A conceptual framework for the study of humanecosystems in urban areas

STEWARD T. A. PICKETT*Institute of Ecosystem Studies, Millbrook, NY 12545-0129, USA

WILLIAM R. BURCH, Jr.School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA

SHAWN E. DALTON and TIMOTHY W. FORESMANDepartment of Geography, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA

J. MORGAN GROVEUSDA Forest Service, Northeastern Forest Experiment Station, 271 Mast Road, Durham, NH 03824, USA

ROWAN ROWNTREEUSDA Forest Service, Pacific Southwest Research Station, P.O. Box 245, Berkeley, CA 94701-0245, USA

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

Keywords: urban ecosystems; human ecology; human ecosystem; patch dynamics; gradient analysis

Introduction

Clear concepts are needed to motivate and support research into the patterns and processes of urban andhuman-occupied ecosystems. These concepts must be capable of satisfying both natural and socialscientists and, therefore, of promoting integrated research (Berk, 1994; Pacala, 1994) into the ecology ofcities, suburbs, exurbs, and the rural hinterlands of cities. We will use the term ‘‘urban’’ or alternatively‘‘metropolitan’’ to refer to this entire range of locations and the rich variety within them.

Integrated research into urban areas is required for two reasons. First, urban areas represent novel

* To whom correspondence should be addressed, at Institute of Ecosystem Studies, Box AB (Route 44A), Millbrook, NY12545-0129.

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combinations of stresses, disturbances, structures, and functions in ecological systems (McDonnell andPickett, 1990; Sukopp, 1990; Waring, 1991). Therefore, understanding how urban ecosystems work, howthey change, and what limits their performance, can add to the understanding of ecosystems in general.Second, the spread of urbanization into agricultural lands and in some cases into relatively wild forest,chaparral, or steppe, is one of three major global impacts of humans (Vitousek, 1994). The process ofurbanization in its broadest sense converts land that has been studied by traditional ecological approachesinto systems that require a clearer appreciation of the roles of humans (Thomas, 1955; Forman, 1995).For example, in the United States, although the population is growing relatively slowly, the proportionof the population inhabiting land classified as urban has increased dramatically (Frey, 1984; Davey,1993). In spite of these needs, ecologists in many places, but especially in North America, have avoidedfocusing on urban areas (Pickett et al., 1994a).

Furthermore, a management strategy that does not take into account the connections of ecologicalsystems to urban areas is not capable of producing sustainable results (Forman, 1995). Ecosystemmanagement calls for planning, implementation, and mitigation that goes beyond the boundaries of aspecific ecosystem supporting production of commodities or other ecological goods and services. Thepatterns and processes in the observed behavior of people in metropolitan areas are now highly linkedwith the degree of direct pressure for exploiting natural resources and responding to direct and indirectimpact from environmental insults such as air-borne toxicants and exotic species. Such linkages areanother motivation for integrating social and ecological sciences in sustainable ecosystem management(Lee, 1992). Therefore, several research initiatives have been articulated to encourage the integration ofhuman and natural components of ecosystems, landscapes, and regions (Machlis et al., 1997; Grove,1996; Burch and DeLuca, 1984; Rambo and Sajise, 1984).

This paper lays out a conceptual framework that can improve understanding of the role of humans inecosystems in general, and can support integrated research in metropolitan areas in particular. A frame-work that encompasses the strong human effects that are evident in urban areas presents the extremes ofhuman impact, which can exist in more subtle form in rural or even apparently pristine ecosystems(Russell, 1993). We first articulate the modern ecosystem concept, and then enumerate key componentsthat must be added to integrate humans with other ecological processes and phenomena. Next, we assessthe role of spatial heterogeneity in human ecosystems. We close with an analysis of the watershedconcept as an especially useful tool for examining spatial heterogeneity in urban ecosystems and forlinking such systems with larger regional systems to meet management and environmental quality goals.

The ecosystem concept

The ecosystem is a multifaceted concept that can be applied to a variety of different situations. Centralto all uses, however, is the core requirement that a physical environment and organisms in a specified areaare functionally linked (Likens, 1992; Fig. 1). Upon the basic foundation of the functional linkage oforganisms and physical environment in a spatial area, many conceptual structures can be built. Ecosys-tems can be large or small, so that both the entire biosphere and a rotting log on a forest floor can bedelimited as ecosystems. Ecosystems can be founded on terrestrial substrates, or occupy volumes ofwater. Hence, the assemblage of organisms and physical environment of a desert shrubland and of amountain stream are both examples of ecosystems.

Perhaps the most powerful use of the ecosystem concept as a scientific tool is as a quantitativedescription of specific parts of the world. Models of ecosystems built under this approach typicallyquantify the amounts of living tissue, dead organic matter stored in the soil and above ground, and keynutrients or toxins in the system. Furthermore, ecosystem ecologists are keen to understand what physicalenvironmental processes control and limit the transformation of energy and materials in ecosystems. Tothis end they quantify the fluxes of energy and matter entering and leaving the stated boundaries of the

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system, as well as the movement of energy and matter between different components of an ecosystem(Schlesinger, 1997).

Boundaries of ecosystems may be set for convenience or at geographic locations where the rates of keyecological processes change. Both approaches are used in ecology (Likens, 1992). However, it is im-portant to recognize that ecosystem boundaries will not contain all the influences that are important toan ecosystem. This is especially true of urban ecosystems. However, every ecosystem study, whether ofa pristine, modified, or built system, must set boundaries. It is necessary to assess inputs of materials,energy, and influences that can affect the structure and function of an ecosystem (Likens, 1984).Boundaries can be useful when considered to be hypotheses that are amenable to test, or as spatial limitsacross which fluxes must be monitored. Many important fluxes and influences in urban ecosystems willrange well beyond any statutory or socially recognized boundary (Cronon, 1991). It is important not tolet local or regional boundaries obscure the effects and flows from a distance.

Increasing attention is being paid to the role of species richness in ecosystems (Jones and Lawton,1995). Of course, organismal ecologists have long focused on the behavior and growth of individuals, thegrowth, limitation, and evolution of populations, and the organization of groups of organisms. However,there has been little focus on how the identity of organisms, the structure of populations, or the diversityof communities affect ecosystem function as measured by material and energy fluxes.

The various chemical components of ecosystems, the processes that operate within them, and the

Figure 1. A simple, general model of the components of an ecosystem. This standard view emphasizes thatecosystems can be decomposed into components linked by flows of matter and energy, and that they are open tofluxes from and to the outside. Boundaries need not imply internal regulation or exclude crucial resource fluxes fromthe outside. Modified with permission from Likens (1992).

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organisms that reside and participate in them are the ecological services and resources on which humanitydepends (Harte, 1997). Therefore, the study of ecosystem processes (i.e., air, water, soil health, andproductivity) and biological diversity in urban areas has significance well beyond the curiosity ofecologists (Boyden and Millar, 1978).

Because the ecosystem concept has been so richly and variously applied, there can be confusion aboutwhat assumptions are central to the concept, and which ones are the legacy of particular kinds ofapplications (Trepl, 1994). For example, ecosystems have been assumed by some to be necessarily selfregulating and homeostatic. Similarly, assumptions can be made about whether the boundaries of eco-systems are relatively impermeable to matter, and whether recycling is efficient. All such assumptionsmay be considered issues for empirical test in different situations, including urban ecosystems, ratherthan a priori requirements of the ecosystem concept itself.

Recognizing the human component

In order to support integrated understanding and research on urban ecosystems, the basic concept of theecosystem must be put into practice in ways that allow the human component to play a functional role(Blood, 1994). The section title suggests that the task facing ecologists is to recognize humans ascomponents of ecosystems. The phrasing is deliberate, because whether to add humans as componentsof a large number of the world’s ecosystems, or not, is a moot point (Turner et al., 1990). Humans, andtheir products and effects, are in fact already part of many component systems of the biosphere. Ex-traordinary efforts must be made to find systems that do not experience human-enhanced atmosphericdeposition of pollutants, altered disturbance regimes, introduced exotic species and diseases, or moresubtle human influences (Russell, 1993).

So the question is not whether to literally add humans to ecosystems, but rather, how to add humansexplicitly to the models ecologists use so that the substantial and subtle effects of humans are not missedor misinterpreted (McDonnell and Pickett, 1993; Blood, 1994). Simply inserting humans in the organ-ismal component of the ecosystem concept is correct, but hardly adequate to understand their role inecosystems. This is because humans are social creatures with large manipulative capacities, whoseprimary means of adaptation is by learning. Hence, they create institutions to regulate the productivity,storage, and distribution of knowledge. Such institutions are the means by which order and behavioralpredictability can contain a relatively free-ranging species. Human institutions bring vast energy subsi-dies to bear on ecosystems, and generate extensive alterations of land forms and community compositions(Cotrell, 1955). Such physical and biological changes necessarily alter ecosystems. The second featureof humans that makes viewing them merely as biological agents inadequate to the task of understandingurban ecosystems is that humans are self aware and can learn individually, as groups, and as institutions(Lee, 1993).

Because human roles in ecosystems depend on so much more than their raw biological density orpopulation characteristics (Machlis et al., 1994), it seems wise to learn from social scientists what theinstitutional, organizational, and interactive features of humans are that should be added to ecosystemmodels to make them more complete and useful.

The human ecosystem model

There is an abundance of simple conceptual models that attempt to unify human and ecological ap-proaches to ecosystems. Perhaps the most common approach is to note the existence of a humanecosystem and a natural ecosystem, and to insert reciprocal connections between them (Boyden, 1993;Blood, 1994; Fig. 2). This captures certain basic features of the human ecological union, but it fails tohighlight some key features.

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First, it fails to note the hybrid characteristic of most ecosystems in which humans play a role (Cronon,1995). Some components, fluxes, regulators, and processes in such systems retain many of their ‘‘natu-ral’’ behaviors, whereas others may be entirely altered or constructed by humans. The combination of thenatural and the constructed is called ‘‘second nature’’ by William Cronon (1991).

Second, the simple connecting of ecological and social systems as though they were independentsystems fails to accommodate the richness of mechanistic connections between the human and the natural(Vayda, 1993; Machlis et al., 1994). The model we present identifies many social components andprocesses in which connections to ecological fluxes, processes, and structures can exist (Fig. 3). There-fore, the model is rich in research implications. Furthermore, the model explicitly recognizes the keyhierarchies of social organization. Because those widely recognized rank hierarchies of wealth, educa-tion, status, property, and power (Logan and Molotch, 1987) can be expressed at different spatial scales,the model can accommodate the rich spatial heterogeneity that so characterizes urban landscapes (Hamm,1982).

The model is organized to group similar functions. It emphasizes that the basic resources on whichhumans and their societies depend are the fruits of ecological processes (Boyden and Millar, 1978). Wedo not subdivide the ecological components of the model that much because such patterns and processesare quite familiar to ecologists. We divide the social structures and processes more completely becausethey are less familiar in ecosystem research, but the relationships between them and ecological processesneed to be clearly documented and better assessed (Boyden, 1977; Blood, 1994; Picket et al., 1994a).

The model we present is a framework to help organize specific studies. Therefore, not all patterns andprocesses suggested by the general model will be used in any given study. Rather, specific interactionsor structures will be chosen as the key ones in specific situations, or for comparison over certain regions.Still more specific, mechanistic hypotheses can be generated to explore the processes included in aquantitative model. In fact, the general framework (Fig. 3) outlines a broad theory of human ecosystems,and within that framework, hierarchies of more specific quantitative and specialized models will cometo exist (Pickett et al., 1994b).

Figure 2. The minimalist view of how to link human and biological-physical systems in the biosphere. This isessentially an overspecified, nonmechanistic model useful only in calling attention to the need for linkage betweenhuman and nonhuman ecological phenomena, and for recognizing that there are feedbacks between the human andnonhuman components of an implied, more inclusive larger system.

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A final but important point about the framework is that it can incorporate dynamics. Human populationdensity and location and human institutions and organizational relationships are notoriously dynamic(Burch, 1971). In fact, many social relationships can be considered to ‘‘cycle’’ or undergo episodicincreases and decreases, just as do natural processes such as successions, climatic periodicities, andpopulation densities. Examples of social ‘‘cycles’’ include individual life cycles, the rise and fall ofinstitutions, the shifting seats of power and influence, the development and deterioration of neighbor-hoods, and economic cycles, among others. The basic point is that various patterns and processes in theframework are subject to change through time. Such changes may be driven by external factors, or by

Figure 3. A conceptual model indicating the kinds of phenomena that constitute the human component of biosphericsystems, and suggesting the general linkages between the human and natural components. The entire figure repre-sents a human ecosystem. The subsystems are a human social system, a resource system comprising both humanresources and ecosystems. All the major subsystems are functionally linked. The ecosystem serves as the foundationfor the human ecosystem. Although based on Machlis et al. (1994), the model is considerably modified, and containsexemplary detail in the bio-physical ecological component. In any application of this high level theoretical model tospecific systems, questions such as which are the dominant social and natural phenomena and structures, whichfeedbacks operate effectively, which feedbacks operate with time lags, and so on, indicate that the overspecificationof the model is resolved by operationalization in specific cases, and is therefore obviated by empirical study. Thephenomena, structures, or interactions listed in each major category are meant to indicate variables rather thanconstants or absolutes. For example, institutions promoting human health can be absent, or if present, be effectiveto different degrees. Used with permission.

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some combination of interactions within a particular model. Changes of this sort are driven by the abilityof humans and their institutions to perceive changes in the built and natural environments, the fact thatthey attach values to such changes and, as a result, attempt to change the environment.

Spatial heterogeneity

Heterogeneity is one of the most important features influencing structure and processes in ecosystems atall scales (Kolasa and Pickett, 1991; Huston, 1994). Heterogeneity creates or closes opportunities fororganisms and hence helps control biological diversity (Caswell and Cohen, 1991). Spatial heterogeneitygenerates barriers or pathways to the flow of materials and energy, and this influences ecosystem nutrientcycles (Gosz, 1991).

Natural sources of heterogeneity in ecosystems include the physical setting, biological agents, and pro-cesses of disturbance and stress (Pickett and Rogers, 1997). The physical template is fundamentally set by thesubstrate, which reflects geology, geomorphology, and soils. Soils are one of the most important sources ofenvironmental heterogeneity because they directly affect so much of the ecosystem structure and function.Soils are notoriously patchy from the smallest to the largest ecological scales. The biological sources ofheterogeneity include the growth, interactions, and legacies produced by organisms. Natural disturbances,which are sudden disruptions of the structure of a system by some physical force, are major sources ofheterogeneity. Examples at relatively coarse scales include some hurricanes, fires, and floods.

Human sources of heterogeneity are analogous to the natural ones, although the causes and extent maydiffer greatly. Humans create or alter physical heterogeneity in ecosystems via resource extraction,introduction of new organisms, modification of landforms and drainage patterns, control or modificationof natural disturbance agents, and the construction of massive infrastructure (Turner et al., 1990).Important to understanding the location and changes in human-generated heterogeneity is a knowledgeof the basis for the spatial differentiation that humans create. The reason lies in the five major rankhierarchies that operate in human societies: wealth, knowledge, status, territory, and power (Logan andMolotch, 1987). These rank hierarchies exist because some people or groups have more of or betteraccess to each of the five resources, whereas others have less. Differentiation of individuals and groupswithin these hierarchies is commonly observed in social science research (Grusky, 1994). Often, differentranks in various hierarchies will occupy different spatial locations (Bogue, 1984). In the dense urban andsuburban landscapes, this spatial differentiation is especially conspicuous. However, even in sparselypopulated rural areas, residents of different rank are spatially differentiated.

Just as natural patchiness affects the generation, flow, and concentration of resources in landscapes, sotoo does human-generated heterogeneity affect resource fluxes (Cronon, 1991; Rebele, 1994). Naturalresources, capital, and labor, among other things, can move differently through patches in human-modified or managed landscapes. For example, wealth resides in particular neighborhoods or commercialdistricts, and labor commutes along certain corridors and avoids certain patches. Likewise, waste andtoxins tend to accumulate or be released in certain districts (Burch, 1976). The variety of patchiness incities and suburbs is legion (Choldin, 1984).

Neither natural nor human-generated patchiness is permanent. Natural causes of change in patchinessinclude vegetation succession, plant and animal dispersal, climate change, and physical disturbance,among others (Pickett and White, 1985). Such processes generate new patches in landscapes, causeexisting patches to move or change shape, or patch structure and composition to change. Together, suchchanges are called patch dynamics. The theory and data that have emerged under this rubric have helpedexplain the coexistence of a diverse array of species, and the evolution of different successional strategies(Pickett and White, 1985). Similarly, the regional movement and concentration of nutrients can be betterunderstood using a patch-dynamics perspective than a static approach to ecosystems and landscapes(Likens and Bormann, 1995).

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Patch dynamics is an appropriate description of human ecological systems as well as natural ones(Frisbie and Kasarda, 1988; Rebele, 1994). The change in a neighborhood from occupancy by upper classowners to working class renters, or from one immigrant group to another, or the conversion of a ruralvillage to a bedroom community, are a few examples. Sometimes such changes are the result of aginginfrastructures or housing stock. Sometimes they are driven by economic decisions made by externalmarkets or stockholders. The opening of new transportation corridors and the superseding of others isanother cause of change in the identity and arrangement of patches defined by human use in urbanlandscapes. The annals of city, town, and human community history are replete with changes that easilyfit the definition of patch dynamics (Bertrand, 1991).

One useful feature of patchiness is that it can be applied to various spatial scales (Wu and Loucks,1995). Vegetation in urban regions may be divided into, among others, wetland marshes, ruderal com-munities, asphalt savannas, mown lawns, and closed canopy forests. A forest patch may be subdividedinto wetland and upland patches, and within each forest type treefall gaps, early successional and latesuccessional patches may appear. Within gaps, there may be patches defined by downed woody debris,upturned soil at the exhumed roots of a fallen tree, and so on.

Similarly, a metropolitan area can be divided into hinterland and city, the city can in turn be dividedinto land use types, neighborhoods, blocks, and so on. Importantly, human patchiness includes not onlystructural differences but also behavioral patch types. Each of these nested hierarchies of patches is morethan a convenient way to organize spatial heterogeneity. Rather, researchers ask what factors influencethe patterns and processes observed at each nested spatial scale. Although the functional relationshipsamong systems or patches at adjacent scales or levels may not be known, hierarchy theory predicts thatsuch relationships should exist (Allen and Hoekstra, 1992). The search for functional relationships withinnested hierarchies is a major task for ecological research (Levin, 1993). The fact is that both social andnatural systems can be nested hierarchies that can exhibit patch dynamics. These commonalities stronglysuggest that integrated study of urban ecosystems can exploit nested spatial patchiness and patch dy-namics. The study is ecological because it uses approaches common to the science of ecology, andbecause some of the components of the system are natural entities and processes. The guiding questionsfor such an integration become: (1) what are the reciprocal, functional relationships between the twokinds of patch patterns – human and natural – and their dynamics; (2) do the social and natural processesoccur at the same scale and are they localized in congruent patches; and (3) are there temporal lags inthe feedbacks between the natural and social components of the systems? These questions are as yetunanswered, and can guide integrated research on human ecological systems and urban areas.

The ecological and social concepts presented so far suggest two limited but different approaches to thestudy of urban ecosystems. The first is the classical ecosystem approach, which focuses on fluxes ofmatter and energy, suggests a concern with the budgets of energy and matter in urban systems. Under thisparadigm, a major concern would be with the magnitude and control of the flows of nutrients, toxins,wastes, and assimilated and thermal energy in urban systems.

The second approach is the spatially focused approach of patch dynamics. The generation of patchinessvia organismal performance and interaction, or by the behavior and interaction of various social units andinstitutions, includes a concern with populations or organisms. Under this paradigm, major concern iswith the cause, structure, and change of spatial patterns and the processes that are affected by the spatialdynamics. Research in urban ecosystems can profitably combine the two approaches, and the watershedconcept provides a foundation for combining them.

Watersheds as integrators

Here we present a concrete spatial setting for application of the human ecosystem model and patchdynamics approaches that may be useful in urban areas. The watershed has been a compelling integrating

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concept in ecological and geological sciences (Hornbeck and Swank, 1992). Hydrological catchmentshave been used to understand the water cycle, geomorphological dynamics, soil formation, and otherprocesses (Band et al., 1993). They have also been used to understand water yield and quality, interac-tions between terrestrial and aquatic systems, biomagnification, and successional relationships. Theyhave been employed as powerful comparative and experimental tools that have contributed to basicscientific understanding and the solution of key practical problems (Hedin and Campos, 1991). Therecognition of and approach to nonpoint source pollution and to acid deposition are examples of thepractical application of watersheds (Likens et al.,1996), and exemplify the power of the watershedexperimental approach (Golley, 1993) even in open systems. Another practical benefit is that the wa-tershed perspective ensures that key functional aspects of metropolitan areas – hydrology and wetlands– are not neglected in cities and suburbs.

In metropolitan regions, the drainage patterns are usually substantially modified (Wolman, 1967).However, in some cities there is a remarkable preservation of the pattern of presettlement catchments.Baltimore, Maryland is such a case. Although the storm sewer and road network must have clear andsubstantial impact on the behavior of the watersheds, the fact is that structure remains at the coarse scalecatchment, even in some of the most highly modified urban zones, and thereby permits a ready frame-work for comparison with less physically modified rural zones. In fact, it becomes a key question tounderstand exactly how the various urban, suburban, exurban, and rural land uses, management practices,and social behaviors affect watershed structure and function. In addition, the power of watersheds as anexperimental unit in natural sciences (Likens, 1992) suggests that they may also play an organizing rolein the study of urban ecosystems (Lee, 1992). Patches of social features (e.g. Fig. 3) at scales similar tophysical catchments can provide potential causal hypotheses for watershed function in urban regions.Here development, restoration, planning, and management practices and scenarios can be applied on awatershed or nested subwatershed level to test the efficacy of different strategies.

A further benefit of the watershed approach is that it can accommodate both the ecosystem flux andthe patch dynamics perspectives. Thus, the budgets of both material and energy, and the study oforganisms, populations, and communities, can be dealt with in the same dynamic and open-ended naturalscheme (Fisher and Grimm, 1991; Fisher et al., 1997). The dynamics of patch mosaics nested within thecoarse scale units, such as the kilometer grid cells often used to study ecosystem fluxes in settledlandscapes (Costanza and Greer, 1995), may provide the mechanistic basis of the larger ecosystem fluxes.In other words, the spatial heterogeneity established by human action and the natural conditions withina metropolitan area may determine the functioning of the larger urban system as a whole. Ecologicalexperience with the openness of watersheds to atmospheric inputs (Hedin and Likens, 1996) is animportant insight to retain in studies of watersheds in densely settled environments.

Finally, the watershed approach can be used to link contrasting catchments with estuaries and coastalwaters. Because of the great significance of nonpoint source pollution, understanding the role of catch-ments of various sizes and degrees and kinds of human use becomes crucial (Costanza and Greer, 1995).The concentration of urban zones in coastal regions, such as Baltimore and the Chesapeake, means thatthe watershed provides a tool to understand the impacts of the urban ecosystem on the adjacent estuaryand coastal zone (Peierls et al., 1991). Based on that understanding, effective education and managementof coastal water quality can follow.

An approach for comparison

Human ecosystem theory and the watershed approach can be applied to research in two ways. The firstis experimental and the second is comparative. Of course, it is possible to imagine planned experimentson urban systems. However, experiments in urban areas are likely to be serendipitous occurrences, inwhich ecologists take advantage of new suburban or urban development, or exploit urban revitalizationefforts as experimental treatments. Therefore, the second comparative approach is more likely to bear

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fruit in urban systems. Here, it is possible to seek out sites with basically the same original physicalenvironment, but that now differ in key, measurable features of urbanization.

The collection of sites can be conceptually ordered to yield a gradient of urbanization to provide thebackground for questions of ecological structure and function. The background can be called an urban-rural gradient (McDonnell and Pickett, 1990; McDonnell et al., 1997). Or similarly, it could be called a‘‘population density gradient’’ from many persons per kilometer to fewer and fewer. The point is thatgradient theory is fundamental in ecology (Pickett et al., 1994b), and provides a powerful analytical toolfor comparing ecosystem structure and function (Vitousek and Matson, 1991). Combining the classicalecosystem approach in ecology with the expanded human ecosystem concept illustrates the urban-ruralgradient (Fig. 4). The rural end of the gradient can be depicted using a standard configuration ofecosystem models in ecology (Likens and Bormann, 1995). Adding the structures that humans build insuburbs and cities to the basic model indicates a gradient of increasingly modified ecosystems comparedwith the rural case (Fig. 4).

How these ecosystems in suburban and central city districts differ from the forests, fields, grasslands,shrublands, and even production ecosystems that ecologists are used to studying is a central issue for thestudy of urban ecosystems. In cases where watersheds can be used to help delimit the ecosystems foundalong such a gradient for quantitative study, the application of two standard ecological approaches to theurban system becomes clear and compelling.

Conclusion

Ecology has struggled since its inception with the issue of how to deal with humans. They have beenconsidered on one side to be just another animal and therefore appropriate for inclusion in ecology. Onthe other side, they have been treated as so obviously different and socially complex as to be avoided atall costs.

The study of urban ecosystems requires a middle course. Humans have to be considered importantecological agents whose effects are embraced and studied within the prevailing conceptual constructs ofecology, whereas at the same time recognizing their powerful capacities for social and spatial organi-zation and for individual and group learning.

This paper has summarized a conceptual framework that can organize the papers to follow, andmotivate integrated research in urban ecosystems. The framework consists of the human ecosystem

Figure 4. General ecosystem models appropriate to three segments of a conceptual gradient from the central city toa forested rural area. The forested or rural end (A) of the gradient is represented by a typical ecosystem model usedin ecological studies that may not directly address the role and effects of humans. However, now even models in suchregions recognize the large role that atmospheric inputs usually have. The expected ecological goods and servicescontributing to quality of human life are exemplified as outputs on the right hand boundary of the ecosystem diagram.(B) Ecosystems in the midrange of the conceptual urban-rural gradient will have a substantial mix of natural orseminatural ecosystem cover and built components supporting high levels of human activity. The specific goods andservices are intentionally left blank for this and the following segments of the urban-rural gradient because they area subject for empirical investigation. (C) The central city or dense urban segment of the conceptual urban-ruralgradient indicates a substantially larger contribution of built components, energy subsidies, and heterotrophy. Themaintenance of the same classes of process from one end of the gradient begs the question of whether those processesindeed are conserved across the gradient, and to what degree control of the metabolism of the ecosystem at eachsegment is internal versus external, resilient versus fragile, and so on. Ultimately, ecosystem ‘‘outputs’’ are theecological goods and services that affect quality of life in each segment of the gradient and in the metropolitan areaas a whole. The rural ecosystem panel (A) is from Bormann and Likens (1979). Used with permission from the ruralpanel.

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model, which combines the ecosystem concept derived from biologically based ecology with insightsabout social institutions and interactions. Furthermore, the framework recognizes the openness of eco-systems, and does not assume equilibrium or self-regulation of urban ecosystems. The framework iscompleted by recognizing spatial heterogeneity in both the natural and social components of the urbanecosystems. The spatial heterogeneity of both components can be dynamic. In addition, the heterogeneityof both social and natural components can be hierarchically organized, and the framework posits awatershed approach as a potential starting point for assessing the interaction of the social and ecologicalcomponents of the integrated system. The framework suggests questions about the patterns and processesof the social-ecological components and their interactions as motivation for future research.

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