Responses of medium- and large-sized bird diversity to irrigation in dry cereal agroecosystems across spatial scales

Conserving biological diversity under agriculture intensification is a global challenge given that the intensification process erodes biodiversity worldwide. Recently, efforts have begun to move towards conserving biodiversity in intensified farmland ecosystems, but there is little understanding of how some kinds of intensification (e.g., irrigation) affect biodiversity. We aimed to assess the responses of farmland bird diversity along an agricultural intensification gradient from dry to irrigation across spatial scales. We covered 10-km-survey transects by car to census medium- and large-sized farmland birds through a decreasing gradient of agricultural intensification ranging from irrigated to dry cereal farming systems in North-western Spain. We evaluated diversity of breeding and winter birds using different measures (species richness, abundance, and true Shannon diversity), and partitioning diversity into spatial components (α, β, and γ). We used turnover and variation partitioning to assess the variation in species composition. Changes from dry to irrigated farmland had no effects on (total) species richness, total abundance, and partitioning of farmland bird diversity in any season. However, irrigation led to changes in species composition, severely affecting open-habitat specialist species, most of which were threatened steppe birds. Βeta(β)-diversity at landscape scale (between-transects) contributed to total diversity as much or more than β-diversity at regional scale (between-farming sub-areas with different degrees of intensification) in both seasons (i.e. spring and winter). Our study suggests a homogenization of the farmland bird community at regional scale driven by intensification. Promoting landscape-scale habitat heterogeneity could be an effective management measure to improve bird diversity in intensified farmland ecosystems, as long as requirements of open-habitat specialists are met.