ABSTRACT
Most accounts of urban ecosystems emphasize their harshness and inhospitability to native biodiversity: as one moves from the outskirts of a city to the core, the replacement of natural substrates with hard surfaces lowers the number of indigenous species (Kowarik 1990), weeding out all but “urban exploiters” (McKinney 2002) that are presumed highly adapted to this novel environment. The inner city features extremes of moisture availability and temperature which causes stress to plants (Spirn 1984; Whitlow and Bassuk 1988). The biological effects of urbanization on non-human nature are thought to represent a consequence of extreme novelty relative to the evolutionary history of the local biota (Hobbs et al. 2006): humans create new situations that many species have not experienced, causing extirpations or genetic evolution in response to these selection pressures. More recently, globalization has extended a homogenizing force into urbanization such that modern cities are increasingly self-similar throughout the globe (Pyšek 1998; McKinney and Lockwood 1999). In parallel, humans are viewed as the ultimate generalist: we can live anywhere, eat anything and we interact with others primarily by engineering ecosystems to suit our own needs. This composite view of urban humanity is admittedly bleak, but contains some fundamental truths about the role of the built environment as an increasingly dominant force in shaping global ecosystems. Urbanization is proceeding rapidly in most of the world, and the built environment is profoundly different from many of the original, native habitats that it replaced. This chapter presents an alternative view of the process of urbanization that emphasizes the biological evolution of urban habitats, the similarities between built and natural forms, and the role of humans as ecological specialists. The contrarian view presented here is called the Urban Cliff Hypothesis and developed as part of a research program on naturally occurring cliffs and rock outcrops by a team of biologists (Larson et al. 2000). While cultural differences among human groups are an obvious consequence of cultural evolution, this account centralizes the biological similarities among human populations and their common evolutionary origins in Africa. It also presents a biologist’s attempt to see urban habitats from the perspective of the other organisms that live there.
