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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.

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.

There are urgent calls for developing a comprehensive and globally-relevant urban science that emphasizes convergence among disciplines and practice. Advancing theory and conceptual frameworks is critical to developing a new urban systems science. We synthesize five frameworks that address features identified in calls for global urban science. The frameworks address the overarching urban conditions of complexity, diffuseness, connectivity, and diversity of cities across the globe. The frameworks also help evaluate how a project or study may advance sustainability. The metacity concept, a spatially scalable representation of mosaic change in urban systems, demonstrates how the frameworks apply to increasingly extensive, spatially heterogeneous, and dynamic urban regions. The metacity concept helps avoid static and isolated plans and management approaches and provides a conceptual foundation for an interdisciplinary urban systems science. The frameworks suggest a practical checklist that may help interventions, strategies, and research better align with goals for transforming urban systems toward sustainability.

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.

Leaders of sustainability research organizations need to provide an environment where interdisciplinary and transdisciplinary science flourish. Developing the necessary leadership skills and attributes requires new, targeted training programmes.

Orexins/hypocretins are neuropeptides implicated in numerous processes, including food intake and cognition. The role of these peptides in the psychopathology of anorexia nervosa (AN) remains poorly understood. The aim of the current study was to evaluate the associations between plasma orexin-A (OXA) concentrations and neuropsychological functioning in adult women with AN, and a matched control group. Fasting plasma OXA concentrations were taken in 51 females with AN and in 51 matched healthy controls. Set-shifting was assessed using the Wisconsin Card Sorting Test (WCST), whereas decision making was measured using the Iowa Gambling Task (IGT). The AN group exhibited lower plasma OXA levels than the HC group. Lower mean scores were obtained on the IGT in AN patients. WCST perseverative errors were significantly higher in the AN group compared to HC. In both the AN and HC group, OXA levels were negatively correlated with WCST non-perseverative errors. Reduced plasma OXA concentrations were found to be associated with set-shifting impairments in AN. Taking into consideration the function of orexins in promoting arousal and cognitive flexibility, future studies should explore whether orexin partly underpins the cognitive impairments found in AN.

Research on urban ecosystems rapidly expanded in the 1990s and is now a central topic in ecosystem science. In this paper, we argue that there are two critical challenges for ecosystem science that are rooted in urban ecosystems: (1) predicting or explaining the assembly and function of novel communities and ecosystems under altered environmental conditions and (2) refining understanding of humans as components of ecosystems in the context of integrated social-ecological systems. We assert that these challenges are also linchpins in the further development of sustainability science and argue that there is a strong need for a new initiative in urban systems science to address these challenges and catalyze the next wave of fundamental advances in ecosystem science, and more broadly in interdisciplinary and transdisciplinary science.

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.

Urban greenspace provides myriad ecosystem services. To fully understand the ecological services provided by urban greenspace, it is crucial to first accurately quantify the composition and configuration, and change of urban greenspace. (1) Quantify the dynamics of greenspace in highly urbanized areas. (2) Compare and evaluate the efficacy of medium and high resolution image data on quantifying urban greenspace dynamics. Using two very well-developed urban districts of Beijing, we compare and contrast the changes in urban greenspace from 2005 to 2009 based on two different datasets, the most commonly used Landsat TM data with 30 m resolution, and 2.5 m high spatial resolution imagery. We found urban greenspace in the two well-developed urban districts of Beijing to be very dynamic, in contrast to findings from previous research that greenspace in inner cities tends to remain largely unchanged. Such dynamics, however, could only be revealed by high spatial resolution imagery because medium resolution data, such as TM data greatly underestimated the percent cover of greenspace. The underestimate neglects smaller elements of greenspace as well as changes in configuration of larger patches, limiting the ability of TM data to detect such changes. Our results underscore the importance and necessity of using high spatial resolution data to adequately quantify the distribution of urban greenspace and its change. Results from this study have important implications for urban greenspace management and planning. In addition, the dynamics revealed support emerging conceptions of urban areas as hierarchical patch mosaics.

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.

A century ago, stewards were responsible for managing estates or for keeping order at public events. Today, the Earth is one global estate, and improved stewardship is vital for maintaining social order and for preserving life on Earth. In this paper, we describe Earth Stewardship, a social–ecological framework for sustaining life in a rapidly changing world. The paper defines the components of Earth Stewardship, characterizes the scientific needs for its agenda, and discusses initial efforts to engage multiple disciplines and segments of society in its application. As a beginning, new knowledge for global stewardship must be generated by teams of physical, biological, and social scientists. However, other stakeholders are needed for generating and applying such knowledge, including people in communities of faith, professions involved in design, planning and restoration, and policymakers and managers. Communicating environmental problems and solutions must take into account the psychology of how people perceive problems, promoting positive stances toward the actions needed for an adaptive approach to Earth Stewardship. Successful long-term stewardship of the Earth will require a global partnership linking researchers, managers, policymakers, and citizens.

The Humane metropolis is a rubric to summarize and promote environmental and social quality in contemporary urban mosaics. Because cities, suburbs, and exurbs, as spatially extensive and connected socio-ecological systems, exhibit many negative features, the humane metropolis identifies a strategy to combat the ills and instill more positive and sustainable features and processes in urban systems. Because the humane metropolis as a program has arisen primarily from social motivations, there is the opportunity to articulate more explicitly the role that science can play in addressing the humane metropolis program and evaluating its success. A humane metropolis can be summarized as one that 1) protects and restores ecological services in cities and suburbs, 2) promotes physical and mental health and safety of residents, 3) enhances efficiency by conserving energy, matter, water, and time, 4) facilitates equity by being inclusive, as well as socially and environmentally just, and 5) maintains a sense of community and a sense of place. We clarify the nature of science as a contributor to the social program, pointing out the social values motivating science, and the role that scientific knowledge and metaphor play in linking science with the social program of the humane metropolis. We further identify roles that socio-ecological research can play in meeting the goals of the humane metropolis. We use examples of environmental history, watershed function and restoration, and environmental justice research and action from the Baltimore Ecosystem Study, Long-Term Ecological Research program. The humane metropolis as a social program benefits from scientific contributions that 1) expose hidden ecological processes in urban systems, 2) generate knowledge connecting people and institutions to the biophysical environment, 3) contribute to the civic dialog, and 4) bring scientific values to the prioritization and balancing of the goals of the humane metropolis.

Urban areas are growing in size and importance; however, we are only beginning to understand how the process of urbanization influences ecosystem dynamics. In particular, there have been few assessments of how the land-use history and age of residential soils influence carbon (C) and nitrogen (N) pools and fluxes, especially at depth. In this study, we used 1-m soil cores to evaluate soil profile characteristics and C and N pools in 32 residential home lawns that differed by previous land use and age, but had similar soil types. These were compared to soils from eight forested reference sites. Residential soils had significantly higher C and N densities than nearby forested soils of similar types (6.95 vs. 5.44 kg C/m2 and 552 vs. 403 g N/m2, P

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.

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.

As defined by Ascher, biocomplexity results from a “multiplicity of interconnected relationships and levels.” However, no integrative framework yet exists to facilitate the application of this concept to coupled human–natural systems. Indeed, the term “biocomplexity” is still used primarily as a creative and provocative metaphor. To help advance its utility, we present a framework that focuses on linkages among different disciplines that are often used in studies of coupled human–natural systems, including the ecological, physical, and socioeconomic sciences. The framework consists of three dimensions of complexity: spatial, organizational, and temporal. Spatial complexity increases as the focus changes from the type and number of the elements of spatial heterogeneity to an explicit configuration of the elements. Similarly, organizational complexity increases as the focus shifts from unconnected units to connectivity among functional units. Finally, temporal complexity increases as the current state of a system comes to rely more and more on past states, and therefore to reflect echoes, legacies, and evolving indirect effects of those states. This three-dimensional, conceptual volume of biocomplexity enables connections between models that derive from different disciplines to be drawn at an appropriate level of complexity for integration.

The ecosystem is a fundamental ecological concept that is not as simple as it first appears. We explore three key dimensions of the concept that make it both complex and broadly useful—its basic definition, its application via models to concrete or specific situations, and its metaphorical connotations as used in general communication within the domain of science and with the public at large. Clarity in identifying what the dimensions are and how they are related can help to maintain the rigor of the concept for specific scientific uses while also allowing enough flexibility for its use in the integration of scientific principles, as well as in public discourse. This analysis of the ecosystem as a multidimensional concept is likely to be generalizable to other important concepts in ecology.

As the density and species composition of insects may change in relation to distance from the forest edge, the role of herbivory in tree establishment may also change across edges. To determine the importance of insect herbivory in tree establishment, insect densities were experimentally altered at different distances from the forest edge. Plots were established at three distances from the edge, with plots located in forest, edge, and field habitats. In half of each plot, insect densities were reduced by insecticide application. Seeds of two tree species, Acer rubrum and Fraxinus americana, were planted into each plot in 1995. The experiment was repeated in 1996 with the addition of Quercus palustris and Quercus rubra. Distance from the forest edge was the most important factor in determining seedling emergence and mortality. Overall seedling performance increased from field to edge to woods, although responses varied among species. In 1995, a drought year, insect removal increased emergence and decreased mortality of tree seedlings. In 1996, a year with normal precipitation, insect removal had much less effect on A. rubrum and F. americana. For the two Quercus species, mortality was reduced by insect removal. The tree species differed in their susceptibility to insect herbivory, with Acer rubrum the most susceptible and Fraxinus americana the least. Herbivory by insects was shown to have the potential to affect both the composition and spatial pattern of tree invasions. Herbivore importance differed greatly between the two years of the study, making the interaction between insects and tree seedlings variable both in space and time.

We approach the benefits and burdens of interdisciplinary research (IDR) from the perspective that science involves both constructive and critical approaches. The constructive aspect generates concepts, theories, and data to understand the observable world, while criticism tests the internal consistency of understanding and its fit to the observable world (Pickett and others 1994). IDR is the linkage of phenomena, research approaches, and conceptual tools that had previously been pursued independently (Parker 1993). Such linkages produce new research questions, new approaches to problems, new theories, and new generalizations. Therefore, IDR emphasizes the constructive aspect of science.

Large, infrequent disturbances (LIDs) can have significant impacts yet seldom are included in management plans. Although this neglect may stem from relative unfamiliarity with a kind of event that rarely occurs in the experience or jurisdiction of individual managers, it may also reflect the assumption that LIDs are so large and powerful as to be beyond the ability of managers to affect. However, some LIDs can be affected by management, and for many of those that cannot be affected, the resilience or recovery of the system disrupted by the disturbance can be influenced to meet management goals. Such results can be achieved through advanced planning that allows for LIDs, whether caused by natural events, human activities, or a combination of the two. Management plans for LIDs may adopt a variety of goals, depending on the nature of the system and the nature of the anticipated disturbance regime. Managers can choose to influence (a) the system prior to the disturbance, (b) the disturbance itself, (c) the system after the disturbance, or (d) the recovery process. Prior to the disturbance, the system can be managed in ways that alter its vulnerability or change how it will respond to a disturbance. The disturbance can be managed through no action, preventive measures, or manipulations that can affect the intensity or frequency of the disturbance. Recovery efforts can focus on either managing the state of the system immediately after the disturbance or managing the ongoing process of recovery. This review of the management implications of LIDs suggests that management actions should be tailored to particular disturbance characteristics and management goals. Management actions should foster survival of residuals and spatial heterogeneity that promote the desired recovery pattern and process. Most importantly, however, management plans need to recognize LIDs and include the potential for such disturbances to occur.

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.

We investigated litter mass loss and soil nitrogen (N)-transformation rates in oak stands along a 130-km, urban-rural transect originating in New York City to examine the relationship between changes in these parameters and previously documented differences in soil temperature, heavy metal and total salt concentrations, and soil biota. Reference litter from a rural site was placed in litterbags, and rates of mass loss and changes in N concentration in litter residues were measured over a 6-month period. Net N-mineralization and nitrification rates were measured in A horizon soils using laboratory incubations under constant moisture and temperature. Both mass loss (76%) and N release (65%) from litterbags reached their maximum in urban stands, whereas net N-mineralization rates were 2.3-fold higher than in rural A horizon soils. Litter fragmentation by earthworms and higher soil temperatures are potential causes of the higher mass loss rates observed in urban stands. The higher releases of N measured in the urban litterbags could be a result of their faster mass loss rates, exogenous inputs of N from atmospheric deposition, a relatively low heterotrophic demand for N, or a combination of these factors. The results of this study suggest that in comparison with rural stands, urban forests are characterized by comparatively high rates of litter decomposition, and may also be characterized by comparatively high rates of N mineralization. Additional studies are needed to test whether these effects are common to urban environments in general.

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 assessment of the ecosystem health and departures from it requires clarity of what the system, its structure, dynamics, and healthy conditions are. Available definitions provide inadequate tools to acquire this clarity and may lead to arbitrary diagnoses of ecosystem health but such diagnoses can be overturned on a variety of scientific, philosophical, or political grounds. Nested hierarchy of ecosystem structure compounds the difficulty in the assessment of stress and health because both states may occur simultaneously at different hierarchical levels: with stress at one level being a necessary condition of health at another. An approach based on a formal definition of system change is advanced. First, a conceptual model identifies a self-maintaining minimum interactive structure (MIS) at each level of ecosystem organization. Components of MIS are complementary, coordinated, and exchanging information - they are integrated. Function is defined as a contribution of a component to the maintenance of the whole. In this context health is viewed as persistence of the system at a given temporal and spatial scale. Impairment of the function is stress and is contrasted with change of system structure (loss, addition, or replacement of components of MIS) which is disturbance. Stress can be measured directly by changes of function or indirectly by changes in integration. Even though undesirable from the human point of view, a changed system may again be considered healthy.

We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems.