While studying the role of waterbirds in the dispersal of plant seeds, I became intrigued by the less expected role of piscivorous birds in the dispersal of aquatic organisms. Together with bachelor students Milou van Veen and Maxime Brugman, we performed some pilot studies to find proof of principle for the dispersal of small aquatic organisms by the Great Cormorant (Phalacrocorax carbo) in the Netherlands.
Cormorants forage on fish, which forage on seeds and invertebrates.
In her project, Milou found no plant seeds, but 42 eggs of invertebrates including crustacea, rotifera and branchiopoda in a small sample of cormorant pellets. She found similar invertebrate eggs in the digestive tracts of several potential prey fish species. Maxime analysed a larger sample of cormorant pellets and did an incubation test after extracting easily visible invertebrate eggs and plant seeds. Plastic beakers with tap water, incubated with fresh cormorant pellets, were shown teaming with invertebrate life after some time in a climate chamber.
Various types of small invertebrates were found after incubation of cormorant pellets in tap water.
Seeds of several plant species, including these Epilobium spec. seeds were germinated from cormorant pellets.
In the conclusion of her bachelor thesis, Maxime wrote:
"Great Cormorants have a high potential as secondary dispersers. The diet of cormorants consists of a large variety of fish that forage on invertebrates, and previous studies have shown that cormorants may fly large distances on a daily basis. Cormorant pellets contain less plant seeds than feaces of granivorous waterfowl, but seeds from pellets have a higher germination rate. Pellets do contain more invertebrate eggs than waterfowl feaces. This could imply that granivorous waterfowl and piscivirous waterbirds complement their respective roles as dispersers of aquatic organisms. Future studies should focus on the scale of secondary dispersal by cormorants in terms of their movement behaviour and the range of organisms dispersed."