Toward a Landscape Ecology of Cities: Beyond Buildings, Trees, and Urban Forests

Human population growth and urbanization are two dominant demographic trends in our time (Brown, 2001). World population has continued to grow exponentially for the past several decades, and reached 6.2 billion in 2002, with a current annual increase rate of almost 80 million (Earth Policy Institute, 2002). The proportion of the total world population that is urban was only a few percent in the 1800s, but it increased to 14% by 1900, rapidly jumped to about 30% in 1950 (Platt, 1994a; Wu and Overton, 2002), and is passing 50% now. Evidently, as the world’s human population has increased exponentially, so has the proportion of people living in cities (Fig. 2.1). It has been projected that 60% of the world’s population will reside in urban areas by 2025 (Platt, 1994a). In 1800, there was only one city, Beijing, in the entire world that had more than a million people; 326 such cities existed 200 years later (Brown, 2001). The urban population is growing three times faster than the rural population (Nilsson et al., 1999), and we are now witnessing a historically unprecedented and monumental, global-scale, rural-to-urban transition. To quote Lester Brown (2001), “For the first time, we will be an urban species!”

At a more regional scale, urban people already account for more than two thirds of the European population today. In the United States, 74% of the population resided in urban areas in 1989, and this number will increase to more than 80% by 2025 (Pickett et al., 2001). The historical record so far has shown that both the number of mega-cities as well as the number of urban dwellers have increased much faster in developing countries than in developed countries. For example, nearly 40% of the population of the Asia-Pacific region is now urban, and the region contains 13 of the 25 largest cities of the world. It has been estimated that by 2015 about 903 million people in Asia will live in cities with a population of over one million people (cf. Wu and Overton, 2002). While the world’s urban population is projected to rise to 60% by 2025, nearly half of these people will reside in the Asia-Pacific region. Undoubtedly, urbanization will continue to have significant impact on the environment as well as on economic, social, and political processes at local, regional, and global scales.