Human-caused impact on preserved vegetation

Climate, soil, air, and water in cities respond sensitively to human impacts that are usually much stronger than in rural areas. In spite of the multitude of impacts cities are quite rich in habitats as well as in plant and animal species. Typically, disturbance of urban ecosystems leads to a decrease in the number of species native to the region and an increase of introduced non-native species. This process is at least partly reversed in the course of succession. Disturbance and the recovery from it during succession are key factors determining the habitat mosaic in urban ecosystems. As an ever increasing percentage of the world's population lives in cities, the conservation of urban biota is of great importance. In many parts of the world urban ecosystems and their impact regimes as a prerequisite of conservation are not sufficiently studied. (C) 2003 Published by Elsevier B.V.