Patch size and landscape effects on pollinators and seed set of the horseshoe vetch, hippocrepis comosa, in an agricultural landscape of central Europe

Agricultural land use intensification has caused a considerable decline of once common semi-natural habitats leaving only small and isolated habitat remnants within a homogeneous landscape matrix. Populations of flowering plant species in these remaining habitat fragments are also declining and can not offer sufficient resources for potential pollinators. The loss of pollinators and consequential pollination limitation is especially severe for obligately out-crossing plant species. In this study, pollination and reproductive success of Hippocrepis comosa was examined. Hippocrepis comosa L is a self-incompatible, declining plant species occurring on fragmented remnants in semi-natural calcareous grassland habitats in an agricultural landscape in Southern Lower Saxony, Germany. Plant-pollinator interactions were analysed in small, medium, and large plant patches in 15 differently sized calcareous grassland fragments surrounded by landscape matrices of differing complexity. Flower visitors were observed during three 15-minute observations, pollen supplementation experiments were performed to test for pollination limitation, and fruits from the plant patches were collected to calculate seed set. Hippocrepis comosa showed evidence of pollination limitation because hand-pollinated flowers set more seeds per inflorescence than open-pollinated flowers and seed set was strongly reduced by decreased visitation rates. Pollinators were most abundant in large patches and in diverse landscapes. Visitation rate was not affected by patch size. Hence, in order to maintain plant-pollinator interactions in an agricultural landscape, conservation efforts should include the preservation of calcareous grasslands that can support sufficiently large patches of a plant species and offer ample rewards for pollinators and the establishment of a diverse agricultural landscape around the semi-natural habitats that will further enhance pollinator abundance.