How Many Principles of Urban Ecology Are There?

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)
  1. Cities are ecosystems.
  2. Cities are spatially heterogeneous.
  3. Cities are dynamic.
  4. Human and natural processes interact in cities.
  5. 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).
  1. Cities and urban areas are human ecosystems in which social-economic and ecological processes feed back to one another.
  2. Urban areas contain remnant or newly emerging vegetated and stream patches that exhibit ecological functions.
  3. Urban flora and fauna are diverse, and this diversity has multiple dimensions (e.g. taxonomy, cladistics, function, geographic origin).
  4. Human values and perceptions are a key link mediating the feedbacks between social and ecological components of human ecosystems.
  5. 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.
  6. Urban form is heterogeneous on many scales, and fine-scale heterogeneity is especially notable in cities and older suburbs.
  7. Urban form reflects planning, incidental, and indirect effects of social and environmental decisions.
  8. Urban form is a dynamic phenomenon and exhibits contrasts through time and across regions that express different cultural and economic contexts of urbanization.
  9. 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.
  10. 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.
  11. Urban comparisons can be framed as linear transects or as abstract gradients, and the abstract comparisons acknowledge the spatial complexity of urban heterogeneity.
  12. Urban land covers and land uses extend into and interdigitate with rural or wild land covers and uses.
  13. 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.
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 differentin 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

Cadenasso, M. L., and S. T. A. Pickett. 2008. Urban principles for ecological landscape design and management: scientific fundamentals. Cities and the Environment 1:Article 4.
Cadenasso, M. L., S. T. A. Pickett, and J. M. Grove. 2006. Integrative approaches to investigating human-natural systems: the Baltimore ecosystem study. Natures Sciences Societes 14:4–14.
Forman, R. T. T. 2016. Urban ecology principles: are urban ecology and natural area ecology really different? Landscape Ecology:1–10. DOI:10.1007/s10980-016-0424-4
Grove, J. M. 2009. Cities: Managing Densely Settled Social–Ecological Systems. Pages 281–294in F. S. Chapin, G. P. Kofinas, and C. Folke, editors. Principles of Ecosystem Stewardship: Resilience-Based Natural Resource Management in a Changing World.
McHale, M. R., S. T. A. Pickett, O. Barbosa, D. N. Bunn, M. L. Cadenasso, D. L. Childers, M. Gartin, G. R. Hess, D. M. Iwaniec, T. McPhearson, M. N. Peterson, A. K. Poole, L. Rivers, S. T. Shutters, and W. Zhou. 2015. The new global urban realm: complex, connected, diffuse, and diverse social-ecological systems. Sustainability 7:5211–5240.
Meiners, S. J., M. L. Cadenasso, and S. T. A. Pickett. 2015. An integrative approach to successional dynamics: Tempo and mode in vegetation change. Cambridge University Press, New York.
Pickett, S. T. A., and M. L. Cadenasso. 2012. Urban ecology. Pages 273–301 in R. Leemans, editor. Ecological systems: selected entries from the encyclopedia of sustainability science and technology. Springer, New York.
Pickett, S. T. A., and J. M. Grove. 2009. Urban ecosystems: what would Tansley do? Urban Ecosystems 12:1–8.
Pickett, S. T. A., J. Kolasa, and C. G. Jones. 2007. Ecological Understanding. Academic Press, San Diego.
Scheiner, S. M., and M. R. Willig, editors. 2011. The theory of ecology. University of Chicago Press, Chicago.
Seto, K. C., A. Reenberg, C. G. Boone, M. Fragkias, D. Haase, T. Langanke, P. Marcotullio, D. K. Munroe, B. Olah, and D. Simon. 2012. Urban land teleconnections and sustainability. Proceedings of the National Academy of Sciences of the United States of America 109:7687–7692.