Urban Systems

Urbanization is a global multidimensional process that manifests itself through rapidly changing human population densities and changing land cover. The growth of cities is due to a combination of four forces: natural growth, rural to urban migration, massive migration due to extreme events, and redefinitions of administrative boundaries. Half of the world’s population today lives in urban areas, a proportion expected to increase by 2/3 within 50 years. Today, over 300 cities have a population of more than 10 and 19 megacities exceed 10. As urbanization is accelerating, the growth of cities forms large urban landscapes, particularly in developing countries. (Urban landscape is here defined as an area with human agglomerations with >50% of the surface built, surrounded by other areas with 30–50% built, and overall a population density of >10 ind. ha .) For example, during the last 20 years in China, clusters of cities have emerged forming at least five mega-urban landscapes. These large and densely urbanized regions have each between 9 and 43 large cities located in close proximity and a population ranging from 27 to 75 million people. This rapid urbanization represents both a challenge and an opportunity to ensure basic human welfare and a viable global environment. The opportunity lies in that urban landscapes also are the very places where knowledge, innovations, human and financial resources for finding solutions to global environmental problems are likely to be found. Since urbanization is a process operating at multiple scales, factors influencing environmental change in urban landscapes often originate far beyond city, regional, or even national boundaries. Fluctuation in global trade, civil unrest in other countries, health pandemics, natural disasters, and possibly climate change and political decisions are all factors driving social–ecological transformations of the urban landscape. Mismatches between spatial and temporal scales of ecological process on the one hand, and social scales of monitoring and decision making on the other have not only limited our understanding of ecological processes in urban landscapes, they have also limited the integration of urban ecological knowledge into urban planning. In ecology there is now a growing understanding that human processes and cultures are fundamental for sustainable management of ecosystems, and in urban planning it is becoming more and more evident that urban management needs to operate at an ecosystem scale rather than within the traditional boundaries of the city. Although studies of ecological patterns and processes in urban areas have shown a rapid increase during the last decade, there are still significant research gaps that constrain our general understanding of the effects of urbanization processes. The vast majority of studies so far have been short term (one to two seasons), conducted in cities in northern Europe or the US, have lacked experimental approaches, focused on either birds or plants, while other taxa are rarely represented and have only included portions of a rural–urban gradient. Most significantly, we nearly completely lack studies in rapidly growing urban landscapes in tropical developing countries that are rich in biodiversity and are just beginning to address the complexity of human settlements in the tropics. Of further significance is that urban landscapes provide important large-scale probing experiments of the effects of global change on ecosystems, since, for example, significant warming and increased nitrogen deposition already are prevalent and because they provide extreme, visible, and measurable examples of human domination of ecosystem processes. Urban landscapes may be viewed as numerous large-scale experiments producing novel types of plant and animal communities and novel types of interactions among species, and as such deserve the full attention of not only evolutionary biologists and ecologists but also of students of social–ecological interactions.