Shifting forward: Urban ecology in perspective.

The world has become urban; cities increasingly shape our worldviews, relation to other species, and the large-scale, long-term decisions we make. Cities are nature, but they need to align better with other ecosystems to avoid accelerating climate change and loss of biodiversity. We need a science to guide urban development across the diverse realities of global cities. This need can be met, in part, by shifts in urban ecology and its linkages to related sciences. This perspective is a "synthesis of syntheses", consolidating ideas from the other articles in the Special Section. It re-examines the role of urban ecology, and explores its integration with other disciplines that study cities. We conclude by summarizing the next steps in the ongoing shift in urban ecology, which is fast becoming an integral part of urban studies.

From urban ecology to urban enquiry: How to build cumulative and context-sensitive understandings.

This paper positions urban ecology as increasingly conversant with multiple perspectives and methods for understanding the functions and qualities of diverse cities and urban situations. Despite progress in the field, we need clear pathways for positioning, connecting and synthesising specific knowledge and to make it speak to more systemic questions about cities and the life within them. These pathways need to be able to make use of diverse sources of information to better account for the diverse relations between people, other species and the ecological, social, cultural, economic, technical and increasingly digital structures that they are embedded in. Grounded in a description of the systemic knowledge needed, we propose five complementary and often connected approaches for building cumulative systemic understandings, and a framework for connecting and combining different methods and evidence. The approaches and the framework help position urban ecology and other fields of study as entry points to further advance interdisciplinary synthesis and open up new fields of research.

The relational shift in urban ecology: From place and structures to multiple modes of coproduction for positive urban futures.

This perspective emerged from ongoing dialogue among ecologists initiated by a virtual workshop in 2021. A transdisciplinary group of researchers and practitioners conclude that urban ecology as a science can better contribute to positive futures by focusing on relationships, rather than prioritizing urban structures. Insights from other relational disciplines, such as political ecology, governance, urban design, and conservation also contribute. Relationality is especially powerful given the need to rapidly adapt to the changing social and biophysical drivers of global urban systems. These unprecedented dynamics are better understood through a relational lens than traditional structural questions. We use three kinds of coproduction-of the social-ecological world, of science, and of actionable knowledge-to identify key processes of coproduction within urban places. Connectivity is crucial to relational urban ecology. Eight themes emerge from the joint explorations of the paper and point toward social action for improving life and environment in urban futures.

A transformative shift in urban ecology toward a more active and relevant future for the field and for cities.

This paper builds on the expansion of urban ecology from a biologically based discipline-ecology in the city-to an increasingly interdisciplinary field-ecology of the city-to a transdisciplinary, knowledge to action endeavor-an ecology for and with the city. We build on this "prepositional journey" by proposing a transformative shift in urban ecology, and we present a framework for how the field may continue this shift. We conceptualize that urban ecology is in a state of flux, and that this shift is needed to transform urban ecology into a more engaged and action based field, and one that includes a diversity of actors willing to participate in the future of their cities. In this transformative shift, these actors will engage, collaborate, and participate in a continuous spiral of knowledge → action → knowledge spiral and back to knowledge loop, with the goal of co producing sustainable and resilient solutions to myriad urban challenges. Our framework for this transformative shift includes three pathways: (1) a repeating knowledge → action → knowledge spiral of ideas, information, and solutions produced by a diverse community of agents of urban change working together in an "urban sandbox"; (2) incorporation of a social-ecological-technological systems framework in this spiral and expanding the spiral temporally to include the "deep future," where future scenarios are based on a visioning of seemingly unimaginable or plausible future states of cities that are sustainable and resilient; and (3) the expansion of the spiral in space, to include rural areas and places that are not yet cities. The three interrelated pathways that define the transformative shift demonstrate the power of an urban ecology that has moved beyond urban systems science and into a realm where collaborations among diverse knowledges and voices are working together to understand cities and what is urban while producing sustainable solutions to contemporary challenges and envisioning futures of socially, ecologically, and technologically resilient cities. We present case study examples of each of the three pathways that make up this transformative shift in urban ecology and discuss both limitations and opportunities for future research and action with this transdisciplinary broadening of the field.

Forging just ecologies: 25 years of urban long-term ecological research collaboration.

We ask how environmental justice and urban ecology have influenced one another over the past 25 years in the context of the US Long-Term Ecological Research (LTER) program and Baltimore Ecosystem Study (BES) project. BES began after environmental justice emerged through activism and scholarship in the 1980s but spans a period of increasing awareness among ecologists and environmental practitioners. The work in Baltimore provides a detailed example of how ecological research has been affected by a growing understanding of environmental justice. The shift shows how unjust environmental outcomes emerge and are reinforced over time by systemic discrimination and exclusion. We do not comprehensively review the literature on environmental justice in urban ecology but do present four brief cases from the Caribbean, Africa, and Asia, to illustrate the global relevance of the topic. The example cases demonstrate the necessity for continuous engagement with communities in addressing environmental problem solving.

Systems in Flames: Dynamic Coproduction of Social-Ecological Processes.

Ecologists who study human-dominated places have adopted a social-ecological systems framework to recognize the coproduced links between ecological and social processes. However, many social scientists are wary of the way ecologists use the systems concept to represent such links. This wariness is sometimes due to a misunderstanding of the contemporary use of the systems concept in ecology. We aim to overcome this misunderstanding by discussing the contemporary systems concept using refinements from biophysical ecology. These refinements allow the systems concept to be used as a bridge rather than a barrier to social-ecological interaction. We then use recent examples of extraordinary fire to illustrate the usefulness and flexibility of the concept for understanding the dynamism of fire as a social-ecological interaction. The systems idea is a useful interdisciplinary abstraction that can be contextualized to account for societally important problems and dynamics.

A framework for research on recurrent acute disasters.

Disaster science examines the causes, behaviors, and consequences of hazardous events, from hurricanes to wildfires, flooding, and major industrial accidents. Individual disasters are recurring more frequently and with greater intensity. Recurrent acute disasters (RADs) are sequential disasters that affect a specific locale over time. While disaster science has matured in recent years, understanding of the distinctive characteristics of RADs varies by discipline and lacks predictive power. A theoretical framework is presented by borrowing in part from mathematical topology and disturbance ecology. The recurrent disasters affecting Puerto Rico 2017-2020 are examined as a case example to test the framework. A key variable is the complex characteristics of legacy conditions created by one disaster that influence the effects of subsequent disasters. Substantial improvements in disaster response, recovery, and preparedness can be gained by adopting a RAD-based approach.

Beyond city expansion: multi-scale environmental impacts of urban megaregion formation in China.

Environmental degradation caused by rapid urbanization is a pressing global issue. However, little is known about how urban changes operate and affect environments across multiple scales. Focusing on China, we found urbanization was indeed massive from 2000 to 2015, but it was also very uneven, exhibiting high internal city dynamics. Urban areas in China as a whole became less green, warmer, and had exacerbated PM2.5 pollution. However, environmental impacts differed in newly developed versus older areas of cities. Adverse impacts were prominent in newly urbanized areas, while old urban areas generally showed improved environmental quality. In addition, regional environmental issues are emerging as cities expand, connect and interact to form urban megaregions. To turn urbanization into an opportunity for, rather than an obstacle to, sustainable development, we must move beyond documenting urban expansion to understand the environmental consequences of both internal city dynamics and the formation of urban megaregions.

Preparing interdisciplinary leadership for a sustainable future.

Urgent sustainability challenges require effective leadership for inter- and trans-disciplinary (ITD) institutions. Based on the diverse experiences of 20 ITD institutional leaders and specific case studies, this article distills key lessons learned from multiple pathways to building successful programs. The lessons reflect both the successes and failures our group has experienced, to suggest how to cultivate appropriate and effective leadership, and generate the resources necessary for leading ITD programs. We present two contrasting pathways toward ITD organizations: one is to establish a new organization and the other is to merge existing organizations. We illustrate how both benefit from a real-world focus, with multiple examples of trajectories of ITD organizations. Our diverse international experiences demonstrate ways to cultivate appropriate leadership qualities and skills, especially the ability to create and foster vision beyond the status quo; collaborative leadership and partnerships; shared culture; communications to multiple audiences; appropriate monitoring and evaluation; and perseverance. We identified five kinds of resources for success: (1) intellectual resources; (2) institutional policies; (3) financial resources; (4) physical infrastructure; and (5) governing boards. We provide illustrations based on our extensive experience in supporting success and learning from failure, and provide a framework that articulates the major facets of leadership in inter- and trans-disciplinary organizations: learning, supporting, sharing, and training.

Theoretical Perspectives of the Baltimore Ecosystem Study: Conceptual Evolution in a Social-Ecological Research Project.

The Earth's population will become more than 80% urban during this century. This threshold is often regarded as sufficient justification for pursuing urban ecology. However, pursuit has primarily focused on building empirical richness, and urban ecology theory is rarely discussed. The Baltimore Ecosystem Study (BES) has been grounded in theory since its inception and its two decades of data collection have stimulated progress toward comprehensive urban theory. Emerging urban ecology theory integrates biology, physical sciences, social sciences, and urban design, probes interdisciplinary frontiers while being founded on textbook disciplinary theories, and accommodates surprising empirical results. Theoretical growth in urban ecology has relied on refined frameworks, increased disciplinary scope, and longevity of interdisciplinary interactions. We describe the theories used by BES initially, and trace ongoing theoretical development that increasingly reflects the hybrid biological-physical-social nature of the Baltimore ecosystem. The specific mix of theories used in Baltimore likely will require modification when applied to other urban areas, but the developmental process, and the key results, will continue to benefit other urban social-ecological research projects.

A multiscale analysis of urbanization effects on ecosystem services supply in an urban megaregion.

The rapid and large-scale urbanization leads to drastic land-use conversion and impacts on ecosystem services. The relationship between urbanization and ecosystem services not only depends on the characteristics of the study area, but is closely related to the selected ecosystem services types and the indicators to measure urbanization level. Exploring the relationship in specific study area is necessary to support regional planning for sustainability. In this study, we analyzed the impacts of urbanization on ecosystem services from 2000 to 2010 in the Beijing-Tianjin-Hebei (BTH) urban megaregion in China. We quantified four critical ecosystem services, food production, carbon sequestration and oxygen production, water conservation, and soil retention, and identified the hotspots of ecosystem service provision. We measured the urbanization level from three aspects, namely, population growth, economic development and developed land expansion. The impacts of urbanization on the selected ecosystem services were examined at the hotspots scale and urban megaregion scale. We found both ecosystem services and urbanization level in the BTH region increased. There was an obvious spatial heterogeneity in the hotspots of ecosystem services, showing hotspots of food production aggregately distributed in the southern plain while hotspots of regulating services mainly located in the north mountainous areas with dense forest. The relationship between population growth, economic development and food production were represented by an inverse U-shaped curve, while it displayed a decreasing trend with regulating services. Both food production and regulating services decreased dramatically with urban land expansion. Additionally, the relationships between urbanization and ecosystem services were consistent across scales. Effective measures should be implemented for the hotspots of different types of ecosystem services to mitigate the loss of ecosystem services during rapid urbanization. The results can provide insights for enhancing urban sustainability in the BTH region, as well other urban megaregion with similar characteristics throughout the world.

Multicontaminant air pollution in Chinese cities.

OBJECTIVE: To investigate multicontaminant air pollution in Chinese cities, to quantify the urban population affected and to explore the relationship between air pollution and urban population size. METHODS: We obtained data for 155 cities with 276 million inhabitants for 2014 from China's air quality monitoring network on concentrations of fine particulate matter measuring under 2.5 µm (PM2.5), coarse particulate matter measuring 2.5 to 10 µm (PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2) and ozone (O3). Concentrations were considered as high, if they exceeded World Health Organization (WHO) guideline limits. FINDINGS: Overall, 51% (142 million) of the study population was exposed to mean annual multicontaminant concentrations above WHO limits - east China and the megacities were worst affected. High daily levels of four-contaminant mixtures of PM2.5, PM10, SO2 and O3 and PM2.5, PM10, SO2 and NO2 occurred on up to 110 days in 2014 in many cities, mainly in Shandong and Hebei Provinces. High daily levels of PM2.5, PM10 and SO2 occurred on over 146 days in 110 cities, mainly in east and central China. High daily levels of mixtures of PM2.5 and PM10, PM2.5 and SO2, and PM10 and SO2 occurred on over 146 days in 145 cities, mainly in east China. Surprisingly, multicontaminant air pollution was less frequent in cities with populations over 10 million than in smaller cities. CONCLUSION: Multicontaminant air pollution was common in Chinese cities. A shift from single-contaminant to multicontaminant evaluations of the health effects of air pollution is needed. China should implement protective measures during future urbanization.

The rapid but "invisible" changes in urban greenspace: A comparative study of nine Chinese cities.

Quantifying the spatial pattern and change of urban greenspace is a prerequisite to understanding the myriad ecosystem services provided by urban greenspace. Previous studies have largely focused on loss of greenspace due to urban expansion, using medium resolution imagery. This paper presents a comparison study on the spatiotemporal patterns of urban greenspace in nine major cities in China, using 2.5m resolution ALOS and SPOT image data collected in 2005 and 2010, respectively. The changes in urban greenspace were further compared with those based on the commonly used 30m Landsat TM data. The results show: 1) Urban greenspace was highly fragmented and heterogeneous, characterized by a mix of a large number of small-sized patches (smaller than 0.1ha) with relatively few dispersed large patches in nine cities. 2) In contrast to findings from previous research that greenspace in inner cities tends to remain largely unchanged, urban greenspace in all nine cities was highly dynamic, experiencing both gain and loss, with net change ranging from 0.51% to 11.26% over five years. Most of the changes in urban greenspace, however, tended to occur as small patches, and could only be revealed by high spatial resolution imagery. 3) Spatial patterns of greenspace varied greatly across cities in terms of patch size, patch and edge density, and shape. Urban greenspace became increasingly fragmented and complex in the southern cities, but the opposite in the northern cities. The high turnover dynamics of urban greenspace in cities proper provide opportunities for better design and planning to achieve urban sustainability, but also call for better protection of small-sized urban greenspaces in Chinese cities.

Spatial-Temporal Variations of Water Quality and Its Relationship to Land Use and Land Cover in Beijing, China.

Rapid urbanization with intense land use and land cover (LULC) change and explosive population growth has a great impact on water quality. The relationship between LULC characteristics and water quality provides important information for non-point sources (NPS) pollution management. In this study, we first quantified the spatial-temporal patterns of five water quality variables in four watersheds with different levels of urbanization in Beijing, China. We then examined the effects of LULC on water quality across different scales, using Pearson correlation analysis, redundancy analysis, and multiple regressions. The results showed that water quality was improved over the sampled years but with no significant difference (p > 0.05). However, water quality was significantly different among nonurban and both exurban and urban sites (p < 0.05). Forest land was positively correlated with water quality and affected water quality significantly (p < 0.05) within a 200 m buffer zone. Impervious surfaces, water, and crop land were negatively correlated with water quality. Crop land and impervious surfaces, however, affected water quality significantly (p < 0.05) for buffer sizes greater than 800 m. Grass land had different effects on water quality with the scales. The results provide important insights into the relationship between LULC and water quality, and thus for controlling NPS pollution in urban areas.

An optimum city size? The scaling relationship for urban population and fine particulate (PM(2.5)) concentration.

We utilize the distribution of PM2.5 concentration and population in large cities at the global scale to illustrate the relationship between urbanization and urban air quality. We found: 1) The relationship varies greatly among continents and countries. Large cities in North America, Europe, and Latin America have better air quality than those in other continents, while those in China and India have the worst air quality. 2) The relationships between urban population size and PM2.5 concentration in large cities of different continents or countries were different. PM2.5 concentration in large cities in North America, Europe, and Latin America showed little fluctuation or a small increasing trend, but those in Africa and India represent a "U" type relationship and in China represent an inverse "U" type relationship. 3) The potential contribution of population to PM2.5 concentration was higher in the large cities in China and India, but lower in other large cities.

A comparison of three empirically based, spatially explicit predictive models of residential soil Pb concentrations in Baltimore, Maryland, USA: understanding the variability within cities.

In many older US cities, lead (Pb) contamination of residential soil is widespread; however, contamination is not uniform. Empirically based, spatially explicit models can assist city agencies in addressing this important public health concern by identifying areas predicted to exceed public health targets for soil Pb contamination. Sampling of 61 residential properties in Baltimore City using field portable X-ray fluorescence revealed that 53 % had soil Pb that exceeded the USEPA reportable limit of 400 ppm. These data were used as the input to three different spatially explicit models: a traditional general linear model (GLM), and two machine learning techniques: classification and regression trees (CART) and Random Forests (RF). The GLM revealed that housing age, distance to road, distance to building, and the interactions between variables explained 38 % of the variation in the data. The CART model confirmed the importance of these variables, with housing age, distance to building, and distance to major road networks determining the terminal nodes of the CART model. Using the same three predictor variables, the RF model explained 42 % of the variation in the data. The overall accuracy, which is a measure of agreement between the model and an independent dataset, was 90 % for the GLM, 83 % for the CART model, and 72 % for the RF model. A range of spatially explicit models that can be adapted to changing soil Pb guidelines allows managers to select the most appropriate model based on public health targets.

Urban ecology in a developing world: why advanced socioecological theory needs Africa.

Socioecological theory, developed through the study of urban environments, has recently led to a proliferation of research focusing on comparative analyses of cities. This research emphasis has been concentrated in the more developed countries of the Northern Hemisphere (often referred to as the "Global North"), yet urbanization is now occurring mostly in the developing world, with the fastest rates of growth in sub-Saharan Africa. Countries like South Africa are experiencing a variety of land-cover changes that may challenge current assumptions about the differences between urban and rural environments and about the connectivity of these dynamic socioecological systems. Furthermore, questions concerning ecosystem services, landscape preferences, and conservation - when analyzed through rural livelihood frameworks - may provide insights into the social and ecological resilience of human settlements. Increasing research on urban development processes occurring in Africa, and on patterns of kinship and migration in the less developed countries of the "Global South", will advance a more comprehensive worldview of how future urbanization will influence the progress of sustainable societies.