In addition to a small-scale structure and varied landscape, time is an essential element for species diversity in a city. Parts of old city parks and unused areas are managed less intensively, so specific biotopes may evolve here over time. In Hamburg, for instance, two-thirds of all special plants occur in only one or two places. Der Naturschutzrat, 2008
Therefore it is important to protect such locales, because these special biotopes may be destroyed forever by a construction activity in the neighbourhood. It is possible to create conditions for the development of specific biotopes, but it is not possible to create a biotope.