Forage use and niche partitioning by non-native bumblebees in New Zealand: implications for the conservation of their populations of origin

Bumblebees were introduced into New Zealand from the UK approximately 120 years ago and four species became established. Two of these, Bombus terrestris and B. hortorum, are common in Europe whilst two, B. ruderatus and B. subterraneus, have experienced declines, and the latter is now extinct in the UK. The presence of these species in New Zealand presents an opportunity to study their ecology in a contrasting environment. Forage visits made by bumblebees in New Zealand were recorded across a season. Ninety-six percent of visits were to six non-native forage plants (Cirsium vulgare, Echium vulgare, Hypericum perforatum, Lotus corniculatus, Lupinus polyphyllus and Trifolium pratense). All but L. polyphyllus are European plant species, and three are noxious weeds in New Zealand. Several of these plants have decreased in abundance in the UK, providing a potential explanation for the declines of B. ruderatus and B. subterraneus in Britain. In contrast to studies conducted elsewhere, B. ruderatus, B. terrestris and B. hortorum did not differ in diet breadth and overlap in forage use was high, probably due to the reduced diversity of bumblebee forage plants present in New Zealand. Diel partitioning of forage use between the species was observed, with foraging activity of B. hortorum greatest in the morning and evening, B. ruderatus greatest in the middle of the day and B. terrestris intermediate between the two. These patterns correspond to the climatic preferences of each species as evidenced by their geographic ranges. Implications for bumblebee conservation in the UK are discussed.