4 Discussion

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

4.1 Flora

The composition of wild flora in the fields was significantly different between the two sampling runs which took place from May to early June and in July, respectively. Due to the progression of the season, more plant individuals could be identified to species in the second run. Also, a later germination of certain species may have influenced the species occurrences recorded. Consequently, the two flora samplings were treated as two separate data sets, for most of the analyses.

For the plant families: Apiaceae, Asteraceae, Lamiaceae and Poaceae, the number of plants decreased with distance to hedge, whereas the number of plants in Brassicaceae, Chenopodiaceae, Scrophulariaceae and Violaceae did not decrease with increased distance to hedge. This difference in effect of distance may be caused by a combination of microclimate, management history such as (ploughing and herbicide applications) and timing of the generative stages of the weed species, all of which have consequences for seed formation and seed dispersal. In surveys of weed abundance and seed banks in arable fields in Southern England, the number of seedlings and number of species also decreased with distance from the hedge in up to 4 m from the hedge, after which the occurrence was stable (Wilson & Aebischer 1995). Similar to Wilson & Aebischer (1995) and Marshall (1989), we found that 21 species (59% of all species in the hedge bottom) were limited to the hedge bottom and absent from the field. 19 species (37% of all species in the field) were limited to the field and absent the hedge-bottom.

The impact on plant diversity of buffer zones was evident, as there generally was a higher density and diversity within a buffer zone than in treated field. Thus, buffer zones have proven to be an important tool to increase biodiversity in agricultural fields. The width of the buffer zone had a significant effect on the number of weeds (especially dicotyledonous weeds), biodiversity of weeds and flowering percentage. Hence, not surprisingly, herbicides, probably in combinations with other agro-chemicals, significantly decreased the floral biodiversity.

In the survey by Marshall (1989), the dicotyledonous species were dispersed with a logistically decreasing distribution pattern for the individual species with increasing distance from the hedgerow (Marshall 1989). In our study, the overall biodiversity index (Shannon’s H) fitted by the logistic model, showed the same pattern (section 3.1.3). The halving distance of Shannon’s biodiversity index comprising of all wild plant species increased with increasing buffer zone width. A significantly higher halving distance was found for buffer 6 compared to buffer 0 at sampling run 2 (July data), indicating that a buffer width of 6 m may significantly improve the biodiversity of wild plants. For a further (significantly) higher halving distance, a buffer width of 24 m was needed.

The wild plants were flowering vividly in the buffer zones, but there was no significant effect of buffer zones on flowering within the hedge bottom. This experiment showed very clearly, that buffer zones will increase the flower resources in the field for pollen and nectar feeding insects such as butterflies and beneficial insects like hoverflies.

The analyses on the marginal gain of increased buffer width (section 3.3) showed that a buffer width of 6 m was sufficient to secure a significantly higher biodiversity in terms of species richness, but also that a wider zone will result in more biodiversity.

In this buffer zone study, with treatment being a combination of fertilizer, herbicide, fungicide and insecticide, we cannot distinguish if the significant effects were a result of one or more of the applied chemicals. However, the effects of herbicides and insecticides have been elucidated in two earlier investigations on effects of reduced dosages (Esbjerg & Petersen eds. 2002) and on conversion to organic farming (Navntoft et al. 2003) and as found for bumblebees in the field in this experiment. These investigations showed very clearly, that herbicides have a combined plant-arthropod effect in three ways: 1) suppression of a number of wild plant species, which in turn exclude presence of insects linked to these plant species, 2) reduction of plant biomass and hence cover, which will affect food quality for herbivorous insects (Kjær & Elmegaard 1996) as well as shelter and microclimate primarily for a number of ground dwelling predators, e.g. ground beetles, rove beetles and spiders (Navntoft et al. 2007). Finally 3), herbicides lead to reduced flowering, which again reduces presence of nectar dependant insects like for instance butterflies (Navntoft et al. 2003). The short term effects of fertilizer on the wild flora in agricultural fields would be increased biomass (Andreasen et al. 2006). Furthermore, the increased biomass of the crop due to fertilization would exert a strong inter-specific competition for water and light, and consequently suppress the wild flora (Andreasen et al. 2006). The application of herbicide will in the short term affect the biomass of the wild flora negatively, as weed biomass correlates with herbicide amount investigated in a similar field experiment (Sønderskov et al. 2006). However, timing, application technology, targeting of the pesticide (mono- dicot) and any herbicide resistance, may affect the response of the wild flora (Kudsk & Streibig 2003). Further investigations of the interactions between wild flora and arthropods are needed if effects of either fertilizer or herbicide reduction should be more precisely clarified.

4.2 Arthropods

4.2.1 Arthropods on woody plants in hedgerows

Hedgerows provide a more stable habitat for arthropods than the field. They provide an overwintering site for many species such as weevils, spiders and ground beetles and a source of food (plant, prey, pollen, nectar) (Maudsley 2000). Particularly the woody plants in the hedgerow are physically removed from fertilizer and pesticide use. Thus a weaker response to buffer zones could be expected in arthropods on woody plants in hedgerow, compared to hedge-bottom and field. Both spiders, Hemiptera and Coleoptera responded positively to increased buffer width, but responses were less pronounced than in hedge-bottom and field and sometimes could only be found on one of the tree species tested. Thus, for spiders a significant response to buffer width was only found on hawthorn. In June (period 2), there was a significantly higher number of Hemiptera in all buffer zones wider than 0 m. Hedgerow dwelling aphids were also significantly affected by buffer zone width in May and June (periods 1 and 2). Heteroptera were only significantly affected by buffer width in blackthorn. Finally, in June there was a significantly higher number of Coleoptera in all buffer zones wider than 0 m. On the family level, the effect of buffer width was significant for Nitidulidae and Curculionidae.

Hedgerow woody plants had significantly different numbers of arthropods, and also the species compositions were different, in accordance with many other studies (Maudsley 2000). Most species were found in blackthorn and hawthorn, and the least in elderberry. While this is not the focus of the current study, tree species value for arthropods and tree species composition may be important in decisions regarding new hedgerow plantings.

Overall, there was a less pronounced response to buffer width in the hedgerow woody plants than in the hedge bottom and field. This is most likely a result of the hedgerow being more distant from the pesticide treated area both in distance and height and in accordance with results of the pesticide drift investigations by Bruus et al. (2008). In addition to species diversity, hedgerows are also a structurally diverse habitat, in which arthropod diversity and abundances are also affected by other management practices, not assessed here, such as plant composition and cutting (Maudsley 2000). The botanical and structural diversity of hedgerows may mean that more hedgerows may need to be assessed for a clearer result. Also no changes in the floral composition of the woody plants would occur in a 1 year study which could drive the change in the fauna composition. Finally, pesticides drift into the hedge is very dependent on wind direction and speed and therefore hedgerows with different orientation may be required for a more complete study on the pesticide effects in the canopy fauna on the woody plants.

4.2.2 Arthropods in hedge-bottom and field

Five out of the nine higher arthropod taxa tested showed significantly higher abundances in hedge-bottom when bordering buffer zones (Lepidoptera, Hemiptera, Coleoptera, Araneae and Thysanoptera). In addition, the higher taxa Hymenoptera (mainly parasitic wasps) and Diptera (hover flies) showed a tendency towards higher abundance in the hedge-bottom at increased buffer width. Only abundances of Carabidae and Staphylinidae within the hedge-bottom were unaffected by buffer zone presence. The protection provided by buffer zones to arthropods in the hedge bottom is an important effect which to our knowledge has not been described before.

The buffer effect on abundance of higher taxa within the hedge bottom depended on the width of the buffer zone. A 4 m buffer was sufficient to benefit both Hemiptera in June and July (periods 2 and 3) and Araneae in July (period 3) (Fig. 3.16 and Fig. 3.24). Thysanoptera within the hedge bottom benefitted from zones of 6 m or wider (Fig. 3.20 - June and July). For Lepidoptera, a 12 m buffer zone was needed to find a higher activity close to the hedge in July (Fig. 3.14). Coleoptera needed a 24 m buffer in order to find a higher activity in the hedge bottom in June to early July (period 2) (Fig. 3.17).

Bruus et al. (2008) found that the hedge bottom is highly exposed to pesticide drift, and the differences found within the hedge bottom are therefore likely caused by direct negative effects of the pesticide applications close to the hedge bottom. An indication of the effect of pesticide drift was found when comparing the diversity in the hedge bottom of beetles with specific plant preferences in relation to plant diversity before and after insecticide applications. There was relatively low plant diversity at the hedge bottom for both buffer 4 and buffer 24 (see Table 3.3) and as could be expected there was also equally low diversity of the herbivorous beetle families Chrysomelidae and Curculionidae in May (before insecticide application). However, after both insecticide applications in July, buffer 24 now had significantly higher herbivorous beetle diversity in the hedge bottom compared to the narrow buffer 4. This may indicate a buffer width effect on the insecticide drift at period 3 (July), with more pesticide drift, and hence deposition, into the hedge bottom at a buffer width of 4 m compared to that of 24 m.

For eight out of nine higher arthropod taxa analyzed (the exception being Staphylinidae), a buffer width of 6 m was the narrowest width to consistently promote a higher abundance or activity of arthropods within the field area (outside the hedge bottom). However, a further increase in buffer width always increased the abundance and activity of arthropods. Buffer zones had a very positive effect on chick-food biomass in June and July (after insecticide applications, Fig. 3.23). As many farmland birds prefer to forage within the first 6 m from the hedge, such a buffer zone width will be of high benefit for many bird populations (Bradbury et al. 2000, B.S. Petersen pers. comm.). A wider buffer zone however, may always be better for birds, as the increases in amount of arthropod food supply seems to be almost proportional with buffer zone width (Fig. 3.23).

As could be anticipated from the pressure of pesticide treatments, there were no significant results that pointed towards enhanced beneficial arthropod activity (Syrphidae, Parasitica, Araneae, Carabidae and Staphylinidae) outside the buffer zones. However, there was a tendency towards higher carabid activity up to 200 m into the field from buffer zone edges. As opposed to spiders, the second insecticide application did not seem to diminish the carabid abundance to the same extent as the first application, probably because of a higher and denser crop cover outside the buffer zones which may provide better microclimatic conditions and protection for most carabid species.

The classical question of buffer zone effects on natural biological control therefore remains open with the present experimental set-up, as not only the aphid pests (the prey), but also the beneficial arthropods outside the buffer strips, may have been severely diminished by the repeated intensive pesticide sprayings. Furthermore, the repeated spraying gave the populations of beneficials in the buffer zones reduced possibility to reinvade the sprayed areas. However, this lack of a measurable recolonisation may biased by the fact, that the samplings were carried out within few days after the pesticide applications.

Biodiversity of most arthropod taxa within the hedge bottom increased when the hedge bottom was protected by a buffer, and biodiversity also increased within the buffer zones themselves. However, the buffer width required for such significant increases varied between taxonomic groups. For Heteroptera, the analysis on accumulated number of species showed, that a buffer width of 4 m was sufficient to secure significantly higher species richness in the hedge bottom compared to buffer 0 (Fig. 3.28). This was indicative supported by the species richness analysis presented in Fig. 3.21. Also for the total plot species richness (when all sampling distances were included in the analysis), buffer 4 significantly increased the species richness of heteropterans The total plot diversity at buffer 4 increased from 9 to 12 species compared to buffer 0 and this difference gradually increased at increased buffer width (Fig. 3.28). The species-area analysis (SPAR), which included all sampling areas and all sampling times, further supported this (Table 3. 7, Fig 3.25). The SPAR analysis also showed that a buffer width of 24 m markedly increased the species richness of Heteroptera compared to all other buffer widths.

For the herbivorous beetle families Chrysomelidae and Curculionidae, a 6 m buffer width was needed to secure a significantly higher plot species richness. A 6 m buffer more than doubled the entire plot species richness of the herbivorous beetles compared to buffer 0 (Fig. 3.28). There was no significant benefit to total species richness in the experimental plots (all sampling distances included) of a wider buffer zone, although it may be an artifact that buffer 6 delivered the highest species richness among all buffer zones. Buffer zones did not significantly increase the species richness within the hedge bottom, although there was a tendency towards higher species richness when buffer zones were present along the hedge bottom. The results of the species-area (SPAR) analysis on the herbivorous beetles, which included all sampling areas and sampling times, supported the results above. Furthermore, the estimates of ß-diversity (a measure of the change in species richness across a spatial scale), increased with increased buffer widths, indicating that more suitable niches for the beetles are created with increased buffer widths.

Among the ground dwelling beneficial arthropods, the order Araneae (spiders) and the family Carabidae (ground beetles) responded to buffer width. Araneae diversity responded positively to a buffer zone of at least 4 m in the field compared to buffer 0 (although such a response was not found in the hedge bottom) (Fig 3.20). An explanation for the lack of differences within the hedge bottom could partly be that many Araneae species overwinter in the hedge bottom and later disperse into the field. For Carabidae, there was a tendency towards increased biodiversity with increased buffer width, a tendency which however was almost eliminated when species-area relationships were considered (Figs. 3.28 and 3.29).

For higher species richness of butterflies, a minimum of 6 m buffer was needed as compared to buffer zone 0 (section 3.2.2.1). 6 m of buffer zone would increase the species diversity of butterflies by 55% on a local scale. When compared to buffer zone 6, a buffer zone of 24 m was needed for a further significantly increase in species richness. When biodiversity of butterflies was measured with Shannon’s H, a minimum of 12 m buffer zone was needed to obtain a significantly higher biodiversity when compared to field with no buffer zone. In addition to the biodiversity analysis presented in section 3.2.2.1, the analysis on accumulated species richness of butterflies at increasing distance from the hedgerow showed a tendency towards more species at increased buffer zone width (Fig. 3.28). The weaker response in the analysis on accumulated species richness may be a result of the statistical method. The accumulation of species over the four distances reduced the number of observations used and hence the degrees of freedom in the accumulated model and therefore also the strength of the model (for model descriptions see Models 2 and 13 in Appendix F). However, the analysis on accumulated species richness showed that a buffer width of 6 m significantly increased the species richness close to the hedge (0-4 m). The species-area (SPAR) analysis showed that a buffer 4 was the narrowest width to deliver significantly higher species richness of butterflies close to the hedge. A buffer 6-24 further increased the species richness along the hedge.

The importance of flowers in the hedge-bottom and field is illustrated by the significant positive correlations between flowering and activity for both butterflies and bumblebees. Also the presence of suitable host-plants seemed to influence the activity of butterflies positively as could be expected (section 3.3.1).

Overall, a 6 m buffer zone is the smallest width to deliver a consistent positive effect on the biodiversity of the arthropod complex studied within the hedge-bottom and field. A wider buffer zone will result in more biodiversity. However, the further increase of biodiversity in response to a wider buffer zone will be relatively small except for a few taxonomic groups. It is noticeable, that the very clear results on biodiversity improvements were obtained instantly with annual buffer zones.

For the monitoring of biodiversity effects of buffer zones, butterfly species richness seems to be a suitable bioindicator. Butterflies responded both to habitat-changes caused by buffer establishment and to buffer zone width. Furthermore, the species richness of butterflies correlated positively and significantly to the species richness of the test taxa Heteroptera and Carabidae, and there was a strong indication of a positive correlation between butterflies and the species richness of dicotyledonous plants. Furthermore, butterfly presence combines several habitat requirements such as suitable host plants and nectar resources as also shown in the present study. This means, that much attention should be paid to butterflies when looking for suitable bioindicators. Observations of butterflies may also be a short cut to disclose the presence of a few locally rare plant species. A draw-back of the transect count method used to sample butterfly activity is that the method is very weather dependent. On the other hand, the method is very cost-efficient (Duelli & Obrist 2003) and may be quite easily adapted by local non-specialists or amateurs for broad-scale monitoring arable landscape (Thomas 2005, Pollard & Yates 1993).

4.3 General discussion

The prime goal of this project was to identify a buffer zone width, which could deliver a marked improvement of biodiversity and still be agriculturally practical. Therefore, effects on plants and arthropods of four different buffer zones (4 m, 6 m, 12 m and 24 m free of fertilizers, herbicides, fungicides and insecticides) and a control (no buffer) were compared. The project only ran for one season, in one crop and at one farm, which slightly limits the general value. However, the design and the limitation in time and space, as well as the use of spring barley crop in the buffer zone (a fairly open crop like some grasses), reduced the possible variables regarding time span, vegetation development etc. ensuring that the main focus of the investigation was the width of the buffer. Though there was some quite foreseeable variation in responses, some interesting and informative general patterns were found.

Both buffer zone width and distance from the hedge significantly influenced the density of wild plants, their flowering and their biodiversity measured as species richness and with Shannon’s biodiversity index. The buffer zone effects on dicotyledonous weeds were the most pronounced. Furthermore, plotting Shannon’s index values for plant diversity against the distance to the hedge indicated that a 6 m buffer zone significantly improved the biodiversity of wild plants compared to field plots without buffer zones but also that 24 m of buffer further improved plant diversity measured by Shannon´s index. However, the analyses on the marginal gain of biodiversity (measured as species richness) at increased buffer area did not show a significant increase in species richness when the buffer width was extended beyond 6 m, although there was a tendency towards higher species richness at increased buffer widths (Fig. 3.28).

Buffer zones had no effect on the flowering within the hedge bottom, however, in the field area the flowering percentages increased markedly within any given combination of buffer and distance to hedge (Fig. 3.7).

For the arthropods, there was a pronounced effect of buffer zones and their width. Eight out of nine higher level taxa: butterflies (Lepidoptera), Hemiptera (such as true bugs and leaf hoppers), foliage dwelling beetles (Coleoptera), parasitic wasps (Hymenoptera), hoverflies (Diptera), thrips (Thysanoptera), spiders (Araneae) and ground beetles (Carabidae) responded very positively to buffer zones in terms of either abundance, biodiversity or both in hedge bottom and/or in the field. Only the rove beetles (Staphylinidae) did not respond to the establishment of buffer zones. In all eight positive cases, a buffer of 6 m was sufficient to secure a significantly higher abundance and/or higher species richness compared to the control (buffer 0).

The positive biodiversity effect of buffer zones is further underpinned by the analyses on the marginal gain of biodiversity at increased buffer widths (Figs. 3.28 and 3.29), which takes into account the general positive correlation between area and species richness. From those analyses it was very clear, that for the majority of the test taxa, a buffer width of 6 m was sufficient to secure a significantly higher species richness compared to field not guarded by a buffer zone.

The butterflies in general showed an interestingly detailed response with significantly effect of buffer zone width on abundance, species diversity and Shannon’s diversity index. Furthermore, their species richness was positively correlated to species richness of most other test taxa (section 3.4.2). This opens for the use of butterflies as an indicator of biodiversity, which can enable non-specialist monitoring of biodiversity. The reason is, that many butterflies are easy to identify and easy to detect because of movement. In addition, their presence reveals the location of certain larval food plants, which may else be more difficult to find. Conversely, the presence of the plants does not necessarily imply the presence of the butterflies. Butterflies may be a more operational indicator than the smaller insects such as bugs (Heteroptera) and the herbivorous beetles Chrysomelidae and Curculionidae which also showed a clear positive response to buffer zones, but which requires more sampling efforts and more taxanomic training to identify.

The high benefits of even a 6 m buffer zone on bird prey quantities will be at a level, which in the light of other investigations (Boatman & Bence 2000, Boatman & Stoate 2000, Esbjerg & Petersen 2002, Navntoft et al. 2003, B.S. Petersen Pers. Comm.), most likely will increase the presence of birds such as the insectivorous Whitethroats.

The buffer zones investigated were also anticipated to yield some protection to fauna on woody plants in hedgerows. However, responses to increasing buffer width within hedgerows were in general weak or inconsistent. Most clear were the positive responses to buffer width by Coleoptera and Hemiptera in period 2, where insecticide had been applied in the field. However, the results on spiders and a number of other insect taxa on woody plants in hedgerows did not give a consistent picture, which could justify an indication of biodiversity improvements due to buffer establishment. However, more studies may be required in order to appreciate buffer zone effects on the arthropod fauna of the woody plants in the hedge.

For the hedge-bottom fauna however, buffer zones generally increased arthropod abundance and diversity. This can presumably be ascribed to the protection against the deposition of agro-chemicals during treatments.

Previous studies of reduced pesticide use on field margins (not hedgerows) have focused on Carabidae, Heteroptera, Staphylinidae, Lepidoptera and grouped chick-food insects (Frampton and Dorne 2007). In these studies, abundance of Heteroptera showed the most pronounced response with up to 12.9 times higher where pesticide use was restricted (Frampton and Dorne 2007). Our findings underpin the effect of buffer zones on Heteroptera. For other invertebrates, earlier studies generally found either increased abundance or no impact with restricted use of pesticides (Frampton & Dorne 2007). Fritz-Kohler (1996) found a correlation of Chrysomelidae and Curculionidae with the prescence of buffer zones in field crops. The presence of a more diverse flora in buffer zones was argued to be the reason for this (Fritz-Kohler 1996). A significant increase of Lepidoptera including Pieridae (whites), in 3-6 m wide unsprayed buffer zones around a winter wheat field was reported by de Snoo et al. (1998). In that study, the number of Lepidopteran species increased by a factor of 2.3 compared to no buffer zone and the number of individuals by a factor of 4.6-4.9 (de Snoo et al. 1998). Chrysomelidae, Curculionidae and Lepidoptera are all sensitive to insecticides. The positive effect of buffer zones on these groups may partly be attributed to this (Wilson et al. 1999). Wilson et al. (1999) found evidence that reversal of intensification especially in arable systems can result in rapid recovery of these groups as well as other bird chick-food resources. Our findings in this only 1-y study confirm this.

It should be noted, that this study is conservative with respect to biodiversity gains from buffer zones, as it only covers one cropping season. Species diversity, species richness and number of individuals after long-term absence of fertilization and pesticide application in buffer zones may contrast this short-term investigation. In the short term, fertilization increases biomass of weeds and crop (Andreasen et al. 2006), while herbicides partly counteract this by decreasing the biomass of the wild flora (Sønderskov et al. 2006). Thus, buffer zones will be expected to have decreasing biomass over time but increasing biodiversity, compared to fertilized and pesticide treated field margins. Conversion to organic farming revealed a differentiation after 3-4 years between plant communities, with stress-tolerant plant species being more abundant in hedge bottom vegetation bordering organic farms and ruderal and nutrient demanding plant species being more abundant in hedge bottoms at conventional farms (Petersen et al. 2006). A comparison of vegetation in hedgerows bordering fields with or without pesticide application through 10-14 years, revealed more species (weed, ruderal and semi-natural) in hedges without pesticide drift (Aude et al. 2003), and the species composition was more similar to semi-natural communities than in conventional hedges (Aude et al. 2004). Long-term buffer zones along hedgerows may thus provide new habitats for plant and arthropod species, due to direct interactions as well as to increased structural diversity and landscape heterogeneity (Benton et al. 2003, Maudsley 2000, Rundlöf et al. 2008). In the UK, the country-wide management practices of the field margins through the last two decades, has brought valuable surveys of effects on biodiversity and resource provision for farmland birds (Douglas et al. 2009, Vickery et al. 2009, Woodcock et al. 2009).

High diversity is not necessarily obtained by no management. Thus a hedgerow which was studied with 27 y interval had reduced plant diversity in the annual vegetation both in parts bordering cultivated fields, managed annually and in unmanaged parts (Garbutt & Sparks 2002). The management of the buffer zone is of importance for plant and resulting arthropod diversity. Mowing without removal of cuttings significantly reduced species richness and yielded more grassy margin strips (de-Cauwer et al. 2005). Annual hay-making, on the other hand, removes excess nutrients, and supports establishment of a diverse more natural flora (Grub et al. 1996, Asteraki et al. 2004). Comparing different management practices in grassy buffer zones, Woodcock et al. (2007) found most beetles in buffer zones with one annual cutting in June, and in uncut buffer zones, compared to other management practices. They also found a higher density of flower feeding and seed feeding beetles in unfertilized grass strips.

Other taxa, not analyzed here, will also be affected by buffer zones. Thus, a diverse flora provides habitat for both soil and herbaceous-living invertebrates. Furthermore, this complexity provides an increased prey accessibility and especially provides key winter resources for seed feeding birds (Vickery et al. 2009).

Buffer zone age, size and connectivity are other important factors. A significant effect of buffer zone age has been found on populations of arthropod predators. Thus, older buffer zones (6 y) have larger populations of predators, especially spiders, and a higher predator:prey ratio than younger bufferzones (Denys & Tscharntke 2002). Fallow field were found to have higher diversity than buffer zones, stressing the importance of size as well as connectivity for biodiversity.

Buffer zones may also favor biodiversity in other crops such as potatoes, sugar beet and brassicas. With such crops, the effect of width may be different from what was found in barley (Zande et al. 2000, Benton et al. 2003). However, these crops should for practical agricultural reasons not be grown in the buffer zone, which should rather be grown with cereals or grass sown at low densities or otherwise remain fallow.

Our results apply only to terrestric systems. For field margins bordering aquatic systems such as streams, a determination of the buffer zone required would depend on effects on the flora and fauna both in the aquatic system and in the terrestric flora and fauna bordering it.

In summary, the present study showed that both along the hedge and in the cereal field quite a high proportion of the investigated flora and fauna benefitted significantly from a buffer zone only 6 m wide. It should be noted the even wider buffer zones (12 and 24 m) would further benefit flora and fauna.