This book describes the challenges and opportunities that urban environments present to the plants and animals that inhabit cities and the ways that those organisms and entire ecosystems respond. The broad outlines of life are always the same: the need to find resources, to avoid being eaten or being killed, and to reproduce successfully. Ecologists have long studied how these factors determine which species live in a particular place and how those species interact with each other and the ecosystem. Only recently, however, has the focus of ecological science turned to life in urban environments.

The science of ecology developed in the late nineteenth century through the integration of three advances: detailed natural history of species and their habits, Darwin's emphasis on species interactions and change over time, and improved understanding of the physiology of plants and animals. The new field struggled to define the very nature of its subject of study, the communities of plants and animals that coexist and interact in one place and time. Was each community a tightly knit whole or merely a loose assemblage? What key factors determine how communities function?

Faced by these fundamental questions, ecologists deferred thinking about the massive disruption that cities bring to natural processes until those processes themselves could be better understood. As that understanding emerged, ecologists began turning their attention to cities. The modern practice of urban ecology grew from several distinct sources. In nineteenth-century Europe, studies of the plants of urban gardens, cemeteries, and highly disturbed building sites established a foundation of natural history information. These studies were among the first to distinguish between introduced and native species, and show how urban climate and urban pollution determine which plant species persist.