Why is Rhododendron ponticum considered invasive in some places?

In the context of the Ericales order, Rhododendron ponticum is considered an invasive species in the British Isles because of its ability to form dense, impenetrable thickets. These thickets are so pervasive that they effectively prevent the growth of almost all other vegetation in the areas they occupy.1

Introduced to the region in the late 18th century from Spain, Portugal, and Turkey, this plant has successfully established itself as a dominant, non-native presence that disrupts local ecosystems by outcompeting native flora.1