UC Berkeley

Crop pollination is an essential ecosystem service predominantly provided by honey bees and native bees. The decline of honey bees due to a variety of maladies, including Colony Collapse Disorder, has prompted increased interest in unmanaged pollinators. Native bees are effective pollinators, but they have specific habitat requirements, namely floral and nesting resources, that can limit their distribution in agricultural landscapes. This dissertation examines how floral availability and nesting habitat influence pollinator communities and pollination services from native bees.

Increasing floral diversity and abundance has been shown to increase native bee richness and abundance, therefore field-scale habitat enhancements, such as hedgerows, have been proposed as a way to attract and support native bee populations. Whether native bees also visit adjacent crop fields, heightening yields, is less well known. I assess the contribution of native bees visiting hedgerows to seed set in cultivated sunflower, a mass-flowering pollinator-dependent crop (Ch. 1). I find that while sunflower specialist bees (bees that forage exclusively on sunflower) are increased in hedgerow plantings, crop pollination in fields adjacent to hedgerows is not increased. Wild bees, however, both directly and indirectly contribute to sunflower seed set though higher richness and through interactions with honey bee crop visitors.

Factors influencing nest-site selection are less well understood, particularly at the community level. Nest locations are challenging to find; therefore proxies have emerged as a means of correlating potential resources to bee species present in a study area. This approach may overestimate nesting bees; therefore I test a method to randomly sample bee nesting using emergence traps, then determine whether nesting proxies correlate to nesting rates (Ch. 2). Emergence traps are effective at capturing bees in their nests. When I compare bees collected in emergence traps to the community of bees found visiting within site floral resources, I see distinct differentiation between the species collected, indicating that bees foraging within a location may not be nesting there. I find that some proportion of bare ground and variability in slope are strongly correlated to nesting incidence and abundance.

I use the emergence trap technique to examine whether hedgerow field-margin enhancements increase nesting resources and subsequent nesting incidence (Ch. 3). While many nesting resources are elevated in hedgerow plantings, I find that this does not translate into increased rates of nesting. This could be attributable to the low site coverage provided by emergence traps, or could indicate that the nesting proxies I am evaluating are not strongly indicative of nesting quality.

Models of pollination services in agricultural landscapes estimate floral and nesting resources. These parameterizations are combined with pollinator foraging ranges to generate predictions of pollination coverage. Nesting suitability is primarily based on expert opinion, which in past studies limited potential nest site availability to field margins. I test this assumption in Ch. 4 by sampling with emergence traps in sunflower fields and along field edges. I then combine this nesting data with direct measurements of foraging distances to simulate pollination within a single crop field. I find that while some bees nest directly within crop fields, in the presence of a mass-flowering crop they only forage a small fraction of their foraging range. Pollination services are thus centralized around nest locations.

While increasing floral resources is important for sustaining native bee populations, I do not find a strong effect of hedgerow restorations on pollination of adjacent crops or on nesting resources. Hedgerows may be more effective in different crop or landscape contexts. Because selected bees nest within crop fields, focusing on management of cropped areas, not just field edges, will likely benefit native bees in agricultural landscapes. In order to sustain pollinator populations and maintain agricultural yields, I suggest incorporating multiple methods of diversification into agricultural landscapes.