Suitability of arable weeds as indicator organisms to evaluate species conservation effects of management in agricultural ecosystems

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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.

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.

Section snippets

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

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)

Criteria to evaluate weed species diversity

The total number of species includes species characteristic for the living conditions in a certain habitat 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

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

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.

To

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

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

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