Competition between managed honeybees and wild bumblebees - Quantifying dietary overlap in semi-natural grasslands

Abstract: The decline of bees, such as bumblebees, over the last decades raises concerns as they are key pollinators in both agricultural and natural ecosystems. One proposed cause for the decline of wild bees is competition for food resources with managed honeybees. Detrimental effects of competition can result where co-existing species exhibit a high degree of shared resource use (i.e., niche overlap). The aim of this study was to assess the degree of niche overlap between honeybees and six bumblebee species in semi-natural grasslands, which are important bee conservation habitats. This study examined the degree of niche breadth (i.e., species richness of foraged plants visited by each bee species) and explored if plant functional richness (flower colour and shape) and bumblebee tongue length structured niche overlap. This was undertaken by analysing data collected from 32 pastures, in 8 different landscapes in Skåne between May and July 2022. Data consisted of all flower visiting honeybees and bumblebees and the visited plant species observed during transect surveys. Differences in niche breadth between honeybees and bumblebees were analysed using generalised linear models. Differences in niche overlap with honeybees between bumblebee species, as well as in relation to plant functional richness and bumblebee tongue length were analysed using linear models. Honeybees and B. terrestris foraged on more plant species than the other bumblebee species. Niche overlap with honeybees varied between bumblebee species. Niche overlap decreased with increasing tongue length and with increasing plant functional richness. Therefore, competition with honeybees is most likely to occur among short-tongued bumblebee species which exhibit the highest niche overlap. Increasing or maintaining plant functional richness in semi-natural grasslands has the potential to reduce competition between honeybees and bumblebees, by decreasing niche overlap and underscores the importance of ongoing conservation of semi-natural grasslands, to help mitigate against ongoing wild bee declines.