Wildflower plantings enhance nesting opportunities for soil-nesting bees.

Ongoing declines of bees and other pollinators are driven in part by the loss of critical floral resources and nesting substrates. Most conservation/restoration efforts for bees aim to enhance floral abundance and continuity but often assume the same actions will bolster nesting opportunities. Recent research suggests that habitat plantings may not always provide both forage and nesting resources. We evaluated wildflower plantings designed to augment floral resources to determine their ability to enhance nesting by soil-nesting bees over 3 study years in Northern California agricultural landscapes. We established wildflower plantings along borders of annual row crops and paired each with an unplanted control border. We used soil emergence traps to assess nest densities and species richness of soil-nesting bees from spring through late summer at paired field borders planted with wildflowers or maintained conventionally as bare or sparsely vegetated areas, as is typical for the region. We also quantified soil-surface characteristics and flower resources among borders. Wildflower plantings significantly increased nest densities and the richness of bee species using them. Such benefits occurred within the first year of planting and persisted up to 4 years post establishment. The composition of nesting bee communities also differed between wildflower and unenhanced borders. Wildflower plantings differed from controls in multiple characteristics of the soil surface, including vegetation cover, surface microtopography and hardness. Surprisingly, only vegetation cover significantly affected nest densities and species richness. Wildflower plantings are a widespread habitat action with the potential to support wild bees. The demonstrated benefit wildflower plantings had for increasing the nesting of soil-nesting bees greatly augments their relevance for the conservation of wild bee communities in agricultural and other landscapes. Identifying soil-surface characteristics that are important for nesting provides critical information to guide the implementation and management of habitats for bees.

Rare and declining bee species are key to consistent pollination of wildflowers and crops across large spatial scales.

Biodiversity promotes ecosystem function (EF) in experiments, but it remains uncertain how biodiversity loss affects function in larger-scale natural ecosystems. In these natural ecosystems, rare and declining species are more likely to be lost, and function needs to be maintained across space and time. Here, we explore the importance of rare and declining bee species to the pollination of three wildflowers and three crops using large-scale (72 sites across 5000 km2 ), multi-year datasets. Half of the sampled bee species (82/164) were rare or declining, but these species provided only ~15% of overall pollination. To determine the number of species important to EF, we used two methods of "scaling up," both of which have previously been used for biodiversity-function analysis. First, we summed bee species' contributions to pollination across space and time and then found the minimum set of species needed to provide a threshold level of function across all sites; according to this method, effectively no rare and declining bee species were important to pollination. Second, we account for the "insurance value" of biodiversity by finding the minimum set of bee species needed to simultaneously provide a threshold level of function at each site in each year. The second method leads to the conclusion that 25 rare and eight declining bee species (36% and 53% of all rare and declining bee species, respectively) are included in the minimum set. Our findings provide some of the strongest evidence yet that rare and declining species are key to meeting threshold levels of EF, thereby providing a more direct link between real-world biodiversity loss and EF.