Cities around the world are facing an ever-increasing variety of challenges that seem to make more sustainable urban futures elusive. Many of these challenges are being driven by, and exacerbated by, increases in urban populations and climate change. Novel solutions are needed today if our cities are to have any hope of more sustainable and resilient futures. Because most of the environmental impacts of any project are manifest at the point of design, we posit that this is where a real difference in urban development can be made. To this end, we present a transformative model that merges urban design and ecology into an inclusive, creative, knowledge-to-action process. This design-ecology nexus—an ecology for cities—will redefine both the process and its products. In this paper we: (1) summarize the relationships among design, infrastructure, and urban development, emphasizing the importance of joining the three to achieve urban climate resilience and enhance sustainability; (2) discuss how urban ecology can move from an ecology of cities to an ecology for cities based on a knowledge-to-action agenda; (3) detail our model for a transformational urban design-ecology nexus, and; (4) demonstrate the efficacy of our model with several case studies.

The urban realm is changing rapidly and becoming increasingly interconnected across continents, and across contrasting types of land covers, while at the same time facing new environmental threats and experiencing new demographic and social pressures. The urban component of the global ecosystem can be made more sustainable by incorporating the ecological understanding of resilience into the discourse. Sustainability is seen as a social, normative goal, which can be promoted using the mechanisms of ecological resilience. Ecological resilience differs from engineering resilience. Ecological resilience emphasizes the capacity of a site to adjust to external shocks and changes in controlling interactions, while engineering resilience emphasizes its ability to return to a state that existed before perturbation. Ecological resilience is particularly appropriate to urban systems, given the extent and open-ended nature of the changes and challenges they face. Adaptive processes are explored as contributions to the achievement of a successful adaptive cycle in urban socio-ecological systems. Key tools for incorporating the ecological thinking about resilience into the social discourse include landscape or patch ecology, the novel idea of the metacity, an assessment of ecological and design models, and the use of designs as experiments.

Urbanization continues to be a transformative process globally, affecting ecosystem integrity and the health and well being of people around the world. Although cities tend to be centers for both the production and consumption of goods and services that degrade natural environments, there is also evidence that urban ecosystems can play a positive role in sustainability efforts. Despite the fact that most of the urbanization is now occurring in the developing countries of the Global South, much of what we know about urban ecosystems has been developed from studying cities in the United States and across Europe. We propose a conceptual framework to broaden the development of urban ecological research and its application to sustainability. Our framework describes four key contemporary urban features that should be accounted for in any attempt to build a unified theory of cities that contributes to urban sustainability efforts. We evaluated a range of examples from cities around the world, highlighting how urban areas are complex, connected, diffuse and diverse and what these interconnected features mean for the study of urban ecosystems and sustainability.

Summary 1. By the end of this decade, the majority of people will live in cities and suburban areas. Urban areas, including suburbs and exurbs, are expanding rapidly worldwide. 2. Plant ecology has largely ignored cities, or has primarily focused on the discrete urban green spaces within cities. 3. Plant ecology is increasingly engaging urban ecosystems as integrated natural-human systems, in which human agency is part of the complex of feedbacks. 4. Linking plant ecology with urban design (architecture, landscape architecture, civil engineering and urban planning) can help to integrate research and understanding of plants into the structure of cities, and to make use of urban design projects as ecological research tools. 5. Synthesis . A cycle of ecological design illustrates the linkage of plant ecological research with the ongoing transformation of urban systems by urban designers and civil society. Quality of life, human health, public appreciation of ecological processes in cities, and scientific understanding can all be enhanced by participating in a cycle of ecological urban design.

Describing and quantifying the spatial heterogeneity of land cover in urban systems is crucial for developing an ecological understanding of cities. This paper presents a new approach to quantifying the fine-scale heterogeneity in urban landscapes that capitalizes on the strengths of two commonly used approaches—visual interpretation and object-based image analysis. This new approach integrates the ability of humans to detect pattern with an object-based image analysis that accurately and efficiently quantifies the components that give rise to that pattern. Patches that contain a mix of built and natural land cover features were first delineated through visual interpretation. These patches served as pre-defined boundaries for finer-scale segmentation and classification of within-patch land cover features which were classified using object-based image analysis. Patches were then classified based on the within-patch proportion cover of features. We applied this approach to the Gwynns Falls watershed in Baltimore, Maryland, USA. The object-based classification approach proved to be effective for classifying within-patch land cover features. The overall accuracy of the classification maps of 1999 and 2004 were 92.3% and 93.7%, respectively. This exercise demonstrates that by integrating visual interpretation with object-based classification, the fine-scale spatial heterogeneity in urban landscapes and land cover change can be described and quantified in a more efficient and ecologically meaningful way than either purely automated or visual methods alone. This new approach provides a tool that allows us to quantify the structure of the urban landscape including both built and non-built components that will better accommodate ecological research linking system structure to ecological processes.