The pollination ecology of co-occuring geographically restricted and widespread species of Astragalus (Fabaceae)

Abstract

Abstract Astragalus linifolius and Astragalus osterhouti are geographically restricted legume species, with total ranges of less than 60 km2. Each of these xerophytic, shrubby perennials co-occurs with a widespread congener: A. linifolius with A. lonchocarpus and A. osterhouti with A. pattersoni. All four species exhibit similar floral size, colour and morphology and reproduce primarily by insect-mediated cross-pollination. These similarities facilitate comparisons of pollinator visitation to the restricted and widespread congeners. The most frequent visitors to the four species are polylectic bees (Bombus spp., Osmia spp., Anthopora spp. and Apis mellifera). Individuals of rare A. linifolius receive significantly lower levels of pollinator visitation than do plants of widespread A. lonchocarpus. The restricted species' poor success in competition for pollinators may have contributed to, and probably helps to maintain, their rarity. Plants of both geographically restricted species may be particularly susceptible to types of human disturbance that would further reduce the frequency of pollinator visitations: pesticide spraying and overgrazing. Plans for management of rare plant populations should take the ecology of associated pollinators into account.