By Steward T.A. Pickett (Cary Institute) & Mary L.
Cadenasso (University of California Davis)
In 2008, we published a short paper on the principles of
urban ecology (Cadenasso and Pickett 2008). It was aimed at landscape architects and
landscaping practitioners as a part of an "Ecological Landscaping"
conference attended mainly by these professionals. Consequently, we wanted to distill key
insights of urban ecological science in digestible form, that could be linked
to landscape design and management.
We generated five general principles to satisfy these
intellectual and practical requirements (Table 1).
Table 1: The Five General Principles from Cadenasso and
Pickett (2008)
- Cities are ecosystems.
- Cities are spatially heterogeneous.
- Cities are dynamic.
- Human and natural processes interact in cities.
- Ecological processes are still at work in cities.
We discussed some design, planning, and management
implications for each of these principles, to make them useful for the
audience. We believe that such
abstractions of the essence of urban ecology have value to researchers and
students within this evolving discipline as well.
What's a Principle, Anyway?
The term, principle, is actually rather vague. A principle can refer to any of the several
conceptually oriented components of theory (Pickett
et al. 2007; Table 1 in Cadenasso and Pickett, 2008). For example, a principle can be a concept, a
definition used to translate concepts to specific cases, a definition of a
quantitative convention within a model, a confirmed empirical generalization,
or a law. Although principles interact
with facts and observations, are not the same as individual facts or even collections
of facts. The construction of principles
involves some degree of abstraction, idealization, summarization, and the use
of relationships among observations or other parts of theory.
To be fair, we didn't specify exactly what kind of
conceptual "thing" our five principles were in 2008. In part, this is because there was no mature,
complete, theory with a clear framework that could be used to identify what
role the principles played in a larger urban ecology theory. We believe that's still the case today.
Later, we expanded the roster of principles by digging
deeper into mechanisms, the cross-disciplinary nature of urban ecology, and
some widespread concerns that would support the linkage of the science to
practices well beyond landscaping. That analysis
appeared in a massive encyclopedia (Pickett and Cadenasso 2012), but is being
refined as part of a chapter we are writing for a book synthesizing the
insights of the Baltimore Ecosystem Study.
In that expanded version, we enumerate 13 principles, providing more
detail in several of the statements (Table 2).
 |
| Research and management in vacant lots (photo: C. Swan) |
Table 2: A more complete and detailed enumeration of
principles. Modified slightly from
Pickett and Cadenasso (2012).
- Cities and urban areas are human ecosystems in which
social-economic and ecological processes feed back to one another.
- Urban areas contain remnant or newly emerging vegetated and
stream patches that exhibit ecological functions.
- Urban flora and fauna are diverse, and this diversity has
multiple dimensions (e.g. taxonomy, cladistics, function, geographic origin).
- Human values and perceptions are a key link mediating the
feedbacks between social and ecological components of human ecosystems.
- Ecological processes are differentially distributed across
the metropolis and the limitation of services and excess of hazards is often
associated with the location of human communities that are poor, discriminated
against, or otherwise disempowered.
- Urban form is heterogeneous on many scales, and fine-scale
heterogeneity is especially notable in cities and older suburbs.
- Urban form reflects planning, incidental, and indirect
effects of social and environmental decisions.
- Urban form is a dynamic phenomenon and exhibits contrasts
through time and across regions that express different cultural and economic
contexts of urbanization.
- Urban designs and development projects at various scales can
be treated as experiments, and used to expose the ecological effects of
different design and management strategies.
- Definition of the boundaries and content of an urban system
model is set by the researchers based on their research questions or the
spatial scope of its intended application.
- Urban comparisons can be framed as linear transects or as
abstract gradients, and the abstract comparisons acknowledge the spatial
complexity of urban heterogeneity.
- Urban land covers and land uses extend into and
interdigitate with rural or wild land covers and uses.
- The flux of water, including both clean water supply and
stormwater management, is of concern to urban and urbanizing areas worldwide,
and connects them explicitly to larger regions.
What do Principles Do?
These 13 more detailed principles (Table 2) explain or nest
within the five general principles laid out in 2008 (Table 1). This suggests that there can be levels of
generality of principles, with some being very "high level" or
general. In the forthcoming book chapter,
we identify a list similar to that in Table 1 as "metaprinciples." The difference between the highest level
metaprinciples, the nested general principles, and then the principles that
operate through specific models suggests that the emerging theory of urban
ecology has -- like so many theories in science -- a nested hierarchical
structure. Examples of hierarchical
theory structures relevant to ecology include that of succession or vegetation
dynamics, and of evolution (Pickett et al. 2007, Meiners et
al. 2015).
Indeed, the theory of ecology in general has been presented
as a hierarchy of eight very general principles, with nested constituent
theories that address more specific ecological topics and processes (Scheiner
and Willig 2011). Within each of
these constituent theories there are one or more models that spell out detailed
mechanisms, temporal and spatial contingencies, rules and exceptions to the
rules. Some of these model components
may legitimately be referred to as principles, though the difference between
those and the most general principles of an area may not always be evident in
the use of the term.
The discussion so far shows that principles can
- Identify the main concerns of a theory.
- Serve to link general and specific components of theory.
- Lay out expectations of mechanism or interaction.
- Summarize patterns of interest to the theory.
How are Principles Arrived At?
Our approach to principles has been from the top down. That is, we have been motivated by
identifying the most general principles of urban theory, and then sorting out
how the mechanistic or pattern details fit within those areas.
A bottom-up approach is also possible. Richard Forman (2016)
identifies urban principles using a primarily bottom-up approach. We interpret his strategy as bottom-up
because his list of the descriptors of science appears in this order: concepts,
principles, laws, models, hypotheses, and theories. Principles appear early in what is tacitly an
inductive series. In addition, his list
of principles that have developed in urban ecology since its 1970s origin amounts
to 90 entries.
The inductive nature of this list is also strongly suggested
by Forman's demonstration that the principles reflect the nature of cities in
which the research supporting them was conducted. Forman lists 16 details of late 19th century
cities, and 15 for late 20th century/early 21st century cities, to illustrate
the contexts in which the 90 urban principles emerged.
One of Forman's main points is that urban ecology, as a
collection of empirical generalizations embodied in his 90 principles, is
different than the principles that have emerged from the study of natural
areas. Thus, urban ecology as a body of knowledge summarized in
inductive principles is (necessarily) different from other kinds of ecology as
bodies of knowledge served by their own inductive principles. The search for urban ecology principles, on
his analysis, appears to be an inductive, place-based pursuit. This is a perfectly legitimate scientific approach,
but the differences between it and the approach of Pickett and Cadenasso (2008; 2012) need to be acknowledged.
The place-based nature of Forman's principles points to an
important opportunity for urban ecology as a global science (McHale
et al. 2015). An important point
about Forman's inductive approach is that it appears to be biased toward cities
of the Global North. Thus, the
trajectories of the industrial revolution, the subsequent development of the
sanitary city (Grove 2009), and the
nodal role of metropolises in colonial and corporate globalization (Seto
et al. 2012) are the boundary conditions for his principles. The rapidly emerging urban realm of the
Global South has a different mixture of contexts (McHale et al. 2015). The areas undergoing the most rapid urban
conversion today may well generate some of the same conditions as have emerged
in the cradle of urban ecological science.
However, it is very likely that today's urbanization in Asia and Africa
will establish some boundary conditions that are quite different. An important task of urban ecology will be to identify these
differences, and to articulate -- from the top down -- or to discover -- from
the bottom up -- empirical generalizations that fit these new situation.
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| Thirteen ways of looking at urban ecology (apologies to Wallace Stevens), arranged as a nested conceptual hierarchy. |
Ecology/Ecologies: One Science, Many Models
The idea of principle is a flexible one, embodying much
power in structuring a science, summarizing its insights, and motivating
application. However, the fact that
principles point to a hierarchy of generality suggests something about what it
means for a subdiscipline or a kind of system to have a "different
ecology."
From the perspective of the definition of ecology as a
scientific pursuit, there is only one thing.
There is only one ecology: The scientific study of the processes
influencing the distribution and abundance of organisms,
the interactions among organisms, and the interactions between organisms and
the transformation and flux of energy and matter.
Urban ecology merely adds an emphasis on
human ecosystems in general, which include organisms, the physical environment
and conditions, the human population and its social structures and processes,
and the built and technological components
(Cadenasso
et al. 2006, Pickett and Grove 2009).
So there is only one ecology -- as a way to pursue science -- although
there are many kinds of system it can apply to.
From the perspective of ecology as a body of knowledge,
however, there can be several kinds.
These "ecologies" are different specific models or families of
model, or sets of generalizations about kinds of systems or historical
periods. This is the analytic home for
Forman's (2016) conclusion that urban ecology is different from the ecology of
wild places. What this means is that the
bodies of facts, empirical generalizations -- or principles in his parlance --
are different when they emerge from urban systems than when they emerge from
wilder systems.
We hope that this discussion clarifies two things. Urban ecology, which focuses on inhabited and
densely built systems, and other kinds of ecology, which focus on uninhabited
but in some cases managed or otherwise anthropogenically impacted, are the same as scientific process, and from the
perspective of generalizable drivers (e.g. Scheiner and Willig 2011). They are different
in terms of the facts and generalizations that apply within their domains. Knowing how to array research and conclusions
across conceptual gradients connecting built and uninhabited landscapes, is as
Forman (2016) suggests, a crucial frontier for ecological science in a changing,
connected world.
And the Envelope, Please!
To answer the question posed in our title, there are roughly
a half dozen principles of urban ecology when one takes a combined top-down and
empirical approach; but there are 90 when one takes a bottom-up approach to
empirical summarization. These
approaches have complementary roles to play in advancing urban ecology, and
reconciling them points to a nested hierarchy as a way to promote integrated
urban ecological theory.
Literature Cited
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Forman, R. T. T. 2016. Urban
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