Two Ways to Discover Disturbance

Ecological disturbance is often defined as an event that disrupts the structure of a specific system (Pickett & White, 1985).  This kind of material or physical disruption is important because it can result in changes in behavior of the system, or leave heterogeneous structural legacies that affect the system in the future (Pickett, Cadenasso, & Jones, 2000; Wiens, 2000).  Such a general and potentially significant ecological process requires conceptual clarity in order to use it successfully (Pickett, Kolasa, Armesto, & Collins, 1989).  As is often the case, seemingly simple definitions actually require great subtlety in their application.  Disturbance invites that kind of attention. 

This essay shows that disturbance can be recognized in two ways.  The first is based on empirical experience with events that have, in the past, commonly acted to disrupt structure of various systems.  This approach can be called event-based detection of disturbance.  The second approach allows disturbance as structural alteration to emerge from the comparison of different kinds of trajectories in a system.  Using such a lens, disturbance shows up as the intersection of long-term data about phenomena or processes with long-term data on system structure or function.  This second approach can be labeled emergent detection of disturbance.  This distinction may be important when disturbance is studied in systems where there is little prior empirical experience, or where interaction of events may be particularly complex.

Background: Disturbance as Process

Although disturbance is one of ecology's fundamental processes, the concept continues to be refined as more examples are brought to bear on understanding disturbance (Peters et al., 2011).  Disturbance can be conceived as a process, of which a conspicuous or powerful event is only a part.  

The process as a whole actually involves interaction between the forces embodied in the event and the characteristics of an ecological system that is exposed to the event.  The system characteristics govern how the forces can affect the system of interest.  In this sense, disturbance can be seen as a complex process because of the multiple interactions between an event and a place. 

The description above requires a caveat.  The term system can be used in different ways in reference to disturbance.  One arises because it may be reasonable to consider disturbance itself as a conceptual system of interacting components and phenomena.  "Disturbance as a system" refers to a conceptual model of the event, forces, and characteristics of places that may be affected by disturbance (Figure 1).  In contrast to a conceptual model involving disturbance, "the system of interest," uses the word system to refer to a concrete location, habitat, place, or ecosystem. 

Figure 1. Disturbance as a complex process (based on Peters et al. 2011 and Grimm et al. 2017)

Event-Based Detection of Disturbance

The refined conceptualization of disturbance, assumed here as background, suggests that there is more than one way to recognize or detect a disturbance.  The first is familiar, and rather intuitive when applied to scales comfortable to humans.  If we can travel through a system, and observe its dynamics at multiple points in time, it is usually easy to identify what a disturbance is.  Walking through a forest after a major wind storm may reveal newly fallen canopy trees, with their upturned roots, and the soil pit from which the roots were wrenched.  In that forest, some trees may have been snapped by wind, and saplings and immature trees may have been broken or bent as canopy trees fell on them.  The scene may be a complex jumble of altered forest structure from the canopy to the subsoil.  This is clearly a disturbance to the formerly intact forest ecosystem, and the motive force of wind equally clear as a driver.  Similarly, walking into a forest some time after a fire, whether one that "crowned" and burned the canopy, or one that was restricted to the litter layer on the ground, shows structural disruption of the prior forest structure.  New seedlings, surviving saplings released from competition with canopy trees, and understory herbaceous plants may respond by faster growth or enhanced reproduction following disturbance.  Such a human-scaled, intuitive recognition of disturbance events has led to familiar, if imprecise, statements that floods, fires, ice storms, landslides, hurricanes, and tornadoes "are disturbances" a priori.

The general model of disturbance (Figure 1) captures these intuitive cases that are linked to human size and experience quite well.  The model suggests though, that understanding exactly how the force of wind, the weight of ice, or the chemistry of combustion affected particular parts of an area require that the nature of the potentially impacted system or area to be known.  This requirement may be realized by rigorous and long-term observation of a system.  But generally, the focus on events matches the requirements of the general model well.  This use of the model is an example of the event-based approach to detecting disturbance.

Emergent Detection of Disturbance

Emergence is a contrasting approach to discovering disturbance.  Not everything that causes disturbance may be the result of a familiar or human-scaled kinds of event, like a hurricane or a flood.  In such cases, the disruption of system structure may result from the application of unexpected or non-intuitive forces.  Non-intuitive forces may exist on scales difficult for individual people to comprehend intuitively.  In addition, such unfamiliar drivers of disturbance may be especially characteristic of social-ecological systems.  The difficulty here is that powerful social-ecological drivers may seem ordinary and unexceptional to people in daily life.  Processes of real estate investment, employment opportunities, or government regulation may not seem at first glance to be the stuff of disturbance.  This invisibility of social-economic drivers is in part a result of the hybrid nature of such systems.  Hybridity or social-ecological-technological system structure means that the forces may have material and social momentum. 

What does such hybridity of forces mean in concrete terms?  If disturbance is an event that disrupts system structure, what counts as an effective event depends very much on what the model of the system is.  The requirement that an explicit model be used to determine what is and what is not a disturbance is an often neglected fundamental of disturbance studies.  Models of hybrid systems can express very different kinds of structures, all of which are important facets of the larger, more inclusive urban ecosystem.  The models state what components the system contains, and how the components of the system are networked together.  For example, social-ecological systems can have structures that serve to transfer information, or transmit social expectations.  Information may include the flows of capital or credit, and expectations may be transmitted in the form of such things as social norms or neighborhood cohesion. 

What can alter the such a socially inflected structure?  Of course, the physical disruption of communication infrastructure can be a disturbance.  This is very much like classical disturbance in ecology.  Alternatively, the physical networks may persist while the capacity of the social network to transfer information may break down due to the removal of an institutional node in the flow of information.  Or restriction of loans in specific areas may disrupt the financial resources that permits people to maintain and refurbish housing stock; ultimately this disruption of the financial system may appear as a material disruption in the urban fabric as buildings are abandoned and perhaps demolished. 

Examples of social features of structure can be labeled a "social contract," or an "ecology of prestige," each of which communicates expectations that influence how people interact in particular places.  A social contract in a African American neighborhood is a structure that can be disrupted by the novel, and perhaps conflicting, expectations about how public space is used and regulated that are put in place by gentrification.  The ecology of prestige is a place-specific social structure expressing a shared aesthetic that directly affects environmental form and management.

Figure 2. Illustration of emergence of disturbance as the intersection of a trajectory of lightning strkes and increasing density of wood stems.  Below a certain threshold density an ignition will not result in fire spread. Hence, there would be no disturbance of the larger landscape.

Disturbances of this kind are particularly complex, and may be more readily discovered by examining the trajectories of change in urban systems than by focusing on specific kinds of physical events (Figure 2).  Trajectories in important biophysical features of urban systems should of course be monitored, as should drivers from outside the system that can cause structural disruption.  However, the events that contribute to disturbance as a process can also arise within the system due to the interaction of changes in various system components. 

The fact that disturbance can arise in two ways in social-ecological-technological systems is a part of the complexity of urban ecology that has helped refine the understanding of one of ecology's basic phenomena.  The fact that the Long-Term Ecological Research program listed disturbance as one of the five core areas for research in its study sites is a symbol of the importance of disturbance across a range of system conceptions including populations, communities, landscapes, and ecosystems.  Disturbance can be hypothesized a priori in for some kinds of models, but must be detected analytically in others.

Steward Pickett

Literature Cited

Grimm, N. B., Pickett, S. T. A., Hale, R. L., & Cadenasso, M. L. (2017). Does the ecological concept of disturbance have utility in urban social-ecological-technological systems? Ecosystem Health and Sustainability, 3(1). doi:10.1002/ehs2.1255

Peters, D. P. C., Lugo, A. E., Chapin, F. S., III, Pickett, S. T. A., Duniway, M., Rocha, A. V., … Jones, J. (2011). Cross-system comparisons elucidate distrubance complexities and generalities. Ecosphere, 2, art 81. doi:10.1890/ES11-00115.1

Pickett, S. T. A., Cadenasso, M. L., & Jones, C. G. (2000). Generation of heterogeneity by organisms: creation, maintenance, and transformation. In M. L. Hutchings, E. A. John, & A. J. A. Stewart (Eds.), Ecological consequences of habitat heterogeneity. Malden, MA: Blackwell.

Pickett, S. T. A., Kolasa, J., Armesto, J. J., & Collins, S. L. (1989). The Ecological Concept of Disturbance and Its Expression at Various Hierarchical Levels. Oikos, 54(2), 129–136. doi:10.2307/3565258

Pickett, S. T. A., & White, P. S. (Eds.). (1985). The Ecology of Natural Disturbance and Patch Dynamics. Orlando: Academic Press.

Wiens, J. (2000). Ecological heterogeneity: an ontogeny of concepts and approaches. In M. J. Hutchins & A. J. A. Stewart (Eds.), The ecological consequences of environmental heterogeneity. Malden, MA: Blackwell.

Ecology Of the City is Twenty Years Old

The phrase "ecology of the city" was introduced in 1997 as a simple rhetorical device to highlight the novelty of the approach to urban ecology adopted in the initial proposal for the Baltimore Ecosystem Study LTER (Pickett et al. 1997).  We and our colleagues in the other urban LTER, located in Phoenix AZ, were anxious to differentiate the proposed work from the usual approach to urban ecology that had been used in the United States, and indeed most studies elsewhere, up to that time (Grimm et al. 2000).   

I have been surprised that the label and its contrast with the ecology in the city has become an organizational and framing tool in many of the contemporary textbooks of urban ecology (Adler and Tanner 2013, Douglas and James 2014).  However, over the intervening 20 years, the label has become more than a superficial framing strategy.  It has become invested with explicit theoretical and empirical content, moving well beyond metaphor (Zhou et al. 2017).  However, it may not be clear to most people that the label in fact now connotes a field of study and a mode of application.  The evolution of how the idea is used also serves as an indicator of how the field of urban ecology itself has developed over that 20 year span.

The predominant approach to urban ecological research in 1997 was called ecology in the city.  It is defined as a research approach focusing on biological organisms and ecological processes that are located in distinct natural, seminatural, or biologically-dominated patches within the fabric of cities, towns, suburbs, and exurbs.  These habitats can be considered to be analogs of those outside of cities, whether those outside locations are rural or wild.  Ecology of the city is defined, in contrast to ecology in the city, as a research approach that integrates biological, social, and technological aspects (Grimm et al. 2016) of urban structures and functions, and focuses on the feedbacks among the components of urban ecosystems that represent these three aspects.  The two approaches share a foundational concern with the spatial structure, heterogeneity, and functioning of urban systems ranging from single neighborhoods to urban megaregions (Figure 1).

Figure 1. Urban megaregions in Asia. The ecology of the city approach applies to all urban scales.

The two approaches can also be differentiated by the way they conceive of spatial heterogeneity, the models they use to represent spatial fluxes, and their implications for management and sustainability.  Such differences have been described in more detail in an earlier post (http://besdirector.blogspot.com/2017/09/ecology-for-city-also-means-with.html).  The contrast can also be exemplified by describing how the nine components of theory (Pickett et al. 2007) differentiate the two approaches (Table 1).

Here, I present a new diagram that may help clarify the relationship of ecology of and ecology in cities (Figure 2).  Ecology in, as a focus on the biological structure and function of "green" patches in cities, is a core and ongoing interest of urban ecology.  This is because such patches are widely recognized as important sources of ecosystem services in the urban landscape (Haase et al. 2014).  They can also be the locus of evolutionary novelty associated with urban environments (Johnson and Munshi-South 2017).  Understanding how these biologically-dominated patches are put together, what biological resources they contain, what ecological and evolutionary functions they support, what benefits and burdens to humans exist within them, or what services emanate from them, are important outcomes of research focusing on ecology in the city.

Figure 2. A conception of the contrast among ecology in as core research, with conceptual and spatial extension to ecology of the city, and ending with the most inclusive approach of ecology for the city, specifying a mode of application.

The contrasting approach of ecology of the city continues to work with biologically-dominated patches, but extends its interest to all habitat types in the urban mosaic (Table 1).  Thus, it asks "what ecological and evolutionary structures and functions, environmental benefits and burdens, exist in and move among all patches in an urban area?"  This inclusive focus means that ecological research under the umbrella of ecology of the city investigates patch types that may not contain obvious biological components.  Ecology of the city must therefore be social-ecological research, rather than only biological research.

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Table 1. The components of theory (see Pickett et al. 2007) and an instance of each in the contrasting approaches of ecology in the city and ecology of the city.  The examples of each component are not complete or comprehensive.  Several of the examples focus on a "filter" of concern with spatial structure as a driver of urban function.  Note that "urban" or "city" here refer to the entirety of urban systems, whether investigated as a whole or not.

Component	In	Of
1.Domain	Biotically-dominated urban patches	Hybrid social-ecological-technological patches
2. Assumptions	Drawn from biology and bio-ecology	Additions from social-ecological science
3. Facts	Biodiversity, traits, genetics, population dynamics	Additions from social-demographic diversity, land cover and institutional attributes, information, organizational dynamics
4. Generalizations	Succession; disturbance; stress; natural selection; stream continuum	Resilience cycle; socio-economic disturbance; cultural selection; engineered stream continuum
5. Laws	Law of succession	Law of adaptive cycle
6. Models	Patch-corridor-matrix; Island biogeography	Landscape mosaic/hybrid patch dynamics; metacity model
7. Translation modes	Science-driven	Engagement-driven
8. Hypotheses	Patterns and mechanisms of biotic impairment	Adaptive capacities and limits
9. Framework	Nested hierarchy of key components to explain biological features and processes in "green" patches in cities	Nested hierarchy of key components to explain hybrid features and processes in all patches in urban mosaics
Application	Biological conservation	Sustainability planning and assessment

 

The third approach, ecology for the city, is defined as the co-production of urban research questions, and the pursuit of social ecological research intended to inform sustainable transformations in cities.  This approach is discussed more fully elsewhere (Childers et al. 2015).  But for this essay, the important idea is that the three approaches to ecological research about cities are not distinct from each other, but in fact interact.  They can be depicted as concentric circles, with ecology in being the core, ecology of being inclusive of in, and the ecology for embracing the knowledge and approaches of the first two.  Ecology in the city supports the social-ecological research exploring the ecology of the city.  Similarly, work pursuant to these two approaches supports the more transdisiplinary, co-produced research of ecology for the city.  Looking in the "opposite direction," each larger circle can be considered to require the input and knowledge provided by the more focused and included domain (Figure 3). 

Figure 3. The conception of ecology in, of, and for as an inclusive theoretical framework, showing their relationship to the disciplinary approach each takes.

The three approaches seen this way become nodes of interest and action in the larger field of urban ecological science.  None is "the" urban ecology.  Rather they are complementary and individual researchers may shift their focus and program among these approaches as time and circumstances permit or require.

The twenty years of research, education, and community engagement motivated by the first expansion of our attention in Baltimore from ecology in to ecology of the city, has continued to invite conceptual clarification.  It also suggests that the empirical content of research of the entire field continues to require understanding the biology within green patches, but also requires understanding how biologically-driven processes contribute to the functioning of patches in which biology may at first seem absent.  The ecology of the city points to the relevance of ecological research and knowledge throughout the city-suburban-exurban mosaic, and demands an interdisciplinary social-ecological stance toward research.  Finally, the necessity and ethical requirement for effective engagement in urban ecological systems has been codified by the ecology for the city approach.   

All Three approaches as defined here make up urban ecology, and together are relevant to the integration of ecological knowledge in urban decision making, ranging from the scale of households to that of entire metropolitan authorities.

Steward Pickett

Literature Cited

Adler, F. R., and C. J. Tanner. 2013. Urban Ecosystems: Ecological Principles for the Built Environment. Cambridge University Press.

Childers, D. L., M. L. Cadenasso, J. M. Grove, V. Marshall, B. McGrath, and S. T. A. Pickett. 2015. An Ecology for Cities: A Transformational Nexus of Design and Ecology to Advance Climate Change Resilience and Urban Sustainability. Sustainability 7:3774–3791.

Douglas, I., and P. James. 2014. Urban Ecology. Routledge, New York.

Grimm, N. B., E. M. Cook, R. L. Hale, and D. M. Iwaniec. 2016. A broader framing of ecosystem services in cities: Benefits and challenges of built, natural, or hybrid system function. Pages 203–212 in K. C. Seto, W. D. Solecki, and C. A. Griffith, editors. The Routledge Handbook of Urbanization and Global Environmental Change. Routledge, New York.

Grimm, N., J. M. Grove, S. T. A. Pickett, and C. Redman. 2000. Integrated Approaches to Long-Term Studies of Urban Ecological Systems. BioScience 50:571–584.

Haase, D., N. Frantzeskaki, and T. Elmqvist. 2014. Ecosystem Services in Urban Landscapes: Practical Applications and Governance Implications. Ambio 43:407–412.

Johnson, M. T. J., and J. Munshi-South. 2017. Evolution of life in urban environments. Science 358:eaam8327.

Pickett, S. T. A., W. R. B. Jr, S. E. Dalton, and T. W. Foresman. 1997. Integrated urban ecosystem research. Urban Ecosystems 1:183–184.

Pickett, S. T. A., J. Kolasa, and C. G. Jones. 2007. Ecological Understanding. Academic Press, San Diego.

Zhou, W., S. T. A. Pickett, and M. L. Cadenasso. 2017. Shifting concepts of urban spatial heterogeneity and their implications for sustainability. Landscape Ecology 32:15–30.