The reproductive success of bees is dependent on the availability of appropriate floral resources.  In arid environments irregular precipitation means that such resources are often unpredictable.  Under such conditions plants with predictable bloom have heightened importance for bees.  Here we explore the value of one such resource, rabbitbrush (Ericameria spp.), on a bee community in the arid Colorado Plateau of western North America.   Data come from a four year study of the native bees of Grand Staircase Escalante National Monument, Utah. We sampled 1 ha plots in diverse habitats biweekly throughout the flowering season.  Bees were collected on all blooming plants in one-hectare plots for 45-minute periods twice a day; an interaction was any visit of a bee to a plant.  Samples included in this analysis are restricted to August through October, the period following a mid summer hiatus in flowering and bee activity.  Interactions are analyzed at the generic level for plants.

Results/Conclusions

Late summer-fall bloom supports a diverse bee fauna.  Samples (n=309) yielded 14,199 interactions for 278 bee species and 62 plant genera.  Plots with Ericameria support larger populations of bees than those without.  Bee visitation to rabbitbrush far exceeds rates to any other flowering plants.  It supports a rich pollinator fauna; over 60% of fall flying bee species were visitors, including a number of specialists.  Bee interactions with rabbitbrush are localized; no bee species was found on more than half of the plots, most were restricted to one or two.  Plant-bee interactions on the Colorado Plateau are highly asymmetrical in the late summer-fall flowering season. Factors that appear to contribute to the dominance of rabbitbrush include predictable bloom even in years of drought, abundant floral resources, and extended bloom.