Heterospecific Pollen Transfer Research Articles

Flower visits and pollinator pollen load networks reveal the effects of pollinator sharing on heterospecific pollen transfer in a subalpine plant community.

The mutualistic network of plant-pollinator also involves interspecific pollination caused by pollinator sharing. Plant-pollinator networks are commonly based on flower visit observations, which may not adequately represent the actual pollen transfer between co-flowering plant species. Here, we compared the network structure of plant-pollinator interactions based on flower visits (FV) and pollen loads (PL) on the bodies of pollinators and tested how the degree of pollinator sharing in the two networks affected heterospecific pollen transfer (HPT) between plant species in a subalpine meadow. The FV and PL networks were largely overlapped. PL network included more links than FV network. The positions of plant and pollinator species in the FV and PL networks were positively correlated, indicating that both networks could detect major plant-pollinator interactions. The degree of pollinator sharing, based on either the FV or the PL network, positively influenced the amount of heterospecific pollen transferred between plant species pairs. However, the degree of pollinator sharing had a low overall explanatory power for HPT, and the explanatory powers of the FV and PL networks were similar. Overall, our study highlights the importance of FV and PL for understanding the drivers and outcomes of plant-pollinator interactions, as well as their relevance to HPT.

Heterospecific pollen deposition in understory plants of a forest in central Amazon

In co-flowering plant communities, pollinator movement can lead to the transfer of heterospecific pollen among species. This can affect the reproductive success of individuals, either by preventing conspecific pollen from reaching the stigma or by allelopathy, but little is known about this process in tropical areas. To analyze heterospecific pollen transfer and deposition, we investigated attributes related to pollinator attraction, such as corolla size, number of flowers per individual, floral arrangement, floral asymmetry, and distance from one flowering individual to another in a Central Amazon forest. The study was carried out in riparian plots of the Adolpho Ducke Forest Reserve during the rainy season. Stigmas and anthers were removed from all individual understory plants found in bloom in permanent plots. Slides were mounted with glycerinated gelatin, and all conspecific and heterospecific pollen grains were identified and counted. We then used a generalized linear mixed model framework to determine how the measured variables were related to the proportion of heterospecific pollen deposition (HPD) on stigmas. We found that the mean proportion of HPD in individuals was 4.7%, and 60% of individuals had no heterospecific pollen; from the 14 species studied, two had no heterospecific pollen. Species with wider flowers received less HPD, while species with more flowers per individual and with solitary flowers received more heterospecific pollen. Despite the significant variation in floral types in the Adolpho Ducke Reserve, there was a low percentage of heterospecific pollen deposition, probably due to specialized plant-pollinator interactions.

Limited reproductive interference despite high rates of heterospecific pollen transfer among co-occurring bat-pollinated Burmeistera.

Many tropical plants are bat-pollinated, but these mammals often carry copious, multispecific pollen loads making bat-pollinated plants susceptible to heterospecific pollen deposition and reproductive interference. We investigated pollen transfer between sympatric bat-pollinated Burmeistera species and their response to heterospecific pollen deposition from each other. We quantified conspecific and heterospecific pollen deposition for two populations of B. ceratocarpa, a recipient species in heterospecific pollen transfer interactions, that co-occur with different donor relatives (B. borjensis and B. glabrata). We then used a cross-pollination scheme using pollen mixtures to assess the species' responses to heterospecific pollen deposition in terms of fruit abortion and seed production. Burmeistera ceratocarpa received significantly more heterospecific pollen from its relatives at both sites than its own pollen was deposited on its relatives. However, heterospecific pollen deposition only affected seed production by B. borjensis and B. glabrata, but not by B. ceratocarpa, suggesting that early acting post-pollination barriers buffer the latter against reproductive interference. Crosses between sympatric and allopatric populations suggest that the study species are fully isolated in sympatry, while isolation between allopatric populations is strong but incomplete. We did not observe evidence of reproductive interference among our study species, because either heterospecific pollen deposition did not affect their seed production (B. ceratocarpa) or they receive heterospecific pollen only rarely (B. borjensis and B. glabrata). Frequent heterospecific pollen deposition might favor the evolution of barriers against foreign pollen (as in B. ceratocarpa) that alleviate the competitive costs of sharing low fidelity pollinators with co-occurring species.

Individual bee foragers are less‐efficient transporters of pollen for plants from which they collect the most pollen in their scopae

Bees provision most of the pollen they remove from anthers to their larvae and transport only a small proportion to stigmas, which can negatively affect plant fitness. Though most bee species collect pollen from multiple plant species, we know little about how the efficiency of bees' pollen transport varies among host plant species, or how it relates to other aspects of generalist bee foraging behavior that benefit plant fitness, such as specialization on individual foraging bouts. We compared the pollen collected and transported by three bee species for 46 co-occurring plant species. Specifically, we compared the relative abundance of pollen taxa in individual bees' scopae, structures where bees store pollen to provision larvae, with the relative abundance of pollen taxa on the rest of bees' bodies, which is more likely to be transferred to stigmas. Bees carried five times more pollen grains in their scopae than elsewhere on their bodies. Within foraging bouts, bees were relatively specialized in their pollen collection, but transported proportionally less pollen for the host plants on which they specialized. Across foraging bouts, two bee species transported proportionally less pollen for some of their host plants than for others, though differences didn't consistently follow the same trend as at the foraging bout scale. Our results suggest that foraging bout specialization, which is known to reduce heterospecific pollen transfer, also results in less efficient pollen transport. Thus, bee foragers that visit predominantly one plant species may have contrasting effects on that plant's fitness. This article is protected by copyright. All rights reserved.

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.

Pollinator visitation closely tracks diurnal patterns in pollen release.

Animal-pollinated plants face a high risk of pollen loss during its transfer. To limit the negative effect of pollen losses by pollen consumption and heterospecific transfer, plant species may adjust and stratify their pollen availability during the day (i.e., "schedule" their pollen presentation) and attract pollinators in specific time frames. We investigated diurnal patterns of pollen availability and pollinator visitation in three coflowering plant species: Succisa pratensis with open flowers and accessible pollen, pollinated mainly by pollen-feeding hoverflies; Centaurea jacea with open flowers and less accessible pollen, pollinated mainly by pollen-collecting bees; and Trifolium hybridum with closed flowers and pollen accessible only after the active opening of the flower, pollinated exclusively by bees. The three plant species differed in the peak pollen availability, tracked by the visitation activity of their pollinators. Succisa pratensis released all pollen in the morning, while pollinator activity was still low and peaked with a slight delay. In contrast, C. jacea and T. hybridum had distinct pollen presentation schedules, peaking in the early afternoon. The pollinator visitation to both of these species closely matched their pollen availability. Stratifying pollen availability to pollinators during the day may be one of several mechanisms that allow coflowering plants to share their pollinators and decrease the probability of heterospecific pollen transfer.

Pollination of urban meadows – Plant reproductive success and urban-related factors influencing frequency of pollinators visits

Pollination is a key ecological service for human food security and the reproduction of the majority of flowering plants. This plant-animal interaction is crucial in both wild and urban habitats, however, increasing urbanization and change in land use alter these interactions. The concerns regarding pollination have led to numerous efforts aimed at improving the quality of wildlife in cities, however, understanding the importance of these activities warrants further research. In Warsaw, the Polish capital, we analyzed the influence of urban-related factors such as floral resource richness, the proportion of green areas in the proximity of foraging sites, air pollution or temperature on plant-pollinator interaction in 14 meadows. We also analyzed the reproductive success and the heterospecific pollen transfer in the case of plants growing in the meadows. Our study revealed that the frequency of visits to flowers was related to city-related factors, but the reaction varied among different groups of pollinators. For example, bumblebees were negatively influenced by higher air pollution, while the rest of the flower visitors were not, Syrphidae flies were more frequent flower visitors when the proportion of green area was higher in the buffer of 150 m, while we did not observe this tendency in other groups of flower visitors. Plants growing in Warsaw meadows were frequently visited and effectively pollinated - urban pollinator assemblages seem to provide sufficient pollination services to plants growing in the city. Species-rich communities were not a threat to pollination; we found mostly conspecific pollen grains on the stigmas of the studied plant species. The results of our study show that species-rich communities make a valuable contribution to pollinator conservation. The conservation activities, however, should be adjusted to the target group of pollinators, since different groups of flower visitors respond differently to urban-related factors studied.

Intraspecific variation of invaded pollination networks – the role of pollen-transport, pollen-transfer and different levels of biological organization

•Pollination networks have long been studied by quantifying plant-flower visitor species interactions. Despite making considerable contributions, this ignores important steps of pollen movement from anthers to receptive stigmas and neglects the intraspecific variation of the interacting partners. Addressing specialization and niche partitioning regarding heterospecific pollen transport and transfer, is fundamental to untangle the mechanisms behind contrasts seen in the impact of alien species on native communities.•We used two well-sampled datasets on pollen-transport and pollen-transfer networks to test how intraspecific variation in interaction specialisation affects invaded pollination networks. We considered different levels of biological organization: from species- to individual-based networks.•We found significant intraspecific variation in the pollen loads and pollen deposition of the invasive plant Impatiens glandulifera; thus only a few individual pollinators and plant stigmas carried large amounts of alien pollen grains, potentially functioning as super-spreaders driving the invading process. Consequently, most individuals carried only a few, or no alien pollen at all, possibly buffering the negative effects of invasive plants at the population and community levels.•Node and structural specialization were higher for individual-based and pollen-transfer networks, suggesting a lack of dominant, highly generalist links when downscaling from pollen-transport to pollen-transfer, and from species to individual-based networks.•The high specialization, selectiveness and niche partitioning of plants, pollinators and their interaction revealed at the different stages of the pollination process and across distinct levels of biological organization, suggest important mechanisms associated with the (re) organization of population niches. Moreover, these mechanisms provide a promising approach towards a more comprehensive understanding of the dynamics of invasion biology from population to community and ecosystem functioning.

The cost of self-promotion: ecological and demographic implications of the mentor effect in natural plant populations.

Under the mentor effect, compatible heterospecific pollen transfer induces self-pollen germination in otherwise self-incompatible plants. The mentor effect could be considered a novel mode of reproductive interference if it negatively impacts fitness. Yet to date, this phenomenon has predominately been investigated under experimental conditions rather than insitu. We address this gap in natural populations of the self-incompatible native dandelion, Taraxacum ceratophorum, where selfing only occurs in association with hybridization from exotic Taraxacum officinale. We tested whether self-fertilization rate increases in the hybrid zone, as predicted due to the mentor effect. Using results from these investigations, we created an exponential growth model to estimate the potential demographic impacts of the mentor effect on T. ceratophorum population growth. Our results demonstrate that the strength of the mentor effect in Taraxacum depends on the prevalence of pollinator-mediated outcross pollen deposition rather than self-pollination. Demographic models suggest that reduced outcrossing in T. ceratophorum under exotic invasion could negatively impact population growth through inbreeding depression. We demonstrate the mentor effect is rare in natural populations of T. ceratophorum due to masking by early life cycle inbreeding depression, prevalent outcrossing, and ovule usurpation by heterospecific pollen.

Relative stable interannual variation in plant–plant pollen transfer rather than the plant–pollinator network of a subalpine meadow

AbstractPrevious studies have shown that plant–pollinator mutualistic interactions experience highly interannual variation. Given that pollinators often move across multiple plant species, the plant–plant interactions that take place via heterospecific pollen (HP) transfer may also vary temporally, which could have important implications for floral evolution and community assembly. Here, we evaluated the interannual variation in plant–pollinator networks and plant–plant heterospecific pollen transfer (HPT) networks of a subalpine meadow community in Southwest China for three consecutive years. The interactions largely varied among years for both network types. The composition of donor-species HP deposited on the plants varied less than did the visit composition of the pollinators, suggesting that HP could be transferred from identical donor species to recipient species through different shared pollinators among years. The plant species were at more similar positions in the HPT network than they were in the plant–pollinator network across years. Moreover, the more generalized plant species in the plant–pollinator network tended to export their pollen grains and more strongly influence HPT. We evaluated the relatively stable structure of the HPT network compared with the plant–pollinator network, which represents an important step in the integration of plant–pollinator and plant–plant interactions.

Heterospecific Pollen Transfer, Self-Assurance, and the Distribution of Dioecy and Hermaphroditism in a Single Species: The Importance of Pollen Landscapes

Heterospecific Pollen Transfer, Self-Assurance, and the Distribution of Dioecy and Hermaphroditism in a Single Species: The Importance of Pollen Landscapes

Goldilocks’ quarter-hectare urban farm: An agent-based model for improved pollination of community gardens and small-holder farms

Industrial agriculture’s expansive monocultures contrast against community farms of diverse crops in small allotments constrained by urban regulations. These human-controlled differences in scale, arrangement and crop diversity, may differently impact insect foraging and pollination. The relationship between human management and insect pollination is under-explored, especially regarding the implications of insect flower constancy–the tendency of insects to favour visits to a single rewarding flower species during a foraging bout. Although high flower-constancy is associated with reliable pollen transport, its impact on pollination depends also on planting regimes, the vicinity of neighbouring crops, weeds and wildflowers. This study explores the potential pollination impacts of insect flower constancy, heterospecific pollen transfer, and human-dictated crop patch size, using an agent-based model of insect pollinators foraging from two flowering plant species. Highly constant pollinators were most effective in the smallest patches when heterospecific pollen transfer was an issue. As patch size increased, pollination rates improved overall, but less constant insects produced better pollination rates for intermediate sized areas because they rapidly switched preferences between flower species. As patch size increased further, the influence of flower constancy on pollination rates was reduced. Study results suggest that typical community farms containing small single-crop patches operated by independent growers within an allotment, may be better pollinated if operated collaboratively to increase single-crop patch size. Crop patches needn’t be large homogeneous agricultural monocultures, but neither should they be so small and heterogeneous as to inhibit pollination. We found a “Goldilocks zone” around 11m x 11m to be a good compromise for pollination, regardless of the level of flower constancy in local insects. As climate and human land use increasingly impact insect populations, the relationships between pollination systems, growers and policy makers must be continually examined to safeguard food supplies and native ecosystems.

High flower richness and abundance decrease pollen transfer on individual plants in road verges but increase it in adjacent fields in intensively managed agroecosystems

Road verges can facilitate the movement of pollinators and pollen in intensively managed agroecosystems, reducing the negative impacts of fragmentation (i.e., isolation of wild pollinator communities) and enhancing the pollination service provided to wild plants. To assess the role of road verges in sustaining the pollination service to wild plants, we studied gardens consisting of two plant species arranged within sixteen road verges and their adjacent fields during two consecutive seasons in the Argentine Pampas. We assessed the influence of attributes of road verges on pollination service at different scales: road verges width, species richness of flowering plant, and flower abundance (patch scale), crops and successional pastures (adjacent land use scale), and landscape heterogeneity (landscape scale). We measured the pollination service provided to two species of potted plants (Hirschfeldia incana and Lobularia maritima) in three different ways: visitation rate, conspecific and heterospecific pollen transfer to stigmas, and distance of pollen transfer from a fixed source. The number of conspecific and heterospecific pollen grains deposited on stigmas decreased with an increased abundance of floral units in associated road verges, which is caused by a dilution effect on individual plant pollination. Also, the pollen transfer into adjacent fields decreased with an increased abundance of floral units in the nearby road verges, which is caused by a decrease of pollen spillover into adjacent crop fields. The visitation rate within adjacent fields increased with higher flowering plant species richness in associated road verges. Also, the number of conspecific pollen grains in the road verges adjacent to successional pastures increased with higher flowering plant species richness in road verges, which results in a concentration effect of pollinator visits and pollen transfer into road verges. We conclude that managing flower abundance and increasing flowering plant species richness in road verges are crucial practices for maintaining the pollination service of entomophilous plants in intensively managed agroecosystems.

Colour similarity to flowering neighbours promotes pollinator visits, pollen receipt and maternal fitness

While co-flowering plants often compete for pollination through pollinator visits and pollen transfer, recent studies reveal potential for the facilitation of pollination. Pollination may be promoted when similarities in the floral traits of co-flowering species enhance pollinator visitation and seed set. Floral similarity, however, could also lead to heterospecific pollen transfer, creating a fitness trade-off that is seldom explored empirically.Here we construct experimental communities of Oxalis to investigate the trade-off between enhanced visitation and heterospecific pollen receipt, which can impact seed set at varying relative plant abundance. Our experimental communities comprise O. purpurea that is matched or mismatched to congeners in flower colour at low and high relative plant abundance. We investigate the consequences of colour overlap for pollinator visitation and the receipt of conspecific, heterospecific, and mixed pollen loads, and assess seed set in natural plots.Flower colour similarity increases visitation by shared pollinators at both low and high relative abundance when O. purpurea occurs in matched flower colour communities. The receipt of pollen analogues from conspecifics is also greater at both abundances when neighbours are matched in floral colour. Although conspecific pollen in matched communities is mostly delivered in mixed pollen loads, hand pollinations confirm the receipt of mixed pollen carries no fitness costs for seed production. Consequently, plants in natural populations plots of O. purpurea experiences increase maternal fitness when growing in matching flower colour communities.We demonstrate a positive net effect of flower colour similarity, relative to flower colour dissimilarity, that can facilitate plant reproduction. Through field experiments we systematically test previously observed patterns of clustered flower colour assembly in Oxalis communities to provide a comprehensive assessment of facilitative co-flowering interactions.

PHENOLOGICAL SENSITIVITIES TO CLIMATE ARE SIMILAR IN TWO CLARKIA CONGENERS: INDIRECT EVIDENCE FOR FACILITATION, CONVERGENCE, NICHE CONSERVATISM, OR GENETIC CONSTRAINTS

To date, most herbarium-based studies of phenological sensitivity to climate and of climate-driven phenological shifts fall into two categories: detailed species-specific studies vs. multi-species investigations designed to explain inter-specific variation in sensitivity to climate and/or the magnitude and direction of their long-term phenological shifts. Few herbarium-based studies, however, have compared the phenological responses of closely related taxa to detect: (1) phenological divergence, which may result from selection for the avoidance of heterospecific pollen transfer or competition for pollinators, or (2) phenological similarity, which may result from phylogenetic niche conservatism, parallel or convergent adaptive evolution, or genetic constraints that prevent divergence. Here, we compare two widespread Clarkia species in California with respect to: the climates that they occupy; mean flowering date, controlling for local climate; the degree and direction of climate change to which they have been exposed over the last ∼115 yr; the sensitivity of flowering date to inter-annual and to long-term mean maximum spring temperature and annual precipitation across their ranges; and their phenological change over time. Specimens of C. cylindrica were sampled from sites that were chronically cooler and drier than those of C. unguiculata, although their climate envelopes broadly overlapped. Clarkia cylindrica flowers ∼ 3.5 d earlier than C. unguiculata when controlling for the effects of local climatic conditions and for quantitative variation in the phenological status of specimens. However, the congeners did not differ in their sensitivities to the climatic variables examined here; cumulative annual precipitation delayed flowering and higher spring temperatures advanced flowering. In spite of significant spring warming over the sampling period, neither species exhibited a long-term phenological shift. Precipitation and spring temperature interacted to influence flowering date: the advancing effect on flowering date of high spring temperatures was greater in dry than in mesic regions, and the delaying effect of high precipitation was greater in warm than in cool regions. The similarities between these species in their phenological sensitivity and behavior are consistent with the interpretation that facilitation by pollinators and/or shared environmental conditions generate similar patterns of selection, or that limited genetic variation in flowering time prevents evolutionary divergence between these species.

Co‐occurrence patterns at four spatial scales implicate reproductive processes in shaping community assembly in clovers

Abstract Competition, niche differences and chance all contribute to community assembly; yet, the role of reproductive interactions between species is often less appreciated. Closely related plant species that share floral form, phenology and habitat often interact through pollination. They potentially facilitate pollinator attraction, compete for pollination services and/or exchange pollen. If reproductive processes are important to co‐occurrence, we predicted that fitness costs of heterospecific pollen transfer or pollen limitation should result in lower rates of co‐occurrence among outcrossing congeners. In contrast, selfers, which may be less exposed to heterospecific pollen, and/or less negatively affected by it, should co‐occur more frequently. Flower size is an excellent proxy for mating system in clovers. Using herbarium records and three independent field datasets, we documented co‐occurrence patterns of Trifolium at 1 m2–1 km2 scales in California. Using a randomization procedure to reshuffle matrices of community membership, we generated null hypotheses for the expected composition of large‐ and small‐flowered species in Trifolium communities of different sizes. Across all spatial scales, large‐flowered outcrossers were over‐represented at sites lacking congeners, but under‐represented in communities with multiple congeners. Conversely, small‐flowered selfers often occupied sites with multiple other Trifolium species. Patterns for plant height and leaf size, which are weakly or strongly correlated with flower size, did not explain co‐occurrence patterns as robustly. Regression analysis and model selection corroborated the null model analyses, indicating that the likelihood of co‐occurrence decreased as flower size, and thus reliance on outcrossing, increased. Synthesis. This study suggests that reproductive traits and processes may be significant contributors to community assembly and co‐occurrence in flowering plants.

The genetic basis of floral mechanical isolation between two hummingbird-pollinated Neotropical understorey herbs.

Floral divergence can contribute to reproductive isolation among plant lineages, and thus provides an opportunity to study the genetics of speciation, including the number, effect size, mode of action and interactions of quantitative trait loci (QTL). Moreover, flowers represent suites of functionally interrelated traits, but it is unclear to what extent the phenotypic integration of the flower is underlain by a shared genetic architecture, which could facilitate or constrain correlated evolution of floral traits. Here, we examine the genetic architecture of floral morphological traits involved in an evolutionary switch from bill to forehead pollen placement between two species of hummingbird-pollinated Neotropical understorey herbs that are reproductively isolated by these floral differences. For the majority of traits, we find multiple QTL of relatively small effect spread throughout the genome. We also find substantial colocalization and alignment of effects of QTL underlying different floral traits that function together to promote outcrossing and reduce heterospecific pollen transfer. Our results are consistent with adaptive pleiotropy or linkage of many co-adapted genes, either of which could have facilitated a response to correlated selection and helped to stabilize divergent phenotypes in the face of low levels of hybridization. Moreover, our results indicate that floral mechanical isolation can be consistent with an infinitesimal model of adaptation.

A quick glance at noteworthy articles for September 2021

A quick glance at noteworthy articles for September 2021

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.

Spatial variation in the intensity of interactions via heterospecific pollen transfer may contribute to local and global patterns of plant diversity

Studies that aim to understand the processes that generate and organize plant diversity in nature have a long history in ecology. Among these, the study of plant-plant interactions that take place indirectly via pollinator choice and floral visitation has been paramount. Current evidence, however, indicates that plants can interact more directly via heterospecific pollen (HP) transfer and that these interactions are ubiquitous and can have strong fitness effects. The intensity of HP interactions can also vary spatially, with important implications for floral evolution and community assembly. Interest in understanding the role of heterospecific pollen transfer in the diversification and organization of plant communities is rapidly rising. The existence of spatial variation in the intensity of species interactions and their role in shaping patterns of diversity is also well recognized. However, after 40 years of research, the importance of spatial variation in HP transfer intensity and effects remains poorly known, and thus we have ignored its potential in shaping patterns of diversity at local and global scales. Here, I develop a conceptual framework and summarize existing evidence for the ecological and evolutionary consequences of spatial variation in HP transfer interactions and outline future directions in this field. The drivers of variation in HP transfer discussed here illustrate the high potential for geographic variation in HP intensity and its effects, as well as in the evolutionary responses to HP receipt. So far, the study of pollinator-mediated plant-plant interactions has been almost entirely dominated by studies of pre-pollination interactions even though their outcomes can be influenced by plant-plant interactions that take place on the stigma. It is hence critical that we fully evaluate the consequences and context-dependency of HP transfer interactions in order to gain a more complete understanding of the role that plant-pollinator interactions play in generating and organizing plant biodiversity.