Gut travellers: internal dispersal of aquatic organisms by waterfowl

Aim Patterns of high biodiversity among less mobile organisms throughout isolated locations suggest that passive dispersal importantly contributes to biodiversity. We examined the contribution of waterbirds to the dispersal of plant seeds and macroinvertebrates between aquatic wetlands. Birds are renowned vectors for seeds of terrestrial plants, but less is known about their role in more dispersal-dependent aquatic systems. We therefore performed a meta-analysis on bird-mediated endozoochorous dispersal of aquatic species. Location Our review included studies that collected data world-wide. Methods We analysed data from 81 peer-reviewed publications on endozoochorous dispersal of aquatic plant seeds and macroinvertebrates by waterbirds. Results In total, 36% of 1581 waterbird droppings collected in the field contained one or more intact propagules, with macroinvertebrates found almost as frequently as plant seeds. Positive droppings contained on average 3.3 intact propagules, of which one-third were viable. In 728 trials from 17 published feeding experiments 24% of the ingested propagules were retrieved intact, with c. 6.5% both viable and intact. As many as 17 species of Anatidae and Rallidae were involved in the dispersal of at least 39 species of macroinvertebrates and seeds from 97 species of plants across a wide taxonomic range. Smaller propagules seemed less affected by digestion than larger ones. We provide a first quantitative model that can be used to estimate waterbird-mediated dispersal of propagules between wetlands. This model indicates that an average waterbird has the potential to disperse five viable propagules after flying more than 100 km, and one additional propagule after flying 300 km. Main conclusions We demonstrate that waterbirds have the potential to transport a wide variety of aquatic plants and animals over several hundreds of kilometres. High survival of propagules might be explained by propagule adaptations or by the digestive adaptations of birds, whereby energy absorption is thought to be maximized rather than assimilation efficiency. Our meta-analysis suggests that waterbirds might contribute significantly to wetland biodiversity around the world, despite several limitations to our current knowledge. We outline avenues for future research to address these knowledge gaps.