Urban Ecosystems, 1997, 1, 185–199
A conceptual framework for the study of human
ecosystems 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. FORESMAN
Department of Geography, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
J. MORGAN GROVE
USDA Forest Service, Northeastern Forest Experiment Station, 271 Mast Road, Durham, NH 03824, USA
ROWAN ROWNTREE
USDA 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 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.
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 and
human-occupied ecosystems. These concepts must be capable of satisfying both natural and social
scientists and, therefore, of promoting integrated research (Berk, 1994; Pacala, 1994) into the ecology of
cities, 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, NY
12545-0129.
1083-8155 © 1997 Chapman & Hall
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combinations of stresses, disturbances, structures, and functions in ecological systems (McDonnell and
Pickett, 1990; Sukopp, 1990; Waring, 1991). Therefore, understanding how urban ecosystems work, how
they 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 of
urbanization in its broadest sense converts land that has been studied by traditional ecological approaches
into 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 proportion
of 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 avoided
focusing on urban areas (Pickett et al., 1994a).
Furthermore, a management strategy that does not take into account the connections of ecological
systems to urban areas is not capable of producing sustainable results (Forman, 1995). Ecosystem
management calls for planning, implementation, and mitigation that goes beyond the boundaries of a
specific ecosystem supporting production of commodities or other ecological goods and services. The
patterns and processes in the observed behavior of people in metropolitan areas are now highly linked
with the degree of direct pressure for exploiting natural resources and responding to direct and indirect
impact from environmental insults such as air-borne toxicants and exotic species. Such linkages are
another motivation for integrating social and ecological sciences in sustainable ecosystem management
(Lee, 1992). Therefore, several research initiatives have been articulated to encourage the integration of
human 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 in
ecosystems in general, and can support integrated research in metropolitan areas in particular. A framework that encompasses the strong human effects that are evident in urban areas presents the extremes of
human 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 components
that must be added to integrate humans with other ecological processes and phenomena. Next, we assess
the role of spatial heterogeneity in human ecosystems. We close with an analysis of the watershed
concept as an especially useful tool for examining spatial heterogeneity in urban ecosystems and for
linking 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. Central
to all uses, however, is the core requirement that a physical environment and organisms in a specified area
are functionally linked (Likens, 1992; Fig. 1). Upon the basic foundation of the functional linkage of
organisms and physical environment in a spatial area, many conceptual structures can be built. Ecosystems can be large or small, so that both the entire biosphere and a rotting log on a forest floor can be
delimited as ecosystems. Ecosystems can be founded on terrestrial substrates, or occupy volumes of
water. Hence, the assemblage of organisms and physical environment of a desert shrubland and of a
mountain stream are both examples of ecosystems.
Perhaps the most powerful use of the ecosystem concept as a scientific tool is as a quantitative
description of specific parts of the world. Models of ecosystems built under this approach typically
quantify the amounts of living tissue, dead organic matter stored in the soil and above ground, and key
nutrients or toxins in the system. Furthermore, ecosystem ecologists are keen to understand what physical
environmental processes control and limit the transformation of energy and materials in ecosystems. To
this end they quantify the fluxes of energy and matter entering and leaving the stated boundaries of the
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Figure 1. A simple, general model of the components of an ecosystem. This standard view emphasizes that
ecosystems can be decomposed into components linked by flows of matter and energy, and that they are open to
fluxes from and to the outside. Boundaries need not imply internal regulation or exclude crucial resource fluxes from
the outside. Modified with permission from Likens (1992).
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 key
ecological processes change. Both approaches are used in ecology (Likens, 1992). However, it is important to recognize that ecosystem boundaries will not contain all the influences that are important to
an ecosystem. This is especially true of urban ecosystems. However, every ecosystem study, whether of
a 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 limits
across which fluxes must be monitored. Many important fluxes and influences in urban ecosystems will
range well beyond any statutory or socially recognized boundary (Cronon, 1991). It is important not to
let 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, the
growth, 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 diversity
of 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
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organisms that reside and participate in them are the ecological services and resources on which humanity
depends (Harte, 1997). Therefore, the study of ecosystem processes (i.e., air, water, soil health, and
productivity) and biological diversity in urban areas has significance well beyond the curiosity of
ecologists (Boyden and Millar, 1978).
Because the ecosystem concept has been so richly and variously applied, there can be confusion about
what assumptions are central to the concept, and which ones are the legacy of particular kinds of
applications (Trepl, 1994). For example, ecosystems have been assumed by some to be necessarily self
regulating and homeostatic. Similarly, assumptions can be made about whether the boundaries of ecosystems are relatively impermeable to matter, and whether recycling is efficient. All such assumptions
may be considered issues for empirical test in different situations, including urban ecosystems, rather
than 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 the
ecosystem 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 as
components of ecosystems. The phrasing is deliberate, because whether to add humans as components
of a large number of the world’s ecosystems, or not, is a moot point (Turner et al., 1990). Humans, and
their products and effects, are in fact already part of many component systems of the biosphere. Extraordinary efforts must be made to find systems that do not experience human-enhanced atmospheric
deposition of pollutants, altered disturbance regimes, introduced exotic species and diseases, or more
subtle human influences (Russell, 1993).
So the question is not whether to literally add humans to ecosystems, but rather, how to add humans
explicitly to the models ecologists use so that the substantial and subtle effects of humans are not missed
or misinterpreted (McDonnell and Pickett, 1993; Blood, 1994). Simply inserting humans in the organismal component of the ecosystem concept is correct, but hardly adequate to understand their role in
ecosystems. This is because humans are social creatures with large manipulative capacities, whose
primary 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 behavioral
predictability can contain a relatively free-ranging species. Human institutions bring vast energy subsidies 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 feature
of humans that makes viewing them merely as biological agents inadequate to the task of understanding
urban 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 or
population characteristics (Machlis et al., 1994), it seems wise to learn from social scientists what the
institutional, organizational, and interactive features of humans are that should be added to ecosystem
models 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 approaches to ecosystems. Perhaps the most common approach is to note the existence of a human
ecosystem 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 to
highlight some key features.
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Figure 2. The minimalist view of how to link human and biological-physical systems in the biosphere. This is
essentially an overspecified, nonmechanistic model useful only in calling attention to the need for linkage between
human and nonhuman ecological phenomena, and for recognizing that there are feedbacks between the human and
nonhuman components of an implied, more inclusive larger system.
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 ‘‘natural’’ behaviors, whereas others may be entirely altered or constructed by humans. The combination of the
natural and the constructed is called ‘‘second nature’’ by William Cronon (1991).
Second, the simple connecting of ecological and social systems as though they were independent
systems 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 and
processes in which connections to ecological fluxes, processes, and structures can exist (Fig. 3). Therefore, the model is rich in research implications. Furthermore, the model explicitly recognizes the key
hierarchies of social organization. Because those widely recognized rank hierarchies of wealth, education, 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 which
humans and their societies depend are the fruits of ecological processes (Boyden and Millar, 1978). We
do not subdivide the ecological components of the model that much because such patterns and processes
are quite familiar to ecologists. We divide the social structures and processes more completely because
they are less familiar in ecosystem research, but the relationships between them and ecological processes
need 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 and
processes suggested by the general model will be used in any given study. Rather, specific interactions
or 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 a
quantitative 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 come
to exist (Pickett et al., 1994b).
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Figure 3. A conceptual model indicating the kinds of phenomena that constitute the human component of biospheric
systems, and suggesting the general linkages between the human and natural components. The entire figure represents a human ecosystem. The subsystems are a human social system, a resource system comprising both human
resources and ecosystems. All the major subsystems are functionally linked. The ecosystem serves as the foundation
for the human ecosystem. Although based on Machlis et al. (1994), the model is considerably modified, and contains
exemplary detail in the bio-physical ecological component. In any application of this high level theoretical model to
specific systems, questions such as which are the dominant social and natural phenomena and structures, which
feedbacks operate effectively, which feedbacks operate with time lags, and so on, indicate that the overspecification
of the model is resolved by operationalization in specific cases, and is therefore obviated by empirical study. The
phenomena, structures, or interactions listed in each major category are meant to indicate variables rather than
constants or absolutes. For example, institutions promoting human health can be absent, or if present, be effective
to different degrees. Used with permission.
A final but important point about the framework is that it can incorporate dynamics. Human population
density 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 episodic
increases and decreases, just as do natural processes such as successions, climatic periodicities, and
population densities. Examples of social ‘‘cycles’’ include individual life cycles, the rise and fall of
institutions, the shifting seats of power and influence, the development and deterioration of neighborhoods, and economic cycles, among others. The basic point is that various patterns and processes in the
framework are subject to change through time. Such changes may be driven by external factors, or by
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some combination of interactions within a particular model. Changes of this sort are driven by the ability
of humans and their institutions to perceive changes in the built and natural environments, the fact that
they 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 at
all scales (Kolasa and Pickett, 1991; Huston, 1994). Heterogeneity creates or closes opportunities for
organisms and hence helps control biological diversity (Caswell and Cohen, 1991). Spatial heterogeneity
generates barriers or pathways to the flow of materials and energy, and this influences ecosystem nutrient
cycles (Gosz, 1991).
Natural sources of heterogeneity in ecosystems include the physical setting, biological agents, and processes of disturbance and stress (Pickett and Rogers, 1997). The physical template is fundamentally set by the
substrate, which reflects geology, geomorphology, and soils. Soils are one of the most important sources of
environmental 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 of
heterogeneity 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 of
heterogeneity. 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 may
differ greatly. Humans create or alter physical heterogeneity in ecosystems via resource extraction,
introduction of new organisms, modification of landforms and drainage patterns, control or modification
of 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 knowledge
of the basis for the spatial differentiation that humans create. The reason lies in the five major rank
hierarchies that operate in human societies: wealth, knowledge, status, territory, and power (Logan and
Molotch, 1987). These rank hierarchies exist because some people or groups have more of or better
access to each of the five resources, whereas others have less. Differentiation of individuals and groups
within these hierarchies is commonly observed in social science research (Grusky, 1994). Often, different
ranks in various hierarchies will occupy different spatial locations (Bogue, 1984). In the dense urban and
suburban landscapes, this spatial differentiation is especially conspicuous. However, even in sparsely
populated rural areas, residents of different rank are spatially differentiated.
Just as natural patchiness affects the generation, flow, and concentration of resources in landscapes, so
too does human-generated heterogeneity affect resource fluxes (Cronon, 1991; Rebele, 1994). Natural
resources, capital, and labor, among other things, can move differently through patches in humanmodified or managed landscapes. For example, wealth resides in particular neighborhoods or commercial
districts, and labor commutes along certain corridors and avoids certain patches. Likewise, waste and
toxins tend to accumulate or be released in certain districts (Burch, 1976). The variety of patchiness in
cities and suburbs is legion (Choldin, 1984).
Neither natural nor human-generated patchiness is permanent. Natural causes of change in patchiness
include vegetation succession, plant and animal dispersal, climate change, and physical disturbance,
among others (Pickett and White, 1985). Such processes generate new patches in landscapes, cause
existing patches to move or change shape, or patch structure and composition to change. Together, such
changes are called patch dynamics. The theory and data that have emerged under this rubric have helped
explain 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 better
understood 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 class
owners to working class renters, or from one immigrant group to another, or the conversion of a rural
village to a bedroom community, are a few examples. Sometimes such changes are the result of aging
infrastructures or housing stock. Sometimes they are driven by economic decisions made by external
markets or stockholders. The opening of new transportation corridors and the superseding of others is
another cause of change in the identity and arrangement of patches defined by human use in urban
landscapes. The annals of city, town, and human community history are replete with changes that easily
fit 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 communities, asphalt savannas, mown lawns, and closed canopy forests. A forest patch may be subdivided
into wetland and upland patches, and within each forest type treefall gaps, early successional and late
successional 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 divided
into land use types, neighborhoods, blocks, and so on. Importantly, human patchiness includes not only
structural differences but also behavioral patch types. Each of these nested hierarchies of patches is more
than a convenient way to organize spatial heterogeneity. Rather, researchers ask what factors influence
the patterns and processes observed at each nested spatial scale. Although the functional relationships
among systems or patches at adjacent scales or levels may not be known, hierarchy theory predicts that
such relationships should exist (Allen and Hoekstra, 1992). The search for functional relationships within
nested hierarchies is a major task for ecological research (Levin, 1993). The fact is that both social and
natural systems can be nested hierarchies that can exhibit patch dynamics. These commonalities strongly
suggest that integrated study of urban ecosystems can exploit nested spatial patchiness and patch dynamics. The study is ecological because it uses approaches common to the science of ecology, and
because some of the components of the system are natural entities and processes. The guiding questions
for such an integration become: (1) what are the reciprocal, functional relationships between the two
kinds of patch patterns – human and natural – and their dynamics; (2) do the social and natural processes
occur at the same scale and are they localized in congruent patches; and (3) are there temporal lags in
the feedbacks between the natural and social components of the systems? These questions are as yet
unanswered, 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 the
study of urban ecosystems. The first is the classical ecosystem approach, which focuses on fluxes of
matter and energy, suggests a concern with the budgets of energy and matter in urban systems. Under this
paradigm, 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 patchiness
via organismal performance and interaction, or by the behavior and interaction of various social units and
institutions, includes a concern with populations or organisms. Under this paradigm, major concern is
with the cause, structure, and change of spatial patterns and the processes that are affected by the spatial
dynamics. Research in urban ecosystems can profitably combine the two approaches, and the watershed
concept provides a foundation for combining them.
Watersheds as integrators
Here we present a concrete spatial setting for application of the human ecosystem model and patch
dynamics 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 catchments
have been used to understand the water cycle, geomorphological dynamics, soil formation, and other
processes (Band et al., 1993). They have also been used to understand water yield and quality, interactions between terrestrial and aquatic systems, biomagnification, and successional relationships. They
have been employed as powerful comparative and experimental tools that have contributed to basic
scientific understanding and the solution of key practical problems (Hedin and Campos, 1991). The
recognition of and approach to nonpoint source pollution and to acid deposition are examples of the
practical application of watersheds (Likens et al.,1996), and exemplify the power of the watershed
experimental approach (Golley, 1993) even in open systems. Another practical benefit is that the watershed 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 and
substantial impact on the behavior of the watersheds, the fact is that structure remains at the coarse scale
catchment, even in some of the most highly modified urban zones, and thereby permits a ready framework for comparison with less physically modified rural zones. In fact, it becomes a key question to
understand 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 an
experimental unit in natural sciences (Likens, 1992) suggests that they may also play an organizing role
in the study of urban ecosystems (Lee, 1992). Patches of social features (e.g. Fig. 3) at scales similar to
physical 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 a
watershed 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 and
the patch dynamics perspectives. Thus, the budgets of both material and energy, and the study of
organisms, populations, and communities, can be dealt with in the same dynamic and open-ended natural
scheme (Fisher and Grimm, 1991; Fisher et al., 1997). The dynamics of patch mosaics nested within the
coarse scale units, such as the kilometer grid cells often used to study ecosystem fluxes in settled
landscapes (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 within
a metropolitan area may determine the functioning of the larger urban system as a whole. Ecological
experience with the openness of watersheds to atmospheric inputs (Hedin and Likens, 1996) is an
important 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 coastal
waters. Because of the great significance of nonpoint source pollution, understanding the role of catchments 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 that
the watershed provides a tool to understand the impacts of the urban ecosystem on the adjacent estuary
and coastal zone (Peierls et al., 1991). Based on that understanding, effective education and management
of 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 first
is experimental and the second is comparative. Of course, it is possible to imagine planned experiments
on urban systems. However, experiments in urban areas are likely to be serendipitous occurrences, in
which ecologists take advantage of new suburban or urban development, or exploit urban revitalization
efforts 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 physical
environment, 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 the
background for questions of ecological structure and function. The background can be called an urbanrural 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 that
gradient theory is fundamental in ecology (Pickett et al., 1994b), and provides a powerful analytical tool
for comparing ecosystem structure and function (Vitousek and Matson, 1991). Combining the classical
ecosystem approach in ecology with the expanded human ecosystem concept illustrates the urban-rural
gradient (Fig. 4). The rural end of the gradient can be depicted using a standard configuration of
ecosystem models in ecology (Likens and Bormann, 1995). Adding the structures that humans build in
suburbs and cities to the basic model indicates a gradient of increasingly modified ecosystems compared
with 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 the
study of urban ecosystems. In cases where watersheds can be used to help delimit the ecosystems found
along such a gradient for quantitative study, the application of two standard ecological approaches to the
urban system becomes clear and compelling.
Conclusion
Ecology has struggled since its inception with the issue of how to deal with humans. They have been
considered on one side to be just another animal and therefore appropriate for inclusion in ecology. On
the other side, they have been treated as so obviously different and socially complex as to be avoided at
all costs.
The study of urban ecosystems requires a middle course. Humans have to be considered important
ecological agents whose effects are embraced and studied within the prevailing conceptual constructs of
ecology, whereas at the same time recognizing their powerful capacities for social and spatial organization and for individual and group learning.
This paper has summarized a conceptual framework that can organize the papers to follow, and
motivate 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 to
a forested rural area. The forested or rural end (A) of the gradient is represented by a typical ecosystem model used
in ecological studies that may not directly address the role and effects of humans. However, now even models in such
regions recognize the large role that atmospheric inputs usually have. The expected ecological goods and services
contributing 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 or
seminatural ecosystem cover and built components supporting high levels of human activity. The specific goods and
services are intentionally left blank for this and the following segments of the urban-rural gradient because they are
a subject for empirical investigation. (C) The central city or dense urban segment of the conceptual urban-rural
gradient indicates a substantially larger contribution of built components, energy subsidies, and heterotrophy. The
maintenance of the same classes of process from one end of the gradient begs the question of whether those processes
indeed are conserved across the gradient, and to what degree control of the metabolism of the ecosystem at each
segment is internal versus external, resilient versus fragile, and so on. Ultimately, ecosystem ‘‘outputs’’ are the
ecological goods and services that affect quality of life in each segment of the gradient and in the metropolitan area
as a whole. The rural ecosystem panel (A) is from Bormann and Likens (1979). Used with permission from the rural
panel.
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model, which combines the ecosystem concept derived from biologically based ecology with insights
about social institutions and interactions. Furthermore, the framework recognizes the openness of ecosystems, and does not assume equilibrium or self-regulation of urban ecosystems. The framework is
completed by recognizing spatial heterogeneity in both the natural and social components of the urban
ecosystems. The spatial heterogeneity of both components can be dynamic. In addition, the heterogeneity
of both social and natural components can be hierarchically organized, and the framework posits a
watershed approach as a potential starting point for assessing the interaction of the social and ecological
components of the integrated system. The framework suggests questions about the patterns and processes
of the social-ecological components and their interactions as motivation for future research.
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