**3. Landscape composition**

Krebs et al. (1999) suggest that biodiversity in agroecosystems depends on both farm management and landscape heterogeneity. Landscape context can modify the influence of organic farming on plants (Roschewitz et al., 2005a) or may be even more important for the diversity of bees, butterflies, carabids and spiders than the local farming system (Kremen et al., 2002; Schmidt et al., 2005; Weibull et al., 2000; Weibull et al., 2003). The contrasting results between organic and conventional fields maybe larger when these fields are isolated in homogeneous landscapes and the species pool may be too small to allow a response in terms of biodiversity to organic farming (Tscharntke et al., 2005).

The landscape context of an agricultural field may make a difference in compensating field isolation or agricultural practices that reduce diversity. Field boundaries, hedges and fallows satisfy a set of wildlife requirements (refuge, food, breeding sites, etc.) that promote species persistence in agricultural landscapes (Benton et al., 2003) facilitating both re-

which no more species will be found (Figure 1). This is because the number of species that can live in a particular habitat is always finite, defined by local climatic and soil conditions and this maximum number is not affected by the way people are handling this habitat: lion will never be found in arctic tundra. On the other hand, even heavily exploited habitats will still harbour some species: the carabid, *Pterostichus melanarius*, is a good example of a species well adapted to intensively managed agroecosystems and was found to be even more numerous in these, compared with more natural habitats (Ameixa & Kindlmann, 2008).

Fig. 1. Hypothetical representation of the diversity expected to be found in agricultural fields. Dmax: Maximum diversity that can be found in agricultural lands; Dmin: minimum

Krebs et al. (1999) suggest that biodiversity in agroecosystems depends on both farm management and landscape heterogeneity. Landscape context can modify the influence of organic farming on plants (Roschewitz et al., 2005a) or may be even more important for the diversity of bees, butterflies, carabids and spiders than the local farming system (Kremen et al., 2002; Schmidt et al., 2005; Weibull et al., 2000; Weibull et al., 2003). The contrasting results between organic and conventional fields maybe larger when these fields are isolated in homogeneous landscapes and the species pool may be too small to allow a response in

The landscape context of an agricultural field may make a difference in compensating field isolation or agricultural practices that reduce diversity. Field boundaries, hedges and fallows satisfy a set of wildlife requirements (refuge, food, breeding sites, etc.) that promote species persistence in agricultural landscapes (Benton et al., 2003) facilitating both re-

diversity that can be found in agricultural lands.

terms of biodiversity to organic farming (Tscharntke et al., 2005).

**3. Landscape composition** 

colonization and maintenance of populations in agricultural landscapes (Duelli & Obrist, 2003). Duelli & Obrist (2003) attribute the lack of effectiveness of AES to the simplification of agricultural landscapes.

However, again we have to take in to account that diversity is expected to increase with complexity of the landscape only above a minimum threshold (Figure 2), as landscape will always harbour some species. Positive effects of landscape complexity will eventually leveloff after a given level of complexity is reached, as the number of species that can live in a particular habitat is always finite, defined by local climatic and soil conditions (Concepción et al., 2008).

Fig. 2. Hypothetical non-linear effects of landscape complexity around cultivated fields on the biological diversity in such fields. Dmax: saturation point of complexity, above which landscapes are so complex that no further effects of complexity are expected; Dmin: minimum threshold of complexity below which landscapes are too simple for maintaining biodiversity (adapted from Conception 2008).
