The improvement in lichen floras in recent years as a consequence of clean air legislation has been demonstrated in many cities. It is apparent from such studies that re-establishing lichen floras differ from those formerly wiped out by air pollution. Work in the West Yorkshire conurbation, U.K. over the past 30 years has documented the distributional and ecological responses of lichens to qualitative and quantitative changes in air pollution. From these longterm data, it has been possible to model biodiversity dynamics in both deteriorating and ameliorating environments