Premise: 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. Methods: 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. Results: 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. Conclusions: 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.

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.

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 and 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.

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.

BACKGROUND: 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. SCOPE: 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. CONCLUSIONS: 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.

Premise of The Study: Studies on the effects of heterospecific pollen (HP) transfer have been focused mainly on insect-pollinated species, despite evidence of insect visitation to wind-pollinated species and transfer of their pollen onto stigmas of insect-pollinated plants. Thus, the potential consequences of HP transfer from wind-pollinated species remain largely unknown. Furthermore, accumulation of pesticide residues in pollen of wind-pollinated crops has been documented, but its potential effects on wild plant species via HP transfer have not been tested. Methods: We evaluated the effect of wind-dispersed Zea mays pollen on pollen tube growth of the insect-pollinated Mimulus nudatus via hand pollinations. We further evaluated whether pesticide-contaminated Z. mays pollen has larger effects on M. nudatus pollen success than non-contaminated Z. mays pollen. Key Results: We found a significant negative effect of Z. mays pollen on M. nudatus pollen tube growth even when deposited in small amounts. However, we did not observe any difference in the magnitude of this effect between pesticide-laden Z. mays pollen and non-contaminated Z. mays pollen. Conclusions: Our results suggest that wind-pollinated species can have negative effects as HP donors on insect-pollinated recipients. Thus, their role in shaping co-flowering interactions for wind- and insect-pollinated species deserves more attention. Although we did not find evidence that pesticide contamination increased HP effects, we cannot fully rule out the existence of such an effect, because pollen load and thus the pesticide dose applied to stigmas was low. This result should be confirmed using other HP donors and across a range of HP loads, pesticide types, and concentrations.

The myriad ways species interact with each other have always captivated biologists. These interactions—predation, competition, parasitism, and mutualism—are fundamental to the stability of ecological communities and drive the evolution of species they contain. Some mutualistic systems consist of mutually dependent partners that strongly influence each other’s survival, while other mutualistic systems consist of many, diffuse relationships between large assemblages of partners. Critical ecological processes like pollination and seed dispersal are prime examples of such complex systems. Plant-pollinator communities are characterized by extensive pollinator sharing among plant species. My dissertation explores some of the consequences of this reliance on shared pollinators on the structure of plant-pollinator interaction networks, the foraging decisions of pollinators, and the fitness outcomes of plant species. Through several comprehensive field studies, I contribute to our understanding of mutualist interaction patterns at multiple levels of biological hierarchy: the community, species, and individuals. My first chapter examines the forces driving the change in interaction patterns of an entire plant-pollinator community and individual species throughout the flowering season. Nearly all studies of plant-pollinator interaction networks ignore potential intra-annual variation, and in doing so may be missing critical mechanisms contributing to overall community stability. I find that the overall turnover of interactions is high and driven by a process of interaction rewiring in which species frequently shuffle between available partners. Furthermore, I distinguish pollinator species whose interactions are driven by an abundance-based neutral process versus those that change their interactions beyond what is predicted by a neutral, abundance-driven null model. My second chapter uses a network-based framework to consider the fitness consequences for plants participating in a diffuse plant-pollinator network. I analyze the relationship between plant species’ network metrics and pollen deposition. Empirical examples that link patterns of interactions and functional outcomes (e.g., pollination) are scarce, but necessary to establish the utility of characterizing species interaction patterns. My final chapter explores how pollinator composition, local floral neighborhoods, and timing of flowering influence the pollination outcomes of individual Oenothera fruticosa flowers. I demonstrate extensive intraspecific variation in receipt of pollen from other species (‘heterospecific pollen receipt’) and find that this heterospecific pollen has a negative fitness effect if present in sufficiently high amounts. Together, the chapters of my thesis provide novel insights into the consequences of pollinator sharing among co-flowering plant species. / Biology

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.

Premise of the Study: 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. Methods: 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. Key Results: 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. Conclusions: 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.

No description available.