Pollinator webs, plant communities and the conservation of rare plants: arable weeds as a case study.
Little is known about the pollinators of rare plants, which is cause for concern given that pollination is essential for the long-term survival of most plant species. The aim of this study was to determine the probable pollinators of three species of rare arable weed: red hemp-nettle Galeopsis angustifolia, small-flowered catchfly Silene gallica and spreading hedge-parsley Torilis arvensis. Species of arable weed are among those suffering the greatest declines in the UK. Five field sites were chosen, two of which were sampled in 2 years. Visitation and pollen transport webs were constructed for the entire plant-pollinator community at each site. Visitation webs described the frequency with which each insect species visited each plant species. Pollen transport webs quantified which insect species transported the pollen of which plant species. A wide range of insect species visited the three plant species. A pollinator importance index was calculated that combined information on both the relative abundance of each insect carrying the pollen of the rare plant and its pollen fidelity. Using this method Galeopsis angustifolia was most likely to be pollinated by Bombus pascuorum at one site and Sphaerophoria scripta at another. Silene gallica was also likely be pollinated by Sphaerophoria scripta. The pollinator fauna of the three plant species varied considerably across their geographical range, but less from one year to the next. Synthesis and applications. All three species of rare plant were linked to other plant species in the community by shared pollinators. In many cases these other plant species constituted the primary food sources for the shared pollinators. Therefore, the long-term survival of rare plant populations is likely to depend on the more common plant species in the community. We recommend that management of the rare plants studied here should also include the protection and management of populations of some of the more common plant species in their respective communities.