Landscape Ecology
Disturbance
Terms/people:
DisturbancePickett & Whitemagnitude
Intensityfrequencyseverity
Predictabilitysynergycontagion
feedbackssuccessionfragmentation
Margaret Davispalynology“balance of nature”
climax communitysere (seral stage)shifting mosaic
intermediate disturbance hypothesis (Connell)
What is Disturbance?
Disturbance has been variously defined by ecologists, and with little consensus. One definition that will suffice for us is:
"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).
The key parts of this definition are that disturbances are discrete in time (vs. chronic stress or background environmental variability) and that they cause a notable change (a perturbation) in the state of the system.
However, not all changes are disturbances (some changes are merely natural variance):
extrinsic or intrinsic to a system -
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
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.
The probability of disturbance occurrence varies spatially, with certain areas being more susceptible (due to elevation, aspect, etc.) than other locales. In addition, the occurrence of a disturbance may make an area more (or less) susceptible to future disturbance events (e.g. fire consumes fuel, reducing probability of a future burn).
Click here for definitions and scale properties of disturbance.
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
Disturbance (of any ilk) may have one or more consequences, including:
-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)
-increase patch number, isolation, edge
-decrease patch size, connectivity, interior
-alter local climate/microclimate, hydrology, biota (in terms of diversity, behavior, health, fitness, and persistence)
Certain variables may show no effect from fragmentation 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. 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
biomass, diversity, life cycles, size of organisms, GPP increase
NPP decreases
nutrient cycles become increasingly closed (little leakage of nutrients)
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 difference species and interspecific interactions lead to changes in community composition over time; seral stages only somewhat predictable/repeatable wrt species present and their abundances; result of interspecific interactions, spatial dynamics, and history.
Succession usually thought of as a vegetative process, but animal communities also undergo succession following disturbance
some animals require certain plant spp. (to eat, nest in, etc.) - obviously affected by seral stage
Kendeigh 1948 - bird diversity increases with successional age in Michigan
Johnston and Odum 1956 - birds and old-field succession in GA
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!
So how do we short-lived humans study succession?
use a “space for time substitution” (an indirect method) = chronosequence or chronoseries
although is confounded by site properties/differences
M. Davis 1976 - U. Minnesota - pollen studies (palynology) from soil/sediment cores from the bottom of various lakes
different plant species’ pollen grains are morphologically distinctive
large-scale changes in biomes (resulting from climate change)
Click here and here for 2 examples.
stability is a matter of scale
Until Davis' work, most studies of succession were conducted within the paradigm of the inherent balance of nature, which holds that if weather flukes were absent, if random disturbances did not occur, then a quantitative knowledge of a site’s initial conditions would permit accurate prediction of climax community because certain species are consistently superior competitors than others and they “win.”
Human Effects on Disturbance Regimes
Land use practices have a variety of effects on natural disturbance regimes:
Rescaling natural disturbances by making them smaller (or larger), less frequent (or more), more or less intense;
Rescaling areas through biogeographic barriers (roads, canals, park boundaries defined by radical change in habitat), which reduces the effective size of the system;
Introducing novel (unprecedented) disturbances, chronic stresses, unnatural shape complexity or degrees of connectedness (which affect disturbance spread);
Homogenizing natural patterns through land use or by suppressing the natural processes that maintain diversity (e.g. natural "old-growth" forests as compared to managed plantations).
Example: introduction of cheatgrass (Bromus tectorum) has changed the fire regime in the Great Basin.
Environmental Variability and "Natural" Vegetation
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 scientists still subscribe to this model, the idea of the “balance of nature” is 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 the Yellowstone area, 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).
Disturbance and Landscape Equilibrium: What is "Natural"?
Turner and colleagues used a simple simulator to address the question of how we might expect systems to behave over time, given a specific disturbance regime and a particular reference area (Turner et al. 1993). They created models that allowed them to simulate system dynamics as an interplay between a simple successional trajectory and system-resetting disturbances of various temporal and spatial scales.
They noted:
Characteristic dynamics can be predicted from the relative scaling of the disturbance regime.
Disturbance-driven landscapes might be equilibrium, quasi-equilibrium, or inherently nonequilibrium (or combinations of these).
Anthropogenic influences may rescale these and change the qualitative dynamics of systems (e.g., fire suppression re-scales a fire regime).
Implications
Although managing for a stationary pattern may seem appealing, the simple fact is that in most real landscapes a stationary pattern in unlikely to be attained, and it certainly cannot be sustained over time.
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. Disturbance may result in landscape fragmentation. There are both natural and anthropogenic factors that generate landscape patterns. Anthropogenic factors are not "modern," for humans have been modifying their environment for as long as humans have existed.
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.
Andrén, H. 1994. Effects of habitat fragmentation on birds and mammals in landscapes with different proportions of suitable habitat: a review. Oikos 71:355-366.
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.
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.
Davis, M.G. 1976. Pleistocene biogeography of temperate deciduous forests. Geoscience and Man 13:13-26.
Johnston, D.W., and E.P. Odum. 1956. Breeding bird populations in relation to plant succession on the piedmont of Georgia. Ecolog 37:50-62.
Kendeigh, S.C. 1948. Bird populations and biotic communities in northern lower Michigan. Ecology 29:101-114.
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. 1997. Ecology: a Bridge between Science and Society. Sinauer, Sunderland, MA.
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.