Effects of mass flowering crops on wild pollinator abundance and species richness : a review of local and landscape effects in the agricultural landscape

Sammanfattning: Agricultural intensification has led to the loss of flowering resources and natural habitats in agricultural landscapes and has been identified as one of the major drivers for the observed decline in wild pollinators in these habitats. It has been suggested that mass flowering crops could be used to counterbalance this negative trend. However, the presence of mass flowering crops is not always beneficial to wild pollinators. This thesis presents a review of the published literature on the effects of mass flowering crops on wild pollinator abundances and species richness and aims to explore the reasons for variation in the observed effects. I found 12 papers that looked at the impact of mass flowering crops on either wild pollinator abundances or species richness in natural habitats. The 12 papers yielded a total of 37 studies: 7 studies on pollinator species richness and 30 studies of wild pollinator abundances. 6 out 7 studies showed negative effects on pollinator species richness. Out of the 30 studies on wild pollinator abundances, 8 studies showed a positive effect of mass flowering crops on wild pollinator abundances, and 18 showed a negative effect. Generally, mass flowering crops had a negative effect on pollinator species richness, possibly due to mass flowering crops benefitting only a few generalist pollinator species. On the other hand, the presence of mass flowering crops appeared to be beneficial for wild pollinator abundances, especially at the local scale. Mass flowering crops seemed to promote positive spill-over of pollinators into the adjacent semi-natural habitats. However, at the landscape scale, when landscapes with and without mass flowering crops were compared, the effect of mass flowering crops was less clear. The effect of mass flowering crops seemed to vary depending on crop type and flowering time. Therefore, I conclude that more research is needed to identify the conditions under which mass flowering crops are beneficial for wild pollinator abundances and species richness.