The role of sustainable urban development in biosphere conservation E 3.8 191 Table E 3.8-1 Differences between non-urban and urban ecosystems with respect to the biosphere. Sources: WBGU, 2000a; Trepl, 1994; Rebele, 1994 Non-urban ecosystem Urban ecosystem System limitation • functionally limited by the • geographically limited, eg town limit, weakest intrasystemic settlement boundary relations • but also geographically limited (forest periphery, bank of a lake) Biogeochemical and energy flows • energy received mainly from • import of fossil fuels, inorganic andthe sun organic resources • self-contained biogeochemical • high biogeochemical turnover and cycles generation of large quantities of wastes and emissions • export of wastes and emissions and import into the environmental media Degree of integration • causal and functional • living creatures in the area not (System interconnection, • heavily integrated ecosystem necessarily linked by causal and whereby each system is • corresponds to functional ecosystem functional relationships functionally linked with the • in an extreme case completely sphere of influence of disintegrated ecosystem feasible another) • system elements do not maintain any ‘ecological’ relationships • corresponds to geographic ecosystem Succession • successions of ecosystems • successions of urban biocenoses (Sequence of various stages mainly caused internally or have a historical character and are of development, succession of controlled subject to anthropogenic influences plant communities in certain • succession deterministic, ie • succession not deterministic, ecosystems over a period targeted, repeatable and not repeatable, not predictable, of time) predictable to a certain extent most on the basis of social science • final community often predictable studies • final community not predictable Invasion • generally relatively • particularly large number of alien (Invasion of living creatures resistant to the invasion of species because of favourable distribution into habitats that they otherwise alien species and naturalisation conditions do not inhabit) • number of species is generally limited to a certain level Stability and equilibrium • estabishment of a dynamic • states of equilibrium in urban equilibrium under natural biotic communities more or less ruled conditions and over a long out because the system will probably be period (decades or centuries) destroyed before the state of equilibrium can be attained Biodiversity • species abundance ‘normal’ • high diversity of sites, organisms and biotic communities can be studied in ecological terms, summarily characterized and demarcated from non-urban ecosystems (Table E 3.8-1). However, a clear separation cannot be made because human settlements vary in density and intensity of use, andthere are transitional forms and overlap areas between ecosystem types in the peripheral areas. E 3.8.3 Importance of a high quality biosphere for the urban environment The following functions of a high quality biosphere can be cited for the urban environment: • Urban climate function: green areas absorb excess precipitation and contribute to cooling by means of evaporation.With the production of oxygen, the absorption of CO 2 andthe removal of dust and pollutants they help to improve air quality (Häckel, 1990). • Function of conserving urban nature: high diversity, eg in the tree population, prevents the loss ofthe populations in the case of disease. In this way, the ecological and climatic functions of urban nature are preserved (Reduron, 1996). • Aesthetic and psychosocial function: urban green areas are used for recreation and contribute to the experience of biodiversity (Gebhard, 1993). Because ofthe increasing urban population, the
192 E Diversity of landscapes and ecosystems biosphere in urban areas is the only way for a large number of people – and in future the majority of people – to experience nature in their everyday lives (Folch, 1996). • Educational function: urban biodiversity is suitable illustrative material for environmental education (Reduron, 1996). E 3.8.4 Function of cities for conserving the biosphere Because ofthe fragmentation of landscapes andthe emission of pollutants, the effects on the biosphere emanating from towns are mainly negative (Chapter C). On the other hand, towns also contribute to the conservation ofthe biosphere and its diversity: • Because ofthe high population concentration and its central functions, towns are usually the key forums for the development of environmentally relevant opinions and consciousness. From the towns, for example, scientific findings are ‘exported’ horizontally into peripheral areas as well as vertically, eg by NGOs, into the political arena (Reduron, 1996). • Furthermore, the heterogeneity of use forms and lifestyles in towns enable the formation of high degrees of ‘socio-diversity’, which provides niches for experimentation with, and subsequent establishment of, unconventional andoften sustainable lifestyles (Feindt, 1997). • Towns directly contribute to the conservation ofthe biosphere through zoological and botanical gardens (LNU, 1997). E 3.8.5 System linkages – the biosphere, towns and global development There are fundamental system linkages between the biosphere, towns and global development (Chapter C). Characterization of important global development trends is the starting point for biosphere-centred system analysis. Many ofthese biosphere trends (conversion, fragmentation, damage and substance overload of ecosystems) also occur in urban systems. Furthermore, the development of transport routes, for example, or the increasing globalization ofthe markets is characterizing the relationship between towns andthe biosphere. An important trend is the increase in alien species in towns, which is encouraged with favourable immigration conditions and low resistance to the infiltration of alien species, as well as advantageous conditions for the spread ofthese species towards the urban biotic communities. ‘Naturalization conditions’ are critically determined by the combination of natural and social factors ofthe location. The natural locational factors (meaning the abiotic environmental factors not modified by living creatures) of most areas on the Earth would allow a higher number of species than in fact are found (Trepl, 1994). Town dwellers promote the increase of alien species by direct import and high demand. In Vancouver, the migration ofthe Chinese led to a complete reshaping of gardens; goat farming is gaining in significance in the Toronto area because ofthe increased demand for products of this kind from immigrants from the Caribbean (Keil, 1998). Barriers to expansion, such as geographical obstacles, missing habitat elements or competing demands among the species, prevent an ‘explosion of species’ in natural ecosystems. However, in urban settlements not only are the geographical barriers to expansion increasingly being removed, but the competition between species is less important.Added to this is the fact that the great number and diversity of crops and ornamental plants constantly create new living conditions for animal species adapted to them. Assessing the increase in species in towns There is a lack of good information for assessing the increase in species in towns in spite ofthe growing number of biotope mapping projects in Germany (Schulte, 1997). In particular, the increasing dismantling of geographical barriers (Rebele, 1994) speaks for the fact that in future there will be a further rise in the number of species in towns (Trepl, 1994). However, the number of species says nothing about the ‘success’ ofthe immigrant species. Thus, for example, the proportion of naturalized species is below 2 per cent ofthe plants introduced by man to semi-natural vegetation (Lohmeyer and Sukopp, 1992), but this proportion is much higher in towns (Kowarik, 1991). On the other hand, towns are characterized by local extinction of species because ofthe frequently dynamic nature of urban development (Rebele, 1991). However, no precise information can be made about these contradictory trends because as far as the human-induced capacity to overcome barriers to dispersal is concerned, there can only be speculation about the duration ofthe migration process. • It is not known whether the introduction of alien species has already reached its peak in urban areas. On the one hand it is noted that the peak ofthe influx was already exceeded in the 19th century (Jäger, 1988). On the other hand, some people are ofthe opinion that invasions will increase due to the better developed transport systems and climate change (di Castri, 1990). • There are differing evaluations ofthe consequences of disturbances to urban ecosystems.