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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 ; Heard, ed., Matthew ( October 2020 , Journal of Ecology)

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

   https://doi.org/10.1111/1365-2745.13776  

   Christie, Kyle ; Harrison, Susan P. ; Friesen, Maren L. ; Strauss, Sharon Y. ( October 2021 , Journal of Ecology)

   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 ofTrifoliumat 1 m²–1 km²scales 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 inTrifoliumcommunities 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 otherTrifoliumspecies. 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.

    

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3. Inter-annual facilitation via pollinator support arises with species-specific germination rates in a model of plant–pollinator communities

   https://doi.org/10.1098/rspb.2022.1485  

   James, Aubrie R. ( January 2023 , Proceedings of the Royal Society B: Biological Sciences)

   Facilitation is likely important for understanding community diversity dynamics, but its myriad potential mechanisms are under-investigated. Studies of pollinator-mediated facilitation in plants, for example, are typically focused on how co-flowering species facilitate each other's pollination within a season. However, pollinator-mediated facilitation could also arise in the form of inter-annual pollination support, where co-occurring plant populations mutually facilitate each other by providing dynamic stability to support a pollinator population through time. In this work, I test this hypothesis with simulation models of annual flowering plant and bee pollinator populations to determine if and how inter-annual pollination support affects the persistence and/or stability of simulated communities. Two-species plant communities persisted at higher rates than single-species communities, and facilitation was strongest in communities with low mean germination rates and highly species-specific responses to environmental variation. Single-species communities were often more stable than their counterparts, likely because of survivorship—persistent single-species communities were necessarily more stable through time to support pollinators. This work shows that competition and facilitation can simultaneously affect plant population dynamics. It also importantly identifies key features of annual plant communities that might exhibit inter-annual pollination support- those with low germination rates and species-specific responses to variation. 

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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)

   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.

    

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5. Experimental shifts in exotic flowering phenology produce strong indirect effects on native plant reproductive success

   https://doi.org/10.1111/1365-2745.13392  

   Waters, Susan M. ; Chen, Wei‐Ling C. ; Hille Ris Lambers, Janneke ; Bonser, ed., Stephen ( April 2020 , Journal of Ecology)

   By causing phenological shifts that vary among species, climate change is altering time envelopes for species interactions, often with unexpected demographic consequences. Indirect interactions, like apparent competition and apparent facilitation, are especially likely to change in duration because they involve multiple interactors, increasing the likelihood of asynchronous phenological shifts by at least one interactor. Thus, we might observe ecological surprises if intermediaries of indirectly interacting species change their mediating behaviour.

   We explored this possibility in a plant–pollinator community that is likely to experience asynchronous phenological shifts. We advanced and delayed the flowering phenology of two ubiquitous exotic plants of western Washington prairies,Hypochaeris radicataandCytisus scoparius, relative to seven native perennial forb species whose phenologies remained unmanipulated. These species interact indirectly through shared pollinators, whose foraging behaviour influences plant reproductive success. We quantified impacts of experimental phenological shifts on seedset, pollinator visitation rates and visiting pollinator composition relative to an unmanipulated control. We first verified that unmanipulated indirect interactions between native and exotic plants were strong, ranging from facilitative to competitive.

   Seedset of native plants was strongly affected by changes in exotic flowering phenology, but the magnitude and direction of effects were not predicted by the nature of the original indirect interaction (facilitative vs. neutral vs. competitive) or the change in interaction duration. The relationship between pollinator visitation and seedset changed for most species, though changes in pollinator visitation rate and pollinator composition were not as widespread as effects on native seedset.

   Synthesis. Changes in pollinator foraging behaviour in response to changes in available floral resources are probably responsible for the unexpected effects we observed. Asynchronous phenological shifts have the potential to produce large and unexpected effects on reproductive success via indirect interactions.

    

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