1 Introduction

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

1.1 Background

In the discussion of the fate of biodiversity in the modern landscape the role of intensified agricultural production and particularly the use of chemical inputs attract much attention. Through analysis of data over 30 years in the UK, Benton et al. (2002) found that the decline in bird populations are correlated with declining insect populations, caused by agricultural intensification. Also in Denmark the improvements of crop yield and quality are at the expense of biodiversity in the arable fields (Andreasen et al. 1996; Kudsk & Streibig 2003; eds. Esbjerg & Petersen 2002, Navntoft et al. 2003), and the use of insecticides has in 1998 (Grell 1998) been suggested as a major factor behind the decline of Danish breeding birds. The British Game Conservancy Trust financed experiments with unsprayed field margins in order to increase the numbers of birds of game. Important effects were demonstrated on bird food insects for the field living birdlife such as Grey Partridge and Pheasant but also butterflies benefitted from non-treated 6 m field margins (Potts 1986, Sotherton 1987, Sotherton et al. 1989). A parallel Danish investigation of effects on flora and insects of 6 m non-sprayed field margins along hedgerows found improvements for both plants and insects (Hald et al., 1988). Later Esbjerg & Petersen, eds. (2002) demonstrated increases of wild flora species, flowering plants, insect and bird abundances at half and particularly quarter dosages of herbicides and insecticides. With conversion to organic farming a further increase in flowering plants and higher presence of butterflies was found, and the concomitant increase of weed seeds and arthropods was followed by a doubling of Skylarks in the organic fields (Navntoft et al. 2003).

The above findings, and the suggestions of Marshall (1989) and Wilson & Aebisher (1995), that hedgerows are important for the wild flora abundance, make hedges and field margins along them an interesting study area for biodiversity improvements. Many studies have looked into different aspects of field margins and others have looked into the potential use of flower strips and beetle banks, mostly with improvement of pest regulation by predators and parasitoids as the focus area.

Despite many demonstrations of predation (e.g. Collins et al. 2002, Collins et al. 2003) the demonstration of direct benefits to farmers at field level have failed except in a very few cases (e.g. Östman et al. 2003).

In contrast to this, the indications of biodiversity improvements are many but the approaches are mostly agriculturally focussed and very mixed in terms of both methodologies and terminologies. This was underlined by a review of buffer zone approaches mainly in Europe (Sigsgaard et al. 2007). Most remarkable was the fact that most buffer zone dimensions seemed to be selected somewhat arbitrarily.

At the administrative level, non-treated field margins is one of the targets of agricultural subsidies in several EU-countries. However, the width of the margin requested varies between countries (Sigsgaard et al. 2007). In this light, and on background of the general concern about biodiversity in farm landscapes, it is interesting that nobody has yet asked if it is possible to find a margin width, which will on one hand ensure a high saving/ improvement of biodiversity, and on the other hand will be tolerable for practical agriculture. Sigsgaard et al. (2007) among others point at the need to further investigate the influence of width and area of buffer zones.

In the current study, we investigated the biodiversity effect of non-fertilized and pesticide free buffer zones bordering hedgerows in order to fulfil the below aims.

1.2 Aims and hypotheses

The project takes some initial methodological steps towards a more systematic analysis of the importance of pesticide and fertilizer free buffer zones along hedgerows, here defined as field margins with one or more rows of woody plants, for improved biodiversity in agricultural landscapes. The project focuses on the impact of a simple set of different buffer widths (4, 6, 12 and 24 m).

AIM AND HYPOTHESES

The aim of the investigation was to identify a buffer zone width which would deliver a significant improvement of biodiversity (measured as species richness and a biodiversity index) from which an additional increase in width would only lead to marginally higher biodiversity. This aim was based on the two hypotheses below, which should be regarded as interconnected:

The biodiversity of plants and arthropods in a buffer zone along a hedgerow will increase with increasing width of the buffer zone, until a substantial saturation level is reached. Further increase of the width will only yield a relatively limited further increase of biodiversity.
It will be possible to identify an agriculturally practicable buffer zone width along hedgerows which will benefit flora and fauna so much, that the abundance and biodiversity will increase significantly.

Furthermore, an important part of this project was to identify organisms which may serve as suitable bioindicators for biodiversity improvements caused by buffer zones in arable fields.