What do you understand by invasion by foreign species? How does it affect biodiversity? 

Sometimes. intentional or non-intentional introduction of a foreign species endangers the existence of indigenous species. This creates an imbalance in the whole ecosystem. Lantana and water hyacinth were introduced for beautification. Lantana was brought by the British in $1807$ and it was first planted in the Botanical Garden in Calcutta. Gradually, this plant proliferated in the entire subcontinent. Now, this plant poses great danger to the local biodiversity because no other plant can grow near it and none of the animals eat this plant. Similarly. water hyacinth was brought from Brazil. Today, it has spread to water bodies throughout India. Some foreign species of grass came along with imported food grains. e.g. Gajar grass (Parthenium) came along with wheat which was imported from America in $1950$. Parthenium is among the most notorious weeds which is not even eaten by animals. This plant has many chemicals which cause allergy. Parthenium has become a threat for our biodiversity.