Seed dispersal by a generalist duck: ingestion, digestion and transportation by mallards (Anas platyrhynchos)

Abstract

Dispersal is key to plant population dynamics and adaptation to environmental changes. In discrete, fragmented habitats, plant propagules need a vector for their transportation, especially in freshwater wetlands, which were characterized by Darwin as ‘islands in a sea of land’. Since seeds dispersed by wind or water have limited potential to reach other catchments, waterbirds may play a major role in dispersal between wetlands. Dabbling ducks in particular have been identified as effective seed dispersers due to their abundance, digestive (in)efficiency and movement behaviour. However, the scale of seed dispersal by waterbirds and the mechanisms behind this mode of dispersal remain largely unknown. By combining lab and field experiments with mallards (Anas platyrhynchos), the most abundant and best studied dabbling duck species worldwide, this project aimed to improve our understanding of (1) the occurrence of seeds in dabbling duck diets, (2) the role of seed and vector traits in the survival of digestive tract passage, and (3) the role of vector animal movements in establishing seed deposition patterns. Firstly, seeds of 81 plant species were retrieved from the digestive tracts of 100 mallards in the Netherlands, only 28% of which were classified as typical wetland plants. Species composition varied strongly between individuals and over time, indicating highly opportunistic foraging on available species. Wetland species did not survive digestive processes better than terrestrial species, suggesting that they are not in general better adapted to internal (endozoochorous) dispersal. Secondly, strong evidence was found for a major role of seed size in resisting digestion. Small seeds survived digestion consistently better than large seeds throughout the experiments. Seed coat permeability was identified as another important factor, especially for resisting chemical processes. Seeds were digested most efficiently when exposed to multiple digestive forces, with a key role for mechanical digestion in the gizzard. Although digestive organ sizes of mallards varied seasonally, no relation was found with seed survival. In contrast, moderate exercise by mallards did affect digestive efficiency. Seed survival was increased up to 80% and gut passage was accelerated in actively swimming mallards, identifying physical activity as a mechanism enhancing the dispersal potential of ingested seeds. Thirdly, GPS tracking of 97 mallards in the Netherlands revealed highly predictable circadian movement patterns, with commuting flights between roost and foraging sites. Movement distances were negatively correlated with the number of water bodies and total shoreline length in the landscape. The affinity of mallards with water results in strongly directed dispersal towards potentially suitable habitat for wetland plants. This implies that mallards become even more important for wetland connectivity in increasingly fragmented landscapes. In conclusion, this study has shown that large numbers of seeds from a wide range of species are being transported by mallards every day. Most of these species at least regularly survive digestive tract passage, facilitated by high variation in digestive strategies and efficiency among mallards. As more freshwater wetlands are being lost by land use change and climate change, waterbirds will become increasingly important for wetland biodiversity and ecosystem functioning.