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

   https://doi.org/10.1111/1365-2745.13520  

   Parra‐Tabla, Víctor; Alonso, Conchita; Ashman, Tia‐Lynn; Raguso, Robert A.; Albor, Cristopher; Sosenski, Paula; Carmona, Diego; Arceo‐Gómez, Gerardo (February 2021, Journal of Ecology)

   Heard, Matthew (Ed.)

   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. 

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2. Floral density and co‐occurring congeners alter patterns of selection in annual plant communities

   https://doi.org/10.1111/evo.13960  

   Eisen, Katherine E.; Wruck, Amy C.; Geber, Monica A. (April 2020, Evolution)

   Although the evolution and diversification of flowers is often attributed to pollinator-mediated selection, interactions between co-occurring plant species can alter patterns of selection mediated by pollinators and other agents. The extent to which both floral density and congeneric species richness affect patterns of net and pollinator-mediated selection on multiple co-occurring species in a community is unknown and is likely to depend on whether co-occurring plants experience competition or facilitation for reproduction. We conducted an observational study of selection on four species of Clarkia (Onagraceae) and tested for pollinator-mediated selection on two Clarkia species in communities differing in congeneric species richness and local floral density. When selection varied with community context, selection was generally stronger in communities with fewer species, where local conspecific floral density was higher, and where local heterospecific floral density was lower. These patterns suggest that intraspecific competition at high densities and interspecific competition at low densities may affect the evolution of floral traits. However, selection on floral traits was not pollinator mediated in Clarkia cylindrica or Clarkia xantiana, despite variation in pollinator visitation and the extent of pollen limitation across communities for C. cylindrica. As such, interactions between co-occurring species may alter patterns of selection mediated by abiotic agents of selection. 

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3. Molecular assays of pollen use consistently reflect pollinator visitation patterns in a system of flowering plants

   https://doi.org/10.5061/dryad.0p2ngf1x0  

   James, Aubrie; Geber, Monica; Toews, David (January 2020, Dryad)

   Determining how pollinators visit plants versus how they carry and transfer pollen is an ongoing project in pollination ecology. The current tools for identifying the pollens that bees carry have different strengths and weaknesses when used for ecological inference. In this study we use three methods to better understand a system of congeneric, co-flowering plants in the genus Clarkia and their bee pollinators: observations of plant-pollinator contact in the field, and two different molecular methods to estimate the relative abundance of each Clarkia pollen in samples collected from pollinators. We use these methods to investigate if observations of plant-pollinator contact in the field correspond to the pollen bees carry; if individual bees carry Clarkia pollens in predictable ways, based on previous knowledge of their foraging behaviors; and how the three approaches differ for understanding plant-pollinator interactions. We find that observations of plant-pollinator contact are generally predictive of the pollens that bees carry while foraging, and network topologies using the three different methods are statistically indistinguishable from each other. Results from molecular pollen analysis also show that while bees can carry multiple species of Clarkia at the same time, they often carry one species of pollen. Our work contributes to the growing body of literature aimed at resolving how pollinators use floral resources. We suggest our novel relative amplicon quantification method as another tool in the developing molecular ecology and pollination biology toolbox. 

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

   https://doi.org/10.1093/aob/mcab082  

   Arceo-Gómez, Gerardo (July 2021, Annals of Botany)

   Abstract BackgroundStudies 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. ScopeInterest 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. ConclusionsThe 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. 

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5. Direct tracking of pollen with quantum dots reveals surprising uniformity in dispersal distance across 11 populations of an annual plant

   https://doi.org/10.1002/ajb2.16201  

   Kern, Brooke R.; Carley, Lauren N.; Moeller, David A. (July 2023, American Journal of Botany)

   Abstract Premise Pollen movement is a crucial component of dispersal in seed plants. Although pollen dispersal is well studied, methodological constraints have made it challenging to directly track pollen flow within multiple populations across landscapes. We labeled pollen with quantum dots, a new technique that overcomes past limitations, to evaluate the spatial scale of pollen dispersal and its relationship with conspecific density within 11 populations of Clarkia xantiana subsp. xantiana , a bee‐pollinated annual plant. Methods We used experimental arrays in two years to track pollen movement across distances of 5–35 m within nine populations and across distances of 10–70 m within two additional populations. We tested for distance decay of pollen dispersal, whether conspecific density modulated dispersal distance, and whether dispersal kernels varied among populations across an environmentally complex landscape. Results Labeled pollen receipt did not decline with distance over 35 m within eight of nine populations or over 70 m within either of two populations. Pollen receipt increased with conspecific density. Overall, dispersal kernels were consistent across populations. Conclusions The surprising uniformity in dispersal distance within different populations was likely influenced by low precipitation and plant density in our study years. This suggests that spatiotemporal variation in the abiotic environment substantially influences the extent of gene flow within and among populations. 

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