Summary

Buffer zones for biodiversity of plants and arthropods: is there a compromise on width?

This report presents the results of a one-season field investigation of plant and arthropod biodiversity, as affected by the width of hedge-bordering buffer zones, maintained without application of fertilizers and pesticides. A review on buffer zones in arable fields (Sigsgaard et al. 2007) pointed at the effect of buffer width on biodiversity in and along agricultural fields as a question calling for attention. The Danish Ministry of Environment made a call for research projects; among other subjects on this aspect of buffer zones. The present project, which incorporated buffer zones of 4, 6, 12 and 24 m and a 0-m control was accepted, and started 2008. It included co-workers from University of Copenhagen (Department of Agriculture and Ecology and Department of Biology) and University of Aarhus (Department of Genetics and Biotechnology).

The aim of the project was to identify a buffer width which would significantly increase biodiversity in the field and in the hedge and which would also be agriculturally acceptable. For this, the effects of buffer zones of different widths were compared in order to investigate whether there is a compromise on width with respect to the increase in biodiversity and the agricultural feasibility. The buffer zones were placed along hedges in four large fields with spring sown barley at Gjorslev Estate on Eastern Zealand. In these zones, the hedge plant composition (woody species and dominant herbs) and their flowering was registered. This was followed by further plant species and plant density counts in the field. The plants’ flowering and generative stage were also noted. Insects and spiders were recorded by four methods three times during the season: beating tray sampling in hedges, transect counts of flying insects, sweep net sampling and pitfall trapping in the hedge-bottom and field areas.

Plants were identified mainly to species, and this was also the case for a considerable quantity of insects (e.g. butterflies, bumblebees, ground and leaf beetles, weevils and true bugs) while others were identified to genus, family or other well defined groups (e.g. small parasitic wasps). The plant and arthropod data were analysed in relation to buffer zone width and distance to the hedge. In addition, the effects of plant abundance and diversity were analysed for some arthropod taxa.

Both buffer zone width and distance to the hedge influenced plants and arthropods significantly. The abundance of wild plants in the field increased significantly and was more than doubled with a 6 m buffer zone compared to sprayed and fertilized field – an effect which to some degree continued with increased buffer width. Also the biodiversity of wild plants was increased with the establishment of buffer zones. 6 m of buffer was the minimum width required in order to significantly increase the plant biodiversity compared to plots without buffer area. There was a tendency towards increased biodiversity of wild plants at a further increased buffer width.

While the buffers only delivered limited protection of the hedge fauna, the buffer zone effects on the arthropod fauna within the hedge bottom (the vegetation beneath the hedge and out to the crop) and in the field were marked both in terms of increased abundance and in terms of increased biodiversity. For the arthropod abundance within the hedge bottom, a buffer width of 24 m delivered the most general increases, although in several cases a narrower buffer also resulted in higher abundances within the hedge bottom.

In the field (outside the hedge bottom) a significantly higher arthropod abundance was generally obtained with a 6 m or wider buffer zone. In addition, a generally and very markedly higher biomass of important bird chick-food items was found within the buffer zones at all distances from the field edge.

The biodiversity of arthropods within the hedge bottom increased consistently with a buffer zone width of minimum 6 m. This result was very clear and for the majority of the analysed taxa, a further increase in buffer width did not result in significantly higher biodiversity. This was further underpinned by the analysis of the marginal gain of biodiversity at increased buffer width, where it was found that the vast majority of the biodiversity increase within hedge and field was obtained already with a 6 m wide buffer zone.

Buffer zones had no effect on the flowering within the hedge bottom. The flowering percentages of wild plants in the field, however, was markedly higher within the buffer zones compared to treated field, and the importance of flowering was underlined by the significant positive correlations between flowering and activity of both butterflies and bumblebees.

An important spin off from this project is that butterflies seem to fulfil the role as a practical indicator for improvement of biodiversity. They responded positively to flowering, and positive correlations were found between biodiversity of butterflies and wild plants and between butterflies and other important arthropod taxa.

It is concluded, that irrespective of the slightly further increases of plant diversity and diversity of some arthropods at buffer zones widths of 12 m and 24 m, a 6 m buffer zone may be seen as a width providing a relatively high proportion of the biodiversity found at broader buffer zones in this one-year study. A 6 m wide buffer zone will also deliver a considerable amount of food resources for higher animals such as birds and small mammals.

For farmers, a 6 m buffer zone along hedges will primarily occupy a part of the field with some yield depression due to hedge competition. Furthermore, such a zone will increase the supply of food for game birds and hence open for an extra income.

For decision makers, the potential of a 6 m wide buffer zone along hedges, as a mean to counteract the negative effects of intensive modern farming on terrestrial biodiversity, should be both acceptable and somewhat attractive. 6 m buffer zones ought to open for subsidised regulation of biodiversity. In addition, monitoring of biodiversity effects should be possible using diversity of butterflies as indicator.

For an assessment of the full potential of buffer zones, future studies should include the performance of buffer zones present in field margins for more than one year. For such more permanent buffer zones, it will be important to include studies on vegetation management, and how vegetation management may further increase biodiversity of plants, insects and spiders, while avoiding that the buffer zones become a source of perennial weeds. It is also highly relevant to consider potential buffer zone effects on landscape connectivity by studying the effect of buffer area and the corridor effect for improved dispersal of flora and fauna by arranging coherent buffer zones over larger areas.