Long-term dynamic of nestedness in bird assemblages inhabiting fragmented landscapes

Context: Nestedness is a common pattern of species assemblages in fragmented landscapes. The spatial pattern and ecological drivers of nested communities have been widely explored, but few studies investigated their long-term variability. Objectives: To investigate the variability of nestedness and species-specific fragment occupancy of forest birds in a fragmented landscape affected by environmental changes over 16 years. Methods: Data were obtained from the Monitoring Program of Breeding Birds in Lombardy (northern Italy). For two study periods (1997–2001, 2013–2017), we tested for overall nestedness and for sites and species nestedness independently using the NODF metric. We tested for nestedness drivers (variable selection on multiple linear regression models) and evaluated the effect of species ecological traits on fragment occupancy changes (variable selection on multiple linear regression model). Results: The community showed a significant nestedness driven by both selective extinction and selective colonization in both study periods. Sites nestedness was significant in the second study period only. Over 16 years the effect of distance from source areas was completely lost and only local isolation conditions drove selective colonization in the second study period. Between the two study periods, we discovered a general occupancy decrease of interior species and a significant occupancy increase of generalist species characterized by large size and large minimum area requirement. Conclusions: Nestedness drivers of the investigated community significantly varied over time probably because of both environmental and demographic changes. Long-term studies are crucial to explore spatial pattern changes and to address management strategies for species conservation in fragmented landscapes.