Wednesday, September 19, 2018

Evolution of the Baltimore Ecosystem Study

Since 1997, the Baltimore Ecosystem Study (BES) has enjoyed the support of the Long-Term Ecological Research Program of the Division of Environmental Biology of the US National Science Foundation. That support is coming to an end, but the Baltimore Ecosystem Study will live on, due to the desire of key partners who have been joined together for 20 years to increase the understanding of Baltimore as a social-ecological system, and to improve its livability, sustainability, and equitability.

The support of the LTER program has been instrumental in helping BES establish a mission devoted to four major goals: 1) Pursue excellence in social-ecological research in an urban system; 2) Maintain positive engagement with communities, environmental institutions, and government agencies; 3) Educate and inform the public, students, and organizations that have need of scientific knowledge; and 4) Assemble and nurture a diverse and inclusive community of researchers, educators, and participants.

Nothing speaks to the future better than BES education.
Although the NSF's LTER support has been crucial in bringing BES to its current maturity, the contributions of the Baltimore Ecosystem Study are too important to urban ecological science, to the jurisdictions of metropolitan Baltimore, to the state of Maryland, and to federal resource management agencies and compacts, to allow the effort to falter.  Consequently, the partners and constituents of BES are reorganizing as a consortium to maintain their shared research and practical efforts, to seek additional sources of support, and to build on the unique foundation of over two decades of interaction and problem solving.

Over the coming months, key partners, including Cary Institute of Ecosystem Studies, the University of Maryland, Baltimore County, and the USDA Forest Service, plan to work with leading non-governmental organizations, local and state  agencies to explore an inclusive governance structure and establish a complementary strategy for building an ongoing funding base for the project.

BES has established and maintained unique, long-term data on watershed function, the land cover of the urban region, the biological diversity and biological community structure of the metropolis, and the role of human decision making at household, community, and governmental levels as interacting controls on the development and change of the urban region.  

The integrated understanding generated by BES is crucial in moving the Baltimore region toward the more sustainable future that jurisdictions and communities so earnestly desire.  Resilience and equitability are important facets of the sustainability visions and plans that exist in the region.  The Baltimore Ecosystem Study is very well poised to help the Baltimore City, the five counties of the metropolitan area, and the larger Chesapeake region to design sustainability goals and to provide the scientific basis for evaluating progress toward meeting these goals.

BES -- sometimes characterized as the "Baltimore School of Urban Ecology" -- is a distinctive, integrative approach to contemporary urban ecology.  The approach provides a powerful platform to advance and apply scientific research, education, and community engagement. Indeed, BES is an acknowledged exemplar for such integration that has informed similar efforts around the world.
BES is adapting to a changing funding regime, but it is also working to remain at the forefront of social-ecological research and application.  The partners making up BES as a consortium are as committed as ever to making it an effective, ground-breaking exploration at the frontier of urban ecological science and application.

Steward Pickett and Emma Rosi

Saturday, April 21, 2018

Why Do Urban Ecology?

The ecological science of understanding the structure, workings, and change of urban places has three main reasons to be (raisons d'ĂȘtre, if you prefer the French).  One is the fact that urban systems are one of the world's most dynamic and increasingly predominant environments.  The second is that ecological understanding can contribute to improving cities and urban regions.  Finally, studying urban systems can improve the science of ecology itself.  These reasons are the universal ones for pursuing urban ecology.

Studying the Urbanizing World. 

Ecology was established as a science when the human population of the world mostly lived in rural places.  They were farmers, harvesters of trees or fish, denizens of villages and small towns closely related to the management of resources and the transport and economy of commodities.  Even during the dawn and consolidation of the industrial era, large, dense cities were unusual places for people as a whole to live.

This rural and natural-resource based residency is now rare for people.  Within the last decade, the addresses of the majority of people have shifted from the countryside and small settlements to cities, suburbs, and urban regions.  More than 50% of the Earth's human population now count as urban dwellers, as defined by the various governments of the world. 

This urbanization of the world's people and places is continuing at a rapid pace.  The United Nations estimates that more than 60% of humans will be urban by 2030.  Converting persons and places from rural and village life to urban involves ever more extensive connections. 

So cities are no longer isolated enclaves, but are connected to rural and wild lands at great distances.  Those connections bring resources from other continents, spread wastes and pollution to other countries, and even affect the lifestyles of those people who are still identified as rural dwellers.  Although cities and their adjacent suburbs only cover about 3% of the land area in even the most densely urban nations, the effects of those areas are almost boundless.  Global climate change, for example, or toxification of global nutrient cycles are widespread outcomes of the intensity and metabolism of human settlements.  Urban is not just the city anymore.

Ecological science has ignored the urban for most of its history.  The oldest professional organization of ecologists dates to 1913.  Exploratory attempts by mainstream ecologists to extend their scope to cities can be dated to the 1970s.  

But substantial attention to the city and the urban didn't really take root in the United States until the late 1990s.  The neglect of urban areas has left ecologists as latecomers to one of the most massive planetary changes in recent history.  Focusing on urban ecology is required because urban systems -- cities, suburbs, and exurbs -- are an important global habitat type.  It is, of course, the habitat type that is now most familiar to people.

Urban Ecology to Improve Cities.

Cities and other urban systems have most often been approached as strictly designed, built, or engineered places.  While they are manifestly intended and constructed to support human habitation, protection, productivity, and delight, they do have ecological components.  Plants, animals, and microbes reside in cities, and contribute to the benefits and hazards that people experience in the urban realm.  Consequently, urban ecological knowledge has as much place as economics, social science, design, political science in assessing and understanding the functioning of cities. 
In particular, urban ecological science is relevant to several key attributes of cities:
  •        Livability, including psychological benefits and human comfort,
  •        Ecological functions as sources of ecosystem services,
  •        The intimate feedbacks among natural and human aspects of cities,
  •        Equity in the free ecological work available to social groups and classes,
  •        Contribution to sustainability and resilience, and
  •        Knowledge to support urban design and planning.

This essay can't detail all the support for these attributes, but the richness of these is widely investigated and ever more firmly supported.

Urban Ecology Improves Ecological Science

The flow of benefits is not just from science to urban systems.  Rather, science itself can reap benefits from investigating the formerly neglected urban systems.  The benefits include at least these:
  • Discover new combinations or levels of environmental factors in cities,
  • Bring insights and methods of other disciplines into ecology,
  • Promote interdisciplinarity and study of social-ecological systems,
  • Improve ecological theory by extending it to a new kind of system,
  • Test existing theories in a new environments, and
  • Link ecology better with design, planning, and engineering professions

Personal Reasons.

The three motivations or outcomes listed above are the public and professional ones.  But the list leaves out an important motivation -- the personal.  There is no way to generalize the personal reasons for pursuing urban ecological research because the mix of reasons is likely to be unique to each researcher.  Similarly, the particular order of events and experiences that draw a person into urban ecological science is likely to be idiosyncratic and specific to where and how they grew up, how they were educated, and who influenced them.  But the existence of those personal reasons is no less important for being individual and unique.

In my own case, my predilection for urban ecology only became apparent long after I had become an ecologist and had studied such places as post-agricultural oldfields, natural disturbance in primeval forest, and spatial heterogeneity in deserts, savannas, and unmanaged forest landscapes.  But thinking back on how I grew up, my personal roots supporting urban ecology reach deep.  I was educated by my Father about the history and complexity of the city he and I grew up in and where our family had lived for 4 generations.  But he also introduced me to the mystery and subtlety of forests at Boy Scout camp.  I must have thought that the woods and the city were both wonderful and interesting places. 
Downtown Louisville, KY viewed across the Ohio River from Southern Indiana.

The first bold steps into urban ecology were guided by colleagues Mark McDonnell and Rich Pouyat.  These steps were reinforced by former student and continuing colleague, Mary Cadenasso, a native of Oakland California and now a professor at UC Davis.  Her long family history in the Bay Area combined deep roots in viticulture and ranching, with the experience of vacationing in the Sierra Nevada.  She echoed my early experience that cities were as cool in their own way as were forests and mountains.

Now perhaps my personal motivation is continuing to change.  A new personal frontier has opened up for me with the desire to find the parallels between Aldo Leopold's "thinking like a mountain," and the need to apply ecology in new habits of thinking like a city.  That is, what are the ethical dimensions of urban ecology?  What are the moral implications of conceiving of linked cities, suburbs, and exurbs as ecosystems -- like Leopold's "mountain?"  In other words, is there an urban land ethic? 

The pleasure I personally derive from understanding ecological complexity, heterogeneity, and dynamism in cities, is married with the desire to advance the human need for safe, delightful, healthful, diverse, and equitable urban places.


Urban ecological science is a required body of knowledge in an unprecedented and increasingly urban world.  It can help guide the way toward desired outcomes such as more livable, sustainable, and equitable cities.  But it can also improve ecological science in general by stretching and testing ecology's theoretical and empirical content.  Finally, urban ecology opens a frontier for exploring and exercising a personal -- and community -- ethical and moral sense. 

Steward Pickett

Wednesday, April 4, 2018

How Does a Long-Term Study Adjust Its Framework while Preserving Data Integrity?

Long-term ecological research is faced with seemingly contradictory constraints: It must maintain a consistent stream of rigorously comparable data over time while at the same time responding to conceptual and theoretical changes in the disciplines underlying those data.  How can such opposing  constraints be reconciled? 

BES has faced this challenge in developing its most recent proposal.  It was required to shift from a framework that -- although it highlighted a frontier topic in the understanding of social-ecological systems -- proved to be problematical for many readers.  In response, members of the project management team took a step back and sought ways to improve the conceptual framework.  First, they wanted to simplify the conceptual framework.  Second, they wanted to increase its parallelism with frameworks of other LTER sites.  Finally, they wanted to emphasize the interactions on multiple scales that caused the changes in urban social-ecological systems over time.

This stepping back took us to the foundations of BES.  The project was founded to examine the basic biophysical structures and functions in an urban system, and how they interacted with social processes, all in a context of change. 

The Founding Ideas of BES

So the founding question of BES asked: How do biological and social patch dynamics combine to shape and change a metropolitan area?  This question was operationalized by applying the watershed concept in a city-suburban-exurban matrix, and by examining nested hierarchies of social, biological, soil, and hydrological processes as potential causes of urban change. 

In revisiting the framework, we looked at what the first three phases of BES had accomplished, and what they suggested about refinements in the concepts.  Furthermore, we looked at the improved understanding of climate change and globalization that had developed over the nearly 20 year history of BES, to see what those insights suggested about revision of our framework. 

New Pressures in the System

It was clear that effects of climate change through sea level rise, increases in storm intensity and frequency, risk of drought, increase in heat waves, and shifts in species ranges were likely to alter the structure and functioning of our urban ecosystem.  Furthermore, globalization was likely to alter human migrations, increase the pressure from introduced and/or invasive plants, animals, pests, and diseases, as well as work continuing social-demographic changes within our region.  These ideas had not played significant roles in the initial conceptualization of BES, which was concerned primarily with testing how well ecological approaches worked in an environment where they had not been tried previously.

Conceptual Refinement about Urban Ecosystems

An additional refinement emerging from BES and other urban social-ecological research also played a role in the evolving framework.  Early on, leaders of BES and its sibling LTER, the Central Arizona Phoenix project, had pointed out the difference in studying ecology in the city versus studying ecology of the city.  The latter approach required an integrative, interdisciplinary stance and investigated all habitats in an urban-suburban-exurban matrix, not just the conspicuously green patches. 

Taking ecology of the city seriously generated a new land cover conceptualization and classification, and required examining the intimate feedbacks between social and biophysical processes over various temporal scales.  Indeed, the urban ecosystem is now seen as "coproduced" by natural and social processes, and consequently, to possess a hybrid social-ecological-technological structure (Rademacher et al. 2018).

Key Features of a New Framework

These insights, empirical advances, and conceptual refinements have led us to propose a new framework to support our continued collection and analysis of long-term data in the urban ecosystem.  The framework divides the research concerns into 1) exogenous drivers of change, 2) the structure of the urban ecosystem, consisting of biological, physical, constructed, and social components, and 3) the functional responses of the urban ecosystem.  All of these aspects -- drivers, structure, and responses -- interact with each other through time.  In addition, the functional responses of the urban ecosystem feed back onto its structure.  In a system that includes humans as individuals, groups, and institutions, the feedbacks may involve learning and adaptation or adjustment.

Exogenous Drivers

Exogenous drivers are those that originate or are controlled from outside of the local or regional urban mosaic.  Climate change is clearly exogenous, as are regional patterns of atmospheric deposition of gasses and particulate pollution.  Much of the economy of urban regions is driven by national and international investments, policy, allocation of jobs, and movement of resources and commodities.  Governance anchored beyond the city, such as requirements of regional compacts, state law and regulation, federal regulations, and private-public interactions, can affect a metropolis or its parts.  Technology emerging elsewhere may also alter the fluxes of matter and energy available to a city, and human population can be altered by regional, national, or international migration numbers and directions. 

Some of the factors enumerated as "exogenous" may, if they are managed or shaped within the city or metropolis, act as local or endogenous factors.  It is the origin and distance which determines exogeneity, not the specific type of flux or influence.  For example, the movement of people within a metropolis could reflect local environmental perceptions, behaviors, and organizational networks.

Ecosystem Structure. 

Exogenous and internal influences come together in the structure of the urban ecosystem.  Like all human ecosystems, urban areas consist of the biological organisms and the physical environment, but also of the various human and social structures, and the constructed environment.  The interactions among these four components drive the functional responses of the urban ecosystem.  All four urban ecosystem components are reflected in the ecosystem function.

Functional Responses. 

The functional responses are divided into three linked process realms, long used to organize BES data collection. Watershed biogeochemistry addresses the amount and content of water flowing through constructed infrastructure and biophysical features of catchments.  Local ecosystem production and nutrient transformations are driven by the biota, represented by plant, animal, and microbial communities. These are indexed by key sentinel species.  Human environmental perceptions, behaviors, and the actions of organizations constitute the social functions of the urban ecosystem.  Clearly, all three functional realms interact with each other.  Equally clearly, the functional interactions feed back on the structural filter by which external drivers impact the system.

Similarity with Other LTER Site Frameworks

This new framework is intended to be readily interpretable by ecologists working outside of urban areas as well as those who focus on urban places.  In fact, the general schema for the framework is very similar to that of the Hubbard Brook Ecosystem Study LTER, located in the forests of the White Mountains of New Hampshire.  Hubbard Brook is one of the oldest LTER projects, and focuses on understanding the dynamics of forested watersheds under the influence of exogenous factors and local management choices and ecological succession.  Exogenous factors are well illustrated by the Hubbard Brook study, where acid rain from distant sources was first identified in North America.  Current examples of climate changes include the role of reduced snow cover, and intensification of winter storms.  Of the existing LTER sites, the vast majority include some sort of exogenous factors in their roster of drivers.  Climate change, sea level rise, and human generated land use change are commonly identified as exogenous drivers.

Accommodating Long-Term Data 

For nearly 20 years, BES has collected continuous or repeated data sets on climate and weather, watershed hydrology, nutrient export,  water quality, biodiversity and key biotic populations, soil processes, land cover and land use, social structures, and social dynamics.  These data sets are arrayed across the seven core areas required of urban LTER sites, as stated in NSF's original request for proposals in 1997 (Table 1).

Table 1. Major BES research areas and their distribution across the LTER core research areas.

BES has revised its conceptual framework by identifying the fundamental ecosystem structures and processes represented by its ongoing, long-term data collection, while at the same time organizing the structures and processes differently than in its original conception.  In this way, we hope to have clarified the big ideas that motivate and tie together the data streams emerging from a still under-studied ecosystem type.  The framework combines some of the most fundamental ideas from ecosystem science with the novel structure and large changes represented by urban systems.

Steward Pickett and Emma Rosi

Background Literature

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.

Pickett, S. T. A., M. L. Cadenasso, E. J. Rosi-Marshall, K. T. Belt, P. M. Groffman, J. M. Grove, E. G. Irwin, S. S. Kaushal, S. L. LaDeau, C. H. Nilon, C. M. Swan, and P. S. Warren. 2017. Dynamic heterogeneity: a framework to promote ecological integration and hypothesis generation in urban systems. Urban Ecosystems 20:1–14. DOI: 10.1007/s11252-016-0574-9

Rademacher, A., M. L. Cadenasso, and S. T. A. Pickett. 2018. From feedbacks to coproduction: Toward an integrated conceptual framework for urban ecosystems. Urban Ecosystems. DOI: 10.1007/s11252-018-0751-0

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. DOI: 10.1007/s10980-016-0432-4

Wednesday, January 3, 2018

Seeking Applicants to Join a Research Project Focused on Green Infrastructure

Our research team has four positions available: Three Postdocs and a Research Support Specialist ("RA") for social-ecological assessment of green infrastructure.

Our interdisciplinary, social-ecological research team is pleased to announce the availability of four research positions to assist with a project entitled, "Environment, Health, and Poverty: is Green Infrastructure a Universal Good?"

We use green infrastructure to investigate the relationships between environment, health, and poverty in cities. Improved scientific knowledge is required to support better use of green infrastructure in light of the needs and constraints in underserved neighborhoods. To support this goal, the project identifies three objectives: 1) to document the biophysical environment and social context of existing and proposed green infrastructure projects in Baltimore, MD, where we have a long history of research and engagement, and which is home to significant underserved populations; 2) to understand how residents, particularly those in underserved neighborhoods, perceive and relate to existing and potential green infrastructure projects; and 3) to evaluate how sustainability plans in various cities conceive of green infrastructure and its relationship to social processes. Our research is intended to support a civic outcome of improved and more equitable use of green infrastructure.

A brief overview of the currently available positions and links to the formal, detailed advertisements with equal opportunity statements, are below.


Three Postdoctoral Associates Sought.

We will employ three postdoctoral associates, one for each of the three objectives:

  • Post Doc I: Postdoctoral Research Associate -Spatial Context of Green Infrastructure in an Urban Landscape (AJL ID# 833942), housed at UC Davis (;

  •  Post Doc II: Social Perceptions of Green Infrastructure in Baltimore Neighborhoods (; and

  • Post Doc III: Document Analysis of the Sustainability or Other Relevant Plans of Baltimore and 15 Additional Cities (  Post Docs II and III will be housed at the Cary Institute of Ecosystem Studies in Millbrook NY.  All will occasionally travel to Baltimore as part of the project.


A Research Support Specialist:

We are currently seeking the first of several Research Support Specialists to assist with Objective III, and this person will be housed at either the Cary Institute in Millbrook, or The New School University in New York City ( 


Join our Team

The senior research team consists of Joshua Ginsberg, Steward Pickett, Emma Rosi, and Shannon LaDeau at the Cary Institute, Timon McPhearson at the Cary Institute and the New School University, Mary Cadenasso at the University of California Davis, Peter Groffman at the Advanced Science Research Center of the City University of New York, and Morgan Grove at the USDA Forest Service.

Posted by Steward Pickett

Wednesday, December 27, 2017

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.