Conspecific Pollen Deposition Research Articles

Differential effects of pollen nutritional quality on male and female reproductive success within a diverse co‐flowering community

Abstract Pollen protein content has been demonstrated to be an essential nutritional component for bees and thus important in mediating plant–pollinator interactions. However, little is known on the drivers and consequences of among‐species variation in pollen protein content and how this can impact male and female reproductive success across plant species. Among‐species variation in resources allocated to pollen nutrition could further be constrained by life‐history strategies (e.g. survival‐reproduction trade‐offs) or evolutionary history. Here, we surveyed pollen protein content for 29 species within a diverse co‐flowering community and evaluated the effect of pollen protein on male and female reproductive success. We also tested the role of life history (annuals vs. perennials) and phylogeny in mediating differences in resource allocation to pollen nutrition. We found that pollen protein content influences components of male (bee visitor abundance and pollen dispersal) but not female (conspecific pollen deposition and pollen tube growth) reproductive success, suggesting this trait affects plants only via male function. This sex‐specific effect further suggests the potential for sexual conflicts driven by differential investment on this trait. We found no phylogenetic signal on pollen protein content. However, pollen protein content was higher in annual compared to perennial species suggesting survival versus reproduction trade‐offs also contribute to variation in pollen protein at the community level. Our study underscores the importance of understanding the ecological and evolutionary drivers of pollen protein content across plant species. Our results further suggest the existence of sexual conflicts and ecological trade‐offs mediated by differential investment in pollen nutritional quality, with important implications for community assembly and the structure of plant–pollinator interactions. Read the free Plain Language Summary for this article on the Journal blog.

Do Silene species with exposed stigmas tolerate interference by heterospecific pollen?

Co-flowering species that have not evolved an avoidance mechanism may have tolerance to heterospecific pollen (HP) deposition as an adaptive strategy to minimize any deleterious effects of HP transfer, but empirical evidence for the tolerance hypothesis remains scarce. To estimate the potential effects of heterospecific pollen deposition (HPD) on female reproductive success, we counted conspecific (CP) and HP pollen grains deposited on stigmas and assessed subsequent seed set of both open- and hand-pollinated flowers in three co-flowering Silene species with exposed stigmas that usually received numerous HP grains on the elongated receptive area. The percentage of HP grains per flower (HP%) varied from 16.6% to 43.0% among three species. Silene chungtienensis had lower HP%, and the CP-HP relationship was neutral; S. gracilicaulis and S. yunnanensis had a relatively higher HP% with a positive CP-HP relationship. The effects of CP and HP number on natural seed set were positive for all three species, but HP% had stronger negative effects in S. chungtienensis and S. gracilicaulis. In hand-pollinated flowers of the three Silene species, seed set did not decrease with HP whether CP was in excess or insufficient, indicating no negative effects of HPD on seed production. Consistent with the tolerance hypothesis, our results indicated that species with higher HP interference are likely to be tolerant to an increase in HP%. These species with generalist-pollinated flowers and exposed large stigmas may benefit from an increase of conspecific pollen deposition, despite the associated increase in heterospecific pollen deposition.

Heterospecific pollen deposition is positively associated with reproductive success in a diverse hummingbird‐pollinated plant community

Heterospecific pollen deposition (HPD) is ubiquitous across plant communities, especially for generalized species which use a diversity of pollinators, and may have negative effects on plant reproduction. However, it is unclear whether temporal changes in the co‐flowering community result in changes in HPD patterns. Moreover, community‐level studies are required to understand which factors influence HPD and how the reproduction of different species is affected.We investigated the temporal variation of HPD, its relationship with level of specialization on pollinators and floral phenotypic specialization, and its association with reproductive success (pollen limitation and fruit set) in 31 hummingbird‐pollinated plant species in a tropical Campo Rupestre.We found seasonality in HPD, with species flowering in the dry season having greater diversity of heterospecific pollen on stigmas and a higher frequency of stigmas containing heterospecific pollen, compared to the rainy season. Stigmas of ecologically generalized species had more heterospecific pollen, while the relationship for ecologically specialized species depended on floral phenotype. Surprisingly, and in contrast to theory, we found a positive relationship between HPD and reproductive success.Our results indicate benefits of generalization and facilitation, in which sharing pollinators brings greater reproductive success via increased conspecific pollen deposition, even if it incurs more HPD. We demonstrated how assessing HPD at a community‐level can contribute to understanding the ecological causes and functional consequences of pollinator sharing.

Spatial variation in bidirectional pollinator-mediated interactions between two co-flowering species in serpentine plant communities.

Pollinator-mediated competition and facilitation are two important mechanisms mediating co-flowering community assembly. Experimental studies, however, have mostly focused on evaluating outcomes for a single interacting partner at a single location. Studies that evaluate spatial variation in the bidirectional effects between co-flowering species are necessary if we aim to advance our understanding of the processes that mediate species coexistence in diverse co-flowering communities. Here, we examine geographic variation (i.e. at landscape level) in bidirectional pollinator-mediated effects between co-flowering Mimulus guttatus and Delphinium uliginosum. We evaluated effects on pollen transfer dynamics (conspecific and heterospecific pollen deposition) and plant reproductive success. We found evidence of asymmetrical effects (one species is disrupted and the other one is facilitated) but the effects were highly dependent on geographical location. Furthermore, effects on pollen transfer dynamics did not always translate to effects on overall plant reproductive success (i.e. pollen tube growth) highlighting the importance of evaluating effects at multiple stages of the pollination process. Overall, our results provide evidence of a spatial mosaic of pollinator-mediated interactions between co-flowering species and suggest that community assembly processes could result from competition and facilitation acting simultaneously. Our study highlights the importance of experimental studies that evaluate the prevalence of competitive and facilitative interactions in the field, and that expand across a wide geographical context, in order to more fully understand the mechanisms that shape plant communities in nature.

Pollinator effectiveness is affected by intraindividual behavioral variation.

Variation in pollinator quality is fundamental to the evolution of plant-pollinator mutualisms and such variation frequently results from differences in foraging behavior. Surprisingly, despite substantial intraindividual variation in pollinator foraging behavior, the consequences for pollen removal and deposition on flowers are largely unknown. We asked how two pollen foraging behaviors of a generalist pollinator (Bombus impatiens) affect removal and deposition of heterospecific and conspecific pollen, key aspects of pollinator quality, for multiple plant species. In addition, we examined how bee body size and pollen placement among body parts shaped pollen movement. We manipulated foraging behavior types using artificial flowers, which donated pollen that captive bees then deposited on three recipient plant species. While body size primarily affected donor pollen removal, foraging behavior primarily affected donor pollen deposition. How behavior affected donor pollen deposition depended on the plant species and the quantity of donor pollen on the bee's abdomen. Plant species with smaller stigmas received significantly less pollen and fewer bees successfully transferred pollen to them. For a single plant species, heterospecific pollen interfered with conspecific pollen deposition, such that more heterospecific pollen on the bee's abdomen resulted in less conspecific pollen deposition on the flower. Thus, intraindividual variation in foraging behavior and its interaction with the amount and placement of acquired pollen and with floral morphology can affect pollinator quality and may shape plant fitness via both conspecific and heterospecific pollen transfer.

The impact of a native dominant plant, Euphorbia jolkinii, on plant–flower visitor networks and pollen deposition on stigmas of co‐flowering species in subalpine meadows of Shangri‐La, SW China

Abstract Anthropogenic activity can modify the distribution of species abundance in a community leading to the appearance of new dominant species. While many studies report that an alien plant species which becomes increasingly dominant can change species composition, plant–pollinator network structure and the reproductive output of native plant species, much less is known about native plant species which become dominant in their communities. Euphorbia jolkinii Boissier (Euphorbia, hereafter) has become a dominant native plant in the over‐grazed meadows of Shangri‐La, SW China. During the flowering season of Euphorbia and over 2 years, we quantified the impact of Euphorbia on plant richness and flower visitor richness in 12 subalpine meadows along a gradient of Euphorbia dominance. We also evaluated the floral preferences of flower visitors, interaction evenness of plant–flower visitor networks and the deposition of pollen on the stigmas of two co‐flowering plant species (Gentiana chungtienensis and Anemone rupestris) in each meadow. The species richness of flower visitors to non‐Euphorbia plants was negatively correlated with Euphorbia dominance. As the proportion of Euphorbia increases, flower visitors to Euphorbia decreased, while flower visitors to other co‐flowering plants increased. Interaction evenness decreased as the proportion of Euphorbia increased. Furthermore, the conspecific pollen deposition of one of the two co‐flowering plant species studied, G. chungtienensis, decreased as the proportion of Euphorbia increased. Synthesis. There appears to be little substantive difference between the impact of a newly dominant native plant and the impacts reported for many alien plants on native plant–pollinator communities. This lack of difference suggests that dominance, in addition to plant origin (alien vs. native), could play an important role in influencing the structure and functioning of native communities. This finding has considerable implications for restoration ecology. Thus, communities where natural dominance order has been changed due to anthropogenic activity may not be considered a problem as all the species are native—in reality though, they may be as damaged as communities invaded by alien species.

Autonomous pollination alleviates pollen limitation in the endemic Cienfuegosia yucatanensis Millsp. (Malvaceae)

Background: Self-compatibility is common on endemic plant species, but pollen limitation and self-pollination could be risk factors.
 Study species: The endemic Cienfuegosia yucatanensis (Malvaceae), whose distribution is mainly restricted to the north coast of the peninsula of Yucatán, México.
 Questions: a) Are flowers of C. yucatanensis autonomous for pollination? b) Are C. yucatanensis fruit or seed-set limited by pollen deposition? and, c) Is there evidence of early inbreeding depression in C. yucatanensis ?
 Study sites and dates: Two sites in the north of the peninsula of Yucatán in a seasonally dry scrubland, in the rainy season of 2013 and 2014.
 Methods: Number of flowers and fruits were weekly recorded. Flowers were collected to count the number of conspecific pollen load and the number of pollen tubes. Autonomous pollination and pollen limitation were evaluated with a hand-pollination experiment. Inbreeding depression on fruit and seed production, and seed weight was evaluated.
 Results: Flower and fruit production occur simultaneously and last from August to October. Conspecific pollen deposition on stigmas occurred through the whole flowering season and a maximum of pollen tubes was observed in August. Autonomous pollination treatment lead to similar fruit and seed production than cross-pollination, but open pollination produced significantly more seeds. No significant differences among self- and cross-pollination treatments on fruit and seed production or seed weight, were found.
 Conclusions: Our results suggest that self-compatibility combined with a relatively efficient autonomous pollination, are suitable mechanisms for the reproductive assurance in C. yucatanensis , with no apparent effects of early inbreeding depression.

Foraging behavior and pollination efficiency of generalist insects in an understory dioecious shrub Helwingia japonica

It has been hypothesized that pollination success in animal-pollinated dioecious plants relies on opportunistic pollinators with no discrimination against female flowers. However, empirical studies of pollinator foraging behavior and pollination effectiveness in dioecious species are few. To investigate potential pollinators in Helwingia japonica, a dioecious shrub with small, inconspicuous flowers, we compared floral visitors and visit frequency to female and male plants for three flowering seasons in two field populations in subtropical forests in southwest China. Pollen placement on the insect bodies of four groups (solitary bees and other bees, fungus gnats, and other flies) was compared, and insect foraging behavior was observed. Pollen removal and conspecific and heterospecific pollen deposition per visit were measured to compare pollination effectiveness among the four groups. Floral visitors usually did not discriminate between male and female flowers and did not gather pollen into collections. Our measurements of pollen transfer efficiency showed that solitary bees were the most effective pollinators with the highest conspecific pollen deposition. These insects seemed to be opportunistic visitors because pollen grains of H. japonica were distributed evenly over different regions of the visitor's body, and heterospecific pollen accounted for over 50% of total pollen loads on stigmas in the two populations. Our investigations indicated that potential pollinators were generalists and did not discriminate against female flowers, as predicted for dioecious species pollinated by insects. A perspective of pollen removal by floral visitors offers insights into the evolution of plant sexual systems.

Impact of Invasive Bees on Plant-Pollinator Interactions and Reproductive Success of Plant Species in Mixed Nothofagus antarctica Forests.

Invasive social bees can alter plant-pollinator interactions with detrimental effects on both partners. However, most studies have focused on one invasive bee species, while the interactions among two or more species remain poorly understood. Also, many study sites had a history of invasive bees, being hard to find sites with historical low abundances. In Patagonia, Bombus ruderatus (F.) invasion begun in 1993 and B. terrestris (L.) in 2006. Though honey bees (Apis mellifera L.) introduction started in 1859, their density is still low in some parts. By experimentally increasing honey bee densities, we evaluated the effect of honey bees and bumblebees floral visitation on native pollinator floral visitation, pollen deposition, and reproductive success of three plant species in mixed Nothofagus antarctica forests of northern Patagonia: Oxalis valdiviensis, Mutisia spinosa and Cirsium vulgare. Our results show that exotic bees became the main floral visitors. No negative association was found between invasive bee and native pollinator visitation rates, but there was evidence of potential competition between honey bees and bumblebees. Floral neighborhood diversity played an important role in pollinator behavior. Conspecific pollen deposition was high for all species, while deposition of heterospecific pollen was very high in M. spinosa and C. vulgare. Not as expected, honey bees visitation rate had a negative effect on heterospecific pollen deposition in C. vulgare. For O. valdiviensis, exotic visitation rates increased conspecific pollen deposition, which was positively related to reproductive success. Although exotic bees became main floral visitors, their contribution to reproductive success was only clear for one species.

Low interspecific pollen transfer between invasive aquatic Ludwigia grandiflora 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.

Pre‐ and post‐pollination interaction between six co‐floweringPedicularisspecies via heterospecific pollen transfer

It remains unclear how related co-flowering species with shared pollinators minimize reproductive interference, given that the degree of interspecific pollen flow and its consequences are little known in natural communities. Differences in pollen size in six Pedicularis species with different style lengths permit us to measure heterospecific pollen transfer (HPT) between species pairs in sympatry. The role of pollen-pistil interactions in mitigating the effects of HPT was examined. Field observations over 2yr showed that bumblebee pollinators visiting one species rarely moved to another. Heterospecific pollen (HP) comprised <10% of total stigmatic pollen loads for each species over 2yr, and was not related to conspecific pollen deposition. Species with longer styles generally received more HP per stigma. The pollen tube study showed that pollen from short-styled species could not grow the full length of the style of long-styled species. Pollen from long-styled species could grow through the short style of P.densispica, but P.densispica rarely received HP in nature. Flower constancy is a key pre-pollination barrier to HPT between co-flowering Pedicularis species. Post-pollination pollen-pistil interactions may further mitigate the effects of HPT because HP transferred to long styles could generally be effectively filtered.

The showy invasive plant Ranunculus ficaria facilitates pollinator activity, pollen deposition, but not always seed production for two native spring ephemeral plants

Showy invasive plants can reduce pollinator visitation to native plants, resulting in a reduction of seed-set in insect-pollinated species. Seed set in native plants may also be reduced due to the presence of foreign pollen. Alternatively, additional floral resources provided by invasive plants may increase pollinator activity, and facilitate pollination of native species. Ranunculus ficaria is a perennial herb invading deciduous forests of the eastern United States. It produces many showy flowers that may influence pollinator activity in heavily invaded habitats. We compared pollinator visitation rates, stigma pollen loads, and seed production of two sympatrically flowering, entomophilous native species (Claytonia virginica and Cardamine concatenata) between habitats invaded by R. ficaria and uninvaded habitats. We found significantly higher pollinator activity (visits to plots) in invaded plots. We also found significantly higher per flower visitation rates to C. virginica in invaded plots. Claytonia virginica stigmas from invaded habitats had more conspecific pollen resulting in more seeds per capsule in invaded areas. There was more conspecific pollen on C. concatenata stigmas in invaded compared to uninvaded habitats, but there were significantly more seeds per silique in uninvaded habitats. Our results indicated that this invasive species increases pollinator activity and conspecific pollen deposition on sympatrically flowering native ephemerals, but may have differential effects on native seed production. Management options should consider the facilitative effects of this species on pollination services, as well as the possible negative competitive effects.

Feedingbehaviour of the dawn bat (&lt;i&gt;Eonycteris spelaea&lt;/i&gt;) promotes cross pollination of economically important plants in Southeast Asia

Eonycteris spelaea is recognized as the principal pollinator of most chiropterophilous plants in SE Asia. The present study describes its feeding behaviour and clarifies its role in cross pollinating these highly self-incompatible plants. Ten individuals of E. spelaea were radio-tracked during the flowering period of durian (Durio zibethinus) and petai (Parkia speciosa) in an agricultural mosaic in southern Thailand. Eonycteris spelaea makes a mean of seven visits per hour to these trees and 80-86% of each feeding bout involves visits to multiple conspecific trees. During each visit, 93% of D. zibethinus stigmas and 50% of P. speciosa stigmas were loaded with conspecific pollen. Eonycteris spelaea was the most common bat visitor to the trees. High visitation frequency and conspecific pollen deposition by E. spelaea to D. zibethinus and P. speciosa indicates that this nectarivorous bat is an effective pollinator. Mixed planting of chiropterophilous trees in fruit orchards is recommended to ensure regular visits of E. spelaea. Protecting natural roost caves of E. spelaea is also essential in order to maintain the vital ecosystem service provided by these bats.

Pollinator-mediated competition influences selection for flower-color displacement in sympatric monkeyflowers.

When coflowering plant species share pollinators, pollinator-mediated competition may favor divergent floral characters associated with pollinator attraction. One potential outcome of this process is that sympatric populations will display increased divergence in floral traits compared with allopatric populations. We developed a new system to study the pattern and process of character displacement. In the central Sierra Nevada of California, USA, Mimulus bicolor is a spring wildflower with two flower-color morphs, one of which resembles coflowering M. guttatus. We documented a fine-scale geographic pattern of character displacement in sympatric and allopatric patches and, using experimental arrays, measured seed set in M. bicolor color morphs in the presence versus absence of M. guttatus. In sympatric arrays yellow, guttatus-like M. bicolor morphs had lower relative fitness (0.35 ± 0.05) and reduced conspecific pollen deposition compared with the distinct alternative morph, whereas in allopatric arrays yellow, guttatus-like morphs were occasionally strongly favored. Pollinator-mediated competition with M. guttatus is consistent with ecological character displacement in M. bicolor and likely contributes to a geographic pattern of character displacement.

Relative floral density of an invasive plant affects pollinator foraging behaviour on a native plant

Interactions between invasive and native plants for pollinators vary from competition to facilitation of pollination of native plants. Theory predicts that relative floral densities should account for some of this variation in outcomes, with facilitation at low floral densities and competition at high floral densities of the invader. We tested this prediction by quantifying pollination and female reproductive success of a native herb, Geranium maculatum, in three experimental arrays that varied in floral density of the invasive shrub Lonicera maackii: control (no L. maackii), low floral density of L. maackii, and high floral density of L. maackii. A low density of L. maackii flowers was associated with an increase in pollinator visitation rate to G. maculatum flowers and an increase in conspecific pollen deposition compared to controls and high density arrays. Increased visitation rates were not associated with an increase in the number of visitors to low density arrays, suggesting instead that a behavioural switch in visitation within the array accounted for increased pollen deposition. In contrast, the only evidence of competition in high density arrays was a shorter duration of visits to G. maculatum flowers relative to the other treatments. The number of seeds per flower did not vary among treatments, although trends in seeds per flower were consistent with patterns of pollinator foraging behaviour. Given increased pollinator visits and pollen deposition at a low density of the invader, our study indicates that complete eradication of invasives as a management or restoration technique may have unintended negative consequences for pollination of native plants.

Relative abundance of an invasive alien plant affects native pollination processes

One major characteristic of invasive alien species is their occurrence at high abundances in their new habitat. Flowering invasive plant species that are visited by native insects and overlap with native plant species in their pollinators may facilitate or disrupt native flower visitation and fertilisation by forming large, dense populations with high numbers of flowers and copious rewards. We investigated the direction of such a proposed effect for the alien invasive Rhododendron ponticum in Irish habitats. Flower visitation, conspecific and alien pollen deposition, fruit and seed set were measured in a self-compatible native focal plant, Digitalis purpurea, and compared between field sites that contained different relative abundances of R. ponticum. Flower visitation was significantly lower at higher alien relative plant abundances than at lower abundances or in the absence of the alien. Native flowers experienced a significant decrease in conspecific pollen deposition with increasing alien abundance. Heterospecific pollen transfer was very low in all field sites but increased significantly with increasing relative R. ponticum abundance. However, lower flower visitation and lower conspecific pollen transfer did not alter reproductive success of D. purpurea. Our study shows that indirect interactions between alien and native plants for pollination can be modified by population characteristics (such as relative abundance) in a similar way as interactions among native plant species. In D. purpurea, only certain aspects of pollination and reproduction were affected by high alien abundances which is probably a result of high resilience due to a self-compatible breeding system. Native species that are more susceptible to pollen limitation are more likely to experience fitness disadvantages in habitats with high relative alien plant abundances.

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.

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. Loniceramaackii 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.

Interspecific pollinator movements reduce pollen deposition and seed production inMimulus ringens(Phrymaceae)

Movement of pollinators between coflowering plant species may influence conspecific pollen deposition and seed set. Interspecific pollinator movements between native and showy invasive plants may be particularly detrimental to the pollination and reproductive success of native species. We explored the effects of invasive Lythrum salicaria on the reproductive success of Mimulus ringens, a wetland plant native to eastern North America. Pollinator flights between these species significantly reduced the amount of conspecific pollen deposited on Mimulus stigmas and the number of seeds in Mimulus fruits, suggesting that pollen loss is an important mechanism of competition for pollination. Although pollen loss is often attributed to pollen wastage on heterospecific floral structures, our novel findings suggest that grooming by bees as they forage on a competitor may also significantly reduce outcross pollen export and seed set in Mimulus ringens.

English

Abstract Many cucurbits, such as cucumbers, squashes and pumpkins, depend on pollinating bees in order to set fruit. However, fruit yield and progeny vigor in these plants generally decreases as heterospecific pollen deposition increases. We studied how the spatial area dedicated to cucumbers (Cucumis sativis), versus other flowering plants, influenced the deposition of conspecific and heterospecific pollen on cucumber plants in New York City community gardens. We also examined the effect of garden size on conspecific and heterospecific pollen deposition on cucumber plants. Female flowers were collected from potted cucumber plants that had been experimentally placed into the gardens, specifically for this study, or that were established in raised beds by members of the community garden. In the laboratory, pollen grains were isolated from the flower by acetolysis, and the number of heterospecific and conspecific cucumber pollen grains were quantified. Conspecific pollen deposition was positively and significantly associated with the size of a community garden, as well as with the area of each garden dedicated to non-cucumber, flowering plants (i.e. floral cover) and the area of each garden dedicated to cucumber plants (i.e. cucumber cover). Although floral cover explained a greater proportion of the variance, cucumber cover had the strongest effect on conspecific pollen deposition. Heterospecific pollen deposition was positively and significantly related to garden area. However, no significant relationship was found between heterospecific pollen deposition and floral cover, or cucumber cover. Based upon these results, we hypothesize that floral cover positively impacts conspecific pollen deposition by attracting a greater number of pollinators into an urban garden, and that total cucumber area positively impacts conspecific pollen deposition when pollinators are locally foraging within a garden. We suggest that the arrangement of plants within a garden can positively influence yield in fruit and vegetable-producing plants within urban community gardens. Due to the low availability of fruits and vegetables within the stores of the neighborhoods where this study was conducted, developing a better understanding of those factors that constrain or foster fruit and vegetable production are important to increasing food security and public health. Keywords Pollination; pollen deposition; urban community gardens; cucumbers; acetolysis; New York City