Evidence that long-distance dispersal of aquatic invertebrates by ducks increases with propagule size

Migratory ducks are key dispersal agents for aquatic organisms, yet differences in their potential for short- and long-distance dispersal are still poorly understood, particularly differences among aquatic invertebrate taxa. Using seven species of aquatic invertebrates and a duck species known to feed on them in the wild (the northern shoveler) as a model system, we evaluated whether their potential for endozoochorous dispersal varies among 5 of the species and scales with propagule size for the 7 species. We also tested the expectation of a lower dispersal potential for invertebrate propagules, as compared to plant seeds; and evaluated whether intra-specific variation (in particular, sexual dimorphism) influences the potential of waterbirds as dispersal vectors. An experiment with 5 invertebrate species demonstrated that most resting eggs (68–95%) were retrieved by 4 h after ingestion, with maximum gut passage times ranging from 16 h for Daphnia magna to 36 h for Artemia salina and Thamnocephalus platyurus. Using models that combine migratory duck movements with gut passage times, we show that aquatic invertebrates may disperse frequently over distances of 15–16 km (median dispersal distance) and regularly over distances up to 110–166 km (Q99 distance). Increasing propagule size resulted in increasing gut passage times, decreasing survival of gut passage and decreasing hatching success. While propagule size had no effects on ‘regular’ dispersal distances (mean, median, Q95 and Q99), the frequency of long-distance dispersal (LDD) increased with it. Increasing propagule size therefore had two contrasting effects on invertebrate dispersal potential, decreasing the frequency of dispersal (less seeds dispersed) but increasing the potential for long-distance dispersal. Conclusions: We provide evidence that endozoochory of invertebrate propagules by waterbirds results in frequent dispersal among wetlands (tens of km) and regular dispersal at regional scale (over a hundred km).