher for native than for alien plant species, but did not differ between invasive and non-invasive alien plant species. Pollinator visitation was on average higher in semi-natural than in urbanized habitats, irrespective of origin or status of the plant species. This might suggest that once an alien species has managed to escape from urbanized into more natural habitats, pollinator limitation will not be a major barrier to establishment and invasion.     

Comparing the reproductive success and pollination biology of an invasive plant to its rare and common native congeners: a case study in the genus <Emphasis Type="Italic">Cirsium</Emphasis> (Asteraceae)

Previous studies have examined an association between reproductive success and pollination biology of rare versus widespread species through pair-wise comparisons of native and invasive congeners or rare and common congeners. To determine the importance of reproductive success and pollination biology for an invasive thistle, Cirsium vulgare, we compared it in its invaded range to five, co-occurring native Cirsium species that range from rare to common. Native study species include C. fontinale var. fontinale, C. andrewsii, C. brevistylum, C. occidentale, and C. quercetorum. We compared all species’ reproductive success, insect visitation rate and composition, autonomous self-pollination, and level of pollen limitation in multiple populations. Species differed in their reproductive success; the invasive C. vulgare produced more flower heads per plant than most native species. C. vulgare attracted more visitors than its congeners. In addition, reproductive success and insect visitation significantly varied between populations within species, mainly due to aphid infestation in one population of C. occidentale. Unlike the rare species (C. fontinale and andrewsii), C. vulgare did not require a pollinator for high-levels of seed production. The remaining native species set fewer seeds than C. vulgare without a pollinator. However, differences in insect visitation and autonomous self-pollination did not lead to differences in pollen limitation across species or between populations. This result suggests that factors other than pollination biology determine the difference in reproductive success of these species. However, high levels of autonomous self-pollination and generalist insect visitation may allow the invasive C. vulgare to easily establish new populations from low numbers of propagules. Our study provides one contrast that should build towards a larger comparative analysis to examine general patterns in the relationship between reproductive success, pollination biology, rare and invasive species, and our ability to predict biological invasions in introduced species.     

Depressed Pollination of Lapageria rosea Ruiz et Pav. (Philesiaceae) in the Fragmented Temperate Rainforest of Southern South America

We studied the pollination and reproductive success in continuous and fragmented populations of Lapageria rosea, a self-compatible plant endemic to temperate forests of Chile. Pollinator abundance, visitation rates, flower abundance, nectar volume and concentration, pollen germination and fruit and seed production, were compared between continuous forest of 145 ha and four forest fragments of 6, 3, 3, and 1 ha respectively, surrounded by mature pine plantations of Pinus radiata. Flower abundance was lower in three out of four forest fragments relative to continuous forest. Nectar volume and sugar concentration did not differ between flowers in the two habitats. Pollinators of L. rosea, the hummingbird Sephanoides sephaniodes and bumblebee Bombus dahlbomii were less abundant and visited flowers of L. rosea at lower rates in fragments than in continuous forest. In addition, in vitro rates of pollen germination were lower for flowers in forest fragments. The number of seeds per fruit was also lower in forest fragments. We suggest that fragmentation affects the reproductive success of L. rosea, lowering the total numbers of seeds produced and possibly compromising long term persistence of fragmented populations.     

High invasive pollen transfer, yet low deposition on native stigmas in a Carpobrotus-invaded community

Background and Aims

Invasive plants are potential agents of disruption in plant–pollinator interactions. They may affect pollinator visitation rates to native plants and modify the plant–pollinator interaction network. However, there is little information about the extent to which invasive pollen is incorporated into the pollination network and about the rates of invasive pollen deposition on the stigmas of native plants.

Methods

The degree of pollinator sharing between the invasive plant Carpobrotus affine acinaciformis and the main co-flowering native plants was tested in a Mediterranean coastal shrubland. Pollen loads were identified from the bodies of the ten most common pollinator species and stigmatic pollen deposition in the five most common native plant species.

Key Results

It was found that pollinators visited Carpobrotus extensively. Seventy-three per cent of pollinator specimens collected on native plants carried Carpobrotus pollen. On average 23 % of the pollen on the bodies of pollinators visiting native plants was Carpobrotus. However, most of the pollen found on the body of pollinators belonged to the species on which they were collected. Similarly, most pollen on native plant stigmas was conspecific. Invasive pollen was present on native plant stigmas, but in low quantity.

Conclusions

Carpobrotus is highly integrated in the pollen transport network. However, the plant-pollination network in the invaded community seems to be sufficiently robust to withstand the impacts of the presence of alien pollen on native plant pollination, as shown by the low levels of heterospecific pollen deposition on native stigmas. Several mechanisms are discussed for the low invasive pollen deposition on native stigmas.Key words: Alien plant, Carpobrotus aff. acinaciformis, competition for pollinators, invasion, Mediterranean shrubland, plant-pollinator network, pollen loads, pollinator visits, stigma     

Low interspecific pollen transfer between invasive aquatic <Emphasis Type="Italic">Ludwigia grandiflora</Emphasis> and native co-flowering plants

Showy invasive alien plants are often integrated in the diet of generalist pollinators and because of the lack of co-evolvement with the native plant community, a high amount of interspecific pollen transfer (IPT) can be expected. We investigated pollinator switching and magnitude plus distance of IPT between the alien aquatic Ludwigia grandiflora and the native Lythrum salicaria in both directions in uninvaded and invaded sites with a different relative abundance of L. grandiflora (% cover of the alien plant: no cover; low cover: <5%; high cover: 50–75%). A field experiment was conducted to include both pollinator interspecific movements and tracking of IPT, using fluorescent dye as a pollen analogue. Despite a substantial overlap in pollinators between L. grandiflora and the native L. salicaria, less than 10% of the observed flights were interspecific. Similar results were found in dye transfer patterns. The proportions of stigmas with conspecific dye were always higher than the proportions of stigmas with heterospecific dye for L grandiflora and L. salicaria. There were no differences in conspecific dye loads for L. salicaria between uninvaded and invaded sites. Conspecific pollen loss (native CPL) and heterospecific pollen deposition (alien HPD) were in general low and species-specific. The distance of HPD ranged respectively from 1.7 to 39 m and from 0.3 to 54.8 m in the low cover and high cover sites while CPL ranged respectively from 6.40 to 68.02 m and from 0.60 to 40.18 m in the low cover and high cover sites. We can conclude that, in this system, CPL and HPD will play a minor role in pollinator-mediated interaction. Furthermore, interspecific competition for pollinators will cover a larger distance than just neighboring individuals. Our results suggest the necessity to consider the combined effect of insect visitation, pollen deposition, relative alien abundance, distance and seed set when investigating pollinator-mediated interactions of invasive plants.     

Pollen transfer in fragmented plant populations: insight from the pollen loads of pollinators and stigmas in a mass‐flowering species

Pollinator and/or mate scarcity affects pollen transfer, with important ecological and evolutionary consequences for plant reproduction. However, the way in which the pollen loads transported by pollinators and deposited on stigmas are affected by pollination context has been little studied. We investigated the impacts of plant mate and visiting insect availabilities on pollen transport and receipt in a mass‐flowering and facultative autogamous shrub (Rhododendron ferrugineum). First, we recorded insect visits to R. ferrugineum in plant patches of diverse densities and sizes. Second, we analyzed the pollen loads transported by R. ferrugineum pollinators and deposited on stigmas of emasculated and intact flowers, in the same patches. Overall, pollinators (bumblebees) transported much larger pollen loads than the ones found on stigmas, and the pollen deposited on stigmas included a high proportion of conspecific pollen. However, comparing pollen loads of emasculated and intact flowers indicated that pollinators contributed only half the conspecific pollen present on the stigma. At low plant density, we found the highest visitation rate and the lowest proportion of conspecific pollen transported and deposited by pollinators. By contrast, at higher plant density and lower visitation rate, pollinators deposited larger proportion of conspecific pollen, although still far from sufficient to ensure that all the ovules were fertilized. Finally, self‐pollen completely buffered the detrimental effects on pollination of patch fragmentation and pollinator failure. Our results indicate that pollen loads from pollinators and emasculated flowers should be quantified for an accurate understanding of the relative impacts of pollinator and mate limitation on pollen transfer in facultative autogamous species.     

EXPERIMENTAL ASSESSMENT OF REPRODUCTIVE INTERACTIONS BETWEEN SYMPATRIC ASTER AND ERIGERON (ASTERACEAE) IN INTERIOR ALASKA

Erigeron glabellus and Aster sibiricus have similar flowers, share pollinators, but bloom sequentially in interior Alaska. Both species depend on insect pollination for seed set: the Erigeron is self-incompatible, and the Aster is apparently self-compatible but allogamous. To test the hypothesis that sequential blooming is maintained by natural selection generated by reproductive interference, we manipulated the flowering time of Erigeron, forcing it to bloom simultaneously with Aster, and measured female fecundity in both species. We found no evidence of reduced female fecundity in either species caused by the presence of the sympatric “competitor” or by artificial pollination with the heterospecific pollen prior to conspecific pollination. Two-species mixtures of simultaneously blooming Aster and Erigeron experienced significant interspecific visitation, which may, under natural conditions, cause loss of pollen to alien stigmas and depressed male fecundity, at least in Erigeron. We found no evidence that sequential blooming in Erigeron and Aster is maintained by depressed female fecundity through pollinator sharing. If sequential blooming is maintained by natural selection, it seems more likely to be the result of selection generated by depressed male fitness through pollen loss to alien stigmas.     

Insect-Flower Interaction Network Structure Is Resilient to a Temporary Pulse of Floral Resources from Invasive Rhododendron ponticum

Invasive alien plants can compete with native plants for resources, and may ultimately decrease native plant diversity and/or abundance in invaded sites. This could have consequences for native mutualistic interactions, such as pollination. Although invasive plants often become highly connected in plant-pollinator interaction networks, in temperate climates they usually only flower for part of the season. Unless sufficient alternative plants flower outside this period, whole-season floral resources may be reduced by invasion. We hypothesized that the cessation of flowering of a dominant invasive plant would lead to dramatic, seasonal compositional changes in plant-pollinator communities, and subsequent changes in network structure. We investigated variation in floral resources, flower-visiting insect communities, and interaction networks during and after the flowering of invasive Rhododendron ponticum in four invaded Irish woodland sites. Floral resources decreased significantly after R. ponticum flowering, but the magnitude of the decrease varied among sites. Neither insect abundance nor richness varied between the two periods (during and after R. ponticum flowering), yet insect community composition was distinct, mostly due to a significant reduction in Bombus abundance after flowering. During flowering R. ponticum was frequently visited by Bombus; after flowering, these highly mobile pollinators presumably left to find alternative floral resources. Despite compositional changes, however, network structural properties remained stable after R. ponticum flowering ceased: generality increased, but quantitative connectance, interaction evenness, vulnerability, H’₂ and network size did not change. This is likely because after R. ponticum flowering, two to three alternative plant species became prominent in networks and insects increased their diet breadth, as indicated by the increase in network-level generality. We conclude that network structure is robust to seasonal changes in floral abundance at sites invaded by alien, mass-flowering plant species, as long as alternative floral resources remain throughout the season to support the flower-visiting community.     

Spatially contingent interactions between an exotic and native plant mediated through flower visitors

Exotic plants can negatively impact the fitness of native plants by changing the behavior of flower visitors and thus affecting pollen transfer. The presence of an exotic plant may decrease the visitation rate to native plants and thus increase pollen limitation. Flower visitors may also switch between exotic and native plants and if pollen from an exotic plant is transferred to native plant stigmas this may impede siring by conspecific pollen. As flower visitors forage within a spatial context, the distribution of plants may affect the type and magnitude of pollinator‐mediated competition. In this study we examined two questions: 1) does the exotic plant, Carduus nutans (Asteraceae) interact with the native Monarda fistulosa (Lamiaceae) through flower visitors by changing visitation rate and/or through heterospecific pollen transfer, and does this affect seed set of the native plant? 2) Does spatial context affect how the native and exotic plants interact through flower visitors? We created plots containing potted M. fistulosa with and without the presence of potted C. nutans. In the presence of C. nutans, M. fistulosa stigmas had significantly fewer conspecific and more C. nutans pollen grains. Visitation rate and seed set tended to be lower in these invaded plots, however they were not significant. In a second experiment, we examined whether changes in visitation rate to M. fistulosa due to the presence of C. nutans was a function of M. fistulosa distance from C. nutans. We found that visitation rate did not decrease in the presence of C. nutans when M. fistulosa were adjacent to C. nutans or 15 meters from C. nutans. However, floral visitation rate to M. fistulosa decreased at 1 and 5 meters from C. nutans. Our results suggest interactions between plant species through flower visitors may depend on spatial scale.     

Timing of invasive pollen deposition influences pollen tube growth and seed set in a native plant

Invasive plants may threaten the reproductive success of native sympatric plants by modifying the pollination process. One potential mechanism takes place through the deposition of invasive pollen onto native stigmas when pollinators are shared among species. We explore how pollen from the invasive plant Brassica nigra influences pre- and post-fertilization stages in the native plant Phacelia parryi, through a series of hand pollination experiments. These two species share pollinators to a high degree. P. parryi flowers were hand-pollinated with either pure conspecific pollen (the control) or with B. nigra pollen applied prior to, simultaneously with, or following conspecific pollen. Application of B. nigra pollen lowered seed set, with the simultaneous application resulting in the highest reduction. Pollen tube growth was also influenced by the presence of invasive pollen, with fewer conspecific pollen tubes reaching the base of P. parryi styles in treatments where B. nigra pollen was applied prior to or simultaneously with conspecific pollen. The deleterious effects of invasive pollen on native seed set in this study are likely not due to loss of stigmatic receptivity since seed set was less affected when heterospecific pollen was applied prior to conspecific pollen, but may instead involve interactions between interspecific pollen grains on the stigma or within the style. Our study highlights the importance of timing of foreign pollen deposition on native stigmas and suggests that interspecific pollen transfer between native and exotic plants may be an important mechanism of competition for pollination in invaded plant communities.     

Increased relative abundance of an invasive competitor for pollination, Lythrum salicaria, reduces seed number in Mimulus ringens

When exotic plant species share pollinators with native species, competition for pollination may lower the reproductive success of natives by reducing the frequency and/or quality of visits they receive. Exotic species often become numerically dominant in plant communities, and the relative abundance of these potential competitors for pollination may be an important determinant of their effects on the pollination and reproductive success of co-occurring native species. Our study experimentally tests whether the presence and abundance of an invasive exotic, Lythrum salicaria L. (Lythraceae), influences reproductive success of a co-flowering native species, Mimulus ringens L. (Phrymaceae). We also examine the mechanisms of competition for pollination and how they may be altered by changes in competitor abundance. We found that the presence of Lythrum salicaria lowered mean seed number in Mimulus ringens fruits. This effect was most pronounced when the invasive competitor was highly abundant, decreasing the number of seeds per fruit by 40% in 2006 and 33% in 2007. Reductions in the number of seeds per fruit were likely due to reduced visit quality resulting from Mimulus pollen loss when bees foraged on neighboring Lythrum plants. This study suggests that visit quality to natives may be influenced by the presence and abundance of invasive flowering plants.     

Comparisons among populations and individuals to evaluate pollen–pistil interaction as a mechanism of reproductive interference in Taraxacum

Reproductive interference (RI), an interspecific mating interaction that reduces the fitness of at least one of the species involved, can lead to exclusive distributions in closely related species. A hypothesis previously proposed is that RI in plants may occur by ovule usurpation, in which pistils lack interspecific incompatibility and mistakenly accept heterospecific pollen, thereby losing an opportunity for conspecific pollen fertilization. However, few comparative studies have evaluated the consistency of the inferred mechanism within and among individuals and populations. We conducted hand-pollination experiments in six populations of three native Taraxacum species that suffered from different levels of RI from an alien congener, T. officinale, and compared pollen–pistil interactions among populations. We also investigated the interactions for eight individual T. japonicum plants whose response to heterospecific pollen deposition had been previously measured. Our results revealed that pollen tubes often penetrated native ovaries following heterospecific pollination in populations suffering from strong RI, whereas they seldom did in populations suffering from marginal RI. However, the relative frequency of the pollen tube penetration was not significantly related to the strength of alien RI. Not all pistils on an individual plant showed the same pollen receptivity following heterospecific pollination; rather, some accepted and some refused the pollen tubes. The relationship between pollen tube penetration following heterospecific pollination and the strength of the alien RI was also not significant among individuals. Our present results generally support the ovule usurpation hypothesis, but suggest that other factors, such as competition for pollinator services, variation in the effects of heterospecific pollen donors, and condition of the native inflorescences, might also affect the observed RI strength.

     

A meta-analysis of impacts of alien vs. native plants on pollinator visitation and reproductive success of co-flowering native plants

Alien plant species can alter pollinator visitation and, in turn, the sexual reproduction of natives. Using a conventional and a phylogenetically controlled meta-analytical approach on a data set of 40 studies, we evaluated the effect of alien neighbour plant species (aliens) on visitation to and reproduction of native co-flowering focal species (focals), and compared such effect to that of native neighbours (natives). An overall significantly negative effect of aliens on visitation to and reproduction of focals was confirmed. Interestingly, aliens differed from natives in their effect on visitation, but not on reproductive success. The negative effect of aliens on visitation and reproductive success increased at high relative alien plant abundance, but this increase was proportionally lower than the increase in relative plant abundance. Likewise, effect of aliens on visitation and reproductive success was most detrimental when alien and focal species had similar flower symmetry or colour. The phylogenetic relatedness between alien neighbours and focals influenced the reproductive success effect size. Results of the phylogenetic meta-analysis were only partly consistent with those of the conventional meta-analysis, depending on the response variable and on whether we controlled for the phylogeny of neighbour or focal species, which calls for special attention to control for species relatedness in this type of review. This study demonstrates the predominant detrimental impact of alien plants on pollination and reproduction of natives, and highlights the importance of phenotypic similarity to the outcome of the interaction.     

The presence of the invasive plant Solanum elaeagnifolium deters honeybees and increases pollen limitation in the native co-flowering species Glaucium flavum

Invasive plants can impact biodiversity and ecosystem functioning by displacing native plants and crop species due to competition for space, nutrients, water and light. The presence of co-flowering invasives has also been shown to affect some native plants through the reduction in pollinator visitation or through the deposition of heterospecific pollen on the native’s stigmas leading to stigma clogging. We examined the impact of the invasive plant Solanum elaeagnifolium Cavanilles (silver-leafed nightshade), native to South and Central America and South-western parts of North America, on the seed set of the native Glaucium flavum Crantz (yellow-horned poppy) on Lesvos Island, Greece. To do this we measured seed set and visitation rates to G. flavum before and after the placement of potted individuals of the invasive near the native plants. In addition, we hand-crossed G. flavum flowers with super-optimal amounts of conspecific pollen, bagged flowers to measure the rate of spontaneous selfing, and applied self-pollen to measure self-compatibility of G. flavum. The hand-selfing treatment resulted in very low seed set, which indicates that G. flavum is to a large degree self-incompatible and highlights the plant’s need for insect-mediated outcrossing. We show that the presence of the invasive significantly enhanced pollen limitation, although the overall visitation rates were not reduced and that this increase is due to a reduction in honeybee visitation in the presence of the invasive resulting in reduced pollination.     

The evolutionary potential of invasive Carpobrotus (Aizoaceae) taxa: are pollen-mediated gene flow potential and hybrid vigor levels connected?

Few studies have addressed the importance of native pollinators in shaping the breeding systems and evolutionary potential of invasive plants. We examined the pollination and gamete production of Carpobrotus affine acinaciformis (L.) L. Bol. and C. edulis (L.) N. E. Br. invading the coasts of southeast France (Provence), and found preliminary evidence that the pollen-mediated gene flow potential (PMGFP) within the four studied populations is positively correlated with their Relative Performance in Hybridization indices (RP_H), suggesting a link between pollinator services, gene flow, and Carpobrotus hybridization. Flower density (FD) may be a driver behind pollinator abundance and visitation patterns since it shows a significant, positive relationship with relative pollinator abundance (FR) and a significant negative relationship with per flower visit frequencies (FR/F). In the populations with the highest RP_H indices, and therefore high hybrid vigor, both taxa produce similar quantities of ovules and pollen per flower, whose ratios further indicate (facultative) xenogamy. Pollen diameter distributions include ∼25% micro-pollen, and overall viability is less than one half. Viable pollen profiles consist of 3.5% micro, 85.5% normal and 10.9% macro pollen (considered as diplogametes) on average per flower for C. affine acinaciformis, and 0.7% micro, 73.0% normal and 26.2% macro pollen for C. edulis. Given the co-occurrence of (1) (facultative) xenogamic breeding strategies, (2) a significant, positive relationship between RP_H and PMGFP, and (3) frequent pollen abnormalities, it is probable that hybrid dysgenesis mediated by the local pollinators occurs in these populations. Furthermore, the unusually high frequency of potentially viable, diploid macro-pollen underlines the evolutionary/polyploid potential of these invasive, introgressed populations. Native pollinator interactions may greatly affect the taxonomic status and evolutionary potential of invasive plant complexes.