Is didymo an exotic species?

Didymo is an exotic organism to New Zealand, which means it is non-native and was introduced from another geographic region.  Exotic species are not necessarily harmful. In fact New Zealand has many exotic valued species such as apples, maize, and sheep.  However, when a non-native species occupies lands or waters, particularly in natural communities, it can often cause negative ecological or economic effects because the insects, diseases, or other grazers/predators that help control it in its native range may not be present in the new location.