Impact of landscape improvement by agri-environment scheme options on densities of characteristic farmland bird species and brown hare (Lepus europaeus)

• Bird and hare densities increased over time in an ecologically improved landscape.
• The major factor influencing species densities is the amount of high-quality habitat.
• We conclude that 14% of arable farmland should be covered by high-quality habitat.

Causes for farmland bird declines are well studied, but on landscape level agri-environment schemes (AES) often show limited gains for biodiversity. We analysed population trends of nine farmland bird species and the brown hare in a Swiss arable landscape. Further, we focused on the impact of the quantity and quality of different ecological compensation area (ECA) options on densities of the study species, as well as on the extent of ECAs required on arable farmland to stop population declines of typical farmland species. Densities of bird species and hare counts were positively correlated with the quantity of ECAs and semi-natural habitat. However, effects of ECA options on birds and hares are species specific. The quantities of wildflower areas and semi-natural habitat strongly enhanced bird and hare numbers. The quality of the ECA options was also important, as densities were positively related to the amount of meadows of high ecological quality, but not to the amount of meadows of low ecological quality. To attain target densities, the required proportion of high-quality AES options and semi-natural habitat has to be at least 14%. This study provides evidence that intensively managed arable farmland can be improved for bird diversity and hare density by the Swiss AES. The amount of AES options of high ecological quality is of major relevance for nature conservation. We estimate that the amount of high-quality options and semi-natural habitats in the Swiss arable lowland must be four times higher than today's area to halt and reverse population declines of farmland species.