Effects of sheep grazing on bumblebee flower visitation rate in an alpine ecosystem

Abstract

A permanent landscape scale grazing experiment was established at Hol, Buskerud, in a lower alpine ecosystem with a recent history of rather low grazing pressure. Since 2002, nine sub-enclosures has been permanently subjected to either of three treatments of sheep grazing pressure: grazing-ceased, grazing-maintained and grazing-increased. By applying a transect based study-design with 15 replicates for each treatment, I investigated how bumblebee abundance responded to the grazing gradient. The effect of grazing treatment on bumblebee flower visitation rate was analysed in a glmmADMB model setting together with a number of other independent variables. The grazing-maintained treatment had the highest flower visit count with 260, while the grazing-ceased treatment had 223, and the grazing-increased treatment had 166. However, the conclusive best model disregarded grazing treatment as an explanatory factor, partly due to naturally occurring variation in in real time bumblebee distribution. I conclude by putting my findings into a grazing ecological context.