Organic versus conventional systems in viticulture: Comparative effects on spiders and carabids in vineyards and adjacent forests

Farming systems and management regimes of vineyards may affect local biodiversity of plants and invertebrates. While most studies have focused on the overall biodiversity of vineyards, there has been little consideration of the response of different ecological guilds to vineyard management, or to how vineyard management affects communities of adjacent semi-natural habitats. We study here two functional guilds of carabids and five of spiders in Langa Astigiana (NW-Italy) with the following aims: (i) to assess the comparative effects of organic and conventional farming systems, along with associated habitat and landscape variables, on species richness and abundance in vineyards; and (ii) to compare the same within forest patches surrounding organic and conventional vineyards. The different guilds exhibited distinct preferences for habitat characteristics (i.e. grass cover), landscape context and farming systems. Generalized Linear Mixed Models showed that spider preferences mostly depended upon habitat variables, while carabid preferences depended on small-scale landscape variables. In general, organic farming increased biodiversity and abundance of arthropod predators, even though different guilds of carabids and spiders responded differently. Brachypterous carabids, ambush spiders, ground-hunter spiders and other hunters preferred organic vineyards, whereas macropterous carabids, specialist spiders (mostly ant-eating spiders) and sheet web weavers selected conventional vineyards. The research we report here shows that preferences for vineyards with different farming systems has been driven not only by farming systems per se (i.e. omission of synthetic pesticides), but also by habitat characteristics and small-scale landscape structure. Arthropod diversity was greater in the forest patches adjacent to organic vineyards than to conventional ones. This suggests that organic systems may sustain a higher diversity of carabids and spiders both in vineyards and in the adjacent forest patches as well. We conclude that although conventional systems may promote the diversity of some guilds, organic systems should take priority.