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Asteroid strike

Source: Eldredge (1998).

Source: Eldredge (1998).

Figure 7.2 Interrelationship between frequencies and magnitudes of perturbations and disturbances, after White and Jentsch (2001).

events, the higher is the probability that the amount of fuel (accumulated burnable organic material) has also increased, and therefore the consequences will be higher if the fire interval has been longer. Similar interrelations can be found concerning the other significant sources of "natural" disturbances, such as volcanoes, droughts, soil erosion events, avalanches, landslides, windstorms, pests, or pathogen outbreaks. The consequences of such rare events can be enormous, and they can be compounded due to human interventions and management regimes. Further information about the hierarchical distinction of rare events included the required time for recovery (Box 7.2).

Table l. 3 Temporal characteristics of some disturbances

Example

Typical temporal scale (orders of magnitude)

Plate tectonics

~ 105 years

Climatic cycles

~ 104 years

Killing frost

~ 102 years

Drought cycles

~ 10 years

El Nino

~ 10 years

Seasonal change

1 year

Source: Di Castri and Hadley (1988), Müller (1992) and Gundersson and Holling (2002).

Source: Di Castri and Hadley (1988), Müller (1992) and Gundersson and Holling (2002).

Box 7.2 Hierarchical distinction of rare events

In Section 2.6, hierarchy theory has been introduced briefly. A key message of this concept is that under steady state conditions the slow processes with broad spatial extents provide constraints for the small-scale processes, which operate with high frequencies. When disturbances occur these hierarchies can be broken and as a consequence (as demonstrated in Section 7.5) small-scale processes can determine the developmental directions of the whole ensemble.

In Figure 7.3 disturbance events are arranged hierarchically, based on quantifications and literature reviews from Vitousek (1994) and Di Castri and Hadley (1988). Here we can also find direct interrelations between spatial and temporal characteristics, i.e., concerning the processes of natural disasters: The broader the spatial scale of a disturbance, the longer time is necessary for the recovery of the system. Furthermore, as shown in Section 7.1, we can assume that events that provoke long recovery times occur with smaller frequencies than disturbances with smaller effects.

Gigon and Grimm (1997) argue that the chain of disturbance effects can also be comprehended from a hierarchical viewpoint. The disturbing event occurs with typical spatio-temporal characteristics, and initially it mainly hits those ecosystem structures that operate on the same scales. Thereafter, an indirect effect chain starts because the internal constraints have changed abruptly. Thus, in the next step, potentially those components should be effected that operate on a lower scale than the initially changed

(continued)

Recovery time (years)

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