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Ecology with the city is a transdisciplinary pursuit, combining the work of researchers, policy makers, managers, and residents to advance equity and sustainability. This undertaking may be facilitated by understanding the parallels in two kinds of coproduction. First, is how urban systems themselves are places that are jointly constituted or coproduced by biophysical and social processes. Second, is how sustainable planning and policies also join human concerns with biophysical structures and processes. Seeking connections between coproduction of place and the coproduction of knowledge may help improve how urban ecology engages with diverse communities and urban interests in service of sustainability.

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.

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.

Redlining was a racially discriminatory housing policy established by the federal government’s Home Owners’ Loan Corporation (HOLC) during the 1930s. For decades, redlining limited access to homeownership and wealth creation among racial minorities, contributing to a host of adverse social outcomes, including high unemployment, poverty, and residential vacancy, that persist today. While the multigenerational socioeconomic impacts of redlining are increasingly understood, the impacts on urban environments and ecosystems remain unclear. To begin to address this gap, we investigated how the HOLC policy administered 80 years ago may relate to present-day tree canopy at the neighborhood level. Urban trees provide many ecosystem services, mitigate the urban heat island effect, and may improve quality of life in cities. In our prior research in Baltimore, MD, we discovered that redlining policy influenced the location and allocation of trees and parks. Our analysis of 37 metropolitan areas here shows that areas formerly graded D, which were mostly inhabited by racial and ethnic minorities, have on average ~23% tree canopy cover today. Areas formerly graded A, characterized by U.S.-born white populations living in newer housing stock, had nearly twice as much tree canopy (~43%). Results are consistent across small and large metropolitan regions. The ranking system used by Home Owners’ Loan Corporation to assess loan risk in the 1930s parallels the rank order of average percent tree canopy cover today.

The addition of two urban sites, based in Baltimore, Maryland, and Phoenix, Arizona, to the Long Term Ecological Research (LTER) program in 1997, posed challenges for creating a truly integrated social-ecological framework. Proposals to include social “core areas” to sites with social science agendas were developed in concert with national environmental priorities being set within the scientific community. Although the National Science Foundation rejected these proposals for LTER as a whole, researchers at the urban sites pursued their goals of developing more sophisticated multi-disciplinary frameworks for the study of urban patterns and processes. The Baltimore Ecosystem Study (BES) illustrates how the conceptual basis of urban ecology evolved over 20 years, developing new strategies to manage cross-disciplinary interactions and relationships. Researchers drew especially on the concept of “boundary objects” as articulated by S. L. Star and J. Griesemer, and on General Stanley A. McChrystal’s conception of the “team of teams” as a way to engage with complex problems. Looking forward, BES is positioned now to expand to a long-term, transdisciplinary science platform which we call the Baltimore Ecosystem Alliance. Its goals include developing an authentic urban ecology on local and regional levels, while also serving as a national and international leader in urban ecology.

Research in urban ecology depends on frameworks that meaningfully integrate our understanding of biophysical and social change. Although the coupled nature of urban ecosystems is widely accepted, the core mechanisms we use to integrate the social and biophysical aspects of urban ecosystems – their social-ecological feedbacks – are poorly understood. This paper considers how feedbacks are used to conceptualize social-ecological change, noting their utility and their limitations. In so doing, we suggest that coproduction provides a meaningful alternative to feedbacks, one that captures not only the structure-function relationships usually assumed in studies of biophysical landscape change, but also the structure-agency relationships that facilitate our most comprehensive understanding of social change. By addressing both the stepwise forms of transformation that a feedback approach captures and the simultaneous forms of transformation captured by a coproduction approach, a more comprehensive assessment of the ways that social and ecological change take place is afforded. We contend that thinking in terms of coproduction is essential for moving beyond the interdisciplinary approach that usually guides urban ecology models, toward a more integrated, trans-disciplinary approach.

A landscape succession paradigm has shaped much of our understanding about the processes of forest emergence and transformation in the United States. Drawing heavily from theory and method in environmental history, this paradigm has focused attention on the role of landscape-scale shifts in land use and land cover in the production of forests. The geography of cities is patchy, dynamic and heterogeneous, with change and differences occurring at much smaller scales (e.g. Jacobs 1961; Clay 1973) compared to coarse scale of stand replacing successions affecting rural forests (Grove et al. Ecosyst Health and Sustain 2(9):e01239, 2016; Pickett et al. Urban Ecosyst 20(1):1–14, 2017). Therefore, trying to understand how urban forests came to be, as well as what they are, requires a research approach that is specific to the land use dynamics of cities and attentive to the social life of urban forests. In response to this methodological gap, this paper describes a research approach called “forest ethnography,” which we are piloting in Baltimore, Maryland as part of the Baltimore Ecosystem Study (BES), one of the National Science Foundation’s urban Long-term Ecological Research Programs (LTER). As we describe, we propose that an urban forest ethnography approach can contribute to our understanding of both forest environmental history and urban political ecology.

Spatial heterogeneity has myriad influences on ecosystem processes, ecosystem services, and thus the sustainability of urban areas. It acts as a medium for urban design, planning, and management to determine how processes affecting sustainability can operate and interact. Therefore, how spatial heterogeneity is conceptualized and measured in cities is crucial for enhancing sustainability. We show that the two most commonly used, but contrasting paradigms of urban ecology, ecology IN versus ecology OF the city, determine how spatial heterogeneity is thought of and used in different ways. We identify the key implications of these theoretical contrasts for the practice and assessment of sustainability in urban areas. We review and compare the different ways in which ecology IN versus ecology OF the city affect how to conceptualize, model and map urban spatial heterogeneity. We present a new framework to guide the comparison of spatial heterogeneity under the two paradigms. The integrative nature of this new framework becomes apparent under the ecology OF the city paradigm, because it recognizes the hybrid social and bioecological nature of heterogeneity in urban ecosystems. The hybrid approach to patchiness resonates with the three pillars of sustainability—environment, society, and economy. We exemplify how the more comprehensive and integrated framework of spatial heterogeneity under the ecology OF the city paradigm (1) supports more effective measurement and integration of the three components of sustainability, (2) improves management of heterogeneous urban ecosystems, and (3) satisfies calls for improved ecological tools to support urban ecosystem design.

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 ecological concept of disturbance has scarcely been applied in urban systems except in the erroneous but commonplace assumption that urbanization itself is a disturbance and cities are therefore perennially disturbed systems. We evaluate the usefulness of the concept in urban ecology by exploring how a recent conceptual framework for disturbance (Peters et al. 2011 , Ecosphere , 2, art 81) applies to these social–ecological–technological systems (). Case studies, especially from the Long‐Term Ecological Research sites of Baltimore and Phoenix, are presented to show the applicability of the framework for disturbances to different elements of these systems at different scales. We find that the framework is easily adapted to urban and that incorporating social and technological drivers and responders can contribute additional insights to disturbance research beyond urban systems.

a b s t r a c t Urban ecology and its theories are increasingly poised to contribute to urban sustainability, through both basic understanding and action. We present a conceptual framework that expands the Indus- trial → Sanitary → Sustainable City transition to include non-sanitary cities, "new cities", and various permutations of transition options for cities encountering exogenous and endogenous "triggers of change". When investigating and modeling these urban transitions, we should consider: (1) the triggers that have induced change; (2) situations where crisis triggers change; (3) why cities transition toward more sustainable states on their own, in the absence of crisis; (4) what we can learn from new city transitions, and non-sanitary city transitions; and (5) how resource interactions affect urban transition s. Several existing theoretical frameworks, including sustainability, resilience, adaptation, and vulnera- bility, may be helpful when considering urban transitions. We suggest that all of these theories interact through inertia in urban systems, and that this multi-faceted inertia—e.g. institutional inertia, infras- tructural inertia, and social inertia—imparts degrees of rigidity that make urban systems less flexible and nimble when facing transitional triggers and change. Given this, solutions to urban sustainability challenges may be categorized as those: (1) that "tweak" the current systems and work with or even take advantage of the inertia in those systems, versus; (2) that are more "transformative", that confront systemic inertia, and that may require new systems. We propose that a model for addressing urban sustainability in the context of relevant theory, and for bridging research and practice, should focus on intercity comparisons. And one mechanism to facilitate this approach is a newly formed interdisci- plinary Research Coordination Network (RCN) that focuses on urban sustainability by integrating urban research while incubating solutions-oriented products and collaborative partnerships with practitioners. The Network includes more than two dozen cities in five continents that are in various degrees of tran- sition. In the true vein of sustainability science, our Network activities are incubating societally-relevant

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.

The contrast between ecology in cities and ecology of cities has emphasized the increasing scope of urban ecosystem research. Ecology in focuses on terrestrial and aquatic patches within cities, suburbs, and exurbs as analogs of non‐urban habitats. Urban fabric outside analog patches is considered to be inhospitable matrix. Ecology of the city differs from ecology in by treating entire urban mosaics as social–ecological systems. Ecology of urban ecosystems incorporates biological, social, and built components. Originally posed as a metaphor to visualize disciplinary evolution, this paper suggests that the contrast has conceptual, empirical, and methodological contents. That is, the contrast constitutes a disciplinary or “local” paradigm shift. The paradigm change between ecology in and ecology of represents increased complexity, moving from focus on biotic communities to holistic social–ecological systems. A third paradigm, ecology for the city, has emerged due to concern for urban sustainability. While ecology for includes the knowledge generated by both ecology in and ecology of , it considers researchers as a part of the system, and acknowledges that they may help envision and advance the social goals of urban sustainability. Using urban heterogeneity as a key urban feature, the three paradigms are shown to contrast in five important ways: disciplinary focus, the relevant theory of spatial heterogeneity, the technology for representing spatial structure, the resulting classification of urban mosaics, and the nature of application to sustainability. Ecology for the city encourages ecologists to engage with other specialists and urban dwellers to shape a more sustainable urban future.

Urban ecology is a field encompassing multiple disciplines and practical applications and has grown rapidly. However, the field is heterogeneous as a global inquiry with multiple theoretical and conceptual frameworks, variable research approaches, and a lack of coordination among multiple schools of thought and research foci. Here, we present an international consensus on how urban ecology can advance along multiple research directions. There is potential for the field to mature as a holistic, integrated science of urban systems. Such an integrated science could better inform decisionmakers who need increased understanding of complex relationships among social, ecological, economic, and built infrastructure systems. To advance the field requires conceptual synthesis, knowledge and data sharing, cross-city comparative research, new intellectual networks, and engagement with additional disciplines. We consider challenges and opportunities for understanding dynamics of urban systems. We suggest pathways for advancing urban ecology research to support the goals of improving urban sustainability and resilience, conserving urban biodiversity, and promoting human well-being on an urbanizing planet.

Landscape structure in the Eastern US experienced great changes in the last century with the expansion of forest cover into abandoned agricultural land and the clearing of secondary forest cover for urban development. In this paper, the spatial and temporal patterns of forest cover from 1914 to 2004 in the Gwynns Falls watershed in Baltimore, Maryland were quantified from historic maps and aerial photographs. Using a database of forest patches from six times—1914, 1938, 1957, 1971, 1999, and 2004—we found that forest cover changed, both temporally and spatially. While total forest area remained essentially constant, turnover in forest cover was very substantial. Less than 20% of initial forest cover remained unchanged. Forest cover became increasingly fragmented as the number, size, shape, and spatial distribution of forest patches within the watershed changed greatly. Forest patch change was also analyzed within 3-km distance bands extending from the urban core to the more suburban end of the watershed. This analysis showed that, over time, the location of high rates of forest cover change shifted from urban to suburban bands which coincides with the spatial shift of urbanization. Forest cover tended to be more stable in and near the urban center, whereas forest cover changed more in areas where urbanization was still in process. These results may have critical implications for the ecological functioning of forest patches and underscore the need to integrate multi-temporal data layers to investigate the spatial pattern of forest cover and the temporal variations of that spatial pattern.

The ecosystem concept was introduced in ecology originally to solve problems associated with theories of succession and ecological communities. It has evolved to become one of ecology’s fundamental ideas, and has proven to be applicable to a wide variety of research questions and applications. However, there is controversy about whether or how well the ecosystem concept is suited to urban habitats. By examining Arthur Tansley’s original presentation of the ecosystem concept, and exploring how the ecological context of the concept has changed, we indicate that the fundamental concept of the ecosystem is well suited to urban ecological studies. The concept can be clarified for urban use by including a social complex and a built complex to insure that human social institutions and actions, and the structures and infrastructure they build are explicitly included in the ecosystem concept. The ecosystem concept is thus seen as clearly robust to use in urban areas.

The emerging discipline of urban ecology is shifting focus from ecological processes embedded within cities to integrative studies of large urban areas as biophysical-social complexes. Yet this discipline lacks a theory. Results from the Baltimore Ecosystem Study, part of the Long Term Ecological Research Network, expose new assumptions and test existing assumptions about urban ecosystems. The findings suggest a broader range of structural and functional relationships than is often assumed for urban ecological systems. We address the relationships between social status and awareness of environmental problems, and between race and environmental hazard. We present patterns of species diversity, riparian function, and stream nitrate loading. In addition, we probe the suitability of land-use models, the diversity of soils, and the potential for urban carbon sequestration. Finally, we illustrate lags between social patterns and vegetation, the biogeochemistry of lawns, ecosystem nutrient retention, and social-biophysical feedbacks. These results suggest a framework for a theory of urban ecosystems.

We propose a framework of key concepts useful in understanding how urban soils can contribute to general ecological theory. The major factors that can cause urban soils to be different from soils in non-urban ecosystems are identified and related to the familiar state factor approach. We evaluate directly altered resource availability, and the role of stress in mediating resource availability in urban ecosystems. Modified groundwater and stream flow, and atmospheric deposition of nitrogen and base cations are particularly important resource fluxes to soils in urban ecosystems. Disturbance can be conceptualized in the same way in urban as in non-urban ecosystems. However, in addition to biophysical disturbances familiar to ecologists studying wild lands, demographically and socially mediated changes in ecosystem structure must also be considered. These changes include human migration and population structure, institutional shifts, and the effects of human health. Finally, spatial heterogeneity, including fragmentation and differential connectivity, integrates the effects of resources and disturbance, and has an effect on subsequent resource availability and susceptibility to disturbance. Layers of heterogeneity include not only the geomorphic template, but urban climate, biotic composition, buildings and infrastructure, and demographic-social patterns. The complex layering of natural and social factors that constitute urban heterogeneity permit the continuation of important ecological processes, as well as modify ecological fluxes involving soils. We present a modification of the state factor approach as an expanded framework for the study of urban soils. The understanding of urban soils can contribute to general ecological theory by testing the generality of important ecosystem drivers and their linkage with social processes in an under investigated ecosystem type that is increasing in extent and impact worldwide.

This paper examines predictors of vegetative cover on private lands in Baltimore, Maryland. Using high- resolution spatial data, we generated two measures: ''pos- sible stewardship,'' which is the proportion of private land that does not have built structures on it and hence has the possibility of supporting vegetation, and ''realized stew- ardship,'' which is the proportion of possible stewardship land upon which vegetation is growing. These measures were calculated at the parcel level and averaged by US Census block group. Realized stewardship was further de- fined by proportion of tree canopy and grass. Expenditures on yard supplies and services, available by block group, were used to help understand where vegetation condition appears to be the result of current activity, past legacies, or abandonment. PRIZM TM market segmentation data were tested as categorical predictors of possible and realized stewardship and yard expenditures. PRIZM TM segmenta- tions are hierarchically clustered into 5, 15, and 62 cate- gories, which correspond to population density, social stratification (income and education), and lifestyle clusters, respectively. We found that PRIZM 15 best predicted var- iation in possible stewardship and PRIZM 62 best predicted variation in realized stewardship. These results were further analyzed by regressing each dependent variable against a set of continuous variables reflective of each of the three PRIZM groupings. Housing age, vacancy, and population density were found to be critical determinants of both stewardship metrics. A number of lifestyle factors, such as average family size, marriage rates, and percentage of sin- gle-family detached homes, were strongly related to real- ized stewardship. The percentage of African Americans by block group was positively related to realized stewardship but negatively related to yard expenditures.

Our understanding of the dynamics of urban ecosystems can be enhanced by examining the multidimensional social characteristics of households. To this end, we investigated the relative significance of three social theories of household structure—population, lifestyle behavior, and social stratification—to the distribution of vegetation cover in Baltimore, Maryland, USA. Our ability to assess the relative significance of these theories depended on fine-scale social and biophysical data. We distinguished among vegetation in three areas hypothesized to be differentially linked to these social theories: riparian areas, private lands, and public rights-of-way (PROWs). Using a multimodel inferential approach, we found that variation of vegetation cover in riparian areas was not explained by any of the three theories and that lifestyle behavior was the best predictor of vegetation cover on private lands. Surprisingly, lifestyle behavior was also the best predictor of vegetation cover in PROWs. The inclusion of a quadratic term for housing age significantly improved the models. Based on these research results, we question the exclusive use of income and education as the standard variables to explain variations in vegetation cover in urban ecological systems. We further suggest that the management of urban vegetation can be improved by developing environmental marketing strategies that address the underlying household motivations for and participation in local land management.

Recent advances in remote sensing and the adoption of geographic information systems (GIS) have greatly increased the availibility of high-resolution spatial and attribute data for examing the relationship between social and vegetation structure in urban areas. There are several motivations for understanding this relationship. First, the United States has experienced a significant increase in the extent of urbanized land. Second, urban foresters increasingly recognize their need for data about urban forestry types, owners and property regimes, and associated social goods, benefits, and services. Third, previous research has focused primarily on the distribution of vegetation cover or diversity. However, little is known about (1) whether vegetation structure varies among urban neighborhoods and (2) whether the motivations, pathways, and capacities for vegetation management vary among households and communities. In this article, we describe novel data and methods from Baltimore, MD, and the Baltimore Ecosystem Study (BES) to address these two questions.

Urban designers, ecologists, and social scientists have called for closer links among their disciplines. We examine a promising new tool for promoting this linkage—the metaphor of “cities of resilience.” To put this tool to best use, we indicate how metaphor fits with other conceptual tools in science. We then present the two opposing definitions of resilience from ecology, and give reasons why one is more appropriate for linking with design. Additional specific tools and insights that are emerging from, or being increasingly used in, ecology can further support the linkage with urban design. These include recognizing the role of spatial heterogeneity in both ecological and social functioning of urban areas, the integrating power of watersheds, social and ecological patch dynamics of cities, the utility of spatial mosaic models to capture function, the use of an integrated “human ecosystem” modeling framework, and the consequent perspective of metropolitan areas as integrated ecological-social systems. Three additional tools are related to the adaptability of people and human institutions. First is the recognition of a “learning loop” in metropolitan ecosystems in which people respond to and affect ecological change, the use of urban design as experiments whose ecological and social outcomes can be measured, and finally the potency of a dialog between professionals and citizens, communities, and institutions, to support both research and design. The metaphor of resilience, and its technical specifications, draw these diverse strands for linking ecology and planning together. © 2003 Elsevier B.V. All rights reserved.

▪ Abstract Ecological studies of terrestrial urban systems have been approached along several kinds of contrasts: ecology in as opposed to ecology of cities; biogeochemical compared to organismal perspectives, land use planning versus biological, and disciplinary versus interdisciplinary. In order to point out how urban ecological studies are poised for significant integration, we review key aspects of these disparate literatures. We emphasize an open definition of urban systems that accounts for the exchanges of material and influence between cities and surrounding landscapes. Research on ecology in urban systems highlights the nature of the physical environment, including urban climate, hydrology, and soils. Biotic research has studied flora, fauna, and vegetation, including trophic effects of wildlife and pets. Unexpected interactions among soil chemistry, leaf litter quality, and exotic invertebrates exemplify the novel kinds of interactions that can occur in urban systems. Vegetation and faunal responses suggest that the configuration of spatial heterogeneity is especially important in urban systems. This insight parallels the concern in the literature on the ecological dimensions of land use planning. The contrasting approach of ecology of cities has used a strategy of biogeochemical budgets, ecological footprints, and summaries of citywide species richness. Contemporary ecosystem approaches have begun to integrate organismal, nutrient, and energetic approaches, and to show the need for understanding the social dimensions of urban ecology. Social structure and the social allocation of natural and institutional resources are subjects that are well understood within social sciences, and that can be readily accommodated in ecosystem models of metropolitan areas. Likewise, the sophisticated understanding of spatial dimensions of social differentiation has parallels with concepts and data on patch dynamics in ecology and sets the stage for comprehensive understanding of urban ecosystems. The linkages are captured in the human ecosystem framework.

Understanding contemporary urban landscapes requires multiple sets of spatially and temporally compatible data that can integrate historical land use patterns and disturbances to land cover. This paper presents three principal methods: (1) core analysis; (2) historic mapping; and (3) gradient analysis, to link spatial and temporal data for urban ecosystems and applies their use in the Baltimore-Chesapeake region. Paleoecological evidence derived from the geochronology of sediment cores provides data on long-term as well as recent changes in vegetative land cover. This information, combined with contemporary vegetation maps, provides a baseline for conducting trend analyses to evaluate urbanization of the landscape. A 200-year historical land use database created from historical maps, census data, and remotely sensed data provides a spatial framework for investigating human impacts on the region. A geographic information system (GIS) integrates core analyses with historic data on land use change to yield a comprehensive land use and land cover framework and rates of change. These data resources establish the regional foundation for investigating the ecological components of an urban ecosystem. Urban-rural gradient analyses and patch analyses are proposed as the most appropriate methods for studying the urban ecosystem as they link ecological and social patterns and processes for varying degrees of urbanization.

The need for integrated concepts, capable of satisfying natural and social scientists and supporting integrated research, motivates a conceptual framework for understanding the role of humans in ecosystems. The question is how to add humans to the ecological models used to understand urban ecosystems. The ecosystem concept can serve as the basis, but specific social attributes of humans and their institutions must be added. Learning and feedback between the human and natural components of urban ecosystems are key attributes of the integrated model. Parallels with familiar ecological approaches can help in understanding the ecology of urban ecosystems. These include the role of spatial heterogeneity and organizational hierarchies in both the social and natural components of urban ecosystems. Although urban watersheds are commonly highly altered, the watershed approach can serve as a spatial basis for organizing comparative studies of ecosystems exhibiting differing degrees of urbanization. The watershed concept can also spatially organize the hierarchically scaled linkages by which the integrated human ecosystem model can be applied. The study of urban ecosystems is a relatively new field, and the questions suggested by the integrated framework can be used to frame ecosystem research in and associated with urban and metropolitan areas.

The development of the science of urban ecology in the United States has not been continuous. We identify three specific periods during the twentieth century when the application of ecological science to understanding the structure and dynamics of urban areas gained momentum. We begin with the Chicago School of the 1920s and 1930s that applied concepts from ecological science to understand the spatial differentiation of people and land uses. The second period considered the city as a system. The third, and contemporary period, advocates an inclusive approach that considers cities as integrated ecological and social systems. We use the metaphor of tides to frame our discussion of the developments that allowed the tide to come in and that perhaps also caused that tide to ebb. Each tide however, contributed approaches and understandings that were ready to be used upon the arrival of the next tide.

There is growing scientific interest, practical need, and substantial support for understanding urban and urbanising areas in terms of their long-term social and ecological trajectories: past, present, and future. Long-term social-ecological research (LTSER) platforms and programmes in urban areas are needed to meet these interests and needs. We describe our experiences as a point of reference for other ecologists and social scientists embarking on or consolidating LTSER research in hopes of sharing what we have learned and stimulating comparisons and collaborations in urban, agricultural, and forested systems. Our experiences emerge from work with two urban LTSERs: the Baltimore Ecosystem Study (BES) and the District of Columbia-Baltimore City Urban Long-Term Ecological Research Area-Exploratory DC-BC ULTRA-Ex project. We use the architectural metaphor of constructing and maintaining a building to frame the description of our experience with these two urban LTSERs. Considering each project to be represented as a building gives the following structure to the chapter: (1) building site context; (2) building structure; and (3) building process and maintenance.

In their recent article in TREE, Ramalho and Hobbs [1] call for a more mechanistic urban ecology that better incorporates the dynamic nature of urban areas in both time and space. We agree with their goals, but we feel that their assessment of urban ecological research was elementary and misrepresents the actual state of the science. Urban ecologists [2] are fully aware that modern cities do not develop as concentric rings, but instead grow in the spatially and temporally complex, non-linear ways that Ramalho and Hobbs emphasize [1].