URBAN ECOLOGY AND SPECIAL FEATURES OF URBAN ECOSYSTEMS

The paper deals with urban ecology as a biological science and applies some of the topics of general importance in ecology to the special conditions found in towns and cities. I consider whether cities should be treated as one integrated ecosystem, or as an assemblage of various ecosystems. In contrast to the holistic, organismic concept of the ecosystem as a new hierarchical level of organization and as an evolving whole which guides the development of the species, I follow the methodological definition of Tansley (1935), who defined ecosystems as 'mental isolates' for 'the purpose of study'. According to Evans (1956) ecosystems can be defined at every level of the biological organization, at the level of the organisms, populations or communities. The introduction of species from other biogeographical regions is a worldwide phenomenon, but the proportion of successfully established introduced species is higher in cities than in rural or forest areas. This is due to numerous colonizing species which fit the anthropogenous habitats. Due to unequal rates of immigration and extinction of species, urban habitats show an imbalanced turnover of species. Another special feature of urban ecology is man-induced disturbance, which initiates the colonization of disturbed or newly created habitats. According to the type of substrate and the availability of diaspores there may be both primary, secondary or intermediate types of succession. Besides disturbance, the main component for structuring communities is biological interactions. In this paper I discuss some aspects of competition, predation and mutualism. The special feature of higher species' richness of cities compared with ecosystems in the countryside can be explained by the high habitat diversity of urban and industrial areas. Although some components which contribute to the complexity of communities, such as competition, are of minor importance in various urban habitats, there may be communities of high complexity. I also consider community characteristics such as stability and productivity. Since most urban communities are in a state of inequilibrium, theories of stability based on equilibrium are inadequate for urban ecosystems. The productivity of the 'ecosystem city' mainly depends on the area of unsealed open space and the successional stage of the plant communities of the various habitats.