Abstract
Organic farming, a low intensity system, may offer benefits for a range of taxa, but what affects the extent of those benefits is imperfectly understood. We explored the effects of organic farming and landscape on the activity density and species density of spiders and carabid beetles, using a large sample of paired organic and conventional farms in the UK. Spider activity density and species density were influenced by both farming system and surrounding landscape. Hunting spiders, which tend to have lower dispersal capabilities, had higher activity density, and more species were captured, on organic compared to conventional farms. There was also evidence for an interaction, as the farming system effect was particularly marked in the cropped area before harvest and was more pronounced in complex landscapes (those with little arable land). There was no evidence for any effect of farming system or landscape on web-building spiders (which include the linyphiids, many of which have high dispersal capabilities). For carabid beetles, the farming system effects were inconsistent. Before harvest, higher activity densities were observed in the crops on organic farms compared with conventional farms. After harvest, no difference was detected in the cropped area, but more carabids were captured on conventional compared to organic boundaries. Carabids were more species-dense in complex landscapes, and farming system did not affect this. There was little evidence that non-cropped habitat differences explained the farming system effects for either spiders or carabid beetles. For spiders, the farming system effects in the cropped area were probably largely attributable to differences in crop management; reduced inputs of pesticides (herbicides and insecticides) and fertilisers are possible influences, and there was some evidence for an effect of non-crop plant species richness on hunting spider activity density. The benefits of organic farming may be greatest for taxa with lower dispersal abilities generally. The evidence for interactions among landscape and farming system in their effects on spiders highlights the importance of developing strategies for managing farmland at the landscape-scale for most effective conservation of biodiversity.
Highlights
Farmland biodiversity in Europe has been declining for some time, with a steep trend in the second half of the twentieth century
More hunting spiders were captured, and more species of hunting spiders captured, on organic compared to conventional farms; the farming system effect was marked in the cropped area before harvest—our analysis suggested that an average of 77% more individuals and 36% more hunting spider species were captured on organic farms
Our results suggest that the farm system differences in the cropped area were probably attributable to crop management differences—including reduced inputs of pesticides and fertilisers and, for hunting spider activity density, plant species richness
Summary
Farmland biodiversity in Europe has been declining for some time, with a steep trend in the second half of the twentieth century. Recent incentives and agri-environment schemes introduced to address this issue have halted and even reversed declines in some groups [7] and, while in some parts of Europe agriculture is not intensified and may not have contributed to substantial biodiversity loss [8], in general, farmland biodiversity continues to decline in Europe [9], [10], amid continued concerns over the potentially harmful effects of pesticides [11]. There is concern that the new agricultural policy of the European Union will not sufficiently address this issue—it may be less beneficial for biodiversity [12]. There is a continuing need to refine initiatives which may benefit farmland biodiversity
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