Plant-pollinator mutualisms are essential to biodiversity and food security, yet pollinators and the plants that depend on them are declining due to anthropogenic stressors. Understanding how multiple interacting anthropogenic stressors affect the structure and dynamics of plant-pollinator mutualisms across spatial scales remains an emerging research area. In particular, evidence remains equivocal as to whether plant invasions have positive or negative effects on plant-pollinator mutualisms. Invasive plants, highly attractive to pollinators, can paradoxically support native pollinator diversity, while in other cases invasive plants can decrease pollinator and create novel communities via invasional meltdowns. California grasslands provide an ideal opportunity to explore these questions and apply the findings to ecological restoration. California grasslands are a hotspot for plant and insect diversity, but are vulnerable to anthropogenic changes, such as climate change, plant invasions, and nutrient deposition. My research occurred in California grasslands in the Coast Range. The area contains a mosaic of heterogenous soils and plant communities: nutrient deficient serpentine soils provide a refugium for a diverse community of native plants, while nutrient-rich non-serpentine soils contain invasive-dominated plant communities. The invasion of exotic annual grasses in what were historically forb-dominated meadows provides a system in which I can examine whether invasive grasses have indirect effects on pollinators via their effects on the forb community. This patchy landscape provides an opportunity to explore questions related to the spillover of pollinators across plant community boundaries and consequent impacts on plant fitness. Moreover, California grasslands are vulnerable to a suite of interacting anthropogenic stressors in addition to plant invasions, particularly climate change, defaunation, and nutrient enrichment. The system has high interannual variability in precipitation and temperature and is sensitive to increased extremes in precipitation under climate change. My doctoral work examines how aridity, plant invasions, and nutrient enrichment alter the structure and diversity of plant communities and their pollinators across space and time. My research combines approaches from a diverse fields including spatial ecology, network theory, community ecology, botany, entomology, restoration ecology, and data science.
My first chapter integrates an invasive grass removal experiment with dynamical consumer resource models to examine whether removing a serpentine-tolerant invasive grass species has downstream effects on pollinators and plant-pollinator interaction networks. While much work has explored the effects of invasive forbs on pollinators, a gap remains as to whether plant-to-plant competition between grasses and forbs for non-pollinator resources (e.g. soil, light) can have consequences for the diversity, structure, and abundance of plant-pollinator interactions. We found that grass removal increased pollinator abundance and diversity, increasing the functional redundancy of the network and enhancing the role of a core generalist wildflower as a hub for pollinators. By comparing these empirical findings to our dynamical model, we demonstrated that consumer-resource theory could benefit from including pollinator spatial ecology and nesting biology to account for increases in diversity that are likely due to pollinator dispersal and increases in bare ground patches for ground-nesting bees.
My second chapter examines whether invasive wildflowers can have cross-ecosystem effects on the pollination, fitness, and plant-pollinator network structure of native wildflowers via the spillover of shared pollinators across ecological boundaries. Patchy mosaics of serpentine and non-serpentine soils provide an opportunity to test these questions. We conducted an observation study of meadows containing serpentine-non-serpentine ecosystem boundaries across a landscape level invasion gradient for two invasive wildflowers that share pollinators with two serpentine wildflowers that are closely related. The relative abundance of invasive and native plants at the boundary influenced whether invasive species had competitive or facilitative boundary effects on their serpentine neighbors. As the ratio of invasive to native wildflowers at the boundary increased, pollinator abundance and diversity to the native plant, native plant seed set, and the role of the native plant as a core hub in the network decreased. This suggests as the landscape context becomes more invaded, the invasive species has a competitive cross-ecosystem effect on the native species via shared pollinators.
For my third chapter, I am collaborating with the global Nutrient Network to ask whether nutrient deposition and large herbivore exclusion have interactive effects on plant communities and their pollinators across biogeographic gradients. I analyzed the global, multiyear data set on plant cover to examine how grasslands forbs have responded to joint effects of fertilization and large mammalian herbivore exclusion. I found that large herbivores can rescue forbs from the negative effects of increased grass competition under nutrient enrichment. This chapter tests whether predictions about the effects of interacting global changes on grassland forbs result in generalizable patterns at a global scale. Losses of forb diversity under nutrient enrichment may have downstream consequences on pollinators.
