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
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
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