The increase in woodland area on our organic farms can be explained by the higher occurrence of tree-planting efforts on organic farms (and not by chance occurrence of larger woodlands on these farms). Spatial analysis of woodland patches indicated that organic farms generally had patches with simpler perimeters (AWM patch fractal dimension closer to one) and consequently less edge than similarly sized patches on conventional farms. The organic farms had more continuous blocks of woodland whereas the woodland on conventional farms often consisted of more linear remnant patches, and this may have implications for biodiversity conservation (Kunin 1997). Usher & Keiller (1998) found that small woods on farms did not support characteristic woodland moth communities. Species richness of the moth family Geometridae was positively related to woodland area, as well as woodland shape, with compact shapes better then elongated shapes. The proportion of woodland habitat on or near farms has been related to the pollination services provided by native bees (Kremen et al. 2002; Kremen et al. 2004) and increases in bird richness (Fuller et al. 2001).
Organic farms supported higher levels of plant abundance, species richness and diversity in arable fields but not in semi-natural areas of vegetation. The differences found in the arable fields were significant despite all of our study farms being embedded in a complex landscape, which might be expected to counteract any effects of farming system on biodiversity (Weibull et al. 2003; Bengtsson et al. 2005; Tscharntke et al. 2005). Furthermore, organic farms contained on average 10 more plant species in arable fields at the whole-farm scale despite having proportionally less area of arable fields. This result is unexpected given that, according to species-area relationships, we would expect fewer plant species in arable fields on organic farms. Clearly, arable field management (both past and present) on organic farms is more important for increasing plant richness. There also appeared to be greater plant abundance and species richness in grass fields on organic farms. However, this is likely to be a product of greater areas of grass fields on organic farms and not necessarily a true increase in plant diversity (Table 4). This suggests that if conventional farmers were to increase their area of grass fields to match those on organic farms they may see a similar increase in plant richness.