Bumblebees in the agricultural landscape of Estonia

Abstract

Bumblebees are valuable pollinators of crops as well as native plants but their populations have declined in many parts of western Europe during recent decades. Their decline has been greatest in intensively managed agricultural areas and associated with the decline in suitable forage plants. The objective of this study was to compare the abundance and diversity of forage plants and bumblebees in agricultural and semi-natural habitats in Estonia, where land management for agriculture is less intensive than in western Europe. Six localities were surveyed, each by two census transects (1000 m long×2 m wide), one passing through a semi-natural habitat and the other through an agricultural habitat. Flower abundance and bumblebee visitors to flowers along the transects were recorded on 4 days per transect during late July/early August in each of 3 years. No significant difference in the abundance of either flowering forage plants or bumblebees between agricultural and semi-natural habitats was found. However, the number of bumblebee species observed in agricultural areas was significantly smaller than the number in adjacent semi-natural areas. The smaller proportion of preferred forage plants in the agricultural habitat was a possible reason for its reduced bumblebee species richness. For example, in the agricultural habitat, the annual and biennial plants tended to be more common and the hemiparasitic Melampyrum nemorosum significantly less common, among the flowers visited by bumblebees, than in the semi-natural habitat. The proportion of stable micro-habitats, which are not subjected to ploughing and root disturbance and therefore allow establishment of highly preferred forage plants, rather than the overall quantity of any kinds of refuge areas between cultivated lands, may be critical for the species richness of bumblebee communities in agro-ecosystems.