The effects of natural disturbances cannot be considered in isolation. Disturbances may interact with one another, such that effects of an initial disturbance alter characteristics and effects of subsequent disturbances (Paine et al. 1998 Robertson and Platt 2001, Platt et al. 2002, Suding et al. 2004, Schroder et al. 2005). As a result, species may invade following sequences of disturbances, especially when de novo disturbances are involved (Kercher and Zedler 2004, Zedler and Kercher 2004).

Natural landscapes can be greatly affected by human-caused alterations of natural disturbance regimes and by de novo anthropogenic disturbances. Altering disturbance regimes changes the environments to which species may have become adapted. Habitat fragmentation as a result of human activity is a major cause of indirect alteration of disturbance regimes, especially those of large-scale disturbances. Fires that otherwise might have swept across large regions of the southeastern U.S., for instance, are contained in much smaller areas by a fragmented landscape (Gilliam and Platt 2006). The result may be less frequent, but more intense fires that are now less dependent on global climate patterns and more dependent on fuel accumulation (Slocum et al. 2007). Similarly, floodplain communities once linked to natural flooding cycles are in altered hydrologic regimes (Sparks 1998, Sparks et al. 1990).

Human disturbances of ecological communities may reduce standing biomass and simplify community structure and composition (Menges and Quintana-Ascencio 2003) – or, on other occasions, they may actually increase biomass by interrupting normal burning cycles. Most significantly, human disturbance regimes typically deviate from historic ecological disturbance regimes and oftentimes result in radical shifts in the ecosystem, such as the introduction of exotic species (Menges and Quintana-Ascencio 2003).

Proceed to Next Section: Military Disturbances and Associated Ecosystem Consequences