There have been many definitions of direct and indirect effects. Information on indirect interactions is scattered in the literature, and may appear under various terms. For example, among ecological phenomena which may (depending on the exact definition) be regarded as indirect effects are exploitative and apparent competition, facilitation, mutualism, cascading effects, tri-trophic-level interactions, higher-order interactions, interaction modification, nonadditive effects, etc.

First of all, it is important to distinguish between direct and indirect effects. Usually, the interactions between two components not involving direct transfer of energy and/or matter are viewed as indirect, while those that involve an explicit direct transaction are viewed as direct. The literature is inconsistent on the definitions of indirect effects, and one way to clarify the problem is to stress the difference between a transaction and a relation. A simple transaction between two ecosystem components is always direct since it is the transfer of matter and/or energy, whereas a relation is the qualitative type of interaction. Relations include predation, mutualism, competition, commensalism, ammensalism, etc. Hence a direct relationship is the one which is based on a direct (i.e., unmediated by another ecosystem component) transaction only. For example, the classic predation (not to be mistaken with, for example, keystone predation, indirect predation, etc.) is direct, and so is the nutrient uptake by plants, algae, and bacteria, whereas mutualism and competition are always indirect, as they result from the combination of a number of simple transactions. It is worth pointing out that the observed patterns of interrelations between ecosystem components (e.g., correlation between abundance indices) frequently result from a combination of direct and indirect effects, as each component is involved in a large number of pathways. Furthermore, if a direct relationship between two ecosystem components (say A and B) is modified by a third ecosystem component, attribute, or forcing function (the two latter notions will include, for example, such modifiers as sunlight, temperature, pH, external and internal concentrations of alternative nutrients) then the indirect relationship between the modifying agent and the first two components (i.e., A and B)

becomes superimposed upon the direct relationship between the components A and B. Consequently, the observed pattern of interrelation (e.g., correlation between the abundance data) between A and B will in this case result from the combination of direct and indirect effects.

Examples of factors known to modify the strength of density-mediated indirect interactions include differences in the specific growth rates (important, for example, for apparent competition), density dependence of the transmitting compartment, and the possibility of stochastic physical disruption. On the other hand, issues important in determining the manifested strength of the behavior-mediated indirect interactions involve ability of a focal species to detect changes in factors which matter for energetic costs and benefits of its behavior, sensitivity of its optimum behavior to these costs and benefits, and available behavioral options.

For density-mediated effects, presence and strength of indirect interactions can be determined by analyzing partial derivatives of the abundance of a species on the abundances of other (not immediately connected) species. However, indirect interaction may involve ecologically important changes other than changes in abundance, for example, demographic changes in the population structure, changes in the genotypic composition, and changes in behavior (e.g., searching rates, antipredator behaviors), morphology, biochemistry (e.g., nutrient content, toxin concentration), or physiology.

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