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Species richness in agro-ecosystems has dramatically declined during the last decades, mainly as a result of the intensification of land-use practices (Krebs et al. 1999; Tilman et al. 2002). On the one hand, intensification has occurred at the field scale through the increased use of pesticides and mineral fertilizers. On the other hand, intensification has also occurred at the landscape scale because of the aggregation of intensively managed arable fields together with land consolidation that has resulted in a transformation of formerly complex landscapes with relatively high proportions of (semi)natural habitats to simple landscapes dominated by arable fields.
Arable fields in complex landscapes should support higher species richness than in simple landscapes as complex landscapes provide alternative habitats and sources for recolonization of fields. The relationship between local species richness and landscape context has been addressed for several groups of arthropods (Menalled et al. 1999; Krauss, Steffan-Dewenter & Tscharntke 2003; Schmidt et al. 2005) but only Weibull, Östman & Granqvist (2003) and Krauss et al. (2004) have investigated this relationship for plants in agricultural landscapes. However, they did not detect an effect of landscape context on plant species richness. Holl & Crone (2004) found only weak importance of landscape-scale variables for the diversity of native riparian understorey plants. Nevertheless, the species diversity of weeds in annual crop fields should depend on both local management and surrounding landscape. Weeds are a highly dynamic group adapted to a frequently disturbed habitat, relying on their seed bank as well as on immigration of seeds through the seed rain from surrounding habitats. Therefore weed species diversity in conventional fields should particularly benefit from landscape complexity as these are more frequently disturbed (e.g. by herbicide use) than organic fields. Knowledge of these relationships is crucial for a better understanding of weed species diversity patterns and should be helpful for future conservation management decisions. However, to our knowledge, this is the first study to analyse how species diversity of arable weeds in the vegetation, seed rain and seed bank responds to the surrounding landscape in organic and conventional fields. In addition, the contribution of the heterogeneity in community composition between weed samples, within and between fields, to field and regional diversity is little known. The concept of additive partitioning of species (Allan 1975; Lande 1996) addresses this problem by dividing total diversity of a given number of samples (gamma diversity) into the additive components alpha (mean diversity) and beta (between sample heterogeneity), thereby allowing species diversity at several spatial scales to be scaled up to whole regions (Wagner, Wildi & Ewald 2000; Gering & Crist 2002; Crist et al. 2003; Gering, Crist & Veech 2003).
In this study, we used this concept to characterize the diversity of arable weeds at two spatial scales. At the regional scale, gamma diversity was the overall number of species found in our study region. Alpha diversity was the average number of species in the individual studied fields within the region, and beta diversity accounted for the within-region heterogeneity (average number of species not found in a field). At the field scale, gamma diversity was the overall number of species found in the samples of one field. Alpha diversity was the average number of species found in the samples of one field, and beta diversity accounted for the within-field heterogeneity (average number of species not found in a sample). At both scales, the relative beta diversity (the percentage of beta contributing to gamma) was also calculated. We analysed the relative importance of local management (organic vs. conventional) and landscape complexity (gradient from simple to complex) on species diversity of arable weeds in the vegetation, seed rain and seed bank (at the field scale) of 24 winter wheat fields.
We hypothesized that the field-scale alpha, beta and gamma diversities of weeds should be higher in organic than in conventional fields and in complex than in simple landscapes, and we tested the idea that landscape complexity may compensate for the reduced diversity in conventional fields. In contrast, the relative within-field heterogeneity, beta (%) was expected to be higher in conventional fields, because these fields should have a low alpha diversity and, consequently, the relative contribution of beta to gamma diversity should be high. In particular, we expected that several species (e.g. threatened species of the Red List of Lower Saxony, Germany; Garve & Letschert 1991; Garve 1993; Korneck, Schnittler & Vollmer 1996) that are more susceptible to disturbances than common species would particularly profit from organic farming and/or from a certain degree of landscape complexity (Korneck & Sukopp 1988; Jedicke 1997; Hofmeister & Garve 1998).
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In this study, local management (organic vs. conventional) and complexity of the surrounding landscape had an influence on alpha, beta and gamma diversities of weeds in 24 winter wheat fields. The arable weed gamma diversity in the whole study region was 153 species. This overall diversity was strongly determined by the heterogeneity between the fields, as beta diversity made up c. 65%. Similarly, Wagner, Wildi & Ewald (2000) described low within-field diversity and large between-field diversity in arable fields and attributed this to crop variability. In our study, the crop species was the same in all analysed fields, which underlines the high variability of weed community structure in the region.
At the field scale, the gamma diversity of weeds in the vegetation was higher in organic than in conventional fields, a finding consistent with the results of other studies (Moreby et al. 1994; Hald 1999; Hyvönen et al. 2003). However, our results showed that this was particularly valid in fields located in simple landscapes with a high percentage of arable land, as gamma diversity in conventional fields was strongly positively affected by landscape complexity, which resulted in nearly similar gamma diversities in organic and conventional fields when landscapes were complex with high percentages of non-crop areas. Gamma diversity of organic fields was only weakly related to landscape complexity, suggesting organic fields to be more or less self-sufficient ecosystems, not depending on species immigration from surrounding habitats in complex landscapes. Plant diversity of organic and conventional farms in Sweden showed a similar tendency: in conventional pastures and field margins, diversity tended to increase more steeply with increasing perimeter-to-area ratio than in organic pastures and field margins (Weibull, Östman & Granqvist 2003). Alpha and beta diversities revealed a similar pattern to gamma diversity.
In contrast to broad-leaf diversity, the gamma and beta diversities of grasses in the vegetation did not differ between the farming systems, but were also negatively related with the percentage of arable land. The number of grass species was generally much lower than that of broad-leaves (in the whole study only 18 grass species were found), which may partly explain why the observed differences between organic and conventional fields in broad-leaf diversity could not generally be found for grasses. As all conventional farmers applied herbicides, broad-leaf species may have been more affected by the applications than grasses, as suggested by Hole et al. (2005).
The reasons for the high importance of landscape complexity for the local weed species diversity, especially in conventional fields, might reflect species entering the fields through the seed rain. Unexpectedly, species diversity in the seed rain appeared to be mainly determined by the local vegetation, as it was higher in organic than in conventional fields and not related to percentage arable land. Many seeds appeared to come from the fields themselves, but not from the landscape. This is supported by several studies showing that seeds of many arable weeds are dispersed only a few metres (Rew, Froud-Williams & Boatman 1996; Bischoff & Mahn 2000). However, Fig. 1 shows a trend that, in complex landscapes, diversity was high in both organic and conventional fields, whereas it decreased with increasing percentage of arable land only in conventional fields. Most of the weed species occurring in the seed rain, but not in the vegetation and seed bank (see the Appendix), should be relatively good dispersers because they are either wind or animal dispersed (Kästner, Jäger & Schubert 2001). Their seeds should have had the potential to reach an arable field from the surrounding habitats, which should provide a higher species pool in complex than in simple landscapes.
The gamma and alpha diversities in the seed bank were generally higher in organic than in conventional fields. Long-term effects of farming systems on weed species richness were also shown by Menalled, Gross & Hammond (2001). Additionally, weed species diversity decreased with increasing percentage of arable land but, in contrast to the diversity of the vegetation, the surrounding landscape affected both organic and conventional fields similarly. So, landscape complexity did not seem to determine the weed species diversity of organic fields immediately (i.e. in the vegetation) but revealed the potential to do so in the following years, expressed via the more diverse germinable seed bank in complex landscapes.
In contrast to the absolute beta diversities, the relative beta diversities of weeds in the vegetation, seed rain and seed bank were higher in conventional than in organic fields, irrespective of landscape complexity. This shows the particular importance of species heterogeneity within a field for the gamma diversity of conventional fields, as the mean (alpha) diversity within a field was comparatively low. Heterogeneity in conventional fields may be the result of less intensive herbicide spraying and fertilization at the field edges. Wilson & Aebischer (1995) showed that several weed species in conventional fields declined with increasing distance from the field edges, and Hald (1999) found a gradient in species density from edge to centre in conventional, but not organic, fields.
The number of Red List species in the vegetation was shown to be higher in complex than in simple landscapes, indicating a high importance of alternative habitats in the surrounding landscape for these species. However, numbers of species did not differ between organic and conventional fields. Thus, our study only partly supports the general opinion that the decline of particular species is caused by intensive agricultural practices at the field scale and the simplification of landscape complexity (Korneck & Sukopp 1988; Jedicke 1997; Hofmeister & Garve 1998). The unexpectedly similar numbers of Red List species in organic and conventional farming may result from a gradual decrease in the land-use intensity of conventional fields, which seemed to be irrespective of landscape complexity (see the Methods). For example, the fertilizer consumption in Germany decreased from c. 3 million t (1991) to c. 2·6 million t (2001) and herbicide consumption from c. 18 000 t (1991) to c. 15 000 t (2001) (Food and Agriculture Organization of the United Nations 2001). However, out of the 23 Red List species 21 were found in organic fields and only half (10 species) in conventional fields.
The results of multiple logistic regressions showed that only very few single species depended on landscape complexity and/or organic farming. Thus, higher species numbers found in complex landscapes and in organic farming cannot only be attributed to single species depending on such landscapes and/or organic farming. Which particular species have contributed to higher species numbers in complex landscapes is more likely to be a matter of chance. This might suggest mass effect (Shmida & Wilson 1985; Auerbach & Shmida 1987; Palmer 1992), which explains high local species richness with continuous immigration from nearby but dissimilar habitats. One characteristic of many arable weed species is their ability to quickly colonize and survive in ruderal and disturbed habitats. Thus, the probability that many species randomly immigrate into a field is obviously greater when the proportion of alternative habitats in the landscape is increased. Eight species could be shown to depend on organic farming. These species were hemicryptophytes or geophytes and some of them were legumes, which may have profited from diverse crop rotations including perennial crops for green manure. Legumes should be more competitive in organic than in conventional fields because of the absent mineral nitrogen input.
Our results have implications for the future management of both arable fields and landscape complexity to conserve species diversity of arable weeds, which are important components of the biodiversity in agricultural landscapes (Marshall et al. 2003). Organic farming generally promoted species diversity of arable weeds and the surrounding landscape was important for the seed bank. In conventional fields, species diversity strongly increased with increasing landscape complexity, thereby generating nearly similar diversity levels as in organic fields when the surrounding landscape was complex. Hence, organic farming contributed most effectively to weed species diversity in simple agricultural landscapes. Therefore, the conversion of conventional to organic farming should be supported in these areas, especially where it will be particularly effective. This is in contrast to the present uptake of agri-environment schemes, which is highest in areas where biodiversity is already relatively high and lowest where biodiversity is low (Kleijn & Sutherland 2003). Promoting non-crop habitats in agricultural landscapes as refugia for weed species appears to be of particular importance for landscape management, especially when organically managed fields are rare. Moreover, differences in species composition (beta diversity) have been shown to be very large within fields (particularly in conventional farming) and between fields, thereby making an important contribution to overall diversity (gamma diversity) at the field and regional scales. Future management policies should therefore take into account the heterogeneity in community composition at different spatial scales.