Early investigations on the ecology of cities were in the tradition of natural history and focused on single biotopes. Of special interest were the plants and animals introduced into newareas directly or indirectly by man. In Central Europe, studies of anthropogenic plant migrations and cultural history were combined in a specific way, the so called Thellungian paradigm. The succession of vegetation on ruins after the bombing during the Second World War was studied in many cities. Ecological studies on whole cities started in the 1970s with investigations on energy flow and nutrient cycling. Today the term urban ecology is used in two different ways: in developing programs for sustainable cities, and in investigation of living organisms in relation to their environment in towns and cities.