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164 7 Reverse Experiments To develop and test a theory of urban ecology and the role that cities play on a planetary scale, we need to redefine research methods and experiments and rewrite the protocols for collecting and synthesizing data. Several methodological challenges have become evident in the study of urban ecosystems: the complex dynamics and multiple confounders in determining causal effects, the difficulty of generalizing across regions and scales, the mismatches of scale across human and ecological system domains, the lack of predictability and certainty, and the problems of defining reference conditions for coupled human-natural systems and of quantifying human well-being. The unprecedented availability of detailed data, increased computing capability, high-resolution real-time sensors, and widespread mobile communication offers unique opportunities to meet these challenges. This chapter discusses the idea of designing studies as reverse experiments through which we can learn how urban ecosystems function, evolve, and succeed. Inverse Problems Studying urban ecosystems poses new challenges to ecology. The complex interplay between natural and human systems is one such challenge. A major problem is that most of the variables and interactions driving eco­ system function in urbanizing regions are not known. Even more challenging is the fact that scientists have yet to define and describe what constitutes a functioning urban ecosystem. Urban ecosystems are complex coupled human-natural systems in which organisms and communities interact according to mechanisms Reverse Experiments 165 and rules not fully explained in ecosystem ecology. Evidence from an increasing number of studies indicates that the assumptions of traditional theories in ecology (e.g., the disturbance hypothesis, in which humans are not included as agents) and social science (e.g., the idea that rational agents interact through efficient and stable markets) do not hold. To understand such systems, it may be necessary to both revise concepts of ecosystem function, stability, and optimality and develop new definitions of the ecosystem concept (O’Neill 2001), its dynamics (Pickett et al. 2011), and evolutionary feedbacks (Alberti 2015). We must redefine our research approaches, methods, and experiments. In urban ecology, we face what geophysicists call inverse problems. An inverse problem is one whose initial inputs are a collection of observed measurements that are used to infer a model (or models) of the governing system that generated the outcomes; this model is the solution to the inverse problem. To explain the nature of inverse problems, the Russian mathematician Sergey Kabanikhin (2008) pointed to the way our brain uses previous experience to reconstruct an image by interpolating the limited information our eyes provide. Similarly, if we have accumulated prior information through experience, we are far less likely to make errors in interpreting and resolving a problem. However, when we attempt to understand complex phenomena or solve problems we have not previously encountered, the probability of error increases; hence, the solution is unstable. These are what mathematicians call ill-posed problems: problems for which more than one solution may exist (Sabatier 2000). Inverse problems are intrinsic to urban ecology. Over the centuries, human societies have experienced and learned to deal with new challenges (e.g., access to clean water, poor sanitation, and air pollution) posed by the transition to urban life. But the scale and pace of current urbanization are unparalleled in the history of humanity and Planet Earth. Most current problems that local communities face in cities are new for humanity. Equally unprecedented are the scientific and policy challenges that the emergent problems pose, especially in the face of rapid climate change. What makes urban communities resilient to extreme climate events? How can we best prepare for, and mitigate, potential impacts? How can we adapt to new conditions? Inverse methods aim to reconstruct phenomena that are difficult to observe or measure directly but which can be inferred from available 166 Chapter 7 observations. Scientists have documented several examples of regime shifts in ecosystems, such as the transition from a coral reef to an algae reef, from a tropical forest to a grassland, or from clear water to eutrophication . We still do not fully understand the emergence of regime shifts and their potential effects on urban ecosystems. What, for ­example, are the processes through which biogeochemical and human activities, coupled with the built infrastructure, lead to urban water eutro­phication or catastrophic flooding events? In studying the Earth, geophysicists seek to determine a continuous function of the space variables representing the Earth’s properties with infinitely many degrees of freedom, as Snieder and Trampert (1999) pointed out. Yet, real experiments...


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