Close neighbourhood of nesting sites and foraging habitats enhances a diverse fauna of native bees

Native bees are essential pollinators of wild and crop plants providing high ecological and economical benefits. However, half of the 600 native bee species of Switzerland are endangered. Ongoing soil sealing and intensification of agricultural land use result in fewer suitable nesting sites and foraging habitats, which is expected to force female bees to cover longer distances between nest and flower-rich patches. In this study, maximum foraging distances of selected solitary bee species were investigated and the effect of increasing foraging distances on their reproduction was analyzed. Bee species, which restrict pollen foraging to a single plant genus, were established in an agricultural landscape lacking their specific host plants. Females were forced to collect pollen on potted host plants in different distances from their nests. Only few individuals of Hoplitis adunca and Hylaeus punctulatissimus covered long distances of more than 1000m to collect pollen. The majority of females already discontinued foraging at a distance of 100-300m, which indicates that long distances between nesting sites and flower resources impose high costs on reproduction. In fact, increased distances by 150m and more substantially reduced the number of progeny produced by females of Hoplitis adunca and Chelostoma rapunculi. Thus, a close neighbourhood of nesting and foraging habitats clearly contributes to a diverse native bee fauna and to an increase of bee population sizes.