Landscape Ecology
Disturbance
Terms/people:
disturbancePickett & White (1985)magnitude
intensityfrequencyseverity
predictabilitysynergycontagion
feedbackssuccessionclimax community
sere (seral stage)shifting mosaiclegacy effect
intermediate disturbance hypothesis (Connell)palimpsest
“balance of nature”
There are 4 main drivers of landscape pattern:
- Abiotic template, e.g. climate, landform, soils
- Biotic factors
- Human land use
Delcourt (1987) discussed the five main ways in which humans have historically affected the Earth’s surface:
- humans have changed the relative abundances of organisms (especially trees) from forest clearing for firewood and making agricultural fields
- humans have extended or truncated the distribution of several species (including apples and olives, deliberately spread, and bald cypress, overused by Native Americans and becoming locally extinct as a result)
- opportunities were created for ruderal species to invade at the edges of human-disturbed areas
- nutrient status of soils was altered both thru ag depletion and fertilization
- landscape mosaic was altered, especially the distribution of forest vs. nonforest
- Disturbance
What is disturbance?
Disturbance has been variously defined by ecologists, with little consensus. One definition that will suffice for us is by Pickett and White (1985):
Because the pattern of disturbance affects its consequences (Zipperer 1993), it is important that we understand the role and outcome of disturbance in the study of pattern-process relationships.
Characteristics of disturbances (click here for definitions and scale properties of disturbance)
Disturbances (and their effects) can be described in terms of a few defining attributes (Pickett and White 1985):
Magnitude-
Intensity-
Severity-
Frequency-
Predictability-
Synergisms-
Contagion-
Feedbacks-
Some of these characteristics are correlated: for example, for many (most?) disturbances, frequency is inversely related to magnitude.
A key question for landscape ecology is whether spatial heterogeneity affects the occurrence and spread of disturbances (e.g. fire, hurricanes, disease, etc.).
Effects of disturbance
Disturbances may destroy or generate spatial pattern (at the same time!).
The ecological effects of disturbances are often determined by legacies (called “the ghost of land use past” by Harding et al. 1998) and residuals that remain after disturbances. These may include biological legacies and residuals (organisms/biotic structures that persist from the predisturbance landscape) or abiotic legacies and residuals (physical modifications of the environment, such as mudslides, lava flows, etc.). These legacies make it difficult to predict landscape change effects, because they are hidden/latent features. Landscapes essentially act as palimpsests.
Effects of disturbance are usually nonlinear. Example: intermediate disturbance hypothesis (Connell 1978):
What happens after disturbance: Succession
2 types: 1) primary succession –
2) secondary succession –
Succession often follows a predictable pattern, with a sequential change in which species are present and their abundances over time until reach “climax” community = self-perpetuating community, not replaced unless disturbed.
each community stage = sere
different seres possess different properties - E. Odum 1969
In many other cases, however, communities are continually dynamic and that there is no predictable pattern of succession nor any stable, equilibrial climax stage.
Currently, succession viewed as a consequence of complex biotic and abiotic interactions initiated by disturbance; life history characteristics of different species and interspecific interactions lead to changes in community composition over time; seral stages are only somewhat predictable/repeatable with respect to species presences and their abundances.
Click here for examples of succession.
Succession illustrates the importance of considering the time scale: succession is an example of an entity that is both a pattern and a process when viewed at different scales!
Environmental variability and the "balance of nature"
Historically, a common perception of nature was that of an equilibrated and equilibrating system that, although occasionally perturbed by various disturbances, tended (and self-regulated) toward some natural balance. Although few scientists still subscribe to this simplistic model, the idea of the “balance of nature” is still a strong one to the general public.
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 Yellowstone, and old-growth forests in the Pacific Northwestern United States. His conclusions were insightful:
"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 (or irrelevant?) to define the "natural state of the system" for many if not most systems.
Disturbance creates a mosaic, thereby promoting landscape diversity. And because disturbances recur over time, undisturbed areas become disturbed, disturbed areas recover, and so forth, creating a shifting mosaic. This dynamic mosaic is, in fact, a steady state (Bormann and Likens 1979)!
Although managing for a stationary pattern may seem appealing, the simple fact is that in most real landscapes a stationary pattern is unlikely to be attained, and it certainly cannot be sustained over time. Real landscapes are typically shifting mosaics of pattern.
Knowing the natural range of variability for a system can put bounds on expected dynamics -- so we can temper our expectations and react appropriately to realistic variability. This knowledge would also provide a reference against which to compare the system, when the system seems to be going "out of normal bounds" and some management intervention seems justified.
Summary
Disturbance is a term that is often used loosely and sloppily. Disturbance creates landscape patterns and affects ecological processes. Various properties of disturbance (e.g. magnitude, intensity, etc.) affect different aspects of pattern-process relationships. Disturbance is scale- and taxon-dependent. Disturbance is the norm in many areas, creating a shifting mosaic and forcing humans to re-think what is normal or undisturbed. Finally, disturbance may result in landscape fragmentation, the topic of the next lecture.
References:
Allen, T.F.H., and E.P. Wyleto. 1983. A hierarchical model for the complexity of plant communities. J. Theor. Biol. 101:529-540.
Bormann, F.H., and G.E. Likens. 1979. Pattern and Process in a Forested Ecosystem. Springer-Verlag, New York, NY.
Connell, J.H. 1978. Diversity in tropical rainforests and coral reefs. Science 199:1302-1310.
Delcourt, H.R. 1987. The impact of prehistoric agriculture and land occupation on natural vegetation. Trends in Ecology and Evolution 2:39-44.
Farina, A. 2006. Principles and Methods in Landscape Ecology: Towards a Science of Landscape. Springer, New York, NY.
Harding, J.S., E.G. Benfield, P.V. Bolstad, G.S. Helfman, and E.B.D. Jones. 1998. Stream biodiversity: the ghost of land use past. Proc. Natl Acad Sci USA 95:14843-14847.
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.
Odum, E.P. 1969. The strategy of ecosystem development. Science 164:262-270.
Pickett, S.T.A., and J.N. Thompson. 1978. Patch dynamics and the design of nature reserves. Biol. Conserv. 13:27-37.
Pickett, S.T.A., and P.S. White. 1985. The Ecology of Natural Disturbance as Patch Dynamics. Academic Press, New York, NY.
Sprugel, D.G. 1991. Disturbance, equilibrium, and environmental variability: what is 'natural' vegetation in a changing environment? Biol. Conserv. 58:1-18.
Turner, M.G., W.H. Romme, R.H. Gardner, R.V. O'Neill, and T.K. Kratz. 1993. A revised concept of landscape equilibrium: disturbance and stability on scaled landscapes. Landscape Ecol. 8:213-227.
Zipperer, W.C. 1993. Deforestation patterns and their effects on forest patches. Landscape Ecology 8:177-184.