Pollinator sharing by three sympatric milkvetches, including the endangered species Astragalus montii

Insects visiting flowers of the endangered Heliotrope milkvetch, Astragalus montii , were compared with those visiting two common sympatric congeners, A. kentrophyta and A. miser , on three sites on the Wasatch Plateau of central Utah for 2 yr. We recorded 27+ species of bees, most of which were uncommon, visiting the three species. All three species were primarily visited by native bees of the genera Osmia (15 species) and/or Bombus (4 species). Most Osmia species visited the three species of Astragalus indiscriminantly; bumblebees preferred A. miser and avoided A. montii . Our hypothesis that A. montii flowers would receive fewer total bee visits and be visited by fewer bee species than their common congeners was rejected: A. montii was intermediate to the two common species in its attractiveness to bees. Also rejected was our hypothesis that the greater similarity between A. montii and A. kentrophyta in flower size, flower morphology, and microhabitat would be associated with greater similarity of flower visitors than either had with A. miser . The data suggest that, rather than competing with each other for pollinators, the three species of Astragalus facilitate each other's visitation rates.