Spatial and temporal patterns of waterbird assemblages in fragmented wetlands of Southern Brazil

Understanding the composition and abundance pattern of species across sites is a central question in community ecology. However, the structure of waterbird assemblages in fragmented wetlands has been poorly documented. We carried out twelve monthly censuses to describe the composition and abundance patterns in 42 wetland fragments and two lagoons in the coastal zone of Rio Grande do Sul, South Brazil. A total of 142,000 birds from 66 species, 18 families and 18 orders were recorded. Most species were either resident (29) or partial migrants (19). All migrant species (8 nearctic, 4 austral and 5 summer breeders) were recorded in small numbers and most of them were restricted to lagoons. The lagoons had more species (60) than the wetland fragments (55), even though the total fragment area (1,426 ha) was about twice the censed area in the lagoons (743 ha). Principal Coordinate Analysis revealed strong temporal and spatial gradients of abundance and composition that were similar in fragments and lagoons. The number of species varied among sites and showed no seasonal pattern. Abundances were higher in the wintering period (min. of 2,500 birds in March and max. of 23,000 in July) due to the increased abundance of Gruiformes and Anseriformes. The White-faced Whistling-duck (Dendrocygna viduata) and the Common Moorhen (Gallinula chloropus) were the most abundant and frequent species over the year, comprising together 69% of the total. The census captured 76% and 60% of the waterbird species listed for the region and the Rio Grande do Sul State, respectively. The study area shared more than 90% of the species with the nearby States and 76% with the Pantanal region. The beta-diversity among sites was 94% when only fragments are considered, and 38% when the lagoons are included in the calculation, showing that a great proportion of the waterbird richness at the landscape scale is accommodated as a beta component. The local abundance and composition of waterbird assemblages seemed to be affected by the interplay of several factors, including the rich regional pool of species, their wide range, the fragment area, the surrounding matrix and the presence of core refuges. A landscape perspective is essential on building sound conservation programs for waterbird assemblages.