Seed dispersal by neotropical waterfowl depends on bird species and seasonality

1 - Waterbird-mediated endozoochory is an essential mechanism for the dispersal of sessile organisms in freshwater ecosystems. However, in the neotropics there are no previous studies of how different waterbird species vary in the dispersal functions they perform, and how seasonality influences endozoochory. In this study, we identified plant diaspores dispersed in faeces of five South American waterfowl (Brazilian teal Amazonetta brasiliensis, yellow-billed teal Anas flavirostris, ringed teal Callonetta leucophrys, coscoroba swan Coscoroba coscoroba and white-faced whistling-duck Dendrocygna viduata). 2 - We collected 165 faecal samples from five wetlands in southern Brazil surrounded by pasture and ricefields, then separated and measured intact seeds and other diaspores. Using Generalized Linear Models, we tested how diaspore abundance and taxonomic richness differed among bird species and between cold (April to September) and warm (October to March) periods. We also analysed bird-specific and seasonal variations in diaspore composition through Principal Coordinates Analysis and PERMANOVA. We used Indicator Species Analysis to determine which diaspore species discriminated between bird species and seasons. Finally, we measured diaspore length in order to analyze differences among waterfowl species in the size of diaspores dispersed. 3 - We found 2,066 intact diaspores from 40 different plant taxa, including seeds of 37 angiosperms and diaspores of Lycophyta (Isoetes cf. maxima), Pterydophyta (Azolla filiculoides) and Charophyceae. There was at least one diaspore in 65% of all faecal samples. Diaspores of native amphibious and emergent plants were dominant. We found 1,835 diaspores (from 33 taxa) in the cold period but only 231 (23 taxa) in the warm period. Seeds of the grass Zizaniopsis bonariensis and of the sedge Rynchospora sp. were the most abundant taxa. A strong interaction between bird species and season was the most important predictor of variation in both taxonomic richness and abundance of diaspores. The taxonomic composition of diaspores differed among waterfowl species and season. Indicator Species Analysis identified 12 plant taxa associated with particular bird species and seasons. Coscoroba swan, the largest bodied species in our study dispersed a higher proportion (8.2%) of large (length >2 mm) seeds. 4 - Despite considerable overlap, there are important differences in the plants dispersed by each species, and the smallest (ringed teal) and largest (coscoroba swan) birds are particularly different. All five waterfowl species are distributed over wide areas of South America and here, we demonstrated that they are likely important plant vectors connecting wetland species at different geographical scales. Many of these plants have previously been assumed to lack mechanisms for long-distance dispersal.Peer reviewe