Shrinking Cities: Effects on Urban Ecology and Challenges for Urban Development

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URBAN ECOLOGY – DEFINITIONS AND CONCEPTS

Wilfried Endlicher, Marcel Langner, Markus Hesse, Harald A. Mieg, Ingo Kowarik, Patrick Hostert, Elmar Kulke, Gunnar Nützmann, Marlies Schulz, Elke van der Meer, Gerd Wessolek, Claudia Wiegand

1. Introduction

Earth’s population more than doubled during the second half of the twentieth century: from approximately 2.5 billion in 1950 to over 6 billion in 2000, and at the time of writing in 2007 has reached a figure of over 6.6 billion. Alongside this exponential growth of population is another important demographic trend: According to the United Nations, the anticipated population growth between 2000 and 2030, approximately 2 billion people, will be concentrated in urban areas (UN 2004). The 21st century will be the century of urbanisation. By the year 2030 more than 60 per cent (4.9 billion) of the estimated world population (8.1 billion) will live in urban settlements, compared to 29 per cent in 1950. The 50 per cent mark is expected to be reached in the year 2007. In 2025, more than a dozen urban agglomerations will have over 20 million inhabitants, and some will have over 30 million. 23 of the 25 biggest urban agglomerations on the planet will be in Africa, Asia, and Latin America, rather than in Europe or North America (KRAAS 2003). These megacities are considered ‘hotspots’ of global change (KRAAS 2007).

Urbanised areas cover between approximately one and six per cent of Earth’s surface, yet they have extraordinarily large ecological ‘footprints’ and complex, powerful, and often indirect effects on ecosystems (REES & WACKERNAGEL 1994).

2. Ways to define urban ecology

The aim of ‘Urban Ecology’ is to study these effects. According to SUKOPP & WITTIG (1998), the term ‘Urban Ecology’ (in German Stadtökologie) can be defined in two ways. Within the natural sciences, urban ecology addresses biological patterns and associated environmental processes in urban areas, as a subdiscipline of biology and ecology. In this sense, urban ecology endeavours to analyse the relationships between plant and animal populations and their communities as well as their relationships to environmental factors including human influences. From this perspective, the research is unconstrained by anthropocentric evaluations. However the second, complementary, definition implies the anthropocentric perspective. Here, urban ecology is understood as a multidisciplinary approach to improving living conditions for the human ← 1 | 2 → population in cities, referring to the ecological functions of urban habitats or ecosystems for people – and thus including aspects of social, especially planning, sciences. From an even broader view, cities can be considered as emergent phenomena of local-scale, dynamic interactions among socio-economic and biophysical forces. These are both complex ecological entities that have their own unique internal rules of behaviour, growth, and evolution, and important global ecological forcing influences (ALBERTI et al. 2003). Urban ecology is the study of ecosystems that includes humans living in cities and urbanising landscapes. It investigates ecosystem services which are closely linked to patterns of urban development (ALBERTI 2005). Urban ecology is an interdisciplinary field that supports societies’ attempts to become more sustainable. It has deep roots in many disciplines including geography, sociology, urban planning, landscape architecture, engineering, economics, anthropology, climatology, public health, and ecology. Because of its interdisciplinary nature and unique focus on humans and natural systems within urbanised areas, ‘urban ecology’ has been used variously to describe the study of humans in cities, nature in cities, and the coupled relationships of humans and nature (MARZLUFF et al. 2008; Fig. 1).

3. Conceptual history of research in urban ecology

Urban ecology has many disciplinary roots. In recent decades, the conceptual approach of the ‘Berlin School of Urban Ecology’, promoted mainly by HERBERT SUKOPP since the 1970s, was influential (WÄCHTER 2003). By this approach, urban habitats and associated environmental processes were analysed at local and regional scales by different disciplines of natural sciences. This includes biodiversity patterns as well as characteristics of urban soils and climate and their variation in time and space due to changing urban land uses. While the contemporary ecosystem approach of DUVI-GNEAUD (1974) mainly addressed fluxes of energy and matters at the city level, the Berlin approach focussed on the explicit spatial variation of ecological components within urban environments. This also led to the first model of a city characterised by idealised variation in climate, soils, terrain, vegetation and fauna along a transect from the densely built-up city centre to the outskirts (SUKOPP 1973; Fig. 2). SUKOPP distinguishes a core surrounded by three rings – the densely built-up central core area, a ring with more open space, where some smaller cores of densely built-up sub-centres may be found and finally the interior and exterior border zones. Concentric models of the spatial organisation of land-uses have existed since VON THÜNEN (1826) and BURGESS (a member of the Chicago School of Social Ecology; BURGESS 1925 and 1929) developed a model of the concentric structure of cities from the perspective of social sciences. SUKOPP was the first to qualify such a model with a broad array of ecological factors. ← 2 | 3 →

Fig. 1: Basic components of the urban ecosystem; this concept is focused on the spheres of the Earth system which are important for cities. The processes between different spheres and the impacts of the anthroposphere (six selected examples) are of special interest (MARZLUFF et al. 2008; modified)

Perhaps the most often reproduced diagram in urban ecology shows a transect through the concentric rings and its consequences for climate, soil and water, topography, vegetation and animal life in the different urban zones (Fig. 2). Many studies of urban ecology follow Sukopp’s transect approach, comparing the specific ecological situation of each zone with the others, and the whole city with its surrounding environment (e.g. the urban heat island characterises the maximum temperature differentiation between a climate station in the core area and another outside the built-up area; ALCOFORADO & ANDRADE 2007). The urban heat island of densely built-up environments is an important factor of additional heat stress in summer months. Lower work efficiency, enhanced morbidity and cardio-vascular diseases ← 3 | 4 → are related to high solar radiation, air temperatures and humidity (KOVATS & JENDRITZKY 2006).

Fig. 2: Transect through the urban built-up structure of Berlin and the ecological consequences for different spheres; this classic concept concentrates on the impacts of urbanisation for five layers: climate, soil and water, relief, vegetation and fauna (adapted after SUKOPP 1973) ← 4 | 5 →

The traditional model of the multinuclear city proposed by HARRIS & ULLMAN (1945) from the Chicago School of Social Ecology is another approach to urban ecology. This includes the classification of built-up structures of cities (Stadtstrukturtypen or Baukörperstrukturen in German). The ecological conditions of each structural type (e.g. industrial area, central business district, suburb with housing function, middle class housing quarter) are investigated and their characteristics may be compared. WITTIG, SUKOPP & KLAUSNITZER (1998) gave a detailed description of the built-up types in German cities. WICKOP et al. (1999) used this model for their ecological studies of Leipzig. This is another widely used method in urban ecology. Urban ecology can be understood as a spatial science in the same way as geography. Therefore the scale of the studies to be carried out is important. Three different scales should be distinguished, especially in larger cities: the micro-scale of the local neighbourhood with its special built-up characteristics where the study or field experiment is carried out, the meso-scale of the district, which features a combination of different land use (built-up) types and finally the macro-scale of the total urban area, sometimes composed of different administrative entities or even cities. The results of the studies may permit a certain generalisation for the three scales and some typical neighbourhoods/districts/cities may be identified, leading to prototypes of a ‘virtual city’ (Fig. 3).

Urban ecology addresses processes in space and time. Besides the spatial dimension, four main processes of change are the focus of recent research: changes in urban biodiversity, climate, human demography and economy:

Urban land use significantly affects biodiversity patterns. Until the 1960s cities were perceived as ‘biological deserts’, whereas they are currently considered as ‘hotspots’ of botanic and animal diversity. Species respond quite differently to urbanisation, with a decline in native species and increase in introduced species as a general trend (KOWARIK 1990). These changes in urban biota are currently regarded as major drivers of global homogenisation (MCKINNEY 2006). However, regional studies have demonstrated that both native and non-native species richness is higher in urban areas than adjacent areas and that non-native species may also contribute to the dissimilarity of urban floras (KÜHN & KLOTZ 2006). Future analysis should thus examine the role of cities in endangering or conserving biodiversity in depth. Cities are important drivers of climate change because about 75 per cent of greenhouse gas emissions are produced in urban territories. Simultaneously, however, cities are especially vulnerable to climate change as Working Group II of the Intergovernmental Panel of Climate Change concludes in its 4th Assessment Report (IPCC 2007). Many components and processes of the earth system are affected, especially the atmosphere (rising temperatures and extreme weather events), the hydrosphere ← 5 | 6 → (rising water levels) and the biosphere (drastic changes to biodiversity). Urbanised areas may serve as subject of field experiments in order to investigate plant responses to climate change, since temperature and C0₂-concentrations are already increased in cities (ZISKA et al. 2003). Impacts are highly variable, but include an increased burden of diseases, increased morbidity and mortality from more frequent and intense heat waves superimposed on the urban heat island. Coastal megacities are particularly at risk from floods, storms and droughts (KRAAS 2003 & 2007).

Fig. 3: Example of an approach in urban ecology that considers three scales, with specific reference to processes of change

Demographic change may also exert an influence on the anthroposphere. In highly industrialised countries people are growing older than ever before, while birth rate is simultaneously decreasing in countries like Germany (KAUFMANN 2005). The proportion of senior citizens is expected to increase, and the pyramid of population is likely to change its shape. This causes modifications of behaviour and demand for living space, for example. However, it is indicative that demographic changes offer potential for improving the ecological conditions of cities, not only due to a reduced number of individuals and therefore demand for water, energy, transport etc., but also in the context of a decreasing pressure on land use and the possibility of alternatives to the classical growth of urban development. Conversely, to ensure costefficient ← 6 | 7 → technical infrastructures, building density should not fall below a certain threshold (see paper of WESTPHAL in this book).

Economic Change is one of the most important factors for the function and development of urban agglomerations. A town’s role in the interregional and supranational network of cities is affected by its economic structure and, in addition, existing economic activity dominates the urban environment. Cities have undergone rapid changes to their economic structures during recent decades: They are becoming increasingly integrated into global supply and demand systems which depends on the process of globalisation. Alongside these developments a key factor in advanced economies for urban agglomerations is the switch from industrial to service based economies; spatial characteristics of this change are the appearance of brownfields on former industrial land and growing demand for spaces for high-ranking services. The four above-mentioned changes are important issues to be taken into account in future urban ecological research and planning processes (STONE 2005).