Connection between ecological niche changes and population trends in the Eurasian Skylark (Alauda arvensis) breeding in lowland and mountain areas of Southern Europe

Population responses to environmental changes can often vary geographically and between environments, potentially as a consequence of differing niche dynamics. The Eurasian Skylark Alauda arvensis is an open-habitat passerine bird that is declining throughout Europe, mainly due to agricultural intensification. We compared population trends (1992-2021) of the species across three sub-regions of northern Italy characterized by different environmental conditions and human pressures: the Alpine sub-region (less anthropized mountains), the Plain (strongly anthropized and intensively cultivated) and the Oltrepo (extensively cultivated hills), with changes in the realized Grinnellian niches over three decades. In each sub-region, niche comparisons did not show divergence over the study. However, we found an overall reduction in niche occupancy over time. In the Alpine sub-region, a reduction in niche occupancy in hayfields and pastures was not associated with population decline; indeed, we found an increasing population (+164%), probably because high availability of natural grasslands counteracted niche contraction. Conversely, in the Plain and Oltrepo sub-regions, the observed population declines (-99% and -36%, respectively) are associated with a general reduction of niche occupancy in arable lands that represented the core of the niche in these ranges. In the Plain, the lack of alternative suitable habitats might have limited any opportunity for the species to colonize new environments. Conversely, in the Oltrepo, the less severe population decline is combined with increased niche occupancy in hayfields and pastures at higher elevations. The joint application of population trend analysis and niche modelling as well as the decomposition of population changes across different environmental contexts can contribute to a better understanding of ecological processes affecting population dynamics, supporting policy-makers to implement targeted conservation strategies.