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There are (per 2018) around 230 causing high risk and 1500 foreign species in total.

The next question can be found by following this hint:

Now you find yourself in the arboretum. Head south-east along the path to make your way to the Blågran (Picea pungens) which plays host to seven species of lichen! Look closely, can you find them?

Eager to learn more?

Invasive species are a problem. There exists an official list from 2018 of alien species and their risk towards the biodiversity in Norway.

If you want to learn more about Sitkagran (Sitka spruce), take a look here.

If you are interested in reading more about Sitkagran (Sitka spruce) in Norway, here is an article conserning alien tree species along the coast of Helgeland.