Degree Name

MS (Master of Science)



Date of Award


Committee Chair or Co-Chairs

Gerado Arceo-Gomez

Committee Members

Darrell Moore, Aruna Kilaru


More than 60% of plant populations experience low pollen availability which limits seed production (pollen limitation). Pollen limitation occurs due to low quantity or quality of pollen delivered to stigmas. Despite its ubiquity in nature, to date we still have limited understanding of the drivers of pollen limitation in plant communities. Plants typically occur in diverse communities where pollinator sharing is common and can influence quality and quantity aspects of pollen limitation. Co-flowering species can attract larger numbers of pollinators or they can compete for pollinators affecting pollen loads on stigmas. Pollinator competition may also lead to higher rates of self-pollination, reducing pollen quality. Knowledge on the relative contribution of pollen quantity and quality to overall pollen limitation, and how it varies with increasing co-flowering diversity, is central for understanding the factors that determine plant reproductive success. Here, we used populations of four Clarkia species to evaluate how changes in co-flowering community diversity impact the contribution of pollen quantity and quality in limiting plant reproduction. We sampled twenty-five communities of 1-4 species. We collected 29-100 styles per site/species (2400 total). Styles were processed and the amount of pollen grains and pollen tubes were counted under a microscope. Differences in the amount of pollen grains received (pollen quantity) and proportion of pollen tubes produced (pollen quality) were estimated across species and populations. The amount of pollen received increased with increasing number of co-flowering species only for C. unguiculata, suggesting pollinator facilitation. The proportion of pollen tubes was not affected by the number of co-flowering species present. All Clarkia species differ in the amount of pollen grains received and the proportion of pollen tubes produced. C. xantiana, received the least amount of pollen but produced the highest amount of pollen tubes. C. speciosa received the highest amount of pollen grains but produced the fewest number of tubes. Pollen quantity and quality varied widely among individuals within a population compared to individuals across populations or species. Pollen quality and quantity are limiting factors in the reproductive success of Clarkia species, however their relative contribution depends on species identity. Overall, differences in pollen limitation may depend on intrinsic species characteristics rather than aspects of the surrounding co-flowering community. Specifically, quantity and quality components of pollen limitation may vary depending on differences in pollinator community composition, level of specialization and efficiency, and differences in plant mating system (self vs outcrossed). This study emphasizes the need to fully evaluate all aspects of pollen limitation and how these vary across space and species. This knowledge is key to understand the processes that mediate plant reproductive success in nature and how plants will respond to human disturbances.

Document Type

Thesis - embargo


Copyright by the authors.

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