Heterospecific Pollen Transfer Research Articles

Pollinator sharing among co-flowering plants mediates patterns of pollen transfer

Co-flowering plant species often share pollinators, which could result in interspecific pollination. Despite some evidence suggesting plant species with overlapping pollinators influence each other’s pollination, the relationship between pollinator sharing and heterospecific pollen transfer (HPT) is not well explained at the community level. Here, we sampled a plant-pollinator visitation network to calculate the degree of pollinator sharing, and we identified pollen deposition on stigmas of co-flowering species to construct a HPT network in a sub-alpine meadow in southwest China. We also analyzed floral colour using reflectance spectra to measure the flower reflectance dissimilarity between each pair species. We found that there was no pollen transfer between most species pairs with shared pollinators. A higher proportion of HPT links between plant species pairs with shared pollinators was observed than between pairs with distinct pollinators. More pollen grains were transferred from donor to recipient species with greater pollinator sharing. Flowers with more dissimilar flower colours exchanged less heterospecific pollen. Our study evaluated the effect of pollinator sharing in shaping pollen transfer patterns among co-flowering species, which may shed light on the ecological implications of plant–pollinator interactions, mediated by pollinator sharing.

Diversity and composition of pollen loads carried by pollinators are primarily driven by insect traits, not floral community characteristics.

Flowering plants require conspecific pollen to reproduce but they often also receive heterospecific pollen, suggesting that pollinators carry mixed pollen loads. However, little is known about drivers of abundance, diversity or composition of pollen carried by pollinators. Are insect-carried pollen loads shaped by pollinator traits, or do they reflect available floral resources? We quantified pollen on 251 individual bees and 95 flies in a florally diverse community. We scored taxonomic order, sex, body size, hairiness and ecological specialization of pollinators, and recorded composition of available flowers. We used phylogenetically controlled model selection to compare relative influences of pollinator traits and floral resources on abundance, diversity and composition of insect-carried pollen. We tested congruence between composition of pollen loads and available flowers. Pollinator size, specialization and type (female bee, male bee, or fly) described pollen abundance, diversity and composition better than floral diversity. Pollen loads varied widely among insects (10-80,000,000 grains, 1-16 species). Pollen loads of male bees were smaller, but vastly more diverse than those of female bees, and equivalent in size but modestly more diverse than those of flies. Pollen load size and diversity were positively correlated with body size but negatively correlated with insect ecological specialization. These traits also drove variation in taxonomic and phylogenetic composition of insect-carried pollen loads, but composition was only weakly congruent with available floral resources. Qualities of pollinators best predict abundance and diversity of carried pollen indicating that functional composition of pollinator communities may be important to structuring heterospecific pollen transfer among plants.

Asymmetrical reproductive barriers in sympatric jewelflowers: are floral isolation, genetic incompatibilities and floral trait displacement connected?

Abstract Floral visitors influence reproductive interactions among sympatric plant species, either by facilitating assortative mating and contributing to reproductive isolation, or by promoting heterospecific pollen transfer, potentially leading to reproductive interference or hybridization. We assessed preference and constancy of floral visitors on two co-occurring jewelflowers [Streptanthus breweri and Streptanthus hesperidis (Brassicaceae)] using field arrays, and quantified two floral rewards potentially important to foraging choice – pollen production and nectar sugar concentration – in a greenhouse common garden. Floral visitors made an abundance of conspecific transitions between S. breweri individuals, which thus experienced minimal opportunities for heterospecific pollen transfer from S. hesperidis. In contrast, behavioural isolation for S. hesperidis was essentially absent due to pollinator inconstancy. This pattern emerged across multiple biotic environments and was unrelated to local density dependence. S. breweri populations that were sympatric with S. hesperidis had higher nectar sugar concentrations than their sympatric congeners, as well as allopatric conspecifics. Previous work shows that S. breweri suffers a greater cost to hybridization than S. hesperidis, and here we find that it also shows asymmetrical floral isolation and floral trait displacement in sympatry. These findings suggest that trait divergence may reduce negative reproductive interactions between sympatric but genetically incompatible relatives.

Flower evolution in the presence of heterospecific gene flow and its contribution to lineage divergence.

The success of species depends on their ability to exploit ecological resources in order to optimize their reproduction. However, species are not usually found within single-species ecosystems but in complex communities. Because of their genetic relatedness, closely related lineages tend to cluster within the same ecosystem, rely on the same resources, and be phenotypically similar. In sympatry, they will therefore compete for the same resources and, in the case of flowering plants, exchange their genes through heterospecific pollen transfer. These interactions, nevertheless, pose significant challenges to species co-existence because they can lead to resource limitation and reproductive interference. In such cases, divergent selective pressures on floral traits will favour genotypes that isolate or desynchronize the reproduction of sympatric lineages. The resulting displacement of reproductive characters will, in turn, lead to pre-mating isolation and promote intraspecific divergence, thus initiating or reinforcing the speciation process. In this review, we discuss the current theoretical and empirical knowledge on the influence of heterospecific pollen transfer on flower evolution, highlighting its potential to uncover the ecological and genomic constraints shaping the speciation process.

Competition for pollination and isolation from mates differentially impact four stages of pollination in a model grassland perennial

Abstract Species that persist in small populations isolated by habitat destruction may experience reproductive failure. Self‐incompatible plants face dual threats of mate‐limitation and competition with co‐flowering plants for pollination services. Such competition may lower pollinator visitation, increase heterospecific pollen transfer and reduce the likelihood that a visit results in successful pollination. To understand how isolation from mates and competition with co‐flowering species contribute to reproductive failure in fragmented habitat, we conducted an observational study of a tallgrass prairie perennial Echinacea angustifolia. We quantified the isolation of focal individuals from mates, characterized species richness and counted inflorescences within 1 m radius, observed pollinator visitation, collected pollinators, quantified pollen loads on pollinators and on Echinacea stigmas, and measured pollination success. Throughout the season, we sampled 223 focal plants across 10 remnant prairie sites. We present evidence that both co‐flowering species and isolation from mates substantially limit reproduction in Echinacea. As the flowering season progressed, the probability of pollinator visitation to focal plants decreased and evidence for pollen‐limited reproduction increased. Pollinators were most likely to visit Echinacea plants from low‐richness floral neighbourhoods with close potential mates, or plants from high‐richness neighbourhoods with distant potential mates. Frequent visitation only increased pollination success in the former case, likely because Echinacea in high‐richness floral neighbourhoods received low‐quality visits. Synthesis. In Echinacea, reproduction was limited by isolation from potential mates and the richness of co‐flowering species. These aspects of the floral neighbourhood influenced pollinator visitation and pollination success, although conditions that predicted high visitation did not always lead to high pollination success. These results reveal how habitat modification and destruction, which influence floral neighbourhood and isolation from conspecific mates, can differentially affect various stages of reproductive biology in self‐incompatible plants. Our results suggest that prairie conservation and restoration efforts that promote patches of greater floral diversity may improve reproductive outcomes in fragmented habitats.

Competition and pollen wars: simulations reveal the dynamics of competition mediated through heterospecific pollen transfer by non-flower constant insects

Heterospecific pollen transfer by insect pollinators has the potential to drive inter-species competition between flowering plants. This phenomenon may newly arise in a region if insect pollinator or flowering plant populations change. An agent-based simulation is presented to assess the potential impact of heterospecific pollen transfer by insects on two co-flowering plant species within an environment consisting of a shared central region and species-specific refugia. Where heterospecific pollen asymmetrically suppressed the reproduction of one competitor, the pollen recipient was rapidly ousted from shared regions. If pollinators made deep, repeated, forays into and out of plant refugia, the clogged species was even unseated from its own refugium. When heterospecific pollen symmetrically suppressed plant reproduction, the same effects were observed, but with one or the other species excluded at random by the pollen clogging mediated interaction. We conclude that both symmetrical and asymmetrical heterospecific pollen transfer may be important elements of inter-species dynamics. In particular, our simulation shows pollen and pollinator visits lost to heterospecific flowers may not always be wasted from the producer’s standpoint. Instead, heterospecific pollen delivery may convey a competitive advantage even when the recipient has a refuge safe from direct invasion. This is possible because the pollen producer may use pollinators to clog a competitor’s stigmas in a refugium without entering into competition there for space, nutrients, light, pollinators, or other resources. Consequently, the evolution of plant signals to promote pollinator constancy may not be the only effective strategy in inter-species competition.

Pollen transfer networks reveal alien species as main heterospecific pollen donors with fitness consequences for natives

Abstract The ecological dynamics of co‐flowering communities are largely mediated by pollinators. However, current understanding of pollinator‐mediated interactions primarily relies on how co‐flowering plants influence attraction of shared pollinators, and much less is known about plant–plant interactions that occur via heterospecific pollen (HP) transfer. Invaded communities in particular can be highly affected by the transfer of alien pollen, but the strength, drivers and fitness consequences of these interactions at a community scale are not well understood. Here we analyse HP transfer networks in nine coastal communities in the Yucatan Mexico that vary in the relative abundance of invasive flowers to evaluate how HP donation and receipt varies between native and alien plants. We further evaluate whether HP donation and receipt are mediated by floral traits (e.g. display, flower size) or pollinator visitation rate. Finally, we evaluated whether post‐pollination success (proportion of pollen tubes produced) was affected by alien HP receipt and whether the effect varied between native and alien recipients. HP transfer networks exhibit relatively high connectance (c. 15%), suggesting high HP transfer within the studied communities. Significant network nestedness further suggests the existence of species that predominantly act as HP donors or recipients in the community. Species‐level analyses showed that natives receive 80% more HP compared to alien species, and that alien plants donate 40% more HP than natives. HP receipt and donation were mediated by different floral traits and such effects were independent of plant origin (native or alien). The proportion of alien HP received significantly affected conspecific pollen tube success in natives, but not that of alien species. Synthesis. Our results suggest that HP transfer in invaded communities is widespread, and that native and alien species play different roles within HP transfer networks, which are mediated by a different suite of floral traits. Alien species, in particular, play a central role as HP donors and are more tolerant to HP receipt than natives—a finding that points to two overlooked mechanisms facilitating alien plant invasion and success within native co‐flowering communities.

Floral traits are associated with the quality but not quantity of heterospecific stigmatic pollen loads

BackgroundIn flowering communities, plant species commonly share pollinators and therefore plant individuals receive heterospecific pollen (HP). However, the patterns of HP transfers can deviate from patterns of plant-pollinator visitations. Although flower-visitor interactions are known to be mediated by floral traits, e.g. floral size or nectar tube depth, the explanatory power of these traits for HP transfer patterns remains elusive. Here, we have explored pollen transfer patterns at three sites in Southern Germany on three dates (early, mid and late summer). At the plant level, we tested whether flower abundance and floral traits are correlated with HP reception and donation. At the community level, we determined whether flower and bee diversity are correlated with network modularity and whether floral traits explain the module affiliation of plant species. We collected the stigmas of flowering plant species, analysed HP and conspecific pollen (CP) loads and measured floral traits, flower and bee diversity.ResultsOur results show that the degree and intensity of HP reception or donation at the plant level do not correlate with floral traits, whereas at the community level, the module affiliation of who is sharing pollen with whom is well-explained by floral traits. Additionally, variation in network modularity between communities is better explained by plant diversity and abundance than by bee diversity and abundance.ConclusionsOverall, our results indicate that floral traits that are known to mediate flower-visitor interactions can improve our understanding of qualitative HP transfer but only provide limited information about the quantity of HP transfer, which more probably depends on other floral traits, flower-visitor identity or community properties.

Flower constancy of the Cape honey bee pollinator of two co-flowering Erica species from the Cape Floristic Region (South Africa)

Erica is the largest genus of the Cape Floristic Region (CFR) of South Africa, and multiple species often coexist. Species that co-flower and share a pollinator may suffer from reproductive interference through heterospecific pollen deposition, especially when such species are closely related. However, little is known about the mechanisms that enable coexistence of co-flowering Erica species that share pollinators. Here we focus on a site where two co-flowering Erica species which are both predominantly pollinated by Cape honey bees grow intermingled, to ask whether this is associated with flower constancy and/or differences in pollen deposition sites. Despite minor differences in sites of pollen deposition on honey bees, most pollen of both Erica species was found on the head and ventral thorax regions. Subsequent visits of honey bees were almost always to the same Erica species from which they departed, and were significantly more constant than expected from random foraging in two consecutive years. Based on differences in pollen types between the two species, we established that pollen loads carried by honey bees were between 67% and 99% species-specific, whereas pollen loads on stigmas were on average over 93% pure. We conclude that despite the superficial similarities of small-flowered Erica species, honeybees can use species-specific cues for constant foraging, which is associated with almost complete absence of heterospecific pollen transfer. Future analyses of pollen carryover rates could help to quantify the benefit of pollinator behaviour for plant fecundity, and inclusion of more study sites sampled throughout the Erica flowering season could shed further light on the importance of flower constancy for facilitating coexistence of Erica species in the CFR.

Effect of heterospecific pollen deposition on pollen tube growth depends on the phylogenetic relatedness between donor and recipient.

Co-flowering plant species may interact via pollinators leading to heterospecific pollen transfer with consequences for plant reproduction. What determines the severity of heterospecific pollen effect on conspecific pollen performance is unclear, but it may depend on the phylogenetic relatedness of the interactors (pollen donors and recipient). The heterospecific pollen effect might also depend on the extent to which plants are exposed to heterospecific pollen over ecological or evolutionary timescales. For instance, generalist-pollinated plant species might tolerate heterospecific pollen more than specialists. Here, we tested whether heterospecific pollen effects are stronger between closely related species than phylogenetically distant ones in a tropical highland community. Then, based on these results, we determined whether responses to heterospecific pollen were stronger in generalized vs. specialized plant species. We applied heterospecific pollen from close (congeneric) or distant (different families) donors alone or with conspecific pollen on stigmas of three recipient species (one generalist, Sisyrinchium wettsteinii; and two specialists, Fuchsia campos-portoi and Fuchsia regia) and scored pollen tube performance in styles. In all species, pollen from closely related donors grew pollen tubes to the base of the style indicating a high potential to interfere with seed set. Conversely, distantly related heterospecific pollen had no effect on either specialist Fuchsia species, whereas enhanced performance of conspecific pollen was observed in generalist S. wettsteinii. The strong effect of phylogenetic relatedness of donor and recipient might have obscured the role of pollination specialization, at least for the three species examined here. Therefore, phylogenetic relatedness mediated the effect of heterospecific pollen on post-pollination success, with possible consequences for reproductive trait evolution and community assembly for further studies to explore.

Frequency-dependent fitness and reproductive dynamics contribute to habitat segregation in sympatric jewelflowers.

Coexistence results from a complex suite of past and contemporary processes including biogeographic history, adaptation, ecological interactions and reproductive dynamics. Here we explore drivers of local micro-parapatry in which two closely related and reproductively isolated Streptanthus species (jewelflower, Brassicaceae) inhabit continuous or adjacent habitat patches and occur within seed dispersal range, yet rarely overlap in fine-scale distribution. We find some evidence for abiotic niche partitioning and local adaptation, however differential survival across habitats cannot fully explain the scarcity of coexistence. Competition may also reduce the fitness of individuals migrating into occupied habitats, yet its effects are insufficient to drive competitive exclusion. Experimental migrants suffered reduced seed production and seed viability at sites occupied by heterospecifics, and we infer that heterospecific pollen transfer by shared pollinators contributes to wasted gametes when the two congeners come into contact. A minority disadvantage may reduce effective colonization of patches already occupied by heterospecifics, even when habitat patches are environmentally suitable. Differential adaptation and resource competition have often been evoked as primary drivers of habitat segregation in plants, yet negative reproductive interactions-including reproductive interference and decreased fecundity among low-frequency migrants-may also contribute to non-overlapping distributions of related species along local tension zones.

Plant-pollinator network structural properties differentially affect pollen transfer dynamics and pollination success.

Plant-pollinator network studies have uncovered important generalities in the structure of these communities, rapidly advancing our understanding of the underlying drivers of such a structure. In spite of this, however, it is still unclear how changes in structural network properties influence overall plant pollination success. One key limitation is the lack of information on the relationship between network structural properties and aspects of pollination and plant reproductive success. Here, we estimate four plant species network structural metrics (interaction strength, weighted degree, closeness centrality, and specialization level), commonly used to describe their importance within plant-pollinator networks, at two different sites, and evaluate their effects on pollen deposition and pollen tube success. We found a positive effect of plant-pollinator specialization and a negative effect of closeness centrality on heterospecific pollen load size. We also found a marginal negative effect of closeness centrality on pollen tube success. Our results suggest that increasing plant-pollinator specialization within nested communities (pollinated by one or very few generalist insect species) may result in high levels of heterospecific pollen transfer. Furthermore, the differential effects of plant-pollinator network metrics on pollination success (pollen receipt and pollen tube success), highlight the need to integrate quantity (e.g. visitation rate) and quality (e.g. pollen delivery) aspects of pollination to achieve a more mechanistic understanding of the relationship between plant-pollinator network structure and function. Such knowledge is key to evaluate the resilience and stability of plant-pollinator communities and the services they provide in the face of increasing human disturbances.

Habitat partitioning in native Geranium species through reproductive interference.

Heterospecific pollen transfer may reduce the fitness of recipient species, a phenomenon known as reproductive interference. A theoretical study has predicted that distributions of species pairs affected by reproductive interference may be syntopic under negligible reproductive interference, sympatric but with partitioning at small spatial scale (i.e. allotopic) under weak interference, or exclusive when reproductive interference is strong. Verifying these predictions is essential for evaluation of the applicability of reproductive interference as a general assembly rule of biological communities. The aim of this study was to test these predictions in two sympatrically distributed wild Geranium species, G. thunbergii and G. wilfordii. To measure the effect of reproductive interference, the associations between the relative abundance of the counterpart species and seed set in the focal species, and seed set reduction following mixed pollination, were analysed. The possibility of hybridization with viable offspring was examined by genotyping plants in the field and after mixed pollination. Fertility of putative hybrids was based on their seed set and the proportion of pollen grains with apertural protrusions. A transect study was conducted to examine spatial partitioning, and possible influences of environmental conditions (canopy openness and soil moisture content) on partitioning between the species were analysed. Neither abundance of the counterpart species nor heterospecific pollen deposition significantly affected seed set in the focal species, and hybridization between species was almost symmetrical. Putative hybrids had low fertility. The two species were exclusively distributed at small scale, although environmental conditions were not significantly different between them. The allotopy of the two species may be maintained by relatively weak reproductive interference through bidirectional hybridization. Re-evaluation of hybridization may allow ongoing or past reproductive interference to be recognized and provide insight into the distributional relationships between the interacting plants.

Effects of invasive Cirsium arvense on pollination in a southern Appalachian floral community vary with spatial scale and floral symmetry

Invasive plants can alter pollination dynamics by disrupting pollinator visitation and pollen transfer dynamics. However, a consensus regarding the direction of their overall effects (competitive vs. facilitative) remains elusive. Here, we evaluate the role of floral traits and spatial scale (community vs. floral neighborhood) in mediating invasive Cirsium arvense effects on resident plant species at multiple stages of the pollination process. C. arvense decreased pollinator visitation rate at the community level only for species with radial floral symmetry. At the floral neighborhood scale, pollinator visitation rate to all resident species was lower in the presence of C. arvense regardless of symmetry. C. arvense altered patterns of conspecific pollen receipt at the floral neighborhood scale, but the direction of the effect varied by plant species. We argue that these scale-dependent effects may be mediated by differences in foraging range and behavior of the main pollinators in the community. C. arvense, however, did not affect the overall reproductive success of resident species at either scale, suggesting that plants at our study sites may not be pollen limited. We further show evidence suggesting that C. arvense may alter the structure of community-level plant–plant interactions via heterospecific pollen transfer by subverting the roles of resident pollen donors within the pollen transfer network. Overall, our results suggest that generalized species (with radial flowers) may be more susceptible to invasive species’ effects than specialized ones (bilateral flowers) and highlight the need to consider scale-dependent effects in order to develop a more predictive understanding of invasive species effects in nature.

Patterns and effects of heterospecific pollen transfer between an invasive and two native plant species: the importance of pollen arrival time to the stigma.

Invasive plant species can integrate into native plant-pollinator communities, but the underlying mechanisms are poorly understood. Competitive interactions between invasive and native plants via heterospecific pollen (HP) and differential invasive HP effects depending on HP arrival time to the stigma may mediate invasion success, but these have been little studied. We evaluated patterns and effects of HP receipt on pollen tube growth in two native and one invasive species in the field. We also used hand-pollination experiments to evaluate the effect of invasive HP pollen and its arrival time on native reproductive success. Native species receive smaller and less-diverse HP loads (5-7 species) compared to invasive species (10 species). The load size of HP had a negative effect on the proportion of pollen tubes in both native species but not in the invasive, suggesting higher HP tolerance in the latter. Invasive HP arrival time differentially affected pollen tube success in native species. Our results highlight the need to study reciprocal HP effects between invasive and native species and the factors that determine differential responses to HP receipt to fully understand the mechanisms facilitating invasive species integration into native plant-pollinator communities.

Floral isolation and pollination in two hummingbird-pollinated plants: the roles of exploitation barriers and pollinator competition

Differences in feeding performance and aggressive abilities between species and sexes of hummingbirds are often associated with the partitioning of their food sources, but whether such partitioning results in floral isolation (reproductive isolation at the stage of pollination) has received little attention. We examined components of floral isolation and pollinator effectiveness of Heliconia caribaea and H. bihai on the island of Dominica, West Indies. The short flowers of H. caribaea match the short bills of male Anthracothorax jugularis, its primary pollinator, whereas the long flowers of H. bihai match the long bills of female A. jugularis, its primary pollinator. In pollination experiments, both sexes of A. jugularis were equally effective at pollinating the short flowers of H. caribaea, which they preferred to H. bihai, whereas females were more effective at pollinating the long flowers of H. bihai. Moreover, an average difference in length of 12 mm between H. caribaea and H. bihai flowers did not prevent heterospecific pollen transfer, and both sexes transported pollen between the two plant species. In field studies using powdered dyes as pollen analogs, however, heterospecific pollen transfer was minimal, with only 2 of 168 flowers receiving dye from the other species. The length of H. bihai flowers acted as an exploitation barrier to male A. jugularis, which were unable to completely remove nectar from 88% of the flowers they visited. In contrast, interference competition combined with high floral fidelity through traplining prevented female A. jugularis from transferring pollen between the two Heliconia species. A combination of exploitation barriers, interference and exploitative competition, and pollinator preferences maintains floral isolation between these heliconias, and may have contributed to the evolution of this hummingbird-plant system.

Global geographic patterns of heterospecific pollen receipt help uncover potential ecological and evolutionary impacts across plant communities worldwide

Species interactions are known to be key in driving patterns of biodiversity across the globe. Plant-plant interactions through heterospecific pollen (HP) transfer by their shared pollinators is common and has consequences for plant reproductive success and floral evolution, and thus has the potential to influence global patterns of biodiversity and plant community assembly. The literature on HP transfer is growing and it is therefore timely to review patterns and causes of among-species variation in HP receipt at a global scale, thus uncovering its potential contribution to global patterns of biodiversity. Here we analyzed published data on 245 species distributed across five continents to evaluate latitudinal and altitudinal patterns of HP receipt. We further analyzed the role of floral symmetry and evolutionary history in mediating patterns of HP receipt. Latitude and elevation affected the likelihood and intensity of HP receipt indicating that HP transfer increases in species-rich communities and in areas with high abundance of vertebrate pollinators. Floral symmetry and evolutionary history determined HP load size across plant communities worldwide. Overall, our results suggest that HP receipt may have the potential to contribute to global geographic patterns of plant diversity by imposing strong selective pressures in species-rich areas across the globe.

Pollinator-mediated interactions between cultivated papaya and co-flowering plant species.

Many modern crop varieties rely on animal pollination to set fruit and seeds. Intensive crop plantations usually do not provide suitable habitats for pollinators so crop yield may depend on the surrounding vegetation to maintain pollination services. However, little is known about the effect of pollinator‐mediated interactions among co‐flowering plants on crop yield or the underlying mechanisms. Plant reproductive success is complex, involving several pre‐ and post‐pollination events; however, the current literature has mainly focused on pre‐pollination events in natural plant communities. We assessed pollinator sharing and the contribution to pollinator diet in a community of wild and cultivated plants that co‐flower with a focal papaya plantation. In addition, we assessed heterospecific pollen transfer to the stigmatic loads of papaya and its effect on fruit and seed production. We found that papaya shared at least one pollinator species with the majority of the co‐flowering plants. Despite this, heterospecific pollen transfer in cultivated papaya was low in open‐pollinated flowers. Hand‐pollination experiments suggest that heterospecific pollen transfer has no negative effect on fruit production or weight, but does reduce seed production. These results suggest that co‐flowering plants offer valuable floral resources to pollinators that are shared with cultivated papaya with little or no cost in terms of heterospecific pollen transfer. Although HP reduced seed production, a reduced number of seeds per se are not negative, given that from an agronomic perspective the number of seeds does not affect the monetary value of the papaya fruit.

Variation in sampling effort affects the observed richness of plant-plant interactions via heterospecific pollen transfer: implications for interpretation of pollen transfer networks.

There is growing interest in understanding plant-plant interactions via pollen transfer at the community level. Studies on the structure and spatial variability of pollen transfer networks have been valuable to this understanding. However, there is high variability in the intensity of sampling used to characterize pollen transfer interactions, which could influence network structure. To date, there is no knowledge of how sampling effort influences the richness of pollen on stigmas and thereby transfer interactions observed, nor how this may vary across species and study sites. We use rarefaction curves on 16 species to characterize the relationship between sampling effort (number of stigmas analyzed) and the richness of pollen transfer interactions recorded. We further assess variability in this relationship among species, plant community types, and sites within a single plant community. We show high among-species variation in the amount of sampling required to sufficiently characterize interspecific pollen transfer. We further reveal variability in the sampling effort-interaction richness relationship among different plant communities and even for the same species growing in different sites. The wide heterogeneity in the sampling effort required to accurately characterize pollen transfer interactions observed has the potential to influence the characterization of pollen transfer dynamics. Thus, sampling completeness should be considered in future studies to avoid overestimation of modularity and specialization in pollen transfer networks that may bias the predicted causes and expected consequences of such processes for plant-plant interactions.

Facilitative interactions among co-flowering Primula species mediated by pollinator sharing

We investigated flowering phenology, pollinator visitation and visitor community composition in communities of self-incompatible sympatric Primula species in a high-elevation Himalayan ecosystem. Within the tight constraints imposed by short growing seasons in such ecosystems, interactions among co-occurring plants for pollinators may vary from competition to facilitation, depending on the specifics of the system. We found that pollinator community composition changed with elevation in this system: lepidopterans were the dominant visitors at lower elevations (2200–3000 masl), bees (other than bumblebees) dominated at mid-elevations (3000–3800 masl) and bumblebees dominated at higher elevations (3800–4600 masl). However, within an elevation zone, there were no significant differences in pollinators amongst co-occurring Primula species. At a focal study site where multiple Primula species co-occurred, our results showed that even while the overall flowering periods of these species broadly overlapped, the peak flowering periods of different Primula species were temporally segregated. Upon further inferring the nature of interaction amongst co-flowering Primula species, we found that plots with higher Primula diversity (≥ 2 species) and density (80–100 individuals) experienced significantly higher pollinator visitation, compared with plots with single species and low flower densities (40–50 individuals). Our results suggest that in this community of sympatric, self-incompatible Primula species, a broadly aggregated, synchronous floral display of multiple species results in pollinator facilitation by attracting a greater number of pollinator visitors. Within this broadly synchronous display, the temporal segregation of peak flowering period of individual species may reduce competition for pollinators and limit heterospecific pollen transfer.