Plant-flower visitor interaction webs: Temporal stability and pollinator specialization increases along an experimental plant diversity gradient

Although most plants benefit from pollen vectors, very little information exists about how plant diversity structures the interactions between plants and their flower visitors. The structure of such interaction webs holds information about specialization and effectiveness of flower visitors in flower resource use. Here, we analyzed 52 plant-flower visitor interaction webs along a gradient of experimentally manipulated plant species richness in a European grassland. The gradient allows testing for effects of the number of flowering plant species per se. Linkage density and interaction diversity between flowering plant species and their visiting insect species increased with higher richness of flowering species. Increased interaction diversity led to smaller temporal variability in the frequency of flower visits. These results suggest higher temporal stability of pollination provided for plants integrated in complex interaction webs with a high number of flowering plant species. Flower resource specialization of solitary bees, but not of honey and bumble bees, increased with increasing flowering plant species richness. Conservation of diverse grasslands can result in high flower specialization and may promote effectiveness of pollination services.