It was Charles Darwin who first mooted the idea that fish-eating birds can play an important part in the dispersal of plant seeds and small invertebrates. After a century and a half, a group of researchers led by NIOO's Casper van Leeuwen has found proof.

It's a well-known fact that birds and fish help to spread seeds by ingesting them and then defecating them more or less intact. But what if a fish that has eaten some seeds is subsequently eaten itself, by a bird or some other predator?


Casper van Leeuwen

Darwin suggested that this won't stop the seeds from being distributed successfully, even to areas that they wouldn't otherwise have reached. The scientific term for this is 'secondary dispersal'.

Notorious fish thieves

Aquatic ecologist Casper van Leeuwen and his international colleagues looked for confirmation for Darwin's theory by studying cormorants. The great cormorant is a widespread waterbird notorious with fishermen because of the large amounts of fish it polishes off.

An average cormorant will eat about 500 grammes of fish per day. Any parts of its prey that it can't digest, it regurgitates in the form of mucus-coated pellets. So studying these pellets will give you a good overview of the birds' diet.

Van Leeuwen and his fellow researchers studied coromorant colonies in seven locations in Europe. Of the pellets they found, one in three contained plant seeds. And one in five contained intact aquatic invertebrates, including bryozoans and even a freshwater sponge.

Surprising diversity

All of these seeds and invertebrates had been ingested by the cormorants' prey, not directly by the birds. The fact that cormorants often roost in overhanging trees near water, provides the seeds and invertebrates with a suitable habitat to establish themselves in.

The researchers were surprised by the diversity of plant seeds in the pellets: 21 species from 16 different plant families, including a number that would not be prime candidates for direct dispersal by ducks or other waterbirds.

"Fish and birds don't always ingest the same kinds of seeds", says Van Leeuwen. "For instance, fish will also eat seeds that birds can't reach because they have sunk." Piscivorous birds such as cormorants allow these seeds to penetrate areas that do not have a direct connection by water to their point of origin.

Climate change

Until now, says Van Leeuwen, the potential of secondary dispersal that Darwin noted had not been systematically researched. But in recent years, there has been growing interest in dispersal mechanisms, and in the role of waterbirds. That interest has been driven to some extent by practical issues such as the spread of invasive species.

Climate change has also been an important factor, according to Van Leeuwen. In situations where species are under pressure from climate change or other anthropogenic stress factors, secondary dispersal could be one of the mechanisms by which they can move their habitat.

"Of course there are many more of Darwin's early insights that researchers could look into", concludes Van Leeuwen. "But this is one theory at least that we've finally been able to get to the bottom of."

 

Photographs: cormorant with prey Helge Sørensen, cormorant colony Maria Ovegård

  • Article: 'Great cormorants reveal overlooked secondary dispersal of plants and invertebrates by piscivorous waterbirds', authors: Casper van Leeuwen, Ádám Lovas-Kiss, Maria Ovegård and Andy J. Green, in Biology Letters vol. 13, nr. 10 (October 2017), DOI: 10.1098/rsbl.2017.0406