Disturbance

Definition: A physical disruption of the structure of a specified system.  Disturbance may also refer to the specific event that causes the disruption (Pickett et al. 1998, Peters et al. 2011).  Such events tend to have sharp onset and short duration, although their effects can be long lasting.

Explanation: For a long time ecologists ignored disruptive events in the systems they studied.  In fact, they would most often seek out ecosystems or communities that were presumably free from disturbance.  However, as more long term data accumulated over time, and as historical studies extended the record into the past, it became apparent that disturbance were a part of ecological systems in many places.

Whether an occurrence of a fire or hurricane is a disturbance or not depends on two things: the nature of the system involved and the intensity of the event.  Floods in general are not disturbances.  Whether any given flood acts as a disturbance requires an investigator to state what the structure, limits, and interactions in a system are.  Only then can it be determined whether some particular event acts as a disturbance (Pickett 2012).

Example: In non-urban systems there are many examples of disruption of system composition or architecture by high winds, ice or snow loading that breaks trees and thins canopies, flooding that kills dominant plants or causes uprooting or breakage of stems, fires, and herbivore outbreaks (Figure 1). 

Figure 1. A treefall gap in forest canopy.  Such openings are created by windstorms, for example.  Light, moisture, and nutrient conditions change in the understory as a result, providing opportunities for suppressed or new plant species to flourish.  Certain animals also often respond to such gaps.  Photo © S.T.A. Pickett.

In urban systems fires, wind damage, riots, and shifts in real estate and other investments can alter the structure of urban systems over relatively short time periods (Figures 2, 3).

Figure 2. Aftermath of the Great Baltimore Fire of 1904.  The fire itself burned 64 acres of downtown Baltimore, but the debris was dumped along the harbor, disturbing the structure of the transitional environments that had existed there previously.

 

Figure 3. Abandonment in residential, commercial, and industrial neighborhoods is a disturbance common to many cities.  This boarded up house in Baltimore represents both a physical disturbance and a rupturing of the social structure of the neighborhood.  Many abandoned houses deteriorate to the extent that they must be torn down, creating a second wave of disturbance in under served neighborhoods.  Photo © Baltimore Ecosystem Study LTER.

Why Important: Disturbance events change the availability of resources, alter the environmental signaling by patterns of temperature or moisture, make space available for establishment or ascendancy of invading or suppressed individual plants, and shift the patterns of dominance by different species. 

Some components and processes within ecological systems depend on the periodic or infrequent structural disruptions generated by disturbance.  So disturbance is “good” for some aspects of systems at the same time it disrupts others (Reice 2001).  There are of course some disturbances that are so intense that they wipe out a system entirely, but many produce gaps within a persistent system.

Disturbance is a part of urban as well as wild and rural systems.  In cities, suburbs, and exurbs, disturbances may include not only naturally generated events like earthquakes, floods, or encroaching wildfires, but also sudden closures of factories and other businesses, shifts in availability of financing or economic investment in particular places, or the migration of different demographic groups into or out of districts or entire urban areas. 

Disturbance is thus potentially a major component of urban ecological structure, and can influence resource availability and stress in ecosystems.  The social and biological resources available and required, the biodiversity and social institutions in urban areas, the fluxes of fresh and waste water, the amount and structure of green spaces, and the vulnerability of people and biological organisms to stress are all features that can be affected by disturbance.

For More Information:

Peters, D. P. C., A. E. Lugo, F. S. Chapin, III, S. T. A. Pickett, M. Duniway, A. V. Rocha, F. J. Swanson, C. Laney, and J. Jones. 2011. Cross-system comparisons elucidate disturbance complexities and generalities. Ecosphere 2:art 81.

Pickett, S. T. A. 1998. Natural processes. Pages 11-19 in M. J. Mac, editor. Status and trends of the nation's biological resources. U.S. Department of Interior, U.S. Geological Survey, Reston, VA.

Pickett, S.T.A. 2012. A disturbance primer for urban systems.  BES Director’s Corner Blog. http://besdirector.blogspot.com/2012/09/a-disturbance-primer-for-urban-systems.html

Reice, S. R. 2001. The silver lining: the benefits of natural disasters. Princeton University Press, Princeton.