The effects of a decade of agri-environment intervention in a lowland farm landscape on population trends of birds and butterflies

The widespread expansion and intensification of farming practices over the 20th century has brought about well-known declines in farmland biodiversity. A key mechanism for attempting to reverse these declines has been agri-environment schemes (AES), which offer financial compensation for taking land out of agricultural production or changing farming practices to achieve environmental targets. AES have been established in many countries for over three decades, with a corresponding history of research into their effectiveness. However, there is still debate regarding their true potential to reverse population declines, especially for more widespread and mobile species of farmland birds and butterflies, which continue to show declines despite over 20 years of AES aiming to support their recovery.

Long-term studies of population trends on farms with known levels of high-quality AES, compared with spatially separated controls, are rare, and limited almost entirely to birds. The study reported here is important in this respect, providing valuable insight for both birds and butterflies from a long-term (10 years) experiment in southern England, within a large, well-characterised farm landscape with high-quality AES.

The study set out to explore whether the results of previous studies demonstrating higher abundances of widespread farmland bird and butterfly species in the parts of the study landscape with higher AES uptake were associated with positive population trends over the 10-year time series following establishment of AES. These trends were then compared with concurrent trends from national monitoring schemes in equivalent control landscapes (i.e. similar climate, soils, topography, land cover composition and landscape structure but without substantial AES).

Monitoring data collected within the study landscape were compared with comparable national data derived from the BTO/JNCC/RSPB Breeding Bird Survey and the Wider Countryside Butterfly Survey. Population trends were analysed by using generalised linear models, revealing that species trends within the study landscape were frequently stable or increasing, in contrast to concurrent declining trends in equivalent landscapes without AES. These differences were found to be statistically significant for the total abundance of granivorous bird species, and for Chaffinch, Blue Tit and Great Tit individually. For the butterfly species, differences in trends were significantly more positive for Gatekeeper and Green-veined White, while the reverse was the case for Small White.

The results demonstrate that, for some bird and butterfly species, the higher abundances associated with areas of AES uptake within a typical commercial farmland landscape can co-occur with positive or stable population trends over long time scales, and that these trends can show significant differences from those in equivalent landscapes without substantial AES interventions. The results underline the importance of delivering and monitoring high-quality AES options if the design and implementation of the next generation of AES is to achieve significant benefits for biodiversity.