Trophic structure

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Trophic structure, the partitioning of biomass between different trophic levels, is affected by both bottom-up (energy and nutrient inputs into primary producers) and top-down (predator consumption suppresses lower trophic levels) factors. Since trophic structure in the broadest sense is ultimately controlled by producer biomass and the ensuing transfer efficiency between trophic levels, both bottom-up and top-down factors represent extremes along a continuum of importance for regulatory control. Within the limits set by energy input into the system, trophic cascades, where predators indirectly benefit plants by suppressing herbivores, is clear evidence of top-down control; however, there is argument as to how widespread such processes are and whether or not they occur primarily in aquatic versus terrestrial systems. As ecosystems increase in potential primary productivity, the forces controlling biomass accumulation at each trophic level may alternate between predation and intraspecific competition for resources. Trophic structure may also differ predictably as a function of trophic position, with low-trophic-level species being primarily controlled by predation and high-trophic-level species being controlled by competition. Heterogeneity in food web structure, which can occur through antiherbivore-or predator defense, predator specialization, or omnivory or intraguild predation, can also affect trophic structure. Finally, there can be substantial effects of temporal or spatial heterogeneity, especially through the use of predator-free refuge habitats by prey.

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Encyclopedia of Ecology