Long-distance pollinator flights and pollen dispersal between populations of Delphinium nuttallianum

Spatial processes in pollination biology are poorly understood, especially at levels above that of the local population. For example, little is known about how pollinators and pollen move among populations, although there is evidence that such movement can exceed what is predicted from intrapopulational movement. We explored pollination success in experimental isolates of the bumblebee- and hummingbird-pollinated wildflower Delphinium nuttallianum. We established a total of 15 arrays of potted plants isolated by 50-400 m from ten natural "source" populations, as well as control arrays embedded within each source. Flowers on potted plants were emasculated, so any pollen received could be assumed to come from source populations. A total of 69 h of observation suggested that pollinators were somewhat less abundant in isolates than in controls, but visited more plants and flowers once within an isolate. Consistent with this, 82.1% of all flowers in isolated arrays received pollen, versus 87.7% in controls. Mean receipt was more than 100 pollen grains per flower in most arrays, and seed set in isolates and controls respectively averaged 69.8% and 74.3% of ovules. Furthermore, pollen receipt in isolates declined relatively slowly with distance from the source. We conclude that pollinators of D. nuttallianum often will fly up to 400 m among populations, and that substantial pollination ensues. Thus isolated populations of this species often belong to metapopulations in terms of pollen dispersal, with important consequences for genetic differentiation, and potential implications for the management of endangered plant species.