Temporal dynamics of communities: disturbance

Community Ecology
Temporal dynamics of communities: disturbance
1. Various definitions of "disturbance"
a. change is not synonymous with disturbance
b. importance of considering whether disturbance is intrinsic or extrinsic to a system
2. Properties of disturbance (Pickett and White 1985)
3. Effects of disturbance
a. fragmentation
b. nonlinear and nonimmediate effects
4. Characteristics of organisms that are especially vulnerable to disturbance
5. Human effects on disturbance regimes
6. Balance of nature?
recurrence interval
time lag
Pickett and White
intrinsic (cf. extrinsic)
Disturbance is a hideously complex and ill-defined concept. For example:
“any event that causes mortality or loss of biomass” (Huston 1994)
“any event that kills, displaces, or damages individuals” (Sousa 1984)
One definition that is commonly used in ecology is from Pickett and White’s classic book, The
Ecology of Natural Disturbance and Patch Dynamics:
"any relatively discrete event in space and time that disrupts ecosystem, community, or
population structure and changes resources, substrate, or the physical environment"
(Pickett and White 1985).
Are some disturbances a part of the system itself? Are they 'inside' or 'outside' the system?
role of the disturbance as being extrinsic or intrinsic to a system.
Characteristics of Disturbances
Disturbances (and their effects) can be described/quantified in terms of a few defining
properties (Pickett and White 1985):
Magnitude --:
 Intensity - Severity -Frequency -Recurrence interval --
Predictability -- related to the variation in frequency or recurrence interval (and perhaps
variance in magnitude). Events with low variance are more predictable.
Synergisms -- interactions with other disturbances, chronic stresses, or other factors (e.g.,
drought, pathogens, and fire, see Knight 1987).
Contagion -Feedbacks -- some disturbances either engender or constrain others. For example, fire may
synchronize other subsequent fires in frequency as well as patch boundaries; reciprocally, lack of
fire can reinforce a system's resistance to fire.
Some of these characteristics are correlated. For example, for many (most?) disturbances,
frequency is inversely related to magnitude.
Effects of Disturbance - depends on the variable, organism, and setting being considered
-total habitat destruction/extinction
-creation of new habitat (i.e., habitat transformation [replacement])
-habitat fragmentation (which implies habitat loss, isolation of habitat remnants, and habitat
-alter local climate/microclimate, hydrology, biota (in terms of diversity, behavior, health,
fitness, and persistence)
-increase patch number, isolation, edge; decrease patch size, connectivity, interior  for more on
the consequences of these things, take my Landscape Ecology (BIOL 4330/5330) course
Certain variables may show no effect from disturbance whereas other variables may show a
pronounced effect:
e.g. Bayne and Hobson (2001) - Ovenbirds in contiguous forest vs. in a forest landscape
fragmented for agriculture - no differences in home range size, mean distance moved per day, or
maximum dispersal difference; however, males were less likely to be paired when close to edges
vs. forest interiors, and the breeding strategy (floater vs. territorial) differed in fragmented vs.
contiguous landscapes
Effects of disturbance are usually nonlinear.
Effects of disturbance may not be felt immediately (owing to time lags).
Vulnerability to Disturbance
Organisms with certain traits are especially vulnerable to disturbance:
Human Effects on Disturbance Regimes:
Human activities (e.g. agriculture, urbanization) are usually considered disturbances. In
addition, however, humans also alter natural disturbances:
Rescaling natural disturbances
Rescaling areas through biogeographic barriers
Introducing novel (unprecedented) disturbances
Homogenizing natural patterns
Environmental Variability and "Natural" Community Structure
Historically, a common perception of nature was that of an equilibrated and equilibrating
system that, although occasionally perturbed by various disturbances, still tended toward some
natural balance. Although few people still subscribe to this model, it remains to be appreciated
just how unusual such an equilibrium state might be in nature.
Sprugel (1991) reviewed several examples of systems thought to be exemplary of the
balance of nature in a "natural" state, including the African savanna, the "Big Woods" of
Minnesota, lodgepole pine landscapes of the Yellowstone area, and old-growth forests in the
Pacific Northwest. His conclusions were:
"Natural" vegetation is far less stable than it may seem to be from our human perspective; in
particular, all of the examples cited are transient or nonequilibrium over timescales measured in
life-times of the dominant organisms.
Vegetation may preserve small or transient effects for a very long time, especially in the case
of forests of long-lived trees.
"Every point in time is special" in that at any time, vegetation has some characteristics that
distinguish it from the same system at any other time.
Thus, it may be impossible (and irrelevant?) to define the "natural state of the system" for
many if not most systems.
State-shifts Implications for conservation and management activities Next lecture: what happens to communities following disturbance: succession
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