Basics
The
fundamental definition of disturbance is a physical disruption of the structure
of a specified system. The term may also
be applied to the specific event that causes that disruption (Pickett et al.
1989, Peters et al. 2011).
The basic
definition of disturbance is neutral. It
is independent of scale, level of organization, or kind of ecological research
approach. Furthermore, it does not
indicate what the subsequent effects of the physical disruption are, or whether
any such effects are “good” or “bad” for the system as a whole or for different
components of the system.
[Box 2
Disturbance Effects: The high rates of
infant mortality experienced in late 19th century Baltimore as a
result of water-borne cholera epidemics led those who could to abandon the
low-lying areas of the city for higher ground and suburban locations. Prior to that migration, poor and minority
Baltimoreans had lived relatively near their places of factory or domestic
work. Especially for domestic workers,
racial and class segregation were very fine scaled, often involving
differentiation between street dwellings for employers and alley residences for
domestics. The suburban migration of the 1880s was thus a cluster of events
that started Baltimore down a road of segregation that was reinforced by policy
and differential access to resources over the next 100 years (Hinman
2002). Migration from cholera-infested
low elevation areas was good for those who could afford it, while putting those
who could not afford or who were not permitted to move at a significant
disadvantage. It would only be in 1911
when Baltimore installed sanitary sewers and prevent waterborne diseases.]
A model of
the system of interest is required to understand what constitutes a disturbance
and what its effects might be (Pickett et al. 2000). An explicit model of the system would specify
such things as the spatial extent, spatial grain, temporal extent, temporal
grain, physical components, functional connections among components, and the
kinds of dynamics involved in those connections. In the general terms of systems models, a
disturbance removes or damages a component, or removes a physical connection
between components.
It is important to separate the causal event or physical
disruption from the effect of the disruption.
An explicit system model helps to accomplish this separation (Peters et
al. 2011).
[Box 3: Response to Disturbance. Post-fire
changes in Baltimore included such things as standardization of fire hose
fittings, zoning changes, and altered construction regulations. Physical environmental changes include the
reshaping of Baltimore’s shorelines by the dumping of fire debris and
subsequent building on those new surfaces.
Indirectly, the need to rebuild downtown led the state of Maryland to
empower Baltimore City in ways that had been withheld previously. These changes suggest a model of the system,
and the isolation of cause and effect promote the analysis of social and
political causes and social, political, and environmental consequences].
Implications
Ecological experience with disturbance suggests features of
the concept that are likely to be equally important in its urban application.
Disturbance is often said to have a sharp onset, or to be a
relatively discrete event. This implies
that events may have sharp, slow, or stepped onset, may have short or long
duration, and may have rapid, slow or stepped decay over time. As the understanding of disturbance has
matured in ecology, gradations among events in terms of onset, duration, or
decay have been recognized and exploited (Pickett and Cadenasso 2009).
No kind of event -- like fire or flood -- is always a
disturbance. This caution is especially
important for complex human activities.
Certainly, considering humans per se, or urbanization, or agriculture as
disturbances a priori, may lead to confusion.
Complex activities, like those mentioned above, must be
disaggregated into separate events so that their effect on systems of various
scales can be determined (Bart and Hartman 2000). For example, farming may include such diverse
activities as site preparation, fertilization, weeding, harvesting, grazing,
and cover cropping. The nature of these
specific component activities may change depending on the crop or kind of
livestock, the market opportunities, and the various sources of income and kinds
of regulatory constraints and opportunities that exist. Furthermore, the specific activities can
change over long time periods as access to markets, technology, and fashion
change.
Even natural occurrences, such as hurricane, are complex
combinations of specific events.
Hurricanes, for example, may act as disturbing events through the direct
effects of wind, the direct and indirect effects of soil saturation by
associated rainfall, riverine flooding, coastal storm surge flooding. Not all hurricanes, even those acting in the
same geography, will possess or combine all those events in the same way.
The specific order of events may be important to the
realization or effect of disturbance.
For example, hurricane winds that act before soils are saturated by wind
may generate different kinds and severity of physical changes in forest canopy
than those which arrive after long periods of rainfall.
The nature of the receiving system can determine the
magnitude and location of disturbance.
For example, the effect of a
hurricane may depend upon its speed of travel over a region, the amount and
timing of rainfall, wind speed, relationship to the tidal cycle, relationship
to prior saturation of the soil, successional stage of vegetation, and slope
and aspect of sites in the path of the storm.
The built components of systems exposed to potentially disturbing events
may suffer different amounts of damage depending upon structural soundness,
orientation, whether the buildings have been secured, and other management and
design decisions in place.
Related Concepts
The term disturbance was introduced in the mid 1980s to
encapsulate the growing appreciation of the ecological community that physical
disruptions of communities and ecosystems were not uncommon. Indeed, the structure and function of many
ecological systems owed a great deal to the spatial and temporal patterns of periodic
events. This was counter to the emphasis
of ecology on systems perceived as climax, stable, or unaffected by
humans. The term was intentionally provocative,
but as its technical use settled in, the need to abandon the vernacular baggage
was emphasized in ecology.
Disturbance Regime. Although the occurrence of any single event
might have stochastic behavior (like weather), the existence of a temporal and
spatial distribution of such events was statistically predictable over some
time and space frame (like climate). The
temporal and spatial pattern of disruptive events is labeled a disturbance
regime. Disturbance regimes, or lack
thereof, are an important part of the environmental drivers in various systems. Examples include the long-term space/time
patterns of crown-destroying wildfires in many coniferous systems, the role of
drought in mesic regions, or riparian-structuring floods. Species adapt to different aspects of the
disturbance regime and aftermath, and the composition and function of
ecosystems adjusts to the patterns and effects of disturbance regimes over the
long term (Pickett 1998).
Perturbation. While this term also refers to a disruption
of system structure, it connotes an equilibrium or linear system dynamic which
is altered by some event. Often return
to that equilibrium or previous trajectory is of interest.
Stress. This concept has be used in ecology to refer
to processes that directly alter the performance or behavior of a system rather
than its structure. Disturbance affects
three dimensional structure, that is anatomy or architecture, of a system. Stress on the other hand affects physiology
or activity of the system of interest.
Stress may ultimately lead to a physical disruption of system structure. The relationship between stress and
disturbance is one reason that this essay has not discriminated between
endogenous and exogenous sources of disturbance.
Press and Pulse. These terms contrast events that have sharp attack and short duration -- Pulses -- with events that establish persistent new conditions. While the term "pulse" may be essentially the same as disturbance, press can have either the direct physical effects like disturbances or the more physiological effects of stresses. Note that the term pulse does not necessarily imply a regular rhythm of events.
Press and Pulse. These terms contrast events that have sharp attack and short duration -- Pulses -- with events that establish persistent new conditions. While the term "pulse" may be essentially the same as disturbance, press can have either the direct physical effects like disturbances or the more physiological effects of stresses. Note that the term pulse does not necessarily imply a regular rhythm of events.
Intervention. In ecology, this neutral term does not have
the same connotations as either perturbation of stress. However, it is not neutral in other
disciplines, where it may stand for certain theoretical approaches, or critical
stances. In ecology, it may be a useful
substitute for disturbance in situations where 1) focus is on the action taken
by humans to achieve some social or institutional goal; 2) either
anatomical/architectural or physiological/functional origin of the change would
be of interest; and 3) complex or indirect effects are envisioned that combine
stress and disturbance or have not yet been successfully disaggregated into the
immediately causal events. Intervention
as a term may be a useful place holder, or a term that invites smoother
interaction with certain other disciplines such as social scientists and urban
designers.
Z. In a model of system i, consisting of specified components, during time period t,and over extent l, let z equal…
The Urban Context
The basics of disturbance must be kept in mind in exploring
how that concept applies in urban ecosystems.
My initial explorations of disturbance took place in systems such as
primary forest in Pennsylvania, old-field succession in New Jersey, riparian
zones in South Africa, and the Negev Desert.
But I am certain that the clarity that has emerged in my mind from
struggling with the need to both generalize across these and other situations,
and to understand the specific mechanisms and constraints that operate in
specific sites will be helpful in urban areas.
Too often, disturbance has been misapplied in urban
contexts. Treating urbanization itself
as a disturbance does not necessarily promote an open minded understanding of
this complex process and its equally complex and mutable product. Nor does considering people to be primarily a
disturbance agent, external to some pristine ecosystem within urban boundaries
or as a disturbance with regional extent, lead to an understanding of people as
parts of ecosystems that combine natural, built, and managed places and
processes.
There is much still to learn about disturbance in the urban
context. Some general questions, which
will have local and thus more specific versions, are these: What disruptions of the physical structure of
urban areas have biophysical effects?
How do they lead back through social processes to affect vulnerability
or resistance to subsequent disturbances?
How are disturbance and stress in cities related? What social, political, and economic
interventions disturb urban systems, either in whole or in part? What peoples and institutions are sensitive
to those disruptions? What features of
urban systems contribute to ecological resilience in the face of disturbance
and stress? Note that this last question
refers to ecological or evolutionary resilience, as defined by (Holling and
Gunderson 2002), and not
“engineering” resilience as capacity to return to a fixed equilibrium point.
“engineering” resilience as capacity to return to a fixed equilibrium point.
Disturbance in urban systems can result from changes in
regulations, shifts in financial investment, changes in policy that affect the
wealth and access of persons to power.
Disturbance in urban systems can also result from certain flood, ice
storm, or other weather events. And of
course, biophysical and socio-economic disturbances can interact, and can each
result in stress in the system as well as alteration of the physical structure
of the system. All of this potential
benefit of applying the disturbance concept in urban areas results from the
clear application of the basics laid out above, and to equal attention to
social and biophysical structures and processes.
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