As shown by the aquatic example in Figure 2, there are numerous ways in which the host, its epiflora and epifauna can potentially interact. Some of the more important interactions are as follows. Sessile epibiota reduce their host's access to light and nutrients, and their additional mass can cause tree branches to break off or aquatic macrophytes to be torn loose by water movement. In an apparent response, some hosts actively discourage epibiota by allelopathy (the release of settlement- or growth-inhibiting chemicals) or by the shedding/sloughing of heavily fouled tissues. There is ample scope for indirect effects, the most important of which is probably the favor that many herbivorous epifauna do their host macrophyte by removing fouling epiphytes - some seaweeds are rapidly smothered by epiphytes in the absence of certain epifaunal grazers. On the other hand, direct grazing by epifauna can seriously
+ve (nutrient source, removal of competitors) -ve (grazing, destruction of host)
Figure 2 Summary of potential interactions between aquatic macrophytes, epiflora, and epifauna. A positive effect of one group of organisms on another is denoted by '+ve', and a negative effect by '-ve'. Superscripted 'S' refers to sessile epifauna.
damage the host macrophyte, especially when vulnerable tissues or microscopic life stages are targeted.
See also: Coral Reefs; Life Forms, Plants; Mutualism; Suspension Feeders; Tropical Rainforest.
Trophic Cascades: The Importance of Predators Further Reading
In aquatic ecosystems, the impact of grazing epifauna on both host macrophytes and their epiflora is often moderated by small predatory fishes (a so-called 'trophic cascade').
Several mesocosm experiments have found that fishes prevent epifaunal overgrazing of seaweeds. Interestingly, for seaweed epifaunal taxa feeding on periphyton the main effect of fish predation is to remove the larger epifaunal individuals, which actually results in higher total numbers of epifauna due to freeing up of the limiting periphyton resource for many more small individuals.
In freshwater macrophyte beds, epifauna appear to have a much greater impact on periphyton than on the host, so by reducing epifaunal densities fish promote periphyton growth and thus decrease host vigor. Fish predation on epifauna may ultimately shift shallow lakes from macro-phyte- to phytoplankton-dominated ecosystems.
Light and nutrients also strongly affect primary productivity in aquatic ecosystems, but it is not yet possible to generalize about the relative importances of bottom-up (resource) and top-down (consumer) controls on community structure.
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