Mind the gap: exploring the usefulness of historical data to assess the impacts of land-use change on bird diversity in a farmland mosaic of Northeastern Portugal (1980-2000)

Agriculture intensification and land abandonment are the main threats to bird communities in farmlands. Although they represent the opposite endpoints of a gradient of land-use change, they are usually examined separately. In this study, we assessed the effects of the occurrence of both drivers in space and in time, on bird assemblages. The study was performed in a Mediterranean vineyard-dominated farmland mosaic located in North-Eastern Portugal, where land abandonment and (mechanized) intensification coexisted for twenty years. The aims of this study were (1) to describe the temporal patterns of land-use change; (2) to investigate whether these changes affected the local bird species composition and ecological guild structure; and (3) to assess which species/ecological traits were more responsive to land-use changes. We compared the occurrence of birds before (1979-84) and after (2002-03) the triggering of the land-use change proccess. Results showed the prevalence of three interacting gradients of land-use change: land intensification, and direct and indirect effects of land abandonment. While the composition and ecological structure of the bird communities remained stable during the period analyzed, trait and species-specific analysis showed significant correlations with the gradients of change. This suggest that the local bird communities were able to cope with the synergic dynamics of the land-use change, possibly due to a combination of a resilient pre-change bird assemblage and the buffering effect of the competing drivers. The pros and cons of the approach adopted and the utility of past datasets as baselines to assess and monitor the impacts of environmental change on farmland bird diversity are discussed.