Introduction
Not so many years ago, most scientists and natural resource managers categorized major natural disturbances as catastrophic events disruptive of otherwise stable states (Clark 1991). Hurricanes, tsunamis, floods, and especially wildfires were thought to produce deviations from otherwise stable ecological systems—interruptions in the progression of species changes and ecosystems toward a climax, or a steady state (Cowles 1899, Clements 1937, Platt and Connell 2003).
These views have changed. Natural disturbances are now recognized as integral and necessary components of ecosystems worldwide. Resource managers who once considered disturbances as deviations from orderly succession now view them as a natural part of ecosystems. Restoration and management actions are planned to include natural disturbances.
For most ecosystems, a self-sustaining “equilibrium” or “climax” state does not occur. Instead, species are recognized as continually responding to changes in environments and to natural disturbances (Nicholson et al. 2002, Platt and Connell 2003). That is not to say that ecosystems cannot be recognized and classified into recurring units, but that ecological classifications increasingly acknowledge the role of natural disturbance in defining the unit (Comer et al. 2003). For example, we now know that fires favor species that survive fires in some life cycle stages and that are adapted for post-fire environments (Platt 1999). Different species thus may be favored under different fire regimes (e.g., Keeley and Zedler 1978, Glitzenstein et al. 1995), and recurring patterns of vegetation can reflect natural dynamics (Leitner et al. 1991). Moreover, some species may engineer disturbance, such as fires, through modification of characteristics and effects of fires, and thus these species influence species composition of ecosystems (Platt 1999). This more current thinking emphasizes the non-equilibrium nature of ecological systems—as a result of ongoing, recurrent, environmental changes, among which are disturbances. These Page 201 of 293 changes are as much a part of biological life on military installations as they are anywhere else.
Disturbance regimes
Natural disturbances vary in duration, scale, intensity, spatial pattern, and return interval in any landscape. Thus, disturbances occurring at different times and different places produce different effects on ecosystems at a landscape scale. An understanding of this is valuable for the military natural resource manager. For example, fires can be patchy and of differing intensities. Not all individuals of a species are affected in the same way by a single fire. Burning at different times of a year may affect species differently. Depending on the time between burns, some species may be able to complete their life cycles or reproduce before the next event. Survivors may be present in some, but not all, areas affected by a disturbance, and the environment may be changed in different ways in different parts of the area affected by a large-scale disturbance. Thus, diversity and heterogeneity at the landscape level are often enhanced by natural large-scale disturbances (Watt 1947, Bratton 1976, Connell 1978, Beatty 1984, Collins and Pickett 1982, Pickett and White 1985, Foster et al. 1998, Platt and Connell 2003).
Ecological disturbances, current thinking holds, are relatively discrete events that affect landscapes in disruptive ways. Each disturbance type and even successive disturbances of the same type are unlikely to affect natural landscapes in precisely the same ways. Thus, it is difficult to predict the exact effects of the next disturbance in any natural landscape. Nonetheless, if similar or different types of disturbances recur with some regularity, then a disturbance regime is produced that may generate predictable consequences. These disturbance regimes often are characterized by the type of disturbance, frequency/return interval, and seasonal timing. Examples could be the intensity of windstorms, duration of floods, or frequency and season of fires. The characteristics of disturbances often vary within landscapes and recurring vegetation types, and may interact with landscape components, like natural “fire breaks,” as well as prior disturbances, to influence the size of the area affected, as well as the intensity, patchiness, and local effects on the animal and plant life and the environment.
Disturbances often are numerous and occur at many different spatial scales. Here, we contrast disturbances at the largest and smallest scales. Disturbances at smaller scales tend not to affect landscapes or even entire patches of vegetation. These disturbances may be important, however, as a result of their combined effects over space and time. Burrowing animals can alter soil structure, for example, and over time change the substrate in ecosystems, as well as directly affect the plant communities where they occur. Likewise, lightning strikes affect individual trees, but consequently influence whole guilds of cavity-nesting birds or wood-consuming insects and their associated predators and parasites. In forested land, a fallen tree can open a gap in the canopy that might produce a sunlit microclimate on the ground below—and this could favor the growth of understory species.
At the other end of the disturbance scale are large-scale disturbances such as large wildfires, hurricanes, and volcanic eruptions. Large-scale disturbances are those that affect entire landscapes and their component ecosystems (Pickett and White 1985). Some examples include disturbances created by fire, wind, ice, and flooding. Invasive species can generate large-scale disturbances. For example, invasive grasses that bring fine fuel into the system may change an ecosystem’s fire frequency (Brown and Lomolino, 1998) or intensity (Platt and Gottschalk 1991). Invasives can also cause profound disturbance to soil—and the biodiversity it harbors—as can pollution, changes in land use, and climate change (Wall et al. in Soulé and Orians 2001).
Any of these large or small-scale disturbances can be as likely to happen on a military base as elsewhere. Numerous types of disturbances occur on military lands. Those introduced by humans are primarily related to land management—forestry, grazing, use of prescribed fire—and military maneuvers.
Variability in ecosystem dynamics
Natural disturbances vary in duration, scale, intensity, spatial pattern, and return interval in any landscape. Thus, disturbances occurring at different times and different places produce different effects on ecosystems at a landscape scale. An understanding of this is valuable for the military natural resource manager. For example, fires can be patchy and of differing intensities. Not all individuals of a species are affected in the same way by a single fire. Burning at different times of a year may affect species differently. Depending on the time between burns, some species may be able to complete their life cycles or reproduce before the next event. Survivors may be present in some, but not all, areas affected by a disturbance, and the environment may be changed in different ways in different parts of the area affected by a large-scale disturbance. Thus, diversity and heterogeneity at the landscape level are often enhanced by natural large-scale disturbances (Watt 1947, Bratton 1976, Connell 1978, Beatty 1984, Collins and Pickett 1982, Pickett and White 1985, Foster et al. 1998, Platt and Connell 2003).
Temporal heterogeneity of disturbances may be predictable or unpredictable (Platt and Connell 2003). If it is predictable, it can thus favor certain types of species. For example, large lightning-initiated fires in the southeastern U.S. tend to occur at certain times of the year and even under certain global weather patterns (Beckage et al. 2003, Slocum et al. 2007). This may favor the growth and survival of some plant species. For example, wiregrass, Aristida beyrichiana (Peet 1993, Kesler et al. 2003) is recognized to flower primarily after growing season fires (Outcalt 1994, Mulligan et al. 2002). In some cases, species may be uncommon because they thrive under certain disturbance regimes that occur rarely, but such species have mechanisms to survive the intervals between successive disturbances (e.g., Sheridan et al. 1997, Schuyler 1999, Norden and Kirkman 2004).
Ecological disturbances can also be categorized in other ways. Exogenous disturbances are external to the communities, ecosystems, or landscapes influenced by those disturbances. Most large-scale disturbances fall into this category. Endogenous or biotic disturbances are internal to the ecosystem affected. Many smaller-scale disturbances fall into this category. Both exogenous and endogenous natural disturbances can be repetitive (recurrent fires or even volcanic eruptions; beaver dams on streams) or de novo (new volcanic eruptions; an invasion of a new species that re-engineers the ecosystem). Human disturbances can be considered as either exogenous (global climate change) or endogenous (clear-cutting forests), but typically are de novo in nature. On military installations, disturbances caused by the military mission are examples of exogenous events. In summary, the role of disturbances (large- and small-scale, exogenous and endogenous; repetitive and as random rare events) is pervasive and of primary importance in natural landscapes.
Conceptual models can be a useful tool for documenting current knowledge of disturbance regimes (Box 8.3). They enable users to state current assumptions about ecosystem dynamics and to consider how changes in certain states or transitions between states may affect conditions on the ground.
Next Page: Box 8.3: Conceptual ecological models to understand ecosystem dynamics
Author
Patrick Comer, Ph.D., Chief Ecologist
NatureServe
William Platt, Professor
Louisiana State University
Steve Orzell, Botanist/Ecologist
Avon Park Air Force Range, United States Air Force
Understanding landscape and ecosystem dynamics
8.1. Understanding landscape and ecosystem dynamics
Box 8.3: Conceptual ecological models to understand ecosystem dynamics