Interaction modification occurs when the relationship between a species pair is modified by a third species (Figure 1f). Examples include positive effects of macro-algae on zooplankton through interference with the hunting potential of fish and changing of a chemical's bioavailability due to the activity of a species, when the chemical in question is important for the functioning of another species (e.g., acids produced by one microbial population may increase bioavailability of compounds that are bound or unaccessible for another microbial population).
It is worth pointing out that 'interaction modification' is often, and quite rightly, considered as a principally different type of indirect effect. By coupling interaction modifications with other types of relationships (e.g., trophic), one may arrive at possibilities of numerous (including very complex) relationships. One of the more simple of such combinations may be exemplified (Figure 2) with an indirect effect of grazers and certain agricultural practices on the population density of foxes (Vulpes vulpes) and the rodent Marmota bobac in Eastern Europe (V. Takarsky, personal communication): lower grazing rates lead to a denser and taller grass cover, enabling more successful hunting of predators. Conversely, higher grazing rates lead to a lower grass cover, thus enhancing the detection of predators by the rodents. As a result, increase in grazing may have an indirect positive effect on the Marmota bobac population, and an indirect negative effect on the population of foxes.
It should also be noted that some of the known examples of ammensalism and commensalism do actually fit in
the description either of a simple interaction modification or interaction modification coupled with a number of tropic relationships. For instance, the bioavailability example described above has been quoted by Atlas and Bartha as an example of commensalism. If, however, the chemical in question is not nutritional, but harmful for the second species, then the relationship fits the criteria for ammensalism. In a similar vein, protocooperative and mutualistic relationships are easily envisaged from certain combinations of interaction modifications and tropic relationships.
It is worth pointing out that although the indirect relationships listed above are mainly studied in relation to pairs of biological species, they are applicable to a wider range of system components. It should also be noted that many more types of indirect effects are easily envisaged from various possible combinations between interacting compartments, and quite a few have indeed been observed in nature. For example, Menge distinguished 83 subtypes of indirect effects. However, an attempt to exemplify every possible type of indirect effects would be outside the scope of this article. The readers could easily construct, for example, many further types of indirect effects combining the most commonly studied ones depicted in Figure 1. In a real world, ecosystem components simultaneously take part in a multitude of interactions, and it is therefore appropriate to name it an interaction web. In fact, the number of possible kinds of indirect effects is likely to be limited only by the number of system components considered.
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