Hurricane Felix was a major disturbance – a category 5 hurricane that came ashore over northeastern Nicaragua on 4 September 2007, with sustained winds of 160 miles per hour. Hurricanes and typhoons provide often devastating illustrations of natural disturbance. (Photo: NASA image by Jeff Schmaltz, Goddard Space Flight Center)

Not so many years ago, scientists and many others 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. Resources 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 so as to include natural disturbances.

Now natural disturbances are considered non-catastrophic by many ecologists. Some individuals of most, if not all, species survive such events (Platt and Connell 2003). For natural communities, a self-sustaining "equilibrium" or "climax" state does not exist, even over a relatively large spatial scale. Moreover, the concept of "climax" states has yet to be demonstrated in the natural world (Sousa 1984). Instead, species are recognized as continually responding to changes in environments and to natural disturbances (Platt and Connell 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). 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 changes are as much a part of biological life on military installations as they are anywhere else.

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 similar 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 periodicity, 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 include be the intensity of windstorms, duration of floods, and the frequency and season of fires. The characteristics of disturbances often vary within landscapes and also may interact with landscape components, as well as prior disturbances, to influence the size of the area affected. Also, local effects may influence the intensity, patchiness, and frequency of gaps or voids on the biota 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 ecosystems. 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 in which they occur. Likewise, lightning strikes affect individual trees, but as a consequence 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 fires, hurricanes, and volcanic eruptions. Large-scale disturbances are those that affect entire landscapes and their component ecological systems (Pickett and White 1985). Some examples include disturbances created by fire, wind, ice, and flooding. Invasive species can generate large-scale disturbances. For example, grasses that easily tolerate fire, may change an ecosystem's fire frequency (Brown and Lomolino, 1998) or intensity (Platt and Gottschalk 1991). Invasives also can wreak profound disturbance on the incalculable value of biodiversity on soil, 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 are as likely to happen on a military base as elsewhere. Numerous types of disturbances occur on military lands. Those induced there by humans are primarily related to land management – forestry, grazing, use of prescribed fire – and military maneuvers.

Proceed to Next Section: Variability