Semi-natural grasslands as population sources for pollinating insects in agricultural landscapes.
In intensively farmed agricultural landscapes, many species are confined to very small uncultivated areas such as field margins. However, it has been suggested that these small habitat elements cannot support viable populations of all the species observed there. Instead, species richness and abundance in these small habitat fragments may, at least partly, be dependent on dispersal from larger semi-natural grassland fragments. We tested this hypothesis for butterflies and bumble bees in 12 independent landscapes in a region of intense agriculture in southern Sweden. In each landscape we surveyed abundance and species richness in one semi-natural grassland, one linear habitat (uncultivated field margin) adjacent to this (called proximate) and one similar linear habitat (called distant) situated at least 1000 m from the semi-natural grassland patch. Both species richness and density (individuals per unit area) of butterflies and bumble bees were significantly higher in proximate linear habitats than in distant ones. Moreover, butterfly species richness was higher for a given area in grasslands than in any of the linear habitat types. Butterfly density in grasslands did not differ from that in proximate linear habitats but was lower in distant linear habitats. The effect of isolation on density was stronger for less mobile butterfly species. For bumble bees there was no difference in species richness between grasslands and proximate linear habitats. For at least some of the butterfly species even these relatively small fragments of semi-natural grasslands act as population sources from which individuals disperse to the surrounding habitats and thereby contribute to higher densities and species richness in adjacent areas. For bumble bees, it is more likely that the grasslands contain a higher density of nests than the surrounding intensively cultivated landscape, and that the density of foraging bumble bees decreases with increasing distance from the nest. Synthesis and application. Habitat fragmentation and intensified agricultural practices are considered to be a threat against services provided by pollinators. In order to sustain the abundance and diversity of insect pollinators in intensively farmed agricultural landscapes, we suggest that preservation of the remaining semi-natural grasslands or re-creation of flower-rich grasslands is essential.