Relationships between the density and diversity of floral resources and flower visitor activity in a temperate grassland community

1. Does the diversity and abundance of one trophic level affect another? Several studies at the landscape level have found a positive relationship between the diversity of floral resources and the diversity and abundance of pollinators. However, little is known about the relationship between these trophic levels on a smaller spatial scale, and the importance of blossom density relative to plant species richness in predicting abundance and richness of different flower visitor groups. 2. This study used a small-scale approach to investigate how, and if, the diversity and abundance of floral resources in study plots affected the visitation activity of different flower visitor groups. During 201 observation periods between late May and mid-August 2003, 3682 visits were observed. Bumblebees (60%), muscoids (17%), syrphids (9%), and beetles (5%) were the most abundant flower visitors. 3. Regression analysis was used to investigate the relationship between blossom density and plant species richness with visitation activity, including the probability of presence in plots, the visits within plots, and the visitor richness of the most abundant pollinator groups. 4. The activity of beetles, bumblebees, and muscoids was positively predicted by the variation in blossom density, while syrphid activity was better predicted by plant species richness. Overall, the models for beetles and bumblebees explained much more of the variation in activity compared with the models for the dipterans, and blossom density was a better predictor of both flower visitor richness and activity than was plant species richness.