The eruption of Mount St. Helens in 1980 was an example of a major and intense natural disturbance. (Photo: U.S. Geological Survey)

Natural disturbances vary in duration, scale, intensity, spatial pattern, and return interval in any landscape. Thus, similar or different 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 resources manager. For example, fires can be patchy and of differing intensities. Not all individuals of a species are affected equivalently 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) is recognized to flower primarily after growing season fires (Outcalt 1994, Mulligan et al. 2002, Peet 1993, Kesler et al. 2003). 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 ecological system affected. Most 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 (clearcutting 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 de novo) is pervasive and of primary importance in natural landscapes.

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