Insect Visitors and Pollination Ecology of Spalding's Catchfly (Silene spaldingii) in the Zumwalt Prairie of Northeastern Oregon

Silene spaldingii S. Watson (Spalding's catchfly) is a threatened wildflower that relies on insect-mediated pollination. However, its pollination ecology is not well understood, particularly in the Zumwalt Prairie of northeastern Oregon, which contains the largest known S. spaldingii population. Our objectives were to: (1) describe the principal insect visitors to S. spaldingii in the Zumwalt Prairie, (2) quantify the available pool of pollinators in the area, and (3) determine whether the visitation rate to individual plants is associated with the density of S. spaldingii at the patch scale, as predicted by the resource concentration hypothesis, and/or by the density and composition of non-S. spaldingii blooming plants, as predicted by the facilitation and competition hypotheses. We recorded insect visits to S. spaldingii during peak bloom at 30 patches and characterized the local bee community using blue vane traps. We quantified the patch-scale density of S. spaldingii and the composition and abundance of other blooming species at each patch. Two bumble bee species comprised all observed visits, although they constituted only 20% of the total bees sampled on the prairie. Bumble bees showed a high degree of host fidelity even when other blooming plants were present. Per capita visitation rates increased with catchfly density and blooming plant abundance at the patch scale, supporting the resource concentration and facilitation hypotheses. Silene spaldingii in the Zumwalt Prairie appears to rely on a narrow pool of pollinators that may preferentially visit it over other blooming plants, and more dense patches of S. spaldingii may increase pollination efficiency.