Long-term trend of the waterbird community breeding in a heavily man-modified coastal lagoon: the case of the important bird area “Lagoon of Venice”

Abstract

The aim of the paper is to examine the temporal and spatial changes observed over a 25-year period in the waterbird community nesting in the largest coastal lagoon around the Mediterranean; to examine driving factors for the observed changes; to address the most urgent conservation actions. Published sources and field surveys made between March and July were used to assess number of breeding pairs of the commonest waterbirds in 1990–1992, 2000–02 and 2012–14. The breeding waterbird community exhibited several changes in its structure, with an overall positive trend; the number of species increased from 14 to 25 and the mean yearly abundance increased from 6155 to 14,008 pairs. The diversity (H′) increased slightly, whereas similarity indices and nMDS ordination both highlighted clear differences between 1990 and 1992 and 2012–2014 communities. The increase in richness and abundance were mostly due to the immigration of birds from nearby wetlands, to the partial recovery of lagoon ecological conditions since the end of the 1980s and to the occurrence of suitable man-made habitats, such as fish farms, dredge islands and a constructed wetland. The fraction of the population nesting at artificial sites and fish farms increased from 50% in 1990–1992 till 80% in 2012–2014, highlighting the importance of artificial breeding sites in costal lagoons. At the opposite natural nesting habitats, such as saltmarshes and beaches are losing importance for breeding waterbirds, thus requiring urgent conservation measures.