Effects of grazing and ant/beetle interaction on seed production in the legume Vicia sepium in a seminatural grassland

1. Seed production can be affected by cattle grazing, insect herbivores, and seed predators. Bush vetch, Vicia sepium L. (Fabaceae), possess extra floral nectaries (EFNs), which can attract ants that act as plant defenders. 2. In this field study, four different manipulations were used to investigate interactions between V. sepium, ants, aphids, and seed predators, mainly Bruchus atomarius. The experiment was set up in grazed and non-grazed plots in a semi-natural grassland in south-central Sweden. 3. Grazing negatively affected number of V. sepium buds, flowers, and pods. Seed set, defined as the number of developed seeds per number of ovules in a pod, was higher in grazed plots. Ant abundance, however, was not affected by grazing. Ten different ant species were found on V. sepium and the number of ants was positively correlated with the presence of EFNs. Aphids were found only when ants were excluded from the plants. Pod and seed production of V. sepium plants was not affected by the presence of ants. The seed predator B. atomarius was not affected by the presence of ants. No support was found for the supposition that V. sepium benefited from attracting ants by producing extra floral nectar. 4. A small observational study of V. sepium plants in shrub (Rosa dumalis) plots compared with grassland plots was added to the experimental study. Vicia sepium plants were longer and had higher seed-set (seeds/ovules) in shrub plots than in grassland plots.