Better out than in: faecal matrix inhibits establishment success after waterfowl endozoochory

Many plant species undergo long-distance dispersal through migratory waterbirds. However, there is little information about the effectiveness of this dispersal, especially regarding the chances of plant establishment, and the impact of gut passage or the faecal matrix on plant germination, growth, and reproductive investment in seeds. In a greenhouse experiment, we addressed these questions using an annual mudflat species, Juncus bufonius (Juncaceae), and a perennial emergent aquatic species, Eleocharis palustris (Cyperaceae), whose seeds are dispersed by many waterbird species in Europe. We planted seeds directly in soil or within mallard faeces placed on soil, using both control seeds and seeds that survived gut passage through mallards. Over the following 11 weeks, we quantified germination and plant performance. Gut passage reduced germination time of J. bufonius when there was no faecal matrix, and increased asymptotic height of E. palustris. Presence of the faecal matrix hindered germinability, plant growth and final biomass for both species, along with total seed production for J. bufonius. Presence of the faecal matrix slowed down germination in E. palustris, yet had the opposite effect for J. bufonius. It was also associated with greater relative investment in seeds in J. bufonius (more seeds per unit biomass), likely as a consequence of later germination. In both species, earlier germination increased final biomass (and seed production in J. bufonius). Our results support the importance of waterbird endozoochory in plant dispersal, but suggest it may be more effective when faeces disintegrate, as when egested into water or disaggregated on land (e.g. by insects). Previous studies with other plants have recorded accelerated germination following waterbird gut passage, and our results show this can benefit plant fitness.