Wild bees are important providers of pollination services to agriculture and should be a focus group for conservation. Recent declines of bumblebees and solitary bees have been attributed to agricultural intensification. To be able to propose adequate conservation strategies, the mechanisms whereby agricultural intensification affects bee populations have to be understood. Through its effects on farming practices and landscape heterogeneity, agricultural intensification may contribute to the decrease in pollinator populations both by reducing the general availability of nectar and pollen resources and by changing the spatial and temporal variation of these. The spatial and temporal scales at which variation acts on individual species may depend on the scales at which these species utilize resources. Therefore landscape complexity may affect bee diversity in predictable ways. By studying the foraging ecology of bumblebee and solitary bee species, the mechanisms whereby farming practices and landscape complexity affect species richness and population densities will be investigated. The focus will be on how spatial and temporal availability of nectar and pollen resources determine bee foraging success and colony growth and as a consequence population persistence and community structure. The results will be used to propose measures to maintain viable bee populations in agricultural landscapes.