Temporal dynamics of communities: disturbance
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a. change is not synonymous with disturbance: importance of considering whether disturbance is intrinsic or extrinsic to a system
2. Properties of disturbance (Pickett and White 1985)
3. Effects of disturbance
b. nonlinear and nonimmediate effects
4. Human effects on disturbance regimes
a. what is natural?
c. implications for conservation and management
disturbance Pickett and White magnitude
frequency intensity severity
recurrence interval predictability synergisms
time lag contagion fragmentation
feedbacks intrinsic (cf. extrinsic)
Disturbance is a hideously complex and ill-defined concept.
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):
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).
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 transformation)
-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).
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.
Implications for conservation and management activities -
Next lecture: what happens to communities following disturbance: succession
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Bayne, E.M., and K.A. Hobson. 2001. Effects of habitat fragmentation on pairing success of Ovenbirds: importance of male age and floater behavior. Auk 118:380-388.
Huston, M.A. 1994. Biological Diversity: The Coexistence of Species on Changing Landscapes. Cambridge University Press, New York, NY.
Knight, D.H. 1987. Parasites, lightning, and the vegetative mosaic in wilderness landscapes. Pp. 59-83 in: Landscape Heterogeneity and Disturbance (M.G. Turner, ed.). Springer-Verlag, New York, NY.
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