Gradients and Landscapes

The intermediate disturbance hypothesis has been applied and tested in larger systems, in particular to spatial patterns of diversity along gradients and within landscapes. Gradients and zonation patterns often have communities dominated by different species at opposite ends with a mixture often with higher diversity at intermediate locations. The diversity pattern is consistent with the effects of intermediate disturbance, but other factors can contribute to this pattern. Gradients can result not only from unidirectional variations in disturbance but from variations in one or more environmental variables. Gradients in factors such as temperature, light, pH, nutrients, soil type, rainfall, or salinity can result in different communities at the extremes based on species tolerances with the potential for a mixture of species or an ecotone between the two. Diversity will likely be higher at intermediate sites, producing a pattern consistent with the intermediate disturbance hypothesis but not resulting from an actual gradient in disturbance. Gradients in diversity with high intermediate values are not by themselves proof of the intermediate disturbance hypothesis.

Ecological landscapes represent complex spatial variation in habitats or communities and disturbance can be an element in producing these patterns. The relationship of landscape pattern to disturbance is clearest when disturbance acts to fragment a mature homogenous system into a terrain of patches in different ages or states of recovery from disturbances of different magnitudes. Some intermediate level of disturbance should produce a landscape of greatest diversity. Given the potentially large spatial extent ofland-scapes, they can be a mixture of habitat types in which overall diversity results from habitat diversity and not disturbance or the landscape can be influenced by multiple types of disturbances that affect only some subset of habitats or fragments. If the landscape consists of habitable fragments or patches imbedded within an unsuitable matrix, such as ecological reserves within an urban area, then the intermediate disturbance hypothesis may not apply.

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