Exploring the relationships between landscape complexity, wild bee species richness and reproduction, and pollination services along a complexity gradient in the Netherlands

Pollinator communities exhibit variable responses to changing landscape composition. A general expectation is that a decreasing cover of semi-natural habitats negatively affects pollinator reproduction, population size and pollination services, but few studies have investigated the simultaneous effects of landscape complexity on different aspects of pollinator communities and functioning. In 20 agricultural landscape plots the size of an average Dutch farm, we studied how changing landscape complexity affected wild bee abundance, species richness and reproduction. To measure pollination, we placed potted strawberry plants as phytometers in landscapes. Landscape complexity was characterized as the area of semi-natural habitats. In addition, we estimated floral resource abundance in each landscape plot. We expected that i) bee species richness, reproduction and pollination would be positively related to area of semi-natural habitats and flower abundance, and that ii) species richness and reproduction would be positively related to pollination. An increase in semi-natural habitats in landscapes increased both the abundance of cavity-nesting bees colonizing trap nests, and the growth rates of experimental Bombus terrestris L. colonies, but not the species richness of wild bees measured by pan traps. There was only a tendency for higher pollination levels of strawberry plants with higher cover of semi-natural habitats. There was no relationship between species richness and bee reproduction in a landscape and the pollination services. Estimated flower abundance in landscape had a positive effect on bumblebee colony growth only and not on the other variables. Our results suggest that, by improving habitat quality on their farms through establishing more semi-natural habitats or enhancing the flower availability in semi-natural habitats, farmers can promote reproduction of a number of functionally important bee species and the pollination services they provide. Bee species richness, however, seems to be more difficult to enhance and requires more than just creating more of the same type of habitats or flowers