Among the possible negative impact on Norwegian biodiversity resulting from the aquatic plant species Crassula helmsii and Myriophyllum heterophyllum is the formation of dense mats, affecting the oxygen levels in the water. This is considered highly problematic for the population viability of local biota (macrophytes, algae, amphibians, invertebrates, even birds).

Seven other species were assessed as posing a moderate risk to Norwegian biodiversity.

Risk in a 50-year perspective

The Norwegian Environment Agency (NEA) asked VKM to assess the risks of negative impacts on biodiversity in Norway resulting from the import and keeping of various species of freshwater plants for aquaria and garden ponds. The assignment comprised aquatic plant species native to both tropical and temperate areas. The assignment also included a range of species already present and established in Norway, including several species listed on the Norwegian Red List of Species.

The assessment adopted a fifty-year perspective and stated for each species whether it can survive below a cut-off temperature of 5° C. This is because current legislation includes an exemption for import permit requirements for species that cannot survive below this cut-off temperature.

VKM concludes that only the species that are distributed in temperate areas and that have exhibited invasive characteristics elsewhere, could potentially cause negative effects on Norwegian biodiversity. This is because the chance of any of the tropical species surviving outdoors in Norway is likely to be rather slim.

Potential damage caused by invasive aquatic plants

In aquariums, plants are kept in closed containers. Garden ponds are, although outdoors, also generally closed systems. Thus theoretically, the chance of plants escaping aquarium and garden ponds should be slim. However, aquarium and garden pond plants may spread to the environment if the aquarium is emptied outside or when unwanted plants are thrown away without any precautionary measures. If such plants escape and spread to the environment they may cause significant problems, including the blocking canals and water control systems.

Assignment

The Norwegian Environment Agency (NEA) requested the risk assessment of selected species, families and genera of aquarium and garden pond plants. The NEA asked VKM to assess the likelihood that alien freshwater plants could survive under Norwegian conditions. For species deemed capable of establishing in Norway, VKM should assess the impact that such species could have on native ecosystems and species. This included the risk of them carrying hitchhiker organisms such as parasites and pathogens.

The NEA intend to use the risk assessment from VKM as a basis when processing import applications.

VKMs panel on Alien organisms and Trade in endangered species (CITES) was responsible for the risk assessment.