The density of a plant population is expected to influence reproductive success through changes in the quantity and quality of pollination service, or because both density and reproduction respond to quality of the local environment. We reported previously that seed set in sparse natural populations of Delphinium nuttallianum and Aconitum columbianum was lower than in nearby dense populations, whereas quantity of pollination service was equivalent. To explore the hypotheses that environmental quality or pollination quality are lower in sparse natural populations, leading to lower seed set, we manipulated density using arrays of potted plants. In three replicate experiments with D. nuttallianum, pollinator visitation rate and seed set were indistinguishable in sparse and dense arrays, consistent with the interpretation that environmental quality contributed to the earlier result in natural populations of this species. In three replicates with A. columbianum, visitation rate tended to increase with density, and seed set increased significantly, in contrast to our earlier result. One element of pollination quality, the degree of within-plant selfing, did not
vary between sparse and dense arrays. These results highlight the complexity of mechanisms by which population parameters may influence plant reproductive success, and the temporal variation that characterizes pollination service.