Stress, Disturbance and Ecological Stability
Disturbance is not necessarily negative for the stability of ecosystems or communities and it can be essential for the maintenance of a dynamic balance. Thus a boreal larch forest would develop into quite a different system without the regular influence of fires. Influences which only transiently cause deviations from the system are called "strain", those that actually change are "stress". The same interfering factors may thus have different consequences for a system. Ultimately, disturbances have different spatial dimensions (Fig. 4.1.55).
Disturbance and strain are generally seen as influences which indicate a "situation deviating from the norm" and which lead to stress situations (Guderian and Braun 1993). To what extent the system can cope with the strain depends on the type, the intensity of the disturbance, the duration and the extent (see Fig. 4.1.55). However, the stability of a system is based on its sensitivity towards disturbance and its characteristics for regeneration after a disturbance. It may be particularly resistant or able to quickly re-
Low local stability High local stability
Low global stability Low global stability
Fig. 4.1.55. Relations between different spatial scales (local, global) and stability. (Begon et al. 1999)
turn to the initial state (i.e. before the disturbance). If a system is able to cope with disturbance it is called resilient.
Disturbance may be inherent in the system, come randomly from outside, or may happen endogenously or exogenously, the latter caused naturally or anthropogenically. The failure for several years of precipitation in desert regions is an internal disturbance to the system, as is the regulation of population density based on predator-prey relationships, or on the dynamics of avalanches or earth flow of montane ecosystems. It is often difficult to distinguish which disturbance is caused by endogenous biological disturbances and which are caused by environmental influences.
Walker (1989) summarised natural disturbance in an hierarchical order. Different spatial as well as temporal scales are taken into account. The scale extends from disturbances by continental drift with a scale of millions of square kilometres and years, to climate change and fluctuations which may only affect one climatic subzone and for a few thousand years, to damage by spring tides, late frosts or extreme her-bivory effecting a few hectares over a very short time span.
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