Waterfowl endozoochory: traits drive plant-bird dispersal interactions in North America

1. Plant biogeography and ongoing changes in plant distributions are influenced by long-distance dispersal (LDD) of seeds, and migratory waterfowl (Anatidae: ducks, geese and swans) are particularly important for the dispersal of plants in and around wetlands through gut passage (endozoochory). However, this dispersal is not contemplated by classical dispersal syndromes, hence currently we have limited understanding of what angiosperm traits predict seed dispersal by waterfowl, and any differences due to avian trait variation among waterfowl species. 2. We conducted a literature search for data on the presence of seeds in the alimentary canal or faeces of North American waterfowl as a proxy for dispersal interactions, since an important fraction of these seeds survive gut passage. We investigate North American flowering plant species (N = 536) from 95 families dispersed by 38 waterfowl species, together with traits of these plants and their vectors. Owing largely to limited sampling effort, 47% of plant species and 35% of plant genera recorded were only found in a single waterfowl species. 3. A suite of analytical approaches revealed major differences in the traits of plants dispersed between foraging groups (guilds) and in relation to other waterfowl traits. Diving ducks and sea ducks are associated with sublittoral plants that have submerged or floating leaves, and hence with the classical hydrochory syndrome. Dabbling ducks and herbivorous waterfowl (notably geese) are associated with terrestrial herbaceous plants, and also with the unspecialized dispersal syndrome common in terrestrial plants. Plants with larger seeds or of greater height are more likely to be dispersed by dabbling ducks, whereas geese dispersed shorter plants than other waterfowl. 4. Network analyses of interactions between waterfowl species and plant traits were generally consistent with these results, but also provided evidence that not all geese, dabbling ducks or sea duck species are alike in their interactions with plants along the terrestrial-aquatic gradient, but instead occupy distinct dispersal niches. The traditional classification of waterfowl into guilds therefore does not accurately reflect their differing roles in plant dispersal. Aquatic plant species had more vector species than terrestrial ones. 5. Detailed studies of waterfowl endozoochory, and more trait determinations for North American plants, are needed to facilitate predictions of LDD as waterfowl population sizes and migration patterns vary in the future with climate change.