Abstract
Wild bees are declining in intensively farmed regions worldwide, threatening pollination services to flowering crops and wild plants. To halt bee declines, it is essential that conservation actions are based on a mechanistic understanding of how bee species utilize landscapes. We aimed at teasing apart how foraging resources in the landscape through the nesting season affected nesting and reproduction of a solitary bee in a farmland region. We investigated how availability of floral resources and potentially resource‐rich habitats surrounding nests affected nest provisioning and reproduction in the solitary polylectic bee Osmia bicornis. The study was performed in 18 landscape sectors dominated by agriculture, but varying in agricultural intensity in terms of proportion of organic crop fields and seminatural permanent pastures. Pasture‐rich sectors contained more oak (Quercus robur), which pollen analysis showed to be favored forage in early season. More oaks ≤100 m from nests led to higher proportions of oak pollen in nest provisions and increased speed of nest construction in early season, but this effect tapered off as flowering decreased. Late‐season pollen foraging was dominated by buttercup (Ranunculus spp.), common in various noncrop habitats. Foraging trips were longer with more oaks and increased further through the season. The opposite was found for buttercup. Oak and buttercup interacted to explain the number of offspring; buttercup had a positive effect only when the number of oaks was above the mean for the studied sectors. The results show that quality of complex and pasture‐rich landscapes for O. bicornis depends on preserving existing and generating new oak trees. Lignose plants are key early‐season forage resources in agricultural landscapes. Increasing habitat heterogeneity with trees and shrubs and promoting suitable late‐flowering forbs can benefit O. bicornis and other wild bees active in spring and early summer, something which existing agri‐environment schemes seldom target.
Highlights
Wild insects, in particular bees, are essential pollinators of crops (Klein et al, 2007) and wild plants (Ollerton, Winfree, & Tarrant, 2011)
Wild bee declines have mainly been attributed to agricultural intensification causing a reduced availability and quality of foraging and nesting habitat, in combination with pesticide use (Potts et al, 2016)
Because bees are central-place foragers, they especially suffer from spatial separation of forage and nesting habitat caused by landscape simplifications through agricultural intensification (Brown & Paxton, 2009; Kremen, Williams, & Thorp, 2002)
Summary
In particular bees, are essential pollinators of crops (Klein et al, 2007) and wild plants (Ollerton, Winfree, & Tarrant, 2011). Resource use by bees in changing landscapes is complex and needs to be elucidated by a mechanistic understanding of how spatial and temporal variation in flower resources affect bee foraging and, as a consequence, their fitness and population dynamics. Such knowledge may in turn be used to inform the design of measures to mitigate bee declines (Wood, Holland, & Goulson, 2015, 2016). We aimed at investigating how availability of flower resources, and of habitats assumed to provide flower resources, affected foraging, reproductive success, and population size of a solitary bee, the polylectic red mason bee Osmia bicornis, throughout its nesting season. We determined the amount of mass-flowering crops (autumn-sown OSR) at both
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