Forest edges influence bumble bee assemblages in populations of Comarum palustre (Rosaceae)

Habitat fragmentation through human activities, resulting in small and isolated plant populations, is a major threat to plant-pollinator networks. Disruptions may arise from effects on either pollinator assemblages or pollinator behaviour, which might alter the quantity and quality of pollination events, thereby decreasing plant reproduction and fitness. Comarum palustre, a perennial herb with a boreo-circumpolar distribution, occurs in different vegetation types across Europe like peat bogs, fens, wet meadows and water body edges where it acts as a pioneer species. This species is considered as rare and threatened throughout Western Europe. As a pioneer species, its occurrence is restricted to very specific biotopes that have largely been fragmented through destruction and forest recolonisation since the middle of the 19th century. We investigated whether the size and isolation of C. palustre populations as well as proportion of forest edges play a major role in its reproductive success. Flower observations and pollen limitation experiments were conducted in sixteen populations in High Belgium, at the peak of flowering seasons from 2009 to 2011. A total of 1 155 visitors were observed during the three years. About 50 different species observed visit C. palustre, mainly Hymenoptera (64%), but also many Diptera as well as Lepidoptera, Coleoptera, Mecoptera, Heteroptera, Hemiptera and Orthoptera. Within the Hymenoptera, Apidae (up to 99% Bombus spp.) were the most abundant with eight species composing 58% of all visitors. Reduced population size, represented as flower density as well as the proportion of surrounding forest had a negative effect on bumble bee visits. Especially parasitic cuckoo bees (B. campestris, B. sylvestris and B. vestalis) disappeared from isolated fragments and those surrounded by high percentages of forest