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Suitability of arable weeds as indicator organisms to evaluate species conservation effects of management in agricultural ecosystems
Agriculture, Ecosystems and Environment 98 (2003) 201–211
Suitability of arable weeds as indicator organisms to evaluate species conservation effects of management in agricultural ecosystems Harald Albrecht∗ Vegetation Ecology, Department of Ecology, TU Muenchen-Weihenstephan, 85350 Freising, Germany
Abstract The overall objective of this study is to examine the application of arable weeds as indicator organisms of biodiversity in agro-ecosystems to evaluate species conservation effects of management practices. Both investigations of interactions between weeds with heterotrophic consumers and strong overall correlations between the number of weed species and the total species diversity indicate that arable weeds are “key species”, the loss of which leads to serious changes in the remaining biocoenosis via habitat and food chain relations. The assessment of the value of management measures for species conservation presupposes a strong relation of target organisms to land use practice. In arable fields, the high percentage of dormant seeds reduces the relevance of single cultivation measures against weed populations and emphasizes the significance of long lasting management including cultivation systems, crop rotations, or field edge effects. A comparison of the number of weed species found at different times and frequencies of sampling indicates—especially in herbicide treated fields—the importance of two recording times—one before the application and one before harvest. This ensures a better estimation of the total species spectrum. To evaluate plant species diversity in fields the number of characteristic arable weeds is proposed. In contrast to the total number of species, the typical arable weeds do not include species frequently occurring outside fields. Thus, several highly noxious species like Cirsium arvense, Elymus repens and Galium aparine are not positively valued. Differences in rarity and usefulness could lead to a more sophisticated evaluation of single weed species and the species spectrum. The use of different number of species as threshold values for different soil types cannot be recommended, however. Programs which fund the results of management necessitate control measures. In Germany, which has an arable area of 11.8 million ha and an estimated average field size of 4 ha, 300 000 sites must be controlled per year when 10% of the farmers were to participate in a corresponding program. Calculating costs of 50 per site, which includes two vegetation relevés, 15 million have to be spent each year. Another possibility to increase agro-biodiversity are programs which pay for the application of specific management practices (reduced fertilization, tillage, and weed control, measures of crop selection and rotation) or management systems like organic farming. As the corresponding control is less time consuming such programs are less expansive. Their positive effects on biodiversity are less specific and less reliable, however. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Weeds; Indicator; Species conservation; Biodiversity
∗ Tel.: +49-8161-713717; fax: +49-8161-714143. E-mail address: [email protected] (H. Albrecht).
0167-8809/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-8809(03)00081-1
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1. Introduction More than 38% of the total area of Germany are arable fields and about 16% is grassland (Statistisches Bundesamt, 1998). The majority of the remainder is used for commercial production forest, settlement, and traffic. Thus, agriculture is the most important type of land use in Germany and the importance of agricultural land for recreation, wellbeing, and species diversity is evident. In the course of intensive farming over the last decades this diversity has suffered a severe decline (Meisel, 1984; Blab et al., 1989; Plachter, 1991; Albrecht, 1995). To counteract this development, most of the German federal states took measures to conserve the organism diversity in agro-ecosystems. Well known examples for these activities were the field margin strip program (Schumacher, 1980) and the program for birds breeding in meadows (Hutterer et al., 1993). On a global scale, governments as well as non-governmental organizations began to develop concepts to conserve and increase biodiversity in agricultural ecosystems, following the environmental summit in Rio de Janeiro in 1992 (Nellinger, 2000). Corresponding indicators proposed by OECD (1999) are the diversity of “domesticated” plants and livestock, as well as “wildlife” biodiversity. For particular agro-ecosystems, however, it was considered that further research is needed to find out representative “key indicator” wildlife species. The aim of the present study is to examine the use of arable weeds as indicator organisms of biodiversity in agro-ecosystems to evaluate species conservation effects of management practices. The ecosystematic relevance, the relation to the management, the influence of temporal and spatial variation on the measurability and the time needed for inventory are examination criteria. In addition, the selection of appropriate methods to evaluate species diversity and socioeconomic effects of different conservation strategies are discussed. 2. Ecosystematic relevance “Key species” are defined as those species, the loss of which leads to serious changes in the remaining biocoenosis (Calow, 1998). Arable weeds belong to the carbon autotrophic producers which provide food
for the consumers in agro-ecosystems. The importance of this relation was impressively demonstrated by Heydemann (1983) who found 1200 phytophageous animal species feeding on the 100 most frequent arable weeds. In search of suitable indicator groups for biodiversity in cultivated ecosystems Duelli and Obrist (1998) proposed to calculate the correlation coefficients between the number of species in every taxonomic group and the total number of species. In an exemplary investigation the number of flowering plant species showed—in contrast to most of the other organism groups tested—a highly significant correlation with the total number of arthropod taxa. The investigators concluded that plant species belong to the best indicators for biodiversity evaluation. In contrast, the correlations among the number of individuals of collembola, carabid beetles, skylarks, and weeds recorded in arable fields by Albrecht et al. (2001) were low. These authors contributed their results to evident differences in habitat and food requirements among the investigated groups. The only highly significant correlation was observed between the weeds and the carabid beetles. This correlation was caused by high number of phyto- and polyphageous beetles predominantly sampled in dense weed stands. Generally, these results show that not all organism groups occurring in arable fields are closely related to each other. Nevertheless, they suggest that weeds belong to the group of species with a high ecosystematic relevance.
3. Relation to management practice As a high density of arable weeds can cause severe problems by reducing the yield and the quality of cultivated crops, weed control is an essential element of arable farming. This obvious impact on farm management may rise the question if such species should be considered at all in nature conservation activities. One reason for such efforts is that only a small percentage of the arable weed species cause remarkable infestation damage. Of 306 plant species listed by Hofmeister and Garve (1998) which occur regularly in arable fields, only 26 are defined problematic. The remaining majority scarcely cause losses in yield and significantly contribute to the species diversity
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of arable landscapes. Consequently, it appears justified to consider the conservation of weeds in future strategies for a sustainable land use. Operations like ploughing or herbicide spraying kill most of the weed plants growing on the soil surface thus suggesting that single management measures can cause a severe decline in population density. That these effects are less critical than it appears is demonstrated by a comparison of the aboveground flora to the weed seed bank in soil. Roberts and Ricketts (1979), Barralis and Chadoeuf (1980), and Albrecht and Pilgram (1997) observed that the number of individual plants on the soil surface represented only 1–10% of the total weed numbers including the
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seeds in soil. Thus, arable weed populations seem to have adapted to such management induced collapses through the development of persistent seed banks. They are characterized by a high percentage of seeds remaining dormant in soil while only a few germinate and establish seedlings (Thompson and Grime, 1979). A canonical correspondence analysis (CCA; ter Braak, 1987–1992) which includes data on the actual and recent management (Fig. 1) shows the long term management practice to be an important factor determining the species composition in the weed seed bank. In ordination plots, the distance of two points to each other indicates the similarity of the sites in terms of species composition. It is evident that even
Fig. 1. CCA ordination of the weed seed bank composition at 256 sites on the FAM research station in Scheyern (southern Bavaria) with environmental variables. Particle size = median soil particle size, pH = pH level (CaCl2 ), P2 O5 = P2 O5 concentration in soil (CAL), Nt = Nt content in soil, hop = previous cultivation of hops (cleared 5 years before sampling) or annual arable crops, user = former land use by different farmers (private farmers/Scheyern Abbey), margin = effect of field margins, organic farming = organic or integrated farming for 2 years. Sampling date: February 1995.
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Table 1 The number of characteristic weed species in cereal and root crops under 4–5 years conventional and organic farming in North Rhine-Westphalia; size of sampling plots 100 m2 (from Frieben, 1998, Table 36) Cereals Management Position in field Median number of characteristic weed species
Organic Margin 13
Root crops Organic Center 11
Conventional Margin 5
4 years after cleaning hop plants the weed seed bank composition of former hop fields still differs clearly from the one found in normal arable fields. Furthermore, the length and the proximity of the vector to the first axis indicates that the former cultivation of hops is still the most important variable to differentiate the weed species composition. Another factor obviously affecting the seed bank composition is the former land use of previous farmers. All sites were brought together under one management regime 4 years before the investigation began. In contrast, the change from conventional to organic or integrated farming which took place 2 years before sampling as well as soil factors seem to be of minor importance. That the management practice affects not only the composition but also the number of species can be seen from results of Frieben (1998) in Table 1. Comparing the number of characteristic weed species in North Rhine-Westphalia fields values from plots which were under organic farming for 4–5 years ranged clearly above the corresponding numbers recorded in fields. In general terms, these results indicate that the arable weeds are an organism group which are significantly affected by the type and intensity of management. A high percentage of dormant seeds reduces the importance of single cultivation measures and emphasizes the significance of long term influences like cultivation systems, crop rotations, or field edges. That chemical weed control impacts not only on plants but also on invertebrates and birds by modifying weed abundance and species assemblages is summarized by Boatman (2002) and Brown (2002).
4. Criteria to evaluate weed species diversity The total number of species includes species characteristic for the living conditions in a certain habi-
Conventional Center 4
Organic Margin 14
Organic Center 11
Conventional Margin 5
Conventional Center 0
tat and ubiquitous generalists. As the characteristic species are greatly affected by changes in land use and intensification than the generalists, they should be considered with more importance in nature conservation issues. Another argument to select only the characteristic species for the evaluation of plant species diversity in arable fields is that highly noxious perennials like Cirsium arvense, Elymus repens and Equisetum arvense which frequently occur outside fields are not positively valued. A list of the “characteristic weeds” of German arable fields is given in Table 2. Vegetation relevés from 130 arable fields in seven different landscapes in Bavaria with a standard sampling area of 100 m2 give an impression of the variation in the number of such characteristic species in winter cereals. There, the median number per site was 11 and the 25 and 75% quartiles were 8 and 14, respectively. In total, the values ranged between a minimum of 2 and a maximum of 23. These numbers were recorded in two vegetation relevés, one before weed control in spring and one before harvest in summer. Compared with “normal” arable fields, management intensity on these sites was low and the spectrum of species well developed. Apart from counting the number of species, diversity can also be estimated by calculating the evenness or combining the number of species with their relative abundance (Magurran, 1988). The Shannon and the Simpson index are the most frequently used indices of the latter type (Usher, 1994). To evaluate species diversity for practical nature conservation issues, these indices are disadvantageous for two reasons. The first is that an accurate record of plant numbers for each species is needed which is very time consuming. The second reason is that these indices favor communities with low number of species and individuals. A corresponding example is given in Table 3 where the Shannon index is calculated for two weed communities
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Table 2 List of higher plant species assigned to the plant-sociological sub-class Violenea asrvensis (Hüppe and Hofmeister, 1990)a Adonis aestivalis L. Adonis flammea Jacq. Aethusa cynapium L. Ajuga chamaepytis (L.) Schreb. Allium vineale L. Alopecurus myosuroides Huds. Anagallis arvensis L. Anagallis foemina Mill. Anchusa arvensis (L.) M. Bieb. Androsace maxima L. Anthemis arvensis L. Anthoxanthum aristatum Boiss. Apera spica-venti (L.) P.B. Aphanes arvensis L. Aphanes inexpectata W. Lippert Arnoseris minima (L.) Schweigg. And Körte Asperula arvensis L. Avena fatua L. Bifora radians M. Bieb. Bunium bulbocastanum L. Bupleurum rotundifolium L. Calendula arvensis L. Caucalis platycarpos L. Centaurea cyanus L. Chenopodium polyspermum L. Chrysanthemum segetum L. Conringia orientalis (L.) Dumort. Consolida regalis Gray Digitaria ischaemum (Schreb.) Muhl. Echinochloa crus-galli (L.) P. Beauv. Erucastrum gallicum (Willd.) O.E. Schulz Euphorbia exigua L. Euphorbia helioscopia L. Euphorbia peplus L. Fallopia convolvulus (L.) A. Löve Filago neglecta (Soy.-Will.) DC. Fumaria officinalis L. Fumaria rostellata Knaf Fumaria vaillantii Loisel. Gagea villosa (M.Bieb.) Sweet
Galeopsis speciosa Mill. Galinsoga ciliata (Raf.) S.F.Blake Galinsoga parviflora Cav. Galium spurium L. Galium tricornutum Dandy Geranium dissectum L. Geranium rotundifolium L. Iberis amara L. Kickxia elatine (L.) Dumort. Kickxia spuria (L.) Dumort. Lamium amplexicaule L. Lamium hybridum Vill. Lamium purpureum L. Lathyrus aphaca L. Lathyrus hirsutus L. Lathyrus nissolia L. Legousia hybrida (L.) Delarbre Legousia speculum-veneris (L.) Chaix Linaria arvensis (L.) Desf. Lithospermum arvense L. Matricaria recutita L. Melampyrum arvense L. Mercurialis annua L. Misopates orontium (L.) Raf. Muscari neglectum Guss. Ex Ten. Myagrum perfoliatum L. Myosotis arvensis (L.) Hill Neslia paniculata (L.) Desv. Nigella arvensis L. Nonea pulla (L.) DC. Orobanche ramosa L. Odontites vernus (Bellardi) Dumort. Orlaya grandiflora (L.) Hoffm. Ornithogalum nutans L. Ornithogalum umbellatum L. Oxalis stricta L. Papaver argemone L. Papaver dubium L. Papaver rhoeas L. Polycnemum arvense L.
Persicaria masculosa Gray Ranunculus arvensis L. Raphanus raphanistrum L. Scandix pecten-veneris L. Scleranthus annuus L. Setaria pumila (Poir.) Roem. and Schult. Setaria viridis (L.) P. Beauv. Sherardia arvensis L. Silene linicola C.C.Gmel. Silene noctiflora L. Sinapis arvensis L. Sonchus asper (L.) Hill Spergula arvensis L. Spergularia segetalis (L.) G. Don. Stachys annua L. Stachys arvensis L. Thlaspi alliaceum L. Thlaspi arvense L. Thymelaea passerina (L.) Coss. and Germ. Torilis arvensis (Huds.) Link Tulipa sylvestris L. Turgenia latifolia (L.) Hoffm. Vaccaria hispanica (Mill.) Rauschert Valerianella carinata Loisel. Valerianella rimosa Bastard Veronica agrestis L. Veronica arvensis L. Veronica hederifolia L. Veronica opaca Fr. Veronica persica Poir. Veronica polita Fr V. triphyllos L. Vicia angustifolia L. Vicia hirsuta (L.) Gray Vicia lutea L. Vicia tetrasperma (L.) Schreb. Vicia villosa Roth ssp. Villosa Viola arvensis Murr.
a
This sub-class comprises 118 plant species which have their main habitat in Germany in arable fields and vineyards (=characteristic arable weeds). It does not include therophytic ruderals as well as perennial species frequently occurring outside arable fields. The original list was given by Hüppe and Hofmeister (1990), several species were added according the sociological classification by Oberdorfer (1990) and Hofmeister and Garve (1998). Names of species accord with Wisskirchen and Haeupler (1998).
with different number of individual plants and species. Although plant community A has only 10 species, its Shannon diversity is as higher than the one in community B where 13 species were found. Obviously, the more even distribution of species abundance compensates the lack in the number of species. That number of individuals are more evenly distributed among
species when the application of herbicides is intensively confirmed by the seed bank investigations of Squire et al. (2000). They observed that the number of species slightly more than doubled as herbicide applications were reduced while the total number of seeds increased by two orders of magnitude. Consequently, indices integrating the evenness of plant species cannot
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Table 3 Shannon index of two differently structured arable weed communities Species
Capsella bursa-pastoris Chenopodium album Tripleurospermum perforatum Taraxacum officinale Myosotis arvensis Polygonum aviculare Galinsoga ciliata Stellaria media Poa annua Viola arvensis Apera spica-venti Matricaria recutita Centaurea cyanus Anthemis arvensis Number of species Number of individual plants Shannon index Evenness
Plant community A (plants m−2 )
B (plants m−2 )
1 1 1 1 1 1 1 2 1 1
2 1 1
10 11 2.27 0.99
1 2 1 2 2 14 1 17 1 1 13 46 1.86
be recommended as a criterion to evaluate biodiversity in this ecosystem type. Another useful criterion for a more sophisticated evaluation of single weed species is their degree of endangerment. As 42 of the 118 species from Table 2 are listed in the red data book of Germany and another 41 species are listed in the corresponding books of single federal states (Korneck et al., 1996), arable weeds belong to the vegetation types with the highest percentage of endangered plant species in Germany. A second criterion for a differentiated evaluation on the species level may be the number of plant species which favor useful insects. Frieben (1998) prepared a list of such species named in literature and found that their numbers varied between 0 and 10 on an area of 100 m2 , depending on the management system. Instead of using “static” criteria like the number of species occurring on a certain area, Pfadenhauer et al. (1997) recommended “dynamic” indicators to evaluate the efficacy of management measures for nature conservation issues. As lots of weeds possess no efficient strategies for an active dispersal over long distances and passive transport by management was greatly reduced by land use changes during the last decades, the dispersal rate could be such an indicator. Thus, farmers
would not be paid for high number of species on their fields but for applying management practices which intensify the spread of species. Unfortunately, knowledge on the efficacy and workability of such measures is poor up to now.
5. Temporal variation Great temporal variation in the apparent part of populations is a factor which makes it generally difficult to record organisms occurring in agricultural ecosystems. One important reason for the differences between the number of weed species found in different arable crops under central European climate conditions is the seasonal variation of temperature. This is caused by cold spells in autumn and early spring favoring weeds with a strong adaptation to germination at low temperatures (Otte, 1996). As these species are unable to germinate in summer, they predominantly occur in winter annual stands. Thus, winter annual crops potentially inhabited by a higher number of species are recommended for sampling. When looking for well developed weed stands within the winter annual crops, autumn sown stands, where weed control is carried out in spring, are favorable as herbicide spraying at sowing time prevents an early development of weeds. In later stages the weeds are suppressed by light and nutrient competition of the crops. This especially applies to oil seed rape where weed sampling is significantly affected by the early herbicide application in autumn. To get representative information on the composition of plant communities, repeated sampling is frequently needed. Fig. 2 illustrates the relation between the frequency of sampling and the number of weed taxa observed in the 130 arable fields in Bavaria, the same which were used for the analysis in Section 4. It can be seen that adding one relevé carried out in early spring to the usual sample made before harvest in summer increased the number of species by almost 30%. Both natural and anthropogenic influences may be the reason behind this increase. Weeds may have faded in the course of weed control after the investigation in spring; thus a human influence. However, natural influences can cause withering of plants long before harvest time in summer. A corresponding
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Fig. 2. Median numbers (with 95% intervals of confidence) of species recorded in 1, 2, 3 and 7 series of vegetation relev´es in 130 winter cereal fields in seven different landscapes in Bavaria (see Fig. 3). The first series was carried out before harvest in summer, the second and third additionally comprise species observed before herbicide application in spring and during the flowering period in early summer, respectively. The right columns record the number of species found in seven relev´es in different crops over 3 continuous years with or without an additional seed bank analysis. The “char” indicates characteristic species according to Table 2, while “total” includes all plant species found at the site.
example is Veronica triphyllos which occurs in German arable fields predominantly on coarse grained soils. This species escapes the summer drought on such substrates by early germination, flowering and fructification (Albrecht et al., 1999). These results demonstrate especially in herbicide treated fields that at least two records—one before the application and one before harvest—are necessary to give a general image of the total species spectrum. As there were no additional species counted in a third series of relevés carried out during the flowering period of cereals suggests that this effort was not worthwhile. In contrast, another significant increase in the number of species was observed when the relevés were repeated seven times over 3 years in different crops and when additional seed bank analyses were made (columns 7–10 in Fig. 2). Thus, sampling in different years and crops in combination with investiga-
tions in the soil seed bank provide very detailed information on the weed flora. Unfortunately, economic circumstances limit such precision in large scale survey programs.
6. Spatial variation Spatial variation is another factor which may aggravate an accurate evaluation of the species diversity in arable weed communities. At a field scale, this variation can be caused by differences in natural site conditions and former management. To overcome this problem using a sophisticated sampling method, Frieben (1998) recommended to record the species observed when crossing the field until no more new species can be found. Doubtlessly, this method is precise but also very time intensive.
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To reduce this effort, most samples for the floristic description of the weed vegetation in arable fields in central Europe were made according to the method of the Zurich-Montpellier school (Braun-Blanquet, 1964). This method is based on the concept of minimal area and species–area curves. It involves doubling the size of the sampling area in a homogeneous stand until no new species are recorded. In practice, this instruction led to remarkable differences in the area researchers used for their vegetation relevés. Sampling areas reported in the corresponding literature vary from a minimum of 2 m2 (Kulp, 1993) to a maximum of 1500 m2 (Plakholm, 1989). As a consequence, the results cannot be compared with each other. To overcome this problem, a standardization of the sampling area is needed. According to Frieben (1998) most of the species occurring under homogeneous site and management conditions in arable fields can be found on an area of 100 m2 . Thus, this sampling area should be used to unify the method for vegetation records. Conditions particularly favorable for high number of species and population densities can be found near field margins. Investigations of Marshall (1989), van Elsen (1994), and Wilson and Aebischer (1995) have shown that the majority of species are most abundant in the outermost meter and thin out sharply within the first 4 m from the field edge. A better supply of
light (van Elsen, 1994, p. 157), lower crop yields and management intensity (Boatman and Sotherton, 1988), and the possibility to survive an to re-invade from neighboring habitats (Wilson and Aebischer, 1995) favor a high species diversity in the margin area. Using this effect for nature conservation issues, programs have been developed in which farmers were paid for limiting herbicide and fertilizer use in field margin strips (Schumacher, 1980). In Germany, these programs were introduced in most of the federal states and covered a total area of approximately 5000 ha in the beginning of the 1990s (Wicke, 1998). This area totaled to 0.04% of the arable land of the FRG. Since the sites included in these programs were continuously controlled and selected for their floristic inventory, many fields with well developed weed communities and with rare species listed in red data books could be included (e.g. Mattheis and Otte, 1994; Frieben, 1995). For farmers, however, managing the field margin and the field center differently is time consuming and can cause crop rotation and weed infestation problems (Boatman and Sotherton, 1988). Thus, the programs were only accepted in conjunction with a suitable compensation payment. As the expenses for the field margin strip programs of the German federal states were reduced since 1992, a severe set back in the area included was observed (Wicke, 1998). To
Fig. 3. Median numbers (with 95% confidence intervals) of characteristic arable weed species in seven different arable regions in Bavaria. Records were made in winter cereal fields with two relev´es per site apart from the field margins. (1) Plant sociological communities as described by Hofmeister and Garve (1998).
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increase the integration into the production process and make it more acceptable for the farmers, an extension of the protected area from the field margin strips to the whole fields could be advantageous. At a large landscape scale arable weed communities are shaped by site conditions and—closely related to the site conditions—by the type and intensity of management. Fig. 3 shows that the median number of characteristic species observed in winter cereal crops in different regions of Bavaria varied between 9 and 15. That sandy and loamy soils did not significantly differ in their number of typical weeds (sandy sites in the Nuremberg basin and in the Tertiärhügelland were compared to the loamy soil type in the Aisch valley and in the Tertiärhügelland using the U-test by Mann–Whitney) is confirmed by the results of Frieben (1998), who investigated corresponding sites in northwestern Germany. The median value for the arable fields on limestone substrates (Lech valley, Main valley and Munich plain), however, significantly exceeded the number of species found on the other soil types. As this observation goes along with unpublished results cited by Frieben (1998), there could be a higher threshold recommended for arable fields on limestone substrates.
7. Economic implications For the realization of nature conservation issues the costs and time expense for corresponding measures are of importance. Programs which pay for the results of management like a high number of species or the presence of rare weeds necessitate control measures. To estimate the costs of such a control, an approximate calculation was drawn up for the Federal Republic of Germany. This country has an arable area of 11.8 million ha and an estimated average field size of 4 ha. Given a participation of 10% in this program, 300 000 arable fields must be controlled. Vegetation ecologists report that one vegetation relevé in a normal arable field takes about 30 min. Thus, the time needed for the two relevés proposed is 60 min. Further reduction of time is possible by leaving out the estimation of cover abundance, using pre-printed survey forms, and finishing the investigation when a certain number of plants is found. Thus, time to reach the sites is often greater than the time
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needed to record information. Calculating the price of two records per site (=field) at 50, 15 million have to be spent each year for the program. Another possibility to increase agro-biodiversity not discussed in detail are programs which pay for the application of specific management practices (reduced fertilization, tillage, and weed control, measures of crop selection and rotation) or management systems like organic farming. As corresponding control is less time consuming such programs cause lower costs. Their positive effects on biodiversity are less specific and less reliable, however (e.g. Albrecht and Mattheis, 1998).
8. Conclusions Both a high sensitivity to cultivation measures and a strong relation to other organism groups make weeds suitable indicators to evaluate management effects on wildlife diversity in arable fields. Considerations presented in Chapter 8 show that recording weed assemblages could feasibly be carried out even in a high number of fields. It remains however uncertain, whether nature conservation authorities are willing to pay several million Euros each year for a system to control the maintenance of biodiversity at a certain level. Thus, a regular weed inventory may remain limited to areas with a high species conservation value (high number of species, occurrence of rare and endangered species). In fields where such a regular control is not possible, specific cultivation measures or management systems would have to take on an indicator function of wildlife biodiversity. This means that applying management systems like organic farming or cultivation patterns like minimum tillage should guarantee a certain level of species diversity. As management systems like organic farming comprise a broad spectrum of diverging cultivation measures, fields of different farms can show a great variation in species diversity (Becker and Hurle, 1998). Thus, single measures like the application of fertilizers or certain herbicides may show a stronger correlation to species diversity. Unfortunately, little is known about these effects on weed and wildlife diversity in arable fields at present Marshall et al. (2002) and should consequently be the focus of future arable ecosystem research. Proposals for evaluating
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the corresponding results were given in the present paper. References Albrecht, H., 1995. Changes in the arable weed flora of Germany during the last five decades. In: Proceedings of the Ninth European Weed Research Society Symposium on Challenges for Weed Science in a Changing Europe, Budapest, pp. 41–48. Albrecht, H., Mattheis, A., 1998. The effect of organic and integrated farming on rare arable weeds on the Forschungsverbund Agrarökosysteme München (FAM) research station in southern Bavaria. Biol. Conserv. 86, 347–356. Albrecht, H., Pilgram, M., 1997. The weed seed bank in a landscape segment in southern Bavaria. II. Relation to environmental factors and to the soil surface vegetation. Plant Ecol. 131, 31–43. Albrecht, H., Mayer, F., Mattheis, A., 1999. Veronica triphyllos L. in the Tertiärhügelland landscape in southern Bavaria—an example for habitat isolation of a stenoeceous plant species in agroecosystems. Zeitschrift für Ökologie und Naturschutz 8, 219–226. Albrecht, H., Kühn, N., Filser, J., 2001. Site effects on plant and animal distribution at the Scheyern experimental farm. Ecol. Stud. 147, 209–227. Barralis, G., Chadoeuf, R., 1980. Etude de la dynamique d’une communauté adventice. I. Evolution de la flore adventice au cours du cycle végétatif d’une culture. Weed Res. 20, 231–237. Becker, B., Hurle, K., 1998. Unkrautflora auf Feldern mit unterschiedlich langer ökologischer Bewirtschaftung. Zeitschrift für Pflanzenkrankheiten und Pflanzenschutz, Sonderheft XVI, 155–161. Blab, J., Bless, R., Nowak, E., Rheinwald, G., 1989. Veränderungen und neue Entwicklungen im Gefährdungs- und Schutzstatus der Wirbeltiere in der Bundesrepublik Deutschland. Schriftenreihe für Landschaftspflege und Naturschutz 29, 9–37. Boatman, N.D., 2002. Relationship between weeds, herbicides and birds. In: Marshall, E.J.P., Brown, V.K., Boatman, N.D., Lutman, P.J.W., Squire, G.R. (Eds.), The Impact of Herbicides on Weed Abundance and Biodiversity. DEFRA Desk Study PN0940, pp. 85–102. Boatman, N.D., Sotherton, N.W., 1988. The agronomic consequences and costs of managing field margins for game and wildlife conservation. Aspects Appl. Biol. 17, 47–56. Braun-Blanquet, J., 1964. Pflanzensoziologie, 3rd ed. Springer, Wien. Brown, V.K., 2002. The impact of herbicides on invertebrates. In: Marshall, E.J.P., Brown, V.K., Boatman, N.D., Lutman, P.J.W., Squire, G.R. (Eds.), The Impact of Herbicides on Weed Abundance and Biodiversity. DEFRA Desk Study PN0940, pp. 59–84. Calow, P. (Ed.), 1998. The Encyclopedia of Ecology and Environmental Management. Blackwell, Oxford. Duelli, P., Obrist, M.K., 1998. In search of the best correlates for local organismal biodiversity in cultivated areas. Biodivers. Conserv. 7, 297–309.
Frieben, B., 1995. Effizienz des Schutzprogrammes für Ackerwildkräuter. Mitteilungen, Schriftenreihe der Landesanstalt für Ökologie, Landschaftsentwicklung und Forstplanung NRW 4/95, pp. 14–19. Frieben, B., 1998. Verfahren zur Bestandsaufnahme und Bewertung von Betrieben des organischen Landbaues im Hinblick auf Biotop- und Artenschutz und die Stabilisierung des Agrarökosystems. D. Köster, Berlin. Heydemann, B., 1983. Aufbau von Ökosystemen im Agrarbereich und ihre langfristigen Veränderungen. Daten und Dokumente zum Umweltschutz 35, 53–83. Hofmeister, H., Garve, E., 1998. Lebensraum Acker, 2nd ed. Parey, Hamburg. Hüppe, J., Hofmeister, H., 1990. Syntaxonomische Fassung und Übersicht über die Ackerunkrautgesellschaften der Bundesrepublik Deutschland. Berichte der Reinhold Tüxen-Gesellschaft 2, 61–61. Hutterer, C.-P., Briemle, G., Fink, C. (Eds.), 1993. Wiesen, Weiden und anderes Grünland. Weitbrecht, Stuttgart. Korneck, D., Schnittler, D.M., Vollmer, I., 1996. Rote Liste der Farn- und Blütenpflanzen (Pteridophyta et Spermatophyta) Deutschlands. Schriftenreihe für Vegetationskunde 28, 21–187. Kulp, K.-G., 1993. Vegetationskundliche und experimentell-ökologische Untersuchungen der Lammkraut-Gesellschaft in Nordwestdeutschland. Dissertationes Botanicae 198. Cramer/ Borntraeger, Stuttgart. Magurran, A.E., 1988. Ecological Diversity and Its Measurement. Chapman & Hall, London. Marshall, E.J.P., 1989. Distribution patterns of plants associated with arable field edges. J. Appl. Ecol. 26, 247–258. Marshall, E.J.P., Brown, V.K., Boatman, N.D., Lutman, P.J.W., Squire, G.R., 2002. The Impact of Herbicides on Weed Abundance and Biodiversity. DEFRA Desk Study PN0940. Mattheis, A., Otte, A., 1994. Ergebnisse der Erfolgskontrollen zum Ackerrandstreifenprogramm im Regierungsbezirk Oberbayern 1995–1991. Schriftenreihe der Stiftung zum Schutz gefährdeter Pflanzen 5, 56–71. Meisel, K., 1984. Landwirtschaft und “Rote Liste”-Pflanzenarten. Natur und Landschaft 59, 301–307. Nellinger, L., 2000. Politische Anforderungen und Vorgaben für Indikatoren im Bereich Landwirtschaft und Biodiversität. Symposium Indikatorenfindung für eine nachhaltige Landwirtschaft in den Bereichen Landschaft und Biodiversität, Freising, 1. Oberdorfer, E., 1990. Pflanzensoziologische Exkursionsflora, 6th ed. Ulmer, Stuttgart. OECD, 1999. Environmental Indicators for Agriculture, vol. 2, Issues and Design. In: Proceedings of the York Workshop. OECD Publications, Paris. Otte, A., 1996. Populationsbiologischen Parameter zur Kennzeichnung von Ackerwildkräutern. Zeitschrift für Pflanzenkrankheiten und Pflanzenschutz, Sonderheft XV, 45–60. Pfadenhauer, J., Albrecht, H., Auerswald, K., 1997. Naturschutz in der Agrarlandschaft - Perspektiven aus dem Forschungsverbund Agrarökosysteme München (FAM). Berichte Reinhold-TüxenGesellschaft 9, 49–59. Plachter, H., 1991. Naturschutz. Gustav Fischer UTB, Stuttgart.
H. Albrecht / Agriculture, Ecosystems and Environment 98 (2003) 201–211 Plakholm, G., 1989. Unkrauterhebungen in biologisch und konventionell bewirtschafteten Getreideäckern Oberösterreichs. Thesis Univ. Agricultural Sciences, Vienna. Roberts, H.A., Ricketts, M.E., 1979. Quantitative relationships between the weed flora after cultivation and the seed population in the soil. Weed Res. 19, 269–275. Schumacher, W., 1980. Schutz und Erhaltung gefährdeter Ackerwildkräuter durch Integration von landwirtschaftlicher Nutzung und Naturschutz. Natur und Landschaft 55, 447–453. Squire, G.R., Rodger, S., Wright, G., 2000. Community-scale seedbank response to less intense rotation and reduced herbicide input at three sites. Ann. Appl. Biol. 136, 47–57. Statistisches Bundesamt, 1998. Land- und Forstwirtschaft, Fischerei. Fachserie 3, Reihe 5.1 Bodenfläche nach Art der tatsächlichen Nutzung 1997. Metzler-Poeschel, Stuttgart. ter Braak, C.J.F., 1987–1992. CANOCO—A FORTRAN Program for Canonical Community Ordination by [partial][detrended][canonical] Correspondence Analysis, Principal Components Analysis and Redundancy Analysis. Microcomputer Power, Ithaca, NY.
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Thompson, K., Grime, J.P., 1979. Seasonal variation in the seed banks of herbaceous species in ten contrasting habitats. J. Ecol. 67, 893–921. Usher, M.B., 1994. Erfassung und Bewertung von Lebnesräumen: Merkmale, Kriterien, Werte. In: Usher, M.B., Erz, W. (Eds.), Erfassen und Bewerten im Naturschutz. Quelle & Meyer/UTB, Heidelberg. van Elsen, T., 1994. Die Fluktuation von AckerwildkrautGesellschaften und ihre Beeinflussung durch Fruchtfolge und Bodenbearbeitungs-Zeitpunkt. Ökologie und Umweltsicherung 9/94. Wicke, G., 1998. Stand der Ackerrandstreifenprogramme in Deutschland. Schriftenreihe Landesanstalt für Pflanzenbau und Pflanzenschutz, Mainz 6, 55–84. Wilson, P.J., Aebischer, N.J., 1995. The distribution of dicotyledoneous arable weeds in relation to distance from the field edge. J. Appl. Ecol. 32, 295–310. Wisskirchen, R., Haeupler, H., 1998. Standardliste der Farn- und Blütenpflanzen Deutschlands. Ulmer, Stuttgart.
Suitability of arable weeds as indicator organisms to evaluate species conservation effects of management in agricultural ecosystems Harald Albrecht∗ Vegetation Ecology, Department of Ecology, TU Muenchen-Weihenstephan, 85350 Freising, Germany
Abstract The overall objective of this study is to examine the application of arable weeds as indicator organisms of biodiversity in agro-ecosystems to evaluate species conservation effects of management practices. Both investigations of interactions between weeds with heterotrophic consumers and strong overall correlations between the number of weed species and the total species diversity indicate that arable weeds are “key species”, the loss of which leads to serious changes in the remaining biocoenosis via habitat and food chain relations. The assessment of the value of management measures for species conservation presupposes a strong relation of target organisms to land use practice. In arable fields, the high percentage of dormant seeds reduces the relevance of single cultivation measures against weed populations and emphasizes the significance of long lasting management including cultivation systems, crop rotations, or field edge effects. A comparison of the number of weed species found at different times and frequencies of sampling indicates—especially in herbicide treated fields—the importance of two recording times—one before the application and one before harvest. This ensures a better estimation of the total species spectrum. To evaluate plant species diversity in fields the number of characteristic arable weeds is proposed. In contrast to the total number of species, the typical arable weeds do not include species frequently occurring outside fields. Thus, several highly noxious species like Cirsium arvense, Elymus repens and Galium aparine are not positively valued. Differences in rarity and usefulness could lead to a more sophisticated evaluation of single weed species and the species spectrum. The use of different number of species as threshold values for different soil types cannot be recommended, however. Programs which fund the results of management necessitate control measures. In Germany, which has an arable area of 11.8 million ha and an estimated average field size of 4 ha, 300 000 sites must be controlled per year when 10% of the farmers were to participate in a corresponding program. Calculating costs of 50 per site, which includes two vegetation relevés, 15 million have to be spent each year. Another possibility to increase agro-biodiversity are programs which pay for the application of specific management practices (reduced fertilization, tillage, and weed control, measures of crop selection and rotation) or management systems like organic farming. As the corresponding control is less time consuming such programs are less expansive. Their positive effects on biodiversity are less specific and less reliable, however. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Weeds; Indicator; Species conservation; Biodiversity
∗ Tel.: +49-8161-713717; fax: +49-8161-714143. E-mail address: [email protected] (H. Albrecht).
0167-8809/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-8809(03)00081-1
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1. Introduction More than 38% of the total area of Germany are arable fields and about 16% is grassland (Statistisches Bundesamt, 1998). The majority of the remainder is used for commercial production forest, settlement, and traffic. Thus, agriculture is the most important type of land use in Germany and the importance of agricultural land for recreation, wellbeing, and species diversity is evident. In the course of intensive farming over the last decades this diversity has suffered a severe decline (Meisel, 1984; Blab et al., 1989; Plachter, 1991; Albrecht, 1995). To counteract this development, most of the German federal states took measures to conserve the organism diversity in agro-ecosystems. Well known examples for these activities were the field margin strip program (Schumacher, 1980) and the program for birds breeding in meadows (Hutterer et al., 1993). On a global scale, governments as well as non-governmental organizations began to develop concepts to conserve and increase biodiversity in agricultural ecosystems, following the environmental summit in Rio de Janeiro in 1992 (Nellinger, 2000). Corresponding indicators proposed by OECD (1999) are the diversity of “domesticated” plants and livestock, as well as “wildlife” biodiversity. For particular agro-ecosystems, however, it was considered that further research is needed to find out representative “key indicator” wildlife species. The aim of the present study is to examine the use of arable weeds as indicator organisms of biodiversity in agro-ecosystems to evaluate species conservation effects of management practices. The ecosystematic relevance, the relation to the management, the influence of temporal and spatial variation on the measurability and the time needed for inventory are examination criteria. In addition, the selection of appropriate methods to evaluate species diversity and socioeconomic effects of different conservation strategies are discussed. 2. Ecosystematic relevance “Key species” are defined as those species, the loss of which leads to serious changes in the remaining biocoenosis (Calow, 1998). Arable weeds belong to the carbon autotrophic producers which provide food
for the consumers in agro-ecosystems. The importance of this relation was impressively demonstrated by Heydemann (1983) who found 1200 phytophageous animal species feeding on the 100 most frequent arable weeds. In search of suitable indicator groups for biodiversity in cultivated ecosystems Duelli and Obrist (1998) proposed to calculate the correlation coefficients between the number of species in every taxonomic group and the total number of species. In an exemplary investigation the number of flowering plant species showed—in contrast to most of the other organism groups tested—a highly significant correlation with the total number of arthropod taxa. The investigators concluded that plant species belong to the best indicators for biodiversity evaluation. In contrast, the correlations among the number of individuals of collembola, carabid beetles, skylarks, and weeds recorded in arable fields by Albrecht et al. (2001) were low. These authors contributed their results to evident differences in habitat and food requirements among the investigated groups. The only highly significant correlation was observed between the weeds and the carabid beetles. This correlation was caused by high number of phyto- and polyphageous beetles predominantly sampled in dense weed stands. Generally, these results show that not all organism groups occurring in arable fields are closely related to each other. Nevertheless, they suggest that weeds belong to the group of species with a high ecosystematic relevance.
3. Relation to management practice As a high density of arable weeds can cause severe problems by reducing the yield and the quality of cultivated crops, weed control is an essential element of arable farming. This obvious impact on farm management may rise the question if such species should be considered at all in nature conservation activities. One reason for such efforts is that only a small percentage of the arable weed species cause remarkable infestation damage. Of 306 plant species listed by Hofmeister and Garve (1998) which occur regularly in arable fields, only 26 are defined problematic. The remaining majority scarcely cause losses in yield and significantly contribute to the species diversity
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of arable landscapes. Consequently, it appears justified to consider the conservation of weeds in future strategies for a sustainable land use. Operations like ploughing or herbicide spraying kill most of the weed plants growing on the soil surface thus suggesting that single management measures can cause a severe decline in population density. That these effects are less critical than it appears is demonstrated by a comparison of the aboveground flora to the weed seed bank in soil. Roberts and Ricketts (1979), Barralis and Chadoeuf (1980), and Albrecht and Pilgram (1997) observed that the number of individual plants on the soil surface represented only 1–10% of the total weed numbers including the
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seeds in soil. Thus, arable weed populations seem to have adapted to such management induced collapses through the development of persistent seed banks. They are characterized by a high percentage of seeds remaining dormant in soil while only a few germinate and establish seedlings (Thompson and Grime, 1979). A canonical correspondence analysis (CCA; ter Braak, 1987–1992) which includes data on the actual and recent management (Fig. 1) shows the long term management practice to be an important factor determining the species composition in the weed seed bank. In ordination plots, the distance of two points to each other indicates the similarity of the sites in terms of species composition. It is evident that even
Fig. 1. CCA ordination of the weed seed bank composition at 256 sites on the FAM research station in Scheyern (southern Bavaria) with environmental variables. Particle size = median soil particle size, pH = pH level (CaCl2 ), P2 O5 = P2 O5 concentration in soil (CAL), Nt = Nt content in soil, hop = previous cultivation of hops (cleared 5 years before sampling) or annual arable crops, user = former land use by different farmers (private farmers/Scheyern Abbey), margin = effect of field margins, organic farming = organic or integrated farming for 2 years. Sampling date: February 1995.
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Table 1 The number of characteristic weed species in cereal and root crops under 4–5 years conventional and organic farming in North Rhine-Westphalia; size of sampling plots 100 m2 (from Frieben, 1998, Table 36) Cereals Management Position in field Median number of characteristic weed species
Organic Margin 13
Root crops Organic Center 11
Conventional Margin 5
4 years after cleaning hop plants the weed seed bank composition of former hop fields still differs clearly from the one found in normal arable fields. Furthermore, the length and the proximity of the vector to the first axis indicates that the former cultivation of hops is still the most important variable to differentiate the weed species composition. Another factor obviously affecting the seed bank composition is the former land use of previous farmers. All sites were brought together under one management regime 4 years before the investigation began. In contrast, the change from conventional to organic or integrated farming which took place 2 years before sampling as well as soil factors seem to be of minor importance. That the management practice affects not only the composition but also the number of species can be seen from results of Frieben (1998) in Table 1. Comparing the number of characteristic weed species in North Rhine-Westphalia fields values from plots which were under organic farming for 4–5 years ranged clearly above the corresponding numbers recorded in fields. In general terms, these results indicate that the arable weeds are an organism group which are significantly affected by the type and intensity of management. A high percentage of dormant seeds reduces the importance of single cultivation measures and emphasizes the significance of long term influences like cultivation systems, crop rotations, or field edges. That chemical weed control impacts not only on plants but also on invertebrates and birds by modifying weed abundance and species assemblages is summarized by Boatman (2002) and Brown (2002).
4. Criteria to evaluate weed species diversity The total number of species includes species characteristic for the living conditions in a certain habi-
Conventional Center 4
Organic Margin 14
Organic Center 11
Conventional Margin 5
Conventional Center 0
tat and ubiquitous generalists. As the characteristic species are greatly affected by changes in land use and intensification than the generalists, they should be considered with more importance in nature conservation issues. Another argument to select only the characteristic species for the evaluation of plant species diversity in arable fields is that highly noxious perennials like Cirsium arvense, Elymus repens and Equisetum arvense which frequently occur outside fields are not positively valued. A list of the “characteristic weeds” of German arable fields is given in Table 2. Vegetation relevés from 130 arable fields in seven different landscapes in Bavaria with a standard sampling area of 100 m2 give an impression of the variation in the number of such characteristic species in winter cereals. There, the median number per site was 11 and the 25 and 75% quartiles were 8 and 14, respectively. In total, the values ranged between a minimum of 2 and a maximum of 23. These numbers were recorded in two vegetation relevés, one before weed control in spring and one before harvest in summer. Compared with “normal” arable fields, management intensity on these sites was low and the spectrum of species well developed. Apart from counting the number of species, diversity can also be estimated by calculating the evenness or combining the number of species with their relative abundance (Magurran, 1988). The Shannon and the Simpson index are the most frequently used indices of the latter type (Usher, 1994). To evaluate species diversity for practical nature conservation issues, these indices are disadvantageous for two reasons. The first is that an accurate record of plant numbers for each species is needed which is very time consuming. The second reason is that these indices favor communities with low number of species and individuals. A corresponding example is given in Table 3 where the Shannon index is calculated for two weed communities
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Table 2 List of higher plant species assigned to the plant-sociological sub-class Violenea asrvensis (Hüppe and Hofmeister, 1990)a Adonis aestivalis L. Adonis flammea Jacq. Aethusa cynapium L. Ajuga chamaepytis (L.) Schreb. Allium vineale L. Alopecurus myosuroides Huds. Anagallis arvensis L. Anagallis foemina Mill. Anchusa arvensis (L.) M. Bieb. Androsace maxima L. Anthemis arvensis L. Anthoxanthum aristatum Boiss. Apera spica-venti (L.) P.B. Aphanes arvensis L. Aphanes inexpectata W. Lippert Arnoseris minima (L.) Schweigg. And Körte Asperula arvensis L. Avena fatua L. Bifora radians M. Bieb. Bunium bulbocastanum L. Bupleurum rotundifolium L. Calendula arvensis L. Caucalis platycarpos L. Centaurea cyanus L. Chenopodium polyspermum L. Chrysanthemum segetum L. Conringia orientalis (L.) Dumort. Consolida regalis Gray Digitaria ischaemum (Schreb.) Muhl. Echinochloa crus-galli (L.) P. Beauv. Erucastrum gallicum (Willd.) O.E. Schulz Euphorbia exigua L. Euphorbia helioscopia L. Euphorbia peplus L. Fallopia convolvulus (L.) A. Löve Filago neglecta (Soy.-Will.) DC. Fumaria officinalis L. Fumaria rostellata Knaf Fumaria vaillantii Loisel. Gagea villosa (M.Bieb.) Sweet
Galeopsis speciosa Mill. Galinsoga ciliata (Raf.) S.F.Blake Galinsoga parviflora Cav. Galium spurium L. Galium tricornutum Dandy Geranium dissectum L. Geranium rotundifolium L. Iberis amara L. Kickxia elatine (L.) Dumort. Kickxia spuria (L.) Dumort. Lamium amplexicaule L. Lamium hybridum Vill. Lamium purpureum L. Lathyrus aphaca L. Lathyrus hirsutus L. Lathyrus nissolia L. Legousia hybrida (L.) Delarbre Legousia speculum-veneris (L.) Chaix Linaria arvensis (L.) Desf. Lithospermum arvense L. Matricaria recutita L. Melampyrum arvense L. Mercurialis annua L. Misopates orontium (L.) Raf. Muscari neglectum Guss. Ex Ten. Myagrum perfoliatum L. Myosotis arvensis (L.) Hill Neslia paniculata (L.) Desv. Nigella arvensis L. Nonea pulla (L.) DC. Orobanche ramosa L. Odontites vernus (Bellardi) Dumort. Orlaya grandiflora (L.) Hoffm. Ornithogalum nutans L. Ornithogalum umbellatum L. Oxalis stricta L. Papaver argemone L. Papaver dubium L. Papaver rhoeas L. Polycnemum arvense L.
Persicaria masculosa Gray Ranunculus arvensis L. Raphanus raphanistrum L. Scandix pecten-veneris L. Scleranthus annuus L. Setaria pumila (Poir.) Roem. and Schult. Setaria viridis (L.) P. Beauv. Sherardia arvensis L. Silene linicola C.C.Gmel. Silene noctiflora L. Sinapis arvensis L. Sonchus asper (L.) Hill Spergula arvensis L. Spergularia segetalis (L.) G. Don. Stachys annua L. Stachys arvensis L. Thlaspi alliaceum L. Thlaspi arvense L. Thymelaea passerina (L.) Coss. and Germ. Torilis arvensis (Huds.) Link Tulipa sylvestris L. Turgenia latifolia (L.) Hoffm. Vaccaria hispanica (Mill.) Rauschert Valerianella carinata Loisel. Valerianella rimosa Bastard Veronica agrestis L. Veronica arvensis L. Veronica hederifolia L. Veronica opaca Fr. Veronica persica Poir. Veronica polita Fr V. triphyllos L. Vicia angustifolia L. Vicia hirsuta (L.) Gray Vicia lutea L. Vicia tetrasperma (L.) Schreb. Vicia villosa Roth ssp. Villosa Viola arvensis Murr.
a
This sub-class comprises 118 plant species which have their main habitat in Germany in arable fields and vineyards (=characteristic arable weeds). It does not include therophytic ruderals as well as perennial species frequently occurring outside arable fields. The original list was given by Hüppe and Hofmeister (1990), several species were added according the sociological classification by Oberdorfer (1990) and Hofmeister and Garve (1998). Names of species accord with Wisskirchen and Haeupler (1998).
with different number of individual plants and species. Although plant community A has only 10 species, its Shannon diversity is as higher than the one in community B where 13 species were found. Obviously, the more even distribution of species abundance compensates the lack in the number of species. That number of individuals are more evenly distributed among
species when the application of herbicides is intensively confirmed by the seed bank investigations of Squire et al. (2000). They observed that the number of species slightly more than doubled as herbicide applications were reduced while the total number of seeds increased by two orders of magnitude. Consequently, indices integrating the evenness of plant species cannot
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Table 3 Shannon index of two differently structured arable weed communities Species
Capsella bursa-pastoris Chenopodium album Tripleurospermum perforatum Taraxacum officinale Myosotis arvensis Polygonum aviculare Galinsoga ciliata Stellaria media Poa annua Viola arvensis Apera spica-venti Matricaria recutita Centaurea cyanus Anthemis arvensis Number of species Number of individual plants Shannon index Evenness
Plant community A (plants m−2 )
B (plants m−2 )
1 1 1 1 1 1 1 2 1 1
2 1 1
10 11 2.27 0.99
1 2 1 2 2 14 1 17 1 1 13 46 1.86
be recommended as a criterion to evaluate biodiversity in this ecosystem type. Another useful criterion for a more sophisticated evaluation of single weed species is their degree of endangerment. As 42 of the 118 species from Table 2 are listed in the red data book of Germany and another 41 species are listed in the corresponding books of single federal states (Korneck et al., 1996), arable weeds belong to the vegetation types with the highest percentage of endangered plant species in Germany. A second criterion for a differentiated evaluation on the species level may be the number of plant species which favor useful insects. Frieben (1998) prepared a list of such species named in literature and found that their numbers varied between 0 and 10 on an area of 100 m2 , depending on the management system. Instead of using “static” criteria like the number of species occurring on a certain area, Pfadenhauer et al. (1997) recommended “dynamic” indicators to evaluate the efficacy of management measures for nature conservation issues. As lots of weeds possess no efficient strategies for an active dispersal over long distances and passive transport by management was greatly reduced by land use changes during the last decades, the dispersal rate could be such an indicator. Thus, farmers
would not be paid for high number of species on their fields but for applying management practices which intensify the spread of species. Unfortunately, knowledge on the efficacy and workability of such measures is poor up to now.
5. Temporal variation Great temporal variation in the apparent part of populations is a factor which makes it generally difficult to record organisms occurring in agricultural ecosystems. One important reason for the differences between the number of weed species found in different arable crops under central European climate conditions is the seasonal variation of temperature. This is caused by cold spells in autumn and early spring favoring weeds with a strong adaptation to germination at low temperatures (Otte, 1996). As these species are unable to germinate in summer, they predominantly occur in winter annual stands. Thus, winter annual crops potentially inhabited by a higher number of species are recommended for sampling. When looking for well developed weed stands within the winter annual crops, autumn sown stands, where weed control is carried out in spring, are favorable as herbicide spraying at sowing time prevents an early development of weeds. In later stages the weeds are suppressed by light and nutrient competition of the crops. This especially applies to oil seed rape where weed sampling is significantly affected by the early herbicide application in autumn. To get representative information on the composition of plant communities, repeated sampling is frequently needed. Fig. 2 illustrates the relation between the frequency of sampling and the number of weed taxa observed in the 130 arable fields in Bavaria, the same which were used for the analysis in Section 4. It can be seen that adding one relevé carried out in early spring to the usual sample made before harvest in summer increased the number of species by almost 30%. Both natural and anthropogenic influences may be the reason behind this increase. Weeds may have faded in the course of weed control after the investigation in spring; thus a human influence. However, natural influences can cause withering of plants long before harvest time in summer. A corresponding
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Fig. 2. Median numbers (with 95% intervals of confidence) of species recorded in 1, 2, 3 and 7 series of vegetation relev´es in 130 winter cereal fields in seven different landscapes in Bavaria (see Fig. 3). The first series was carried out before harvest in summer, the second and third additionally comprise species observed before herbicide application in spring and during the flowering period in early summer, respectively. The right columns record the number of species found in seven relev´es in different crops over 3 continuous years with or without an additional seed bank analysis. The “char” indicates characteristic species according to Table 2, while “total” includes all plant species found at the site.
example is Veronica triphyllos which occurs in German arable fields predominantly on coarse grained soils. This species escapes the summer drought on such substrates by early germination, flowering and fructification (Albrecht et al., 1999). These results demonstrate especially in herbicide treated fields that at least two records—one before the application and one before harvest—are necessary to give a general image of the total species spectrum. As there were no additional species counted in a third series of relevés carried out during the flowering period of cereals suggests that this effort was not worthwhile. In contrast, another significant increase in the number of species was observed when the relevés were repeated seven times over 3 years in different crops and when additional seed bank analyses were made (columns 7–10 in Fig. 2). Thus, sampling in different years and crops in combination with investiga-
tions in the soil seed bank provide very detailed information on the weed flora. Unfortunately, economic circumstances limit such precision in large scale survey programs.
6. Spatial variation Spatial variation is another factor which may aggravate an accurate evaluation of the species diversity in arable weed communities. At a field scale, this variation can be caused by differences in natural site conditions and former management. To overcome this problem using a sophisticated sampling method, Frieben (1998) recommended to record the species observed when crossing the field until no more new species can be found. Doubtlessly, this method is precise but also very time intensive.
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To reduce this effort, most samples for the floristic description of the weed vegetation in arable fields in central Europe were made according to the method of the Zurich-Montpellier school (Braun-Blanquet, 1964). This method is based on the concept of minimal area and species–area curves. It involves doubling the size of the sampling area in a homogeneous stand until no new species are recorded. In practice, this instruction led to remarkable differences in the area researchers used for their vegetation relevés. Sampling areas reported in the corresponding literature vary from a minimum of 2 m2 (Kulp, 1993) to a maximum of 1500 m2 (Plakholm, 1989). As a consequence, the results cannot be compared with each other. To overcome this problem, a standardization of the sampling area is needed. According to Frieben (1998) most of the species occurring under homogeneous site and management conditions in arable fields can be found on an area of 100 m2 . Thus, this sampling area should be used to unify the method for vegetation records. Conditions particularly favorable for high number of species and population densities can be found near field margins. Investigations of Marshall (1989), van Elsen (1994), and Wilson and Aebischer (1995) have shown that the majority of species are most abundant in the outermost meter and thin out sharply within the first 4 m from the field edge. A better supply of
light (van Elsen, 1994, p. 157), lower crop yields and management intensity (Boatman and Sotherton, 1988), and the possibility to survive an to re-invade from neighboring habitats (Wilson and Aebischer, 1995) favor a high species diversity in the margin area. Using this effect for nature conservation issues, programs have been developed in which farmers were paid for limiting herbicide and fertilizer use in field margin strips (Schumacher, 1980). In Germany, these programs were introduced in most of the federal states and covered a total area of approximately 5000 ha in the beginning of the 1990s (Wicke, 1998). This area totaled to 0.04% of the arable land of the FRG. Since the sites included in these programs were continuously controlled and selected for their floristic inventory, many fields with well developed weed communities and with rare species listed in red data books could be included (e.g. Mattheis and Otte, 1994; Frieben, 1995). For farmers, however, managing the field margin and the field center differently is time consuming and can cause crop rotation and weed infestation problems (Boatman and Sotherton, 1988). Thus, the programs were only accepted in conjunction with a suitable compensation payment. As the expenses for the field margin strip programs of the German federal states were reduced since 1992, a severe set back in the area included was observed (Wicke, 1998). To
Fig. 3. Median numbers (with 95% confidence intervals) of characteristic arable weed species in seven different arable regions in Bavaria. Records were made in winter cereal fields with two relev´es per site apart from the field margins. (1) Plant sociological communities as described by Hofmeister and Garve (1998).
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increase the integration into the production process and make it more acceptable for the farmers, an extension of the protected area from the field margin strips to the whole fields could be advantageous. At a large landscape scale arable weed communities are shaped by site conditions and—closely related to the site conditions—by the type and intensity of management. Fig. 3 shows that the median number of characteristic species observed in winter cereal crops in different regions of Bavaria varied between 9 and 15. That sandy and loamy soils did not significantly differ in their number of typical weeds (sandy sites in the Nuremberg basin and in the Tertiärhügelland were compared to the loamy soil type in the Aisch valley and in the Tertiärhügelland using the U-test by Mann–Whitney) is confirmed by the results of Frieben (1998), who investigated corresponding sites in northwestern Germany. The median value for the arable fields on limestone substrates (Lech valley, Main valley and Munich plain), however, significantly exceeded the number of species found on the other soil types. As this observation goes along with unpublished results cited by Frieben (1998), there could be a higher threshold recommended for arable fields on limestone substrates.
7. Economic implications For the realization of nature conservation issues the costs and time expense for corresponding measures are of importance. Programs which pay for the results of management like a high number of species or the presence of rare weeds necessitate control measures. To estimate the costs of such a control, an approximate calculation was drawn up for the Federal Republic of Germany. This country has an arable area of 11.8 million ha and an estimated average field size of 4 ha. Given a participation of 10% in this program, 300 000 arable fields must be controlled. Vegetation ecologists report that one vegetation relevé in a normal arable field takes about 30 min. Thus, the time needed for the two relevés proposed is 60 min. Further reduction of time is possible by leaving out the estimation of cover abundance, using pre-printed survey forms, and finishing the investigation when a certain number of plants is found. Thus, time to reach the sites is often greater than the time
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needed to record information. Calculating the price of two records per site (=field) at 50, 15 million have to be spent each year for the program. Another possibility to increase agro-biodiversity not discussed in detail are programs which pay for the application of specific management practices (reduced fertilization, tillage, and weed control, measures of crop selection and rotation) or management systems like organic farming. As corresponding control is less time consuming such programs cause lower costs. Their positive effects on biodiversity are less specific and less reliable, however (e.g. Albrecht and Mattheis, 1998).
8. Conclusions Both a high sensitivity to cultivation measures and a strong relation to other organism groups make weeds suitable indicators to evaluate management effects on wildlife diversity in arable fields. Considerations presented in Chapter 8 show that recording weed assemblages could feasibly be carried out even in a high number of fields. It remains however uncertain, whether nature conservation authorities are willing to pay several million Euros each year for a system to control the maintenance of biodiversity at a certain level. Thus, a regular weed inventory may remain limited to areas with a high species conservation value (high number of species, occurrence of rare and endangered species). In fields where such a regular control is not possible, specific cultivation measures or management systems would have to take on an indicator function of wildlife biodiversity. This means that applying management systems like organic farming or cultivation patterns like minimum tillage should guarantee a certain level of species diversity. As management systems like organic farming comprise a broad spectrum of diverging cultivation measures, fields of different farms can show a great variation in species diversity (Becker and Hurle, 1998). Thus, single measures like the application of fertilizers or certain herbicides may show a stronger correlation to species diversity. Unfortunately, little is known about these effects on weed and wildlife diversity in arable fields at present Marshall et al. (2002) and should consequently be the focus of future arable ecosystem research. Proposals for evaluating
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