Within the context of the intermediate disturbance hypothesis, disturbances are any events or processes that produce mortality. Disturbances will vary in frequency, magnitude, and spatial extent and they are usually viewed as unpredictable. Frequency is the most common attribute associated with intermediate disturbances and the effects of changes in disturbance frequency are, perhaps, those most easily seen. In part, this results because the rate at which disturbances occur can be imposed directly onto the temporal change in a system's diversity. Disturbance rate translates directly into a mean age of the community and its diversity.
The magnitude of a disturbance is measured in terms of the mortality it causes which can vary between catastrophic, with complete loss of all individuals of all species, and noncatastrophic with the loss of only a few individuals and possibly no loss of species. Secondary succession is often seen as resulting from disturbances of low to intermediate magnitude. Disturbances of intermediate magnitude and frequency may reduce abundances without causing the loss of any species. The lowered abundances of some or all of the species can free resources, reduce or eliminate competition, and allow coexistence and high diversity.
Finally, disturbances can vary in spatial extent, affecting all or only certain parts of a system. Because species are usually not distributed evenly, the spatial extent and location of a disturbance can result in the complete loss of some species regardless of the magnitude or frequency of the disturbance. Spatial extent of disturbances is particularly important in patchy environments. In a patchy system, the community within each patch may develop independently and the age and diversity of communities can differ among patches as a function of their disturbance histories. Within such systems there may be two or more disturbance levels that promote the highest diversity, one for each patch which may vary with size and a mean disturbance rate for the whole system that maximizes the differences in species composition among patches and thus overall diversity. The degree to which the patches are in phase or disturbed at the same time and to the same degree will affect the impact of patchiness on intermediate disturbance.
Disturbances can result from physical or biological processes and the source of a disturbance will determine its impact on diversity. Physical disturbances are the most common ones associated with intermediate disturbance. A long list of physical phenomena including wind, flooding, waves, fire, exposure to extreme heat or cold, increased UV radiation, low dissolved oxygen or anoxia, drought, extreme pH, etc., can cause various degrees of mortality and disturbance. Although the probability of any of these occurring may vary predictably with location or season, we usually see physical disturbances as unpredictable in the degree to which they affect populations and as being fairly unselective in terms of which species are affected. Species will differ in their tolerances to any of these and other physical phenomena, but these disturbances do not target particular species. Biological disturbances most commonly result from disease, herbivory, or predation all of which may target particular species and result in selective disturbance. If the targeted species are dominants then an intermediate level of disturbance is likely to produce higher diversities by making resources available to other species. If biological disturbances cause a disproportionate loss of rare or competitively inferior species, diversity could be lowered by exacerbating the rate at which dominants monopolize resources. This is not to say that biological disturbances are all selective. For example, a grazing limpet might remove all species from all or part of rock, regardless of whether they are food items. This disturbance would not differ from a similar removal through physical scraping.
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