New evidence on the linkage between long-term niche changes and population trends in birds

Despite the assessment of long-term dynamics of ecological niche could provide crucial information for investigating species responses to environmental changes, it is a poorly investigated topic in ecology. Here, we present a case study of multi-species niche analysis for 71 common breeding birds in Northern Italy, exploring long-term niche changes from 1992 to 2017 and their relationship with population trends. We (i) quantified the realized Grinnellian niche in the environmental space, (ii) compared variations in niche breadth and centroid, (iii) tested niche divergence and conservatism hypotheses through equivalency and similarity tests, (iv) calculated niche overlap, expansion and unfilling indices, and (v) investigated the association between niche metrics and population trends. Results showed niche conservatism for 56% of the species, while 10% of them highlighted evidence of niche divergence. We detected a general tendency to adjust the niche centroids towards warmer thermal conditions. Increasing populations were positively correlated with the niche expansion, while negatively correlated with the niche overlap, albeit in the latter case at the limit of the significance threshold. We encourage studies correlating long-term population trends and niche changes with a specific focus on cause-effect relationship at both single and multiple-species level, also combining species traits’ information.