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Biodiversity is the sum of all living organisms including plants, animals and microorganisms in the world or in a particular area (Raven 1994). An additional strength of organic farming systems is their diversity - including the diversity of crops, fields, rotations, landscapes and farm activities (mix of various farm enterprises). Positive effects of enhanced biodiversity on pest prevention have been shown by several authors (Pfiffner and Luka 2003; Wyss et al. 2005; Zehnder et al. 2007). Similar effects of diversified agroecosystems on diseases and better utilization of soil nutrients and water are also likely to occur (Altieri et al. 2005).

In sustainable agricultural systems, biodiversity has fundamental importance by providing a range of biological services including natural enemies. In conventional farming systems, these services are effectively substituted by external inputs.

As biodiversity and consequently genetic diversity are reduced, the integrity of the agro-ecosystem in terms of disease resistance and optimal resource cycling is eroded. The most extreme loss of biodiversity is represented in monocultures. The inherent genetic uniformity in monocultures, especially those with a single uniform variety, is highly susceptible to and unstable against pests, diseases, weeds and all environmental stresses (Geier 2000). Therefore, from a yield point of view, crop diversity is an important tool to minimize crop losses due to diseases, pests, droughts, floods and other adverse external factors and significantly reduces the risk of food shortage in case of crop failure of a particular species within a rotation or mixed-crop stand. Most diseases and pests affect only one crop, and often propagate faster and more extensively if this crop is grown on large, continuous areas. For soil-borne pests and diseases, it is well known that the best prevention is simply to avoid growing the same plant species on the same field too often and the same applies to some pests and diseases that affect the foliage. Such well-established practices within farming systems have long contributed to biodiversity, sustainabil-ity, protection of the abiotic resources and nature preservation, but the effectiveness of other practices is often unknown (Oppermann 2003). For example, the potential risks of transgenic crops, which are also called genetically modified organisms [GMOs] for biodiversity and the environment were overshadowed by the potential benefits in the early phases of commercialization. However, recent scientific assessments concluded that some risks posed by transgenic crops are unique, and that the regulatory system has not been functioning effectively. The major risks include increased resistance to particular pesticides, gene flow into related plant species, and negative effects on non-target organisms. Significant gaps in knowledge, often stemming from missing markets for ecological services, warrant a cautious environmental regulatory approach for transgenic crops (Ervin et al. 2003).

Creating biodiversity within a crop is an organic cropping technique that improves the reliability of food supply. Some communities that traditionally depend on vegetatively propagated root crops such as potatoes, e.g. in the Andes mountains of South America, carefully mix many different genotypes in the field. The most popular ones that give the highest yields or the most palatable tubers are usually the most susceptible to diseases and pests and hence crop failure. However, by mixing them with resistant but lower yielding or less desirable genotypes, a reliable food supply is ensured. The same applies to mixtures of other crop genotypes, which usually have less disease and higher average yield than the same genotypes grown separately (Wolfe 1997). In practice, however, seed is rarely sold as mixtures of species or varieties. Most conventional seed is sold as single genotypes primarily to ensure that intellectual property rights of the breeder and phytosanitary regulations can be regulated and controlled. Production and processing is also simplified and using a single variety ensures completely uniform ripening in the field, which is particularly important for large-scale mechanical harvest, but more difficult to achieve with mixtures. In contrast, for the subsistence farmer, who does not purchase new seed every year anyway, complete genetic uniformity is neither realistic nor particularly desirable; in fact the most important characteristics are local adaptation to the prevailing conditions of soil and climate (Brandt and Kidmose 2002).

Mixture cropping may provide both organic and conventional producers with a more sustainable approach in reducing weed pressure, crop rotation flexibility, improved yield stability, buffering against pests and diseases, minimizing soil variability and increasing animal feed value (Kaut et al. 2008). Intercropping, multiple cropping and other interspecies biodiversity such as the number of different crops grown in the rotation within and between years could encourage higher numbers of related micro-organisms, insects, worms, weeds and soil fauna. This is not the case in intensive, conventional systems, which can lead to extreme losses of biodiversity and to combat this trend, agri-environment schemes have been introduced, in which farmers are paid to modify their farming practice to provide diversity and ecological benefits.

Organic agriculture is an ecological production management system that promotes and enhances biodiversity, biological cycles and soil biological activity (Haas et al. 2001; Vetterli et al. 2003) and organic growers promote diversity at all levels (Liebhardt 2003). There are evidences showing that insect pest control is enhanced as a consequence of greater biodiversity on organic farms, and an increase in the diversity of insect predators and parasitoids can have positive or negative effects on prey consumption rates (Letourneau and Bothwell 2008). By adopting mixed cropping, applying organic fertilizers such as composts and farmyard manures, using mulches and cover cropping and avoiding synthetic chemicals, habitats are provided for a variety of macro- and micro-organisms. Some of these may be beneficial and keep pest and disease damages below economically damaging levels (Liebhardt 2003). Therefore, the organic farming systems regard biodiversity as an irreplaceable production factor or even a driving force at different levels of the farming system, and as an instrument for preventing pests, diseases and weeds (Geier 2000). Such a self-regulating, stabilizing force in agroecosystems provided by biodiversity is not simply governed by the number of species involved, but mostly by a selective number of specific, functional species in an appropriate ratio. Therefore, depending on the quantity and quality of species in the agroecosystems, the organic farmer faces the challenge of managing site-specific diversity and identifying the correct combinations of species (in time and space) that through their biological synergism achieve the self-regulating capacity of his individual farm ecosystem (Lammerts-van-Bueren et al. 2002).

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