This is a pressed specimen of yellow sweet clover (Melilotus officinalis). It probably came from central and southern Europe and western Asia, and was first found in the Oslofjord in 1861. Yellow sweet clover is what is known as an alien species in Norway, but it is also called a “ballast plant”. How did it get here?
From the world to the Oslofjord
The Oslofjord is full of ballast plants, plants that were introduced to Norway as seeds in ships’ ballast. Ballast is heavy material loaded onto an otherwise empty ship to stabilise it. For ships built of timber, stone was a common ballast, but earth and gravel were also used. To create space for cargo, the ballast had to be dumped. Ballast plants started growing where the ballast was dumped.
Alien species
Ballast plants are alien species in Norway, so they are not naturally adapted to the ecosystem here. Some alien species are not a problem, but others can cause imbalance in the ecosystem and displace species that grew here previously.
Ballast water
Today, empty ships use ballast water to improve stability. Large tanks are filled with water containing vast numbers of living organisms as stowaways. As a rough estimate, each ship’s ballast water tanks contain about 7,000 species at any given time. The introduction of such alien species is considered to be the biggest threat to biodiversity in our seas and rivers. An international ballast water management convention came into force in 2017 to ensure that such water is cleaned before being discharged.
Chosen by Anita, museum staff (34)
On loan from the Natural History Museum, University of Oslo