From Soil Fungi to Bees: Fertile Grounds for Agroecology in California’s San Joaquin Valley

All the ecological components of a farm are tightly interlinked – influencing each other, negatively orpositively, through multiple divergent pathways. Belowground, roots can form a well-developed network with arbuscular mycorrhizal fungi (AMF) for greater nutrient uptake. Aboveground, floral resources (pollen and nectar) influence bee visitation needed for crop pollination. In this way, AMF could positively influence crop pollination through improving nutrient uptake. However, agricultural intensification, characterized by high nutrient inputs, low crop diversity, and high tillage frequency, have been linked to biodiversity loss, negatively impacting multiple species interactions. While diversifying crops could help to promote these beneficial ecological interactions, it is poorly understood how changes in crop diversity impact bees that specialize on limited floral resources or mycorrhizal fungi that exhibit preferential host associations. By partnering with small-scale farmers of color, my dissertation focused on understanding the mechanisms in which crop diversity influences below- and above-ground interactions, and their connections (AMF to pollination)./nThe first part of my dissertation examined how crop diversity (monoculture versus polycultures)affected AMF and pollinator communities in the intensively managed agricultural region in California’s San Joaquin Valley. I examined how crop diversity influences several aspects of AMF communities: (1) richness and diversity, (2) composition, and (3) colonization. I demonstrate that crop diversity enriches AMF communities, counteracting the negative effects of agricultural intensification, providing the potential to increase agroecosystem functioning and sustainability. Next, to investigate the impact of on-farm diversification on pollinator communities, I focused on how specialist squash bees (Peponapis genera) responded to crop diversity (monoculture versus polyculture) using squash (Cucurbita pepo) as the focal crop. I hypothesized that squash bees would be more abundant on monocultures, due to greater numbers of squash flowers, than polycultures. Despite this, I found that increasing non-squash floral resource diversity in polycultures supports a greater abundance of squash bees. On-farm diversification may be an important refuge for specialist bees that are vulnerable in landscapes dominated by agriculture./nThe second part of my dissertation sought to more closely characterize mechanisms that enhancepollination services critical to promoting food security. While aboveground floral visitors provide pollination services to plants, belowground AMF could enhance the quality of floral resources (pollen and nectar). Yet, the effect of the AMF community composition on pollination is still unclear because existing studies consist of a single AMF species or disregard the interacting effect of soil nutrient availability. Therefore, in a controlled greenhouse and field experiment, I investigated how different levels of AMF diversity and nutrient availability impacted floral resources and pollinator visitation. Our results demonstrate that greater AMF diversity, regardless of nutrient availability, has the largest positive effect on floral resource quality and pollinator visitation, an indicator of pollination services./nAltogether, this dissertation points to the effects of crop diversity on belowground and abovegroundbiodiversity that could bolster ecosystem functions. By working directly with small-scale farmers of color, this research also outlines potential pathways that farmers can take towards harnessing beneficial ecosystem functions. Importantly, the results demonstrate that crop diversification could help to recover biodiversity from below- to above-ground even in the intensively managed region in California’s San Joaquin Valley.

Advisor: Kremen, Claire
Bowles, Timothy M