In and around London, over the past few years, I have noticed the sudden population increase of the Rose-Ringed Parakeet (Psittacula krameri). Needless to say, this is an invasive species of bird. This parakeet species is indigenous to Sub-Saharan African Rain forests, but it is the sub species P. krameri manillensis from the Indian Subcontinent that has managed to populate the city of London.
The rapid deforestation and urbanization of the forests was devastating for most of the parot species found in this area, whom have been pushed into the small pockets of remaining forests. In contrast, P. krameri manillensis has remained and populated the new urban habitat, demonstrating their ability to adapt.
Usually, invasive species are regarded as detrimental to the habitats they are introduced to, as well as the biggest cause of lose of biodiversity. In most cases this is true (Grey Squirrel, Signal Crayfish and Harlequin Ladybird), as most replace native species of a similar niche. The parakeets do not seem to have significant overlaps in the niche they have taken up in London.
So it would appear that these colourful, yet noisy, new residents of London are welcome as they appear not to be a problematic invasive species.