n. An exogenous ecological factor that disrupts ecosystem function and structure.
In its natural role, fire should not be considered a disturbance that impacts ecosystems, but rather an incorporated ecological process that is as much a part of the environment as wind, flooding, soil development, erosion, predation, herbivory, carbon and nutrient cycling, and energy flow. Fire resets vegetation trajectories, sets up and maintains a dynamic mosaic of different vegetation structures and compositions, and reduces fuel accumulations. Humans have often disrupted these processes, and the result can be that fire behavior and fire effects are outside of their range of natural variation. At that point, fire is considered an exogenous disturbance factor (Sugihara and others 2006).
Traditional theories of natural disturbance classify disturbance as a major catastrophic event that must originate in the physical environment (Agee 1993). Much discussion has centered on these points, and various definitions and thresholds have been applied to distinguish disturbances from processes. White (1979) urged that the concept of disturbance not be limited to large, catastrophic events that originate from within the physical environment but also include small events and external factors. White and Pickett (1985) defined disturbance as “any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.” Fire was specified as a source of natural disturbance. Walker and Willig (1999) follow the terminology of White and Pickett (1985) and treat fire as a natural disturbance. They go on to state that disturbances originating inside the system of interest are considered to be endogenous. Fire is driven by an interplay of exogenous factors from outside of the system, such as climate and topography, and endogenous factors, such as soil and biota. In this sense, Walker and Willig (1999) consider fire to be an inherent ecological process.
Turner and Dale (1998) state that large, infrequent disturbances are difficult to define because they occur across a continuum of time and space. They propose that disturbances should have statistical distributions of extent, intensity, or duration greater than two standard deviations of the mean for the period and area of interest. Romme and others (1998) distinguish large, infrequent disturbances from small, frequent ones by a response threshold—when the force of the disturbance exceeds the capacity of internal mechanisms to resist disturbance or where new means of recovery become involved. For example, an area that burns with a very high-severity fire as a result of an unnaturally heavy accumulation of fuel would be qualitatively different from an area that burns with frequent, low-severity fires. However, not all high-severity fires cross the response threshold. Romme and others (1998) cite the example of jack pine (Pinus banksiana), an ecological equivalent of lodgepole pine that re-establishes itself after stand-replacing fires, regardless of size, through the dispersal of seed throughout the area from serotinous cones. These criteria (Turner and Dale  and Romme and others ) form a basis for separating endogenous fires from those arising from outside the environment of the ecosystem.