Grand challenges in urban ecology

The field of urban ecology has seen a major expansion in the last two decades. As the world rapidly urbanizes, human settlements constitute one of the few ecosystems on earth that are significantly increasing in their extent. As a result, interest from both the scientific community and society at large has turned its attention to the functioning of cities and their impacts on larger regions, global resources, and human well-being. Yet, urban ecosystems are fundamentally different from their natural counterparts in the dominant influence of human actions, both intentional and unintentional, on ecosystem function. While the term urban ecology is attributed to the Chicago School of sociology (Park et al., 1925), interdisciplinary, ecological studies of human settlements worldwide were facilitated most widely by UNESCO's Man and the Biosphere Program in the 1970's, and two decades later by the establishment of urban sites in the U. The resulting studies of urban-rural gradients, urban mass and energy budgets, and the " ecology of cities " have led to rapid development of methods and conceptual frameworks for integrating the social and natural sciences in the study of cities as ecosystems (Pickett et al. While many early studies of urban ecosystem structure and function were case studies of individual cities and specific urban ecological processes, there are an increasing number of cross-city comparisons that are facilitating a new, synthetic understanding of the causes and consequences of urbanization and the complex interactions among urban ecosystem components (Loram et al. While urban ecology is poised to make new breakthroughs in the functioning of complex, human-dominated ecosystems, the potential to translate scientific advances to practical applications has never been greater. Cities worldwide are struggling with a myriad of environmental and social problems as growing cities face air, water, and soil pollution, resource depletion, and aging infrastructure. As a result, there is renewed interest in developing and testing new solutions to these problems, with an increasing emphasis on approaches that apply ecological principles such as green infrastructure (Gill et al. The challenge going forward will be to apply an increasingly advanced and nuanced understanding of urban ecology in the practice of planning, designing, and monitoring cities as dynamic ecosystems. Such applications are a test of both the depth and breadth of our understanding of urban ecosystem processes. Unlike natural ecosystems, cities contain many ecosystem components that are almost entirely human-constructed. Building robust ecosystems that can withstand