Urban Ecology
London Biodiversity Action Plan

We need to balance the undoubted problems that some exotic species can create against the rich diversity, historical and cultural interest and the considerable local distinctiveness that the vast majority bring to the Capital. Most introductions are benign, and in urban areas the natural colonisation of wasteland by native and exotic species has formed communities that are distinctive and unique to cities.

Source: English Nature, www.english-nature.org.uk