Trophic cascades involve propagation of the effect along a vertical trophic chain consisting of three or more components connected by grazing or predation. In Figure 1c, an increase/decrease in Component 4 will lead to the decrease/increase in Component 3, increase/decrease in Component 2, and decrease/increase in Component 1. These effects are particularly well studied in aquatic food chains (see examples below), but have also been studied in terrestrial systems.
It is worth pointing out, however, that the structure of real ecosystems hardly ever fits tidily into the concepts of simple trophic levels (e.g., omnivory is widespread in nature), and trophic cascades, therefore, are often complicated by the interlinks within and among trophic levels (e.g., in terrestrial ecosystems insectivorous birds prey on predatory, herbivorous, and parasitoid insects, and the resulting effect of birds on the primary producers and their damage by herbivory may, therefore, depend on the specific species and the conditions involved). In particular, proper consideration of detritus contributions to the energy flows may prove the 'trophic cascade' simplification unsuitable, as the detritus compartment often has direct links to a number of trophic levels.
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