Sunflower (Helianthus annuus) pollination in California's Central Valley is limited by native bee nest site location

Abstract

A major challenge in habitat restoration is targeting the key aspects of a species' niche for enhancement, particularly for species that use a diverse set of habitat features. However, restoration that focuses on limited aspects of a species' niche may neglect other resources that are critical to population persistence. We evaluated the ability of native plant hedgerows, planted to increase pollen and nectar resources for wild bees in agricultural landscapes, to provide suitable nesting habitat and enhance nesting rates of ground-nesting bees. We found that, when compared to unmanaged field edges (controls), hedgerows did not augment most indicators of nest habitat quality (bare ground, soil surface irregularity, and soil hardness), although coarser soils were associated with higher incidence and richness of nesting bees. Hedgerows did not augment nesting rates when compared to control edges. Although all the bee species we detected nesting were also found foraging on floral resources, the foraging versus nesting assemblages found within a site were highly dissimilar. These results may reflect sampling error; or, species found foraging but not nesting in hedgerows could be utilizing hedgerows as "partial habitats," nesting outside hedgerow plantings but foraging on the floral resources they provide. We conclude that although hedgerows are known to provide critical floral resources to wild bees especially in resource-poor intensive agricultural landscapes, simply increasing vegetative diversity and structure may not be simultaneously enhancing nesting habitat for ground-nesting bees.

Hedgerow presence does not enhance indicators of nest‐site habitat quality or nesting rates of ground‐nesting bees

“…Nearby natural or seminatural habitat, such as rangelands, have been suggested as potential nesting habitat (Kremen et al ). Adjacent agricultural fields could have also provided nest site locations (Kim et al ; Sardiñas et al , in press). Alternately, foraging species could have been nesting in our study sites but were undetected by e‐traps.…”

“…For all models we used model validation procedures to ensure that the models were not overdispersed and did not have inflated type I error rates (Ives ; Sardiñas et al , in press). We included only female bees of ground‐nesting species in all analyses (both nesting and foraging) because male bees are not indicative of nesting rates and may only be resting in vegetation (Kim et al ).…”

Irrigation method does not affect wild bee pollinators of hybrid sunflower

“…We used 1.96-square-foot emergence traps (BugDorm, MegaView Science, Taiwan) to sample nesting bees (Sardiñas and Kremen 2014;Sardiñas et al 2016). …”

“…Soil moisture, however, may be correlated to a number of other conditions, including soil compaction (Xie et al 2013). In the sunflower study system, generalist wild bees have been found to nest both within crop fields as well as along un-irrigated field edges (Sardiñas et al 2016). Soil moisture may not exert as strong of effects as other characteristics that affect nest site selection and nesting success of generalists, while sunflower specialists may be better adapted to the within field conditions where sunflowers are grown.…”

“…Sunflower is visited by both specialist and generalist ground-nesting native bee pollinators that nest within crop fields (Hurd et al 1980;Kim et al 2006;Sardiñas et al 2016). Generalist bees visit a variety of plant species, whereas specialists collect only sunflower pollen to provision their nests.…”