Don’t poop in the pool – except if you’re an eider duck

At Tvärminne Zoological Station, researchers have made an unexpected finding about how eider ducks influence water fleas.


Despite their name, water fleas are not insects, but small fresh-water crustaceans. In Tvärminne, they can be found in rain water pools on the islands. The archipelago contains thousands of these pools, and in 1982, the researcher Ilmari Pajunen started keeping track of where the water fleas occur. Professor Dieter Ebert came to Tvärminne in 1997 to study the interaction between the water fleas and their parasites.

– Thirty years of studies on water fleas have given us one of the largest datasets in the world detailing what happens to the populations of a species, says Ebert.

Over the years, the group has studied the water flea metapopulation to find out, among other things, why the fleas occur in some pools and not others. A major factor is the acidity of the pool.

– Some pools are too acidic for the fleas, because both the granite rock and the Finnish soil are acidic. We wanted to know why some pools less acidic than others, Ebert explains.

To solve the mystery, Ebert turned to the most common bird in the area: the eider duck. The eiders leave their droppings all over the islands, and some end up in the rock pools. The droppings contain mussel shells rich in calcium carbonate, a substance which can reduce acidity.

– We thought that this might explain the difference in acidity, says Ebert.

Dieter Ebert

To test the idea, Ebert and his colleagues manipulated natural pools. Without eider droppings, only ten percent of pools were habitable for water fleas. But when eider droppings were added, in eighty percent of cases the water fleas were able to survive. The strong effect was a surprise to Ebert.

– I often talk to the people who study eider ducks here. But I never thought that their work was connected to mine!

The connection between eiders and water fleas also helps explain why water fleas have recently disappeared from some ponds. Because of conservation efforts, the number of sea-eagles has increased in the area. This has made the eider ducks move their nesting sites away from the rocks and inside the forest, where they are less easily preyed on by the eagles.

– This means that the ponds on the bare rocks don’t get any eider droppings anymore. And in recent years, we have indeed seen the water fleas disappear from these ponds – but until now, we didn’t know the reason for it, says Ebert.

The result demonstrates how everything in nature is connected: man, sea-eagles, eiders, water fleas and parasites inside the water fleas.

Dieter Ebert at the University of Basel, Switzerland »»

Eider research at Tvärminne Zoological Station »»

Tvärminne Zoological Station »»

Comment on Facebook »»

Text: Katja Bargum
Image: Dieter Ebert (water flea), Katja Bargum (Dieter Ebert)
University of Helsinki, digital communications

News of the month »»
News archive »»
University of Helsinki