All the relations not restricted to the effects of a direct transaction of matter and energy between the adjacent ecosystem components are treated as indirect. Hence, for the purpose of the forgoing sections, all types of indirect interactions mentioned above will be considered as indirect effects. However, the distinction between directly and indirectly mediated effects will be made where deemed appropriate. The terms 'relationship' and 'interaction' will be used interchangeably. Furthermore, although it is realized that for the purpose of quantitative assessment the distinction between the terms 'effect' and 'interaction' may be helpful, no such distinction has been made in this article, as in many studies addressing indirect effects these terms are used interchangeably.
The definition of indirect effects given above is very encompassing, and will include some of the effects which may fall into the category of 'direct' under a different definition. For example, it is useful to account for the distinction between those effects that are directly and indirectly mediated, since the latter ones are particularly difficult to observe, especially if the cause and effect are substantially separated in time.
The directly mediated effects have previously been regarded as direct (i.e., as regards to the properties of their propagation). Here, however, the directly mediated effects will be treated as indirect, and the definition of indirect effects will, therefore, include such effects as trophic cascades, top-down and bottom-up controls, etc. The classification of indirect effects into directly and indirectly mediated is applicable to a wide range of environmental processes and bears certain similarities with the distinction between 'interaction chains' and 'interaction modifications' earlier recognized for purely biotic relationships.
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