Flower strip networks offer promising long term effects on pollinator species richness in intensively cultivated agricultural areas

Abstract

Background: Intensively cultivated agricultural landscapes often suffer from substantial pollinator losses, which may be leading to decreasing pollination services for crops and wild flowering plants. Conservation measures that are easy to implement and accepted by farmers are needed to halt a further loss of pollinators in large areas under intensive agricultural management. Here we report the results of a replicated long-term study involving networks of mostly perennial flower strips covering 10% of a conventionally managed agricultural landscape in southwestern Germany.

Results: We demonstrate the considerable success of these measures for wild bee and butterfly species richness over an observation period of 5 years. Overall species richness of bees and butterflies but also the numbers of specialist bee species clearly increased in the ecological enhancement areas as compared to the control areas without ecological enhancement measures. A three to five-fold increase in species richness was found after more than 2 years of enhancement of the areas with flower strips. Oligolectic bee species increased significantly only after the third year.

Conclusions: In our long-term field experiment we used a large variety of seed mixtures and temporal variation in seeding time, ensured continuity of the flower-strips by using perennial seed mixtures and distributed the measures over c. 10% of the landscape. This led to an increase in pollinator abundance, suggesting that these measures may be instrumental for the successful support of pollinators. These measures may ensure the availability of a network of diverse habitats and foraging resources for pollinators throughout the year, as well as nesting sites for many species. The measures are applied in-field and are suitable for application in areas under intensive agriculture. We propose that flower strip networks should be implemented much more in the upcoming CAP (common agricultural policy) reform in the European Union and promoted more by advisory services for farmers.

Keywords: Agri-environmental schemes; Bees; Butterflies; CAP reform; Floral resources; Flower-strips; Fragmentation; Long-term field experiment; Pollinators; Specialist species.

Fig. 1

Location of the study regions near Dettenheim and Rheinmünster in Southern Germany in Central Europe. The study areas are marked in red. Larger cities are marked in blue. Flower strips located in the enhancement areas are marked in orange

Fig. 2

Change through time of bee species richness and abundances between 2010 (before the enhancement measures) and 2015. Upper graphs refer to all species, lower graphs refer to oligolectic species, only. The green line marks the year 2011 when the enhancement measures were initiated; blue symbols: control area; red symbols: enhancement area. The clipart is freely available via https://openclipart.org

Fig. 3

Differences D of species numbers and abundance of bees between enhancement area and control area. Comparison between the enhancement areas (EA) and the control areas (CA) in 2010 (before the enhancement measures started) and in the year 2015 (in the fifth year of enhancement measures). The values present the mean difference D between the enhancement area and the control area ± standard error, therefore slightly negative values or overlapping standard errors are possible. Significant differences between the years 2010 and 2015 are marked with * if p < 0.05; ** if p < 0.01 and *** if p < 0.001. The clipart is freely available via https://openclipart.org

Fig. 4

Difference D of the Chao 1 Diversity of bee species. Comparison between the enhancement areas (EA) and the control areas (CA) in 2010 (before the enhancement measures started) and in the year 2015 (in the fifth year of enhancement measures). The values present the mean difference D between the enhancement area and the control area ± standard error. Significant differences between the years 2010 and 2015 are marked with * if p < 0.05; ** if p < 0.01 and *** if p < 0.001. The clipart is freely available via https://openclipart.org

Fig. 5

Difference of species numbers and Chao2 Diversity of butterflies. Comparison between the enhancement areas (EA) and the control areas (CA) in 2010 (before the enhancement measures) and in the year 2015. The values present the mean difference D between the enhancement area and the control area ± standard error. Significant differences between the years 2010 and 2015 are marked with * if p < 0.05; ** if p < 0.01, *** if p < 0.001 and (*) if p < 0.01. The clipart is freely available via https://openclipart.org

Fig. 6

Summary of relevant aspects of flower strip networks that increase pollinator species richness. The described set of easily applicable in-field measures builds upon the experiences of existing theoretical and applied studies and was evaluated within this study