Diversity of flower-visiting bees in cereal fields: effects of farming system, landscape composition and regional context
Published source details
Holzschuh A., Steffan-dewenter I., Kleijn D. & Tscharntke T. (2007) Diversity of flower-visiting bees in cereal fields: effects of farming system, landscape composition and regional context. Journal of Applied Ecology, 44, 41-49.
Published source details Holzschuh A., Steffan-dewenter I., Kleijn D. & Tscharntke T. (2007) Diversity of flower-visiting bees in cereal fields: effects of farming system, landscape composition and regional context. Journal of Applied Ecology, 44, 41-49.
Some European agri-environment schemes (AES) promote organic farming in an attempt to reduce the negative effects of agricultural intensification on farmland biodiversity and promote ‘ecosystem services’ such as pollination. A study was performed in three regions (each 400-500 km² in area) in Germany differing in land use intensity. Within each region, seven pairs of conventionally and organically cultivated wheat fields were selected to encompass a gradient from heterogeneous to homogeneous landscapes within a 1 km radius around each field. The objective was to examine if organic farming is efficient in promoting bee (Apiformes) diversity in agricultural landscapes.
Study sites: The study was conducted in 2003 in three regions about 150 km apart: Soester Boerde (51º35'00"N, 008º07'00"E) - characterized by intensive agriculture, mainly wheat Triticum aestivum and sugar beet Beta vulgaris; Leine Bergland (51º32'00"N, 09º56'00"E) - productive land with more diverse agricultural landscapes in hilly parts; Lahn-Dill-Bergland (50º49'00"N, 08º46'00"E) a hilly area with a high proportion of grassland and small fields, and a flat and homogeneous area with intensive agriculture.
In total, 42 winter wheat fields were studied (7 organic and 7 conventional fields per region). Each organic field was paired with the closest conventional wheat field. Distances between fields within a pair ranged from 0 to 600 m, field sizes were similar (average 4 ha).
Bee and flowering plant surveys: Bees were recorded for 15 min/transect (95 × 1 m) on four occasions between May and July. Transects were located in the field centre and edge (edge transects 1 m into the field parallel to the boundary). Bees were collected for identification. Fields of a pair were sampled onthe same day, between 10:00 and 18:00 in warm, sunny, calm weather. Plant species flowering and cover (percentage cover of flower corollas per ground surface area) were recorded during each sampling period in a 1 m wide band along the transects.
Quantifying landscape context: The surrounding landscape was characterized for each field within a 1 km radius of the field centre (chosen as solitary bees, most of the recorded bee species, are known to be influenced by the landscape at scales up to 1 km). Areas of different habitat types were recorded, and the proportions of different habitat types and Shannon index of habitat diversity calculated. Sectors of different field pairs did not overlap as the distance between pairs ranged from 3 to 45 km. The proportion of annual crop fields was used as a homogeneous indicator for landscape complexity because of its negative correlation with the Shannon index and the proportions of grassland and forest. The proportion of crop fields did not differ significantly between the study regions nor between landscape sectors around organic and conventional fields. The proportion of crop fields was negatively related to the altitude of study sites, but analyses did not show any relationship between altitude and bee diversity or flower availability.
In total, 1,507 bees were recorded (167 solitary bees, 693 bumblebees Bombus spp.; 647 honeybees), representing 37 species from 12 genera. The most species-rich genera were Andrena (15 spp.), Bombus (7 spp.), Nomada (4 spp.) and Lasioglossum (3 spp.). In organic fields 1,326 individuals of 31 species (average abundance 63.1 ) and in conventional fields 181 individuals of 16 species (average abundance 8.6) were recorded. At the landscape scale, average bee species richness per field was 2.1 species for conventional fields and 6.9 for organic fields. Bee diversity was positvely correlated with flower cover and flowering plant diversity (51 spp. recorded).
As the proportion of arable crops in the surrounding landscape increased, differences in bee diversity between organic and conventional fields decreased. Bee diversity increased with increasing landscape heterogeneity. Bee diversity in the field centre and edge in organic fields was similar, whilst in conventional fields diversity was higher at the edge. Bee diversity differed between the regions, but the effects of farming systems and landscape context were similar.
Conclusions: Bee diversity was mainly, and consistently in the three regions, influenced by farming system i.e. organic farmed fields had higher diversity. Whilst organic farming may help to sustain pollination services by generalist bees in agricultural landscapes, the authors indicate that other measures are required to conserve more specialized species in semi-natural habitats.